WO2025108575A1

WO2025108575A1 - Methods of selective deprotection and synthesis of transhydrindane- skeleton-based compounds

Info

Publication number: WO2025108575A1
Application number: PCT/EP2024/054386
Authority: WO
Inventors: Florian Mauritius Erasmus HUBER
Original assignee: Roche Diagnostics GmbH
Current assignee: Roche Diagnostics GmbH
Priority date: 2023-11-21
Filing date: 2024-02-21
Publication date: 2025-05-30
Anticipated expiration: 2026-05-21

Abstract

The present invention relates to methods of selectively deprotecting a transhydrindane-skeleton-based compound comprising at least two different silyl ether groups. The present invention further relates to methods of synthesizing a Vitamin D molecule and to Vitamin D molecules obtainable by such processes. Furthermore, the present invention relates to a compound according to formula (I) comprising two different silyl ether groups, its use in the synthesis of Vitamin D molecules and to methods of producing such a compound.

Description

METHODS OF SELECTIVE DEPROTECTION AND SYNTHESIS OF TRANSHYDRINDANE- SKELETON-BASED COMPOUNDS Field of the Invention The present invention relates to methods of selectively deprotecting a transhydrindane-skeleton-based compound comprising at least two different silyl ether groups. The present invention further relates to methods of synthesizing a Vitamin D molecule and to Vitamin D molecules obtainable by such processes. Furthermore, the present invention relates to a compound according to formula (I) comprising two different silyl ether groups, its use in the synthesis of Vitamin D molecules and to methods of producing such a compound. Background of the Invention Several economically or biologically important compounds have transhydrindane- based skeletons as part of their molecular framework and several hydoxyl groups that are either important for their activity or employed during synthesis. Examples of such important compounds are steroids and derivatives thereof. Some of these derivatives can be used as pharmaceuticals, for example, as corticosteroids or analogues of Vitamin D molecules. One example of such a derivative is 24,25- dihydroxyvitamin D3 (24,25(OH)2-D3). 24,25(OH)2-D3 is a common known metabolite of Vitamin D3.24,25(OH)2-D3 has a low affinity to the Vitamin D receptor and a circulating half-life of about seven days. Although 24,25(OH)2-D3 itself seems to be biologically inactive, it is an important biomarker, for example, for chronic kidney disease (CKD) in human diagnostics (Bosworth et al., Kidney Int.2012, 82, 693-700; Dusso et al., Am. J. of Physiol. Renal Physiol.2005, 289, F8-F28). Several different routes for the synthesis of 24,25(OH)2-D3 or other Vitamin D molecules, like 24,25-dihydroxyvitamin D2, are known (Journal of Steroid Biochemistry & Molecular Biology 2010, 121, 43–45; Steroids 1989, 54(2), 145- 157; US 2011/0213168A1; Chem. Eur. J.2002, 8(12), 2747-2752; Tetrahedron Lett. 1980, 21, 5027-5028; Tetrahedron Lett. 1987, 28(15), 1685-1688; ChemBioChem 2021, 22, 2896–2900; US4344888, 1982, A; Int. J. Mol. Sci. 2021, 22, 11863). However, they have the disadvantage that they are lengthy, have a low yield and/or employ too many or too much of hazardous substances. Some have the additional disadvantage to be unsuitable for the incorporation of ¹³C or other stable isotopes in sufficient amounts. Synthesis routes toward specifically ¹³C labelled versions of 24,25(OH)₂-D3 or other Vitamin D molecules have to be short and efficient, as they involve the use precious ¹³C labelled building blocks. Thus, any shortening of an existing preparation method would increase the overall yield of the synthesis, which is especially advantageous when referring to precious ¹³C labelled building blocks. Furthermore, any shortening of a potential preparative method will safe energy, waste and costly manpower, thus reducing the price of Vitamin D molecules, which is particularly pronounced for stable isotope labelled versions of Vitamin D molecules. To date, still mainly immunoassays are used for routine testing in the clinical field. Due to the structural similarity of most Vitamin D congeners and metabolites, antibody related technologies - like immunoassays - usually exhibit a variety of cross-reactions and are not able to discriminate well enough between vitamin D3 and D2 and their respective metabolites. In addition, matrix effects and the presence of other lipids can falsify the measurements even more. The establishment of an LC-MS method for the determination of Vitamin D led to a more accurate measurement technique superior in flexibility, sensitivity, and specificity (Tai et al., Anal. Chem.2010, 82, 1942-1948). Regarding LC-MS based methods, native and stable isotopically labelled materials are necessary for referencing and calibrating the experimental methods to generate quantitative data for human diagnostics. In Vitamin D analytics, deuterated internal standards are widely used (Švarc et al., Food Chem. 2021, 357, 129588). However, labelling with deuterium poses its challenges. When situated on activated sites, deuterium can be lost from the internal standard during the assay due to an exchange with hydrogen. Such instability can alter the detected amount of the quantifier mass of the internal standard, leading to an overestimation of analyte concentrations (IsoSciences, Philadelphia, USA, 2016). In addition, the deuterated internal standard exhibits a lower lipophilicity than the corresponding analyte, leading to a slightly shorter retention time in reversed-phase chromatography systems as used in almost all LC-MS-based methods. The difference in retention time can result in a different degree of ion suppression, which in turn affects the determined content of the two analogues (Švarc et al., Food Chem. 2021, 357, 129588). The use of ¹³C labelled internal standards on the contrary avoids these problems. Exchange with ¹²C is impossible without breaking up the molecular framework of the analyte and any isotope effect is significantly smaller compared to H and D. They coelute with the target analyte and matrix effects are sufficiently lower than with deuterated stable isotope labelled internal standards (SIL-ISs), leading to a more accurate measurement (Švarc et al., Food Chem.2021, 357, 129588). All of the known synthesis routes have the disadvantage that they are lengthy, have a low yield or employ too many hazardous substances. These disadvantages are even more pronounced when employing precious materials, such as ¹³C. Some of the known synthesis routes are even unsuitable for incorporating ¹³C or incorporating sufficient ¹³C. Therefore, the provision of a synthesis route for compounds with a transhydrindane- based skeleton as a precursor, for example, for Vitamin D molecules, that is shorter than the known synthesis routes provides several advantages. The yield can be higher, less hazardous materials can be employed and/or the synthesis route can be more economic. Any shortening of a potential preparative method will safe energy, waste, and costly manpower. Transhydrindane-skeleton-based compounds comprising at least two hydroxyl groups at opposite sides of the indane structure are interesting precursor compounds for different bio-active molecules. However these hydroxyl groups, depending on the synthesis, need to react differently. The C/D-ring fragment (with respect to the steroid nomenclature, IUPAC, Nomenclature of steroids, Pure & Appl. Chem., Vol. 61, No.10, pp.1783-1822, 1989), as a precursor of Vitamin D molecules, is such a transhydrindane molecule comprising a hydroxyl group at the C-ring and at least one further hydroxyl group at the side chain attached at the opposite side (e.g. a hydroxyl group connected to each of carbon 8 and the side chain of carbon 17 with respect to steroid nomenclature). The hydroxyl group at the C-ring is further reacted to attach the A-ring fragment and thus complete the secosteroid structure, while the hydroxyl groups at the side chain need to remain protected. Silyl groups are particularly suitable protecting groups for hydroxyl groups, because they readily react with hydroxyl groups, can be easily and most often orthogonally removed and are inert to strong bases and oxidants. As is the case for the C/D-ring fragment, sometimes different hydroxyl groups on a given molecule comprising several hydroxyl groups need to be reacted differently. As the deprotection of silyl ether groups commonly is non-discriminatory, i.e. removing different silyl groups like TES and TBDMS concurrently, it is difficult to protect both hydroxyl groups with silyl ether groups. This might force the skilled person to employ only one silyl ether group to protect one hydroxyl group and another protecting group to protect the other hydroxyl group, thus losing the advantage of employing a silyl ether group for at least one hydroxyl group. One example for this can be found in Ono et al., Steroids 2006, 71, 529–540, in which a synthesis route for 1,25(OH)₂-D3 is disclosed, wherein one silyl group, TES, is first elaborately replaced with an acetate at the C-ring of the C/D-ring fragment, before using OTBDMS for hydroxyl group protection at the side chain of the D-ring. However, this requires further synthesis steps and therefore requires more energy, manpower, produces more waste, but results in less yield. Therefore, there remains a need for a suitable process for selectively deprotecting silyl ether groups from transhydrindane-skeleton-based compounds, in particular for precursors of Vitamin D molecules, such as a compound according to formula (I). SUMMARY OF THE INVENTION In the following, the present invention relates to the following items: In a first aspect, the present invention relates to a method of selectively deprotecting a transhydrindane-skeleton-based compound comprising at least two different silyl ether groups comprising the steps of: (a) providing a compound comprising at least one first silyl ether group and at least one second silyl ether group, wherein the at least one first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS, and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group; (b) selectively deprotecting the transhydrindane-skeleton-based compound wherein the TES or TMS group of the at least one first silyl ether group is replaced with hydrogen to obtain a hydroxyl group and wherein the silyl group of the at least one second silyl ether group is not replaced. In a second aspect, the present invention relates to a method of synthesizing a Vitamin D molecule comprising the steps of: (a) providing a transhydrindane-skeleton-based compound according to formula (I)

wherein R1 is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X1 is H, OH, R2 or not present, X2 is H, OH, R3 or not present, X3 is H, OH, R4 or not present, X4 is methyl, ethyl, H or not present, wherein R2, R3 and R4 are a second silyl ether group; wherein X5 comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X1 is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X5 is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X5; and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group; and an A-ring fragment; (b) selectively deprotecting the compound according to formula (I), wherein the TES or TMS group of the first silyl ether group is replaced with hydrogen to obtain a hydroxyl group, while the second silyl ether group of X₅, R₂, R₃ and/or R₄ is not replaced; and (c) reacting the A-ring fragment with the oxygen of R₁ of a derivative of the compound of formula (I) obtained in step (b) to obtain a Vitamin D molecule. In a third aspect, the invention relates to a compound according to formula (I)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁ is H, OH, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group. In a fourth aspect, the present invention relates to the use of compounds according to the second aspect for the synthesis of a Vitamin D molecule. In a fifth aspect, the present invention relates to a Vitamin D molecule obtainable, in particular obtained, from a method according to the first aspect. In a sixth aspect, the present invention relates to a method of producing a compound according to the third aspect comprising the steps of (A) providing a compound according to formula (VII)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁* is H, OH or not present, X₂* is H, OH or not present, X₃* is H, OH or not present, X₄ is methyl, ethyl, H or not present; wherein X₅* comprises an alkyl moiety, which is optionally substituted with a hydroxyl group; wherein Y is C^b or O, wherein X₁* is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅* is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; and wherein at least one hydroxyl group is connected to any one of C^b, C^c or C^d, or is comprised in X₅*; and at least one second silyl ether group triflate, wherein the second silyl ether group is selected from a group of silyl groups being different to the first silyl ether group; (B) substituting the hydrogen of each hydroxyl group of the compound according to formula (VII) with a silyl group to obtain a compound according to the third aspect. The following embodiments are provided to aid the understanding of the present invention, the true scope of which is set forth in the appended claims. It is understood that modifications can be made in the procedures set forth without departing from the spirit of the invention. DETAILED DESCRIPTION OF THE INVENTION Before the present invention is described in detail below, it is to be understood that this invention is not limited to the particular embodiments and examples described herein as these may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention which will be limited only by the appended claims. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Several documents are cited throughout the text of this specification. Each of the documents cited herein (including all patents, patent applications, scientific publications, manufacturer's specifications, instructions etc.), whether supra or infra, is hereby incorporated by reference in its entirety. In the event of a conflict between the definitions or teachings of such incorporated references and definitions or teachings recited in the present specification, the text of the present specification takes precedence. In the following, the elements of the present invention will be described. These elements are listed with specific embodiments, however, it should be understood that they may be combined in any manner and in any number to create additional embodiments. The various described examples and preferred embodiments should not be construed to limit the present invention to only the explicitly described embodiments. This description should be understood to support and encompass embodiments which combine the explicitly described embodiments with any number of the disclosed and/or preferred elements. Furthermore, any permutations and combinations of all described elements in this application should be considered disclosed by the description of the present application unless the context indicates otherwise. The following definitions and embodiments apply to the present disclosure in its entirety, especially to all aspects and embodiments of the invention. As used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents, unless the content clearly dictates otherwise. The word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The use of the alternative (e.g. “or”) should be understood to mean either one, both or any combination thereof of the alternatives. The term “and/or” should be understood to mean either one, or both of the alternatives. Percentages, concentrations, amounts and other numerical data may be expressed or presented herein in a “range” format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. As an illustration, a numerical range of "4 % to 20 %" should be interpreted to include not only the explicitly recited values of 4 % to 20 %, but to also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 4, 5, 6, 7, 8, 9, 10, … 18, 19, 20 % and sub-ranges such as from 4-10 %, 5-15 %, 10-20 %, etc. This same principle applies to ranges reciting minimal or maximal values. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described. As used herein and unless stated otherwise, it is to be understood that the term “about” is used synonymously with the term “approximately”. Illustratively and unless stated otherwise, the use of the term “about” when used in conjunction with a stated numerical value or range denotes somewhat more or somewhat less than the stated value or range, to within a range of ±15% of that stated, ±10% of that stated, ±5% of that stated, or conveniently ± 2% of that stated. Such values are thus encompassed by the scope of the claims reciting the terms “about” or “approximately”. In the context of the present invention, the term “compound” refers to a chemical substance having a specific chemical structure. In the context of the present invention, the term “compound” preferably refers to a transhydrindane-skeleton- based compound, in particular a compound according to formula (I) or (VII). Said compound may comprise two or more hydroxyl groups or silyl ether groups. The terms "sample" or "sample of interest" are used interchangeably herein, referring to a part or piece of a tissue, organ or individual, typically being smaller than such tissue, organ or individual, intended to represent the whole of the tissue, organ or individual. Upon analysis a sample provides information about the tissue status or the health or diseased status of an organ or individual. Examples of samples include but are not limited to fluid samples such as blood, serum, plasma, synovial fluid, spinal fluid, urine, saliva, and lymphatic fluid, or solid samples such as dried blood spots and tissue extracts. Further examples of samples are cell cultures or tissue cultures. The term "¹³C" refers to the stable isotope of carbon with a nucleus containing six protons and seven neutrons. The term "A-ring fragment" refers to a compound that will form the A-ring of a Vitamin D molecule, with respect to steroid-nomenclature (IUPAC, Nomenclature of steroids, Pure & Appl. Chem., Vol. 61, No. 10, pp. 1783-1822, 1989), upon attaching the A-ring fragment to a derivative of a compound according to formula (I), e.g. a compound according to formula (III) or (VI). The term "acidic solution" refers to a solution comprising an acid. Preferably an acidic solution has a pH of 7.0 or less. The term "internal standard" (ISTD) refers to a known amount of a substance which exhibits similar, preferably identical, properties as the analyte of interest when subjected to the mass spectrometric detection workflow (i.e. including any pre- treatment, enrichment and actual detection step). Although the ISTD exhibits similar properties as the analyte of interest, it is still clearly distinguishable from the analyte of interest upon detection. Exemplified, during chromatographic separation, such as gas or liquid chromatography, the ISTD has about the same elution properties, in particular retention time, as the analyte of interest from the sample. Thus, both the analyte and the ISTD enter the mass spectrometer at the same time. The ISTD however, exhibits a different molecular mass than the analyte of interest from the sample. This allows a mass spectrometric distinction between ions from the ISTD and ions from the analyte by means of their different mass/charge (m/z) ratios. Both are subject to fragmentation and provide daughter ions. These daughter ions can be distinguished by means of their m/z ratios from each other and from the respective parent ions. Consequently, a separate determination and quantification of the signals from the ISTD and the analyte can be performed. Since the ISTD has been added in known amounts, the signal intensity of the analyte from the sample can be attributed to a specific quantitative amount of the analyte. Thus, the addition of an ISTD allows for a relative comparison of the amount of analyte detected, and enables unambiguous identification and quantification of the analyte(s) of interest present in the sample when the analyte(s) reach the mass spectrometer. Typically, but not necessarily, the ISTD is a stable isotopically labelled variant (SIL-IS), e.g. comprising ²H, ¹³C, or ¹⁵N etc. label) of the analyte of interest. The term "SIL-IS" refers to stable isotope labelled internal standard. The term "stable isotope" refers to a non-radioactive isotope of an element, in particular an isotope with an atomic mass greater than the standard atomic weight, or where applicable the conventional atomic weight, as established by the Commission on Isotopic Abundances and Atomic Weights of the IUPAC. For example, ¹³C is a stable isotope of the element carbon or deuterium is a stable isotope of the element hydrogen. The terms "selective deprotection" or “selectively deprotecting” refer to a process or method of removing at least one first protecting group from a compound, which comprises at least two different protecting groups, while after the process is finished the at least one second protecting group does not react and remains attached to the compound. The term "side chain starting at C^a" refers to the group of atoms directly attached to the cyclopentyl ring of the transhydrindane skeleton of the compound of formula (I) or (VII). For example, with respect to steroid nomenclature, the side chain is attached to C¹⁷ and comprises the carbon atoms C²⁰ to C²⁷. In the present invention, the side chain starting at C^a includes C^a and the methyl group attached to it, Y, C^c, C^d and X₁ to X₅ of the compounds of formula (I), (II), (III), (VI) and (VII). The term “side chain starting at C^c” refers to a part of the group of atoms indirectly attached to the cyclopentyl ring of the transhydrindane skeleton of the compound of formula (I) or (VII) and excludes C^a, the methyl group attached to C^a and Y. For example, with respect to steroid nomenclature, the side chain is attached to C²² and comprises the carbon atoms C²³ to C²⁷. In the present invention, the side chain starting at C^c includes C^c, C^d and X₂ to X₅ of the compounds of formula (I), (II), (III), (VI) and (VII). The term "side chain starting at C^d" refers to a part of the group of atoms indirectly attached to the cyclopentyl ring of the transhydrindane skeleton of the compound of formula (I) or (VII) and excludes C^a, the methyl group attached to C^a, Y and C^c. For example, with respect to steroid nomenclature, the side chain is attached to C²³ and comprises the carbon atoms C²⁴ to C²⁷. In the present invention, the side chain starting at C^d includes C^d and X₃ to X₅ of the compounds of formula (I), (II), (III), (VI) and (VII). The terms "TBDMS" or “TBS” refer to tert-butyldimethylsilyl. The term "TBDPS" refers to tert-butyldiphenylsilyl. The term "TES" refers to triethylsilyl. The term "TIPS" refers to triisopropylsilyl. The term "TMS" refers to trimethylsilyl. The terms "OTBDMS" or “OTBS” refer to tert-butyldimethylsilyl ether. The term "OTBDPS" refers to tert-butyldiphenylsilyl ether. The term "OTES" refers to triethylsilyl ether. The term “OTIPS” refers to triisopropylsilyl ether. The term "OTMS" refers to trimethylsilyl ether. The term "transhydrindane-skeleton-based compound" refers to a compound comprising a transhydrindane and further residues attached to it. The term "v(acid):v(organic solvent):v(water)" refers to the volume ratio of acid, organic solvent and water in a solution, in particular excluding other components of the solution, e.g. salts. The term "v(AcOH):v(THF):v(water)" refers to the volume ratio of acetic acid, tetrahydrofuran and water in a solution, in particular excluding other components of the solution. The term "VDR" refers to the Vitamin D receptor. The term "Vitamin D" refers to a group of secosteroid molecules. The group includes naturally occurring molecules, e.g. cholecalciferol, and to synthetic molecules, e.g. calcipotriol. Common to all these molecules is the fission of the B-ring, with respect to steroid nomenclature, between C⁹ and C¹⁰. The term "Vitamin D molecule" refers to a molecule of the Vitamin D group of molecules. It may refer to naturally occurring molecules, e.g. cholecalciferol, and to synthetic molecules, such as Vitamin D analogues, e.g. calcipotriol. Vitamin D molecules may have various biological effects or functions: for example, calcitriol may affect blood calcium levels by increasing the uptake of calcium in the intestine or tacalcitol or calcipotriol may be used to treat psoriasis. Vitamin D molecules act through binding to Vitamin D receptor (VDR), which belongs to the nuclear receptor superfamily of transcription factors. Through this receptor Vitamin D molecules may regulate, for example, calcium and/or phosphate homeostasis or the differentiation and/or proliferation of cells. Vitamin D molecules include in particular 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3-epi-1,25- dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. The term “Mass Spectrometry” (“Mass Spec” or “MS”) or “mass spectrometric determination“ relates to an analytical technology used to identify and/or quantify compounds by their mass. MS is a methods of filtering, detecting, and measuring ions based on their mass-to-charge ratio, or "m/z". MS technology generally includes (1) ionizing the compounds to form charged compounds; and (2) detecting the charged compounds, relating the detected signals to a mass-to-charge ratio and calculating the molecular weight and intensities. The compounds may be ionized and detected by any suitable means. A "mass spectrometer" generally includes an ionizer and an ion detector. In general, one or more molecules of interest are ionized, and the ions are subsequently introduced into a mass spectrographic instrument where, due to a combination of magnetic and electric fields, the ions follow a path in space that is dependent upon mass ("m") and charge ("z"). The term "ionization" or "ionizing" refers to the process of generating an analyte ion having a net electrical charge equal to one or more electron units. Negative ions are those having a net negative charge of one or more electron units, while positive ions are those having a net positive charge of one or more electron units. The MS method may be performed either in "negative ion mode", wherein negative ions are generated and detected, or in "positive ion mode", wherein positive ions are generated and detected. Determination of an analyte, in particular via mass spectrometry, may include the identification and/or quantification of the analyte. “Tandem mass spectrometry” or “MS/MS” involves multiple steps of mass spectrometry selection, wherein fragmentation of the analyte occurs in between the stages. In a tandem mass spectrometer, ions are formed in the ion source and separated by mass-to-charge ratio in the first stage of mass spectrometry (MS1). Ions of a particular mass-to-charge ratio (precursor ions or parent ion) are selected and fragment ions (or daughter ions) are created by collision-induced dissociation, ion- molecule reaction or photodissociation. The resulting ions are then separated and detected in a second stage of mass spectrometry (MS2). Since a mass spectrometer separates and detects ions of slightly different masses, it easily distinguishes different isotopes of a given element. Mass spectrometry is thus, an important method for the accurate mass determination and characterization of analytes, including but not limited to low-molecular weight analytes, peptides, polypeptides or proteins. Its applications include the identification of proteins and their post-translational modifications, the elucidation of protein complexes, their subunits and functional interactions, as well as the global measurement of proteins in proteomics. De novo sequencing of peptides or proteins by mass spectrometry can typically be performed without prior knowledge of the amino acid sequence. Most sample workflows in MS further include sample preparation and/or enrichment steps, wherein e.g. the analyte(s) of interest are separated from the matrix using e.g. gas or liquid chromatography. Typically, for the mass spectrometry measurement, the following three steps are performed: 1. a sample comprising an analyte of interest is ionized, usually by electron bombardment or by complex formation with cations/anions, often by protonation to cations/deprotonation to anions. Ionization source include but are not limited to electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). 2. the ions are sorted and separated according to their mass and charge. For example, High-field asymmetric-waveform ion-mobility spectrometry (FAIMS) may be used as ion filter. 3. the separated ions are then detected, e.g. in multiple reaction mode (MRM), and the results are displayed on a chart. The term "electrospray ionization" or "ESI," refers to methods in which a solution is passed along a short length of capillary tube, to the end of which is applied a high positive or negative electric potential. Solution reaching the end of the tube is vaporized (nebulised) into a jet or spray of very small droplets of solution in solvent vapour. This mist of droplets flows through an evaporation chamber, which is heated slightly to prevent condensation and to evaporate solvent. As the droplets get smaller the electrical surface charge density increases until such time that the natural repulsion between like charges causes ions as well as neutral molecules to be released. The term "atmospheric pressure chemical ionization" or "APCI," refers to mass spectrometry methods that are similar to ESI; however, APCI produces ions by ion- molecule reactions that occur within a plasma at atmospheric pressure. The plasma is maintained by an electric discharge between the spray capillary and a counter electrode. Then ions are typically extracted into the mass analyser by use of a set of differentially pumped skimmer stages. A counterflow of dry and preheated nitrogen gas may be used to improve removal of solvent. The gas-phase ionization in APCI can be more effective than ESI for analysing less-polar entity. "High-field asymmetric-waveform ion-mobility spectrometry (FAIMS)" is an atmospheric pressure ion mobility technique that separates gas-phase ions by their behaviour in strong and weak electric fields. "Multiple reaction mode" or "MRM" is a detection mode for a MS instrument in which a precursor ion and one or more fragment ions are selectively detected. Mass spectrometric determination may be combined with additional analytical methods including chromatographic methods such as gas chromatography (GC), liquid chromatography (LC), particularly HPLC, and/or ion mobility-based separation techniques. In the context of the present disclosure, the terms “analyte”, “analyte molecule”, or “analyte(s) of interest” are used interchangeably referring to the chemical species to be analysed via mass spectrometry. Chemical species suitable to be analysed via mass spectrometry, i.e. analytes, can be any kind of molecule present in a living organism, include but are not limited to nucleic acid (e.g. DNA, mRNA, miRNA, rRNA etc.), amino acids, peptides, proteins (e.g. cell surface receptor, cytosolic protein etc.), metabolite or hormones (e.g. testosterone, estrogen, estradiol, etc.), fatty acids, lipids, carbohydrates, steroids, ketosteroids, secosteroids, molecules characteristic of a certain modification of another molecule (e.g. sugar moieties or phosphoryl residues on proteins, methyl residues on genomic DNA) or a substance that has been internalized by the organism (e.g. therapeutic drugs, drugs of abuse, toxins, etc.) or a metabolite of such a substance. Such analyte may serve as a biomarker. In the context of present invention, the term “biomarker” refers to a substance within a biological system that is used as an indicator of a biological state of said system. For example, an analyte refers to a Vitamin D molecule, preferably to a Vitamin D2 or Vitamin D3 molecule. The term "chromatography" refers to a process in which a chemical mixture carried by a liquid or gas is separated into components as a result of differential distribution of the chemical entities as they flow around or over a stationary liquid or solid phase. The term “liquid chromatography” or "LC" refers to a process of selective retardation of one or more components of a fluid solution as the fluid uniformly percolates through a column of a finely divided substance, or through capillary passageways. The retardation results from the distribution of the components of the mixture between one or more stationary phases and the bulk fluid, (i.e. mobile phase), as this fluid moves relative to the stationary phase(s). Methods in which the stationary phase is more polar than the mobile phase (e.g. toluene as the mobile phase, silica as the stationary phase) are termed normal phase liquid chromatography (NPLC) and methods in which the stationary phase is less polar than the mobile phase (e.g. water- methanol mixture as the mobile phase and C18 (octadecylsilyl) as the stationary phase) is termed reversed phase liquid chromatography (RPLC). "High performance liquid chromatography" or "HPLC" refers to a method of liquid chromatography in which the degree of separation is increased by forcing the mobile phase under pressure through a stationary phase, typically a densely packed column. Typically, the column is packed with a stationary phase composed of irregularly or spherically shaped particles, a porous monolithic layer, or a porous membrane. HPLC is historically divided into two different sub-classes based on the polarity of the mobile and stationary phases. Methods in which the stationary phase is more polar than the mobile phase (e.g. toluene as the mobile phase, silica as the stationary phase) are termed normal phase liquid chromatography (NPLC) and the opposite (e.g. water-methanol mixture as the mobile phase and C18 (octadecylsilyl) as the stationary phase) is termed reversed phase liquid chromatography (RPLC). Micro LC refers to a HPLC method using a column having a norrow inner column diameter, typically below 1 mm, e.g. about 0.5 mm. “Ultra high performance liquid chromatography" or “UHPLC” refers to a HPLC method using a pressure of 120 MPa. Rapid LC refers to an LC method using a column having an inner diameter as mentioned above, with a short length <2 cm, e.g. 1 cm, applying a flow rate as mentioned above and with a pressure as mentioned above (Micro LC, UHPLC). The short Rapid LC protocol includes a trapping / wash / elution step using a single analytical column and realizes LC in a very short time <1 min. Further well-known LC modi include “hydrophilic interaction chromatography” (HILIC), size-exclusion LC, ion exchange LC, and affinity LC. LC separation may be single-channel LC or multi-channel LC comprising a plurality of LC channels arranged in parallel. In LC analytes may be separated according to their polarity or log P value, size or affinity, as generally known to the skilled person. The term “fragmentation” refers to the dissociation of a single molecule into two or more separate molecules. As used herein, the term fragmentation refers to a specific fragmentation event, wherein the breaking point in the parent molecule at which the fragmentation event takes place is well defined, and wherein the two or more daughter molecules resulting from the fragmentation event are well characterized. It is well-known to the skilled person how to determine the breaking point of a parent molecule as well as the two or more resulting daughter molecules. The resulting daughter molecules may be stable or may dissociate in subsequent fragmentation events. Exemplified, in case a parent molecule undergoing fragmentation comprises a N-benzylpyridinium unit, the skilled person is able to determine based on the overall structure of the molecule whether the pyridinium unit will fragment to release a benzyl entity or would be released completely from the parent molecule, i.e the resulting daughter molecules would either be a benzyl molecule and a parent molecule lacking of benzyl. Fragmentation may occur via collision-induced dissociation (CID), electron-capture dissociation (ECD), electron-transfer dissociation (ETD), negative electron-transfer dissociation (NETD), electron- detachment dissociation (EDD), photodissociation, particularly infrared multiphoton dissociation (IRMPD) and blackbody infrared radiative dissociation (BIRD), surface-induced dissociation (SID), Higher-energy C-trap dissociation (HCD) or charge remote fragmentation. A "kit" is any manufacture (e.g. a package or container) comprising at least one reagent, e.g. a medicament for treatment of a disorder, or a probe for specifically detecting a biomarker gene or protein of the invention. The kit is preferably promoted, distributed, or sold as a unit for performing the methods of the present invention. Typically, a kit may further comprise carrier means being compartmentalized to receive in close confinement one or more container means such as vials, tubes, and the like. In particular, each of the container means comprises one of the separate elements to be used in the method of the first aspect. Kits may further comprise one or more other reagents including but not limited to reaction catalyst. Kits may further comprise one or more other containers comprising further materials including but not limited to buffers, diluents, filters, needles, syringes, and package inserts with instructions for use. A label may be present on the container to indicate that the composition is used for a specific application and may also indicate directions for either in vivo or in vitro use. The computer program code may be provided on a data storage medium or device such as an optical storage medium (e.g. a Compact Disc) or directly on a computer or data processing device. Moreover, the kit may, comprise standard amounts for the biomarkers as described elsewhere herein for calibration purposes. The terms “replacing with hydrogen” or “replaced with hydrogen” refer in particular to substituting a particular residue with hydrogen. In the context of the present invention, this refers to replacing a silyl group, which forms a silyl ether with oxygen, by hydrogen and thereby forming a hydroxyl group. The terms “not replaced” or “not replaced with hydrogen” refer to the fact that a specific residue, such as a silyl group, is not replaced with another residue, in particular with hydrogen, during a specific step, e.g. a situation in which a single processing step takes place, e.g. the compound reacts with another compound or an incubation for a specific time or a drying step with the purpose of changing a physical state in which the compound is in. Particularly, it refers to a situation wherein a specific residue of the compound does not react and no other residue, in particular hydrogen, replaces that specific residue. Other residues of a compound might still be replaced with hydrogen. The specific residue might be replaced with another residue, in particular hydrogen, in a subsequent step nonetheless. The term “secosteroid” refers to a steroid with a fission of a ring - “seco” derives from Latin secare “to cut”. The numbering of the cut of the ring follows IUPAC nomenclature of steroids. For example, the following molecule is a secosteroid with a cut of the A-ring between carbon atom 2 and 3 and is therefore called 2,3-seco-5α- cholestane .

Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. In a first aspect, the present invention relates to a method of selectively deprotecting a transhydrindane-skeleton-based compound comprising at least two different silyl ether groups comprising the steps of: (a) providing a compound comprising at least one first silyl ether group and at least one second silyl ether group, wherein the at least one first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS, and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group; (b) selectively deprotecting the transhydrindane-skeleton-based compound wherein the TES or TMS group of the at least one first silyl ether group is replaced with hydrogen to obtain a hydroxyl group and wherein the silyl group of the at least one second silyl ether group is not replaced, in particular with hydrogen. In embodiments of the present invention, the transhydrindane-skeleton-based compound comprising at least two different silyl ether groups is a steroid, a ketosteroid, a secosteroid or a precursor thereof, preferably a secosteroid or a precursor thereof. In embodiments of the present invention, the transhydrindane-skeleton-based compound comprising at least two different silyl ether groups is a compound according to formula (I)

wherein R₁ is a first silyl ether group; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; and wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅. In embodiments of the present invention, step (b) is a step of selectively deprotecting the silyl ether group of R₁ by replacing the TES or TMS group with hydrogen to obtain a hydroxyl group, while the second silyl ether group of X₅, R₂, R₃ and/or R₄ is not replaced, in particular with hydrogen. In embodiments of the present invention, the silyl group of the at least one second silyl ether group is selected from the group consisting of TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t- butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl and Tris(trimethylsilyl)silyl. In embodiments of the present invention, the silyl group of the at least one second silyl ether group is selected from the group consisting of TBDMS, TBDPS, TIPS. In embodiments of the present invention, the silyl group of the at least one second silyl ether group is TBDMS. In a second aspect, the present invention relates to a method of synthesizing a Vitamin D molecule comprising the steps of: (a) providing a transhydrindane-skeleton-based compound according to formula (I)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein the at least one second silyl ether group comprises a silyl group which is selected from another group of silyl groups compared to the first silyl ether group; and an A-ring fragment; (b) selectively deprotecting the compound according to formula (I), wherein the TES or TMS group of the first silyl ether group is replaced with hydrogen to obtain a hydroxyl group, while the second silyl ether groups of X₅, R₂, R₃ and/or R₄ are not replaced, in particular with hydrogen; and (c) reacting the A-ring fragment with the oxygen of R₁ of a derivative of the compound of formula (I) obtained in step (b) to obtain a Vitamin D molecule. All embodiments mentioned for the first aspect of the invention apply for the second aspect of the invention and vice versa. In embodiments of the present invention, the Vitamin D molecule is selected from the group consisting of 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25- dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi-1,25- dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25- dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3-epi- 1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. In embodiments of the present invention, the Vitamin D molecule is 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi- 1,25-dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3. In embodiments of the present invention, the Vitamin D molecule is 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25- dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3 or (24R)-24,25-dihydroxyvitamin D3. In embodiments of the present invention, the Vitamin D molecule is 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3 or (24R)-24,25- dihydroxyvitamin D3. In embodiments of the present invention, X₁ is H, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, ethyl, H or not present. In embodiments of the present invention, X₁ is H, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, H or not present. In embodiments of the present invention, X₁ is H, R₂ or not present. In embodiments of the present invention, X₁ is H or not present. In embodiments of the present invention, X₁ is H. In embodiments of the present invention, X₁ is not present. In embodiments of the present invention, X₂ is H, R₃ or not present. In embodiments of the present invention, X₁ is H or not present. In embodiments of the present invention, X₂ is H or R_3. In embodiments of the present invention, X₂ is H. In embodiments of the present invention, X₂ is not present. In embodiments of the present invention, X₃ is H, R₄ or not present. In embodiments of the present invention, X₃ is H or R_4. In embodiments of the present invention, X₃ is R_4. In embodiments of the present invention, X₃ is H. In embodiments of the present invention, X₄ is methyl, H or not present. In embodiments of the present invention, X₄ is methyl or H. In embodiments of the present invention, X₄ is methyl. In embodiments of the present invention, X₄ is ethyl. In embodiments of the present invention, X₄ is H. In embodiments of the present invention, the alkyl moiety of X₅ is a linear alkyl, branched alkyl or cycloalkyl moiety. In embodiments of the present invention, the alkyl moiety of X₅ is a linear alkyl or branched alkyl moiety. In embodiments of the present invention, the alkyl moiety of X₅ is a substituted or unsubstituted alkyl. Preferably, the alkyl moiety is substituted with an alkyl, heteroalkyl, alkenyl, heteroalkenyl, aryl, acyl, alkoxy, acyloxy, aryloxy, cycloalkyl or heterocycloalkyl moiety. In embodiments of the present invention, Y is C^b. In embodiments of the present invention, the bond between C^a and C^b is a single, double or triple bond. Preferably, the bond between C^a and C^b is a single or double bond. Most preferably, the bond between C^a and C^b is a single bond. In embodiments of the present invention, the bond between C^b and C^c is a single, double or triple bond. Preferably, the bond between C^b and C^c is a single or double bond. In particular, the bond between C^b and C^c is a single bond. In particular, the bond between C^b and C^c is a double bond. In embodiments of the present invention, the bond between C^c and C^d is a single, double or triple bond. Preferably, bond between C^c and C^d is a single or double bond. Most preferably, the bond between C^c and C^d is a single bond. In embodiments of the present invention, the bond between C^d and X₅ is a single or double bond. Preferably, the bond between C^d and X₅ is a single bond. In embodiments of the present invention, X₅ is cyclopropyl, 1-methylethyl, 1-hydroxyl-1-methylethyl, 1-hydroxyl-1-trichloromethyl-2,2,2-trichloroethyl, or 2-hydroxyl-2-ethylbutyl, wherein in case a hydroxyl group is present in X₅, the hydrogen is replaced by a silyl group forming a second silyl ether group. Preferably, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is H or not present, X₂ is H, R₃ or not present, wherein R₃ is a second silyl ether group, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is methyl or H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single or double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is H, X₂ is H or R₃, wherein R₃ is a second silyl ether group, X₃ is H or R₄, wherein R₄ is a second silyl ether group , X₄ is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is H, X₂ is R₃, wherein R₃ is a second silyl ether group, X₃ is H, X₄ is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is H, X₂ is H, X₃ is R₄, wherein R₄ is a second silyl ether group, X₄ is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. Preferably, the stereocenter of C^d is in the R or S configuration, more preferably in the R configuration. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is H, X₂ is H, X₃ is H, X₄ is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is not present, X₂ is not present, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is methyl; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is not present, X₂ is not present, X₃ is R₄, wherein R₄ is a second silyl ether group, X₄ is methyl; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. Preferably, wherein the stereocenter of C^d is in the R or S configuration, more preferably in the R configuration. In embodiments of the present invention, X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; X₁ is not present, X₂ is not present, X₃ is H, X₄ is methyl; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. Preferably, wherein the stereocenter of C^d is in the R or S configuration, more preferably in the R configuration. In embodiments of the present invention, R₁ is OTES. In embodiments of the present invention, the compound comprises exactly one first silyl ether group. In embodiments of the present invention, the compound comprises exactly one, two or three second silyl ether groups. In embodiments of the present invention, the compound comprises exactly one first silyl ether group and exactly one second silyl ether group. In embodiments of the present invention, the compound comprises exactly one first silyl ether group and exactly two second silyl ether groups. In embodiments of the present invention, the compound is labelled with a stable isotope. A compound of the present invention that is labelled with a stable isotope has the advantage that it can be used as an internal standard for mass spectrometric determination of an analyte (SIL-IS). The labelling is helpful in that it enables the skilled person to distinguish the standard and the analyte when analysing a mixture of standard and analyte, because, besides the isotopic label, standard and analyte have the same structure. The number of atoms in the standard, which are stable isotopes, depends on the ability to distinguish between standard and label. However, it is usually beneficial to not have more stable isotopes in the standard than necessary to distinguish between standard and analyte during mass spectrometry, because the signal intensity reduces with more heavier isotopes at the same concentration of standard. In embodiments of the present invention, at least one hydrogen atom of the compound is deuterium. In embodiments of the present invention, at least one, preferably 2, 3, 4 or 5, hydrogen atoms of the side chain starting at C^a of the compound according to formula (I) are deuterium. In embodiments of the present invention, at least one carbon atom of the compound is ¹³C.¹³C, as compared to deuterium, has the advantage that it is part of the carbon atom skeleton of a molecule and is not easily replaced by unwanted interactions with a solvent or agent, for example, water, because the deuterium might be exchanged for hydrogen. Therefore, such a labelled compound is more stable in solution and in any other step of the mass spectrometry workflow in which proton or deuterium atoms can be abstracted or exchanged. In embodiments of the present invention, at least one, preferably all, carbon atoms of the side chain starting at C^a of the compound according to formula (I) is ¹³C. Preferably, at least three carbon atoms are ¹³C. Alternatively, one, two, three, four, five or six carbon atoms are ¹³C. In embodiments of the present invention, at least one carbon atom of the side chain starting at C^c of the compound according to formula (I) is ¹³C. Preferably, at least three carbon atoms are ¹³C. Alternatively, one, two, three, four, five or six carbon atoms are ¹³C. In embodiments of the present invention, all carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C. In embodiments of the present invention, C^c, C^d, all carbon atoms of X₅ and, in case X₄ is an alkyl, all carbon atoms of X₄ are ¹³C. In embodiments of the present invention, C^d, all carbon atoms of X₅ and, in case X₄ is an alkyl, all carbon atoms of X₄ are ¹³C. In embodiments of the present invention, C^c, C^d and all carbon atoms of X₅ are ¹³C. In embodiments of the present invention, C^d and all carbon atoms of X₅ are ¹³C. In embodiments of the present invention, the compound is not labelled with a stable isotope. In embodiments of the present invention, the silyl group of the at least one second silyl ether group is not TES or TMS. In embodiments of the present invention, the silyl group of the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is not TES or TMS. In embodiments of the present invention, the silyl group in the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl. Preferably, the silyl group in the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is TBDMS, TBDPS or TIPS. More preferably, the silyl group in the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is TBDMS. In embodiments of the present invention, the silyl groups in the second silyl ether groups of X₅, R₂, R₃ and/or R₄ are the same or different, preferably the same. Preferably, the silyl groups in the second silyl ether groups of X₅, R₂, R₃ and R₄ are the same or different. Most preferably, the silyl groups in the second silyl ether groups of X₅, R₂, R₃ and R₄ are the same. In embodiments of the present invention, in step (b) the selective deprotection is carried out by reacting the compound in an acidic solution. Preferably, the acidic solution has a pH of 7.0 or less, 6.0 or less, 5.0 or less, 4.0 or less, preferably 4.0 to 3.5. In embodiments of the present invention, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an acid selected from the group consisting of H₂SiF₆, citric acid, TFA , HClO₄, HCl, HBr, HI, HF, HF*pyridine, HF*NEt₃, HF*urea, pTSA, SiF₄, BF₃*OEt₂, Zn(BF₄)₂, Dowex 50WX8, PPTS, CSA, formic acid, H₄IO₅, H₂SO₄, trichloroacetic acid, tratric acid, benzoic, acid, squaric acid, lactic acid, succinic acid, Hydrogen cyanide, hydrazoic acid, H₂CrO₄, CH₃SO₃H, CF₃SO₃H, B(OH)₃, H₃PO₄, polyphosphoric acid, Nafion-H, SbCl₅, pyridinium tribromide, Sc(OTf)₃, Ce(OTf)₄, CeCl₃, BiCl₃, Bi(OTf)₃, InCl₃, CuCl₂, i-Bu₂AlH, ZrCl₄, FeCl₃, SnCl₂, Me₂BBr, BCl₃, CAN, PdCl₂(MeCN)₂, Al₂O₃, acetic acid (AcOH) and PdO. Preferably, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an acid selected from the group consisting of acetic acid (AcOH), H₂SiF₆, citric acid, trifluoro acetic acid (TFA), HClO₄, HCl, HBr, HI, HF, HF*pyridine, HF*NEt₃, HF*urea, p-toluenesulfonic acid (pTSA), pyridinium p-toluenesulfonate (PPTS), camphorsulfonic acid (CSA), formic acid, H₂SO₄, trichloroacetic acid, CH₃SO₃H, CF₃SO₃H, H₃PO₄, Sc(OTf)₃, CeCl₃, CuCl₂, FeCl₃, Al₂O₃, and combinations thereof. In embodiments of the present invention, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising acetic acid. In embodiments of the present invention, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an organic solvent selected from the group consisting of tetrahydrofuran (THF), 1,4-dioxane, diethylether, dimethoxyethane (DME), methyl tert-butyl ether (MTBE), 2-methyl-tetrahydrofuran, toluene, benzene, ethanol, isopropyl alcohol, 1- butanol, 1-octanol, methanol, n-hexane, n-pentane, n-heptane, diglyme, dichloromethane, 1,2-dichloroethane, acetonitrile, cyclohexane, pyridine and di- isopropyl ether. Preferably, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising tetrahydrofuran (THF). In embodiments of the present invention, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising water. In embodiments of the present invention, the ratio between acid, organic solvent and water in the solution, in which step (b) is carried out, is 1-100:1-100:1, 1-50:1-50:1, 1-20:1-20:1, 1-10:1-10:1, 1-6:1-4:1, preferably 2-6:2-4:1 (v(acid):v(organic solvent):v(water)). In embodiments of the present invention, in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising acetic acid, tetrahydrofuran and water. Preferably, the ratio between acetic acid, tetrahydrofuran and water in the solution is 1-100:1-100:1, 1-50:1-50:1, 1-20:1-20:1, 1-10:1-10:1, 1-6:1-4:1 (v(AcOH):v(THF):v(water)). Preferably, the ratio between acetic acid, tetrahydrofuran and water in the solution is 2-6:2-4:1 (v(AcOH):v(THF):v(water)). Most preferably, the ratio between acetic acid, tetrahydrofuran and water in the solution is 6:4:1 (v(AcOH):v(THF):v(water)). In embodiments of the present invention, in step (b) the solution comprising the compound is heated. In embodiments of the present invention, the solution comprising the compound has a temperature of 0 to 100 °C, 0 to 70 °C, 10 to 70 °C, 20 to 70 °C, 30 to 60 °C. Preferably, the solution comprising the compound has a temperature of 45 to 55 °C. In embodiments of the present invention, in step (b) the solution comprising the compound is reacted for at least 30 min, 1 h, 5 h, 10 h. Preferably, in step (b) the solution comprising the compound is reacted for 30 min to 7 days, 30 min to 24 h, 10 h to 24 h. In embodiments of the present invention, step (b) is performed at 0.5 to 10 bar, 0.5 to 5 bar, 0.5 to 2.5 bar, 0.9 to 1.2 bar (absolute pressure). In embodiments of the present invention, step (b) is performed at atmospheric pressure. In embodiments of the present invention, in step (c) the A-ring fragment is reacted to obtain a compound according to formula (II)

wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein R₅ is a third silyl ether group, R₆ is a third silyl ether group or H, R₇ is a third silyl ether group or H and R₈ is a methylene group or H. In embodiments of the present invention, in a further step (d) any silyl groups on the compound according to formula (II) are replaced with hydrogen or 1-hydroxylpropyl. Preferably, in a further step (d) any silyl groups on the compound according to formula (II) are replaced with hydrogen. In embodiments of the present invention, in a further step (e) a Vitamin D molecule is obtained. In embodiments of the present invention, wherein step (c) comprises a step of oxidizing the hydroxyl-group at the cyclohexyl-ring of the transhydrindane skeleton of the compound of formula (I) to form a keto-group to obtain a compound according to formula (III)

In embodiments of the present invention, the A-ring fragment provided in step (a) is a compound according to formula (IV)

wherein R5 is a third silyl ether group, R6 is a third silyl ether group or H, R7 is a third silyl ether group or H and R8 is a methylene group or H; or a compound according to formula (V)

wherein R₅ is a third silyl ether group, R₆ is a third silyl ether group or H, R₇ is a third silyl ether group or H and R₈ is a methylene group or H. Preferably, the A-ring fragment provided in step (a) is a compound according to formula (IV)

wherein R5 is a third silyl ether group, R6 is a third silyl ether group or H, R7 is a third silyl ether group or H and R8 is a methylene group or H. In embodiments of the present invention, the silyl group in the third silyl ether group of R5, R6 and/or R7 is TES, TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TES, TBDPS or TIPS or TBDMS, preferably TES or TBDMS, preferably TBDMS. Preferably, the silyl group in the third silyl ether group of R5, R6 and R7 is TES, TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl. Preferably, the silyl group in the third silyl ether group of R5, R6 and/or R7 is TES, TBDPS or TIPS or TBDMS. More preferably, the silyl group in the third silyl ether group of R5, R6 and/or R7 is TES or TBDMS. Most preferably, the silyl group in the third silyl ether group of R5, R6 and/or R7 is TBDMS. In embodiments of the present invention, the third silyl ether groups of R5, R6 and/or R7 are the same or different. Preferably, the third silyl ether groups of R5, R6 and/or R₇ are the same. Most preferably, the third silyl ether groups of R₅, R₆ and R₇ are the same. In embodiments of the present invention, the silyl group in the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇, in particular X₅, R₂, R₃, R₄, R₅, R₆ and R₇, is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di- t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl. Preferably, the silyl group in the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇, in particular X₅, R₂, R₃, R₄, R₅, R₆ and R₇, is TBDMS, TBDPS or TIPS. More preferably, the silyl group in the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇, in particular X₅, R₂, R₃, R₄, R₅, R₆ and R₇, is TBDMS. In embodiments of the present invention, the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇ are the same or different. Preferably, the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇, in particular X₅, R₂, R₃, R₄, R₅, R₆ and R₇, are the same. Most preferably, the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and R₇ are the same. In embodiments of the present invention, R₆ is H and R₈ is a methylene group. In embodiments of the present invention, R₅ is a third silyl ether group, R₆ is H, R₇ is a third silyl ether group and R₈ is a methylene group. In embodiments of the present invention, R₅ is a third silyl ether group, R₆ is H, R₇ is H and R₈ is a methylene group. In embodiments of the present invention, step (c) comprises a step of: (i) reacting a compound according to formula (III) with a compound according to formula (IV) to obtain a compound according to formula (II).

In embodiments of the present invention, step (c) comprises the steps of: (i) substituting the keto-group with a methyl-halide-group to obtain a compound according to formula (VI)

wherein X₆ is Br, F, Cl or I, preferably X₆ is Br or F, most preferably X₆ is Br; (ii) reacting a compound according to formula (VI) with a compound according to formula (V) to obtain a compound according to formula (II). In embodiments of the present invention, the compound according to formula (I) is

In embodiments of the present invention, wherein the compound according to formula (II) is

In embodiments of the present invention, the compound according to formula (III) is

In embodiments of the present invention, the compound according to formula (I) is

wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (I) are ¹³C .

, wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (I) are ¹³C .

wherein C^c, C^d and the carbon atoms of the 1-oxy-1-methylethyl residue are ¹³C. In embodiments of the present invention, the compound according to formula (I) is

wherein C^c, C^d and the carbon atoms of the 1-oxy-1-methylethyl residue are ¹³C.

wherein C^d, the methyl group attached to C^d and the carbon atoms of the 1-oxy-1- methylethyl residue are ¹³C. In embodiments of the present invention, the compound according to formula (I) is

wherein C^d, the methyl group attached to C^d and the carbon atoms of the 1-oxy-1- methylethyl residue are ¹³C. In embodiments of the present invention, the compound according to formula (IV)

or the compound according to formula (V) is

Preferably, the compound according to formula (IV) is

In embodiments of the present invention, the compound according to formula (VI) is

In a third aspect, the invention relates to a compound according to formula (I)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁ is H, OH, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein the at least one second silyl ether group comprises a silyl group which is selected from another group of silyl groups compared to the first silyl ether group. All embodiments mentioned for the first and/or second aspect of the invention apply for the third aspect of the invention and vice versa. In particular, every embodiment referring to a compound according to formula (I) apply as well to the compound of the third aspect. In embodiments of the present invention, the compound is for use in the synthesis of a Vitamin D molecule. In a fourth aspect, the present invention relates to the use of compounds according to the second aspect for the synthesis of a Vitamin D molecule. All embodiments mentioned for the first and/or second and/or third aspect of the invention apply for the fourth aspect of the invention and vice versa. In a fifth aspect, the present invention relates to a Vitamin D molecule obtainable, in particular obtained, from a method according to the first aspect. All embodiments mentioned for the first and/or second and/or third and/or fourth aspect of the invention apply for the fifth aspect of the invention and vice versa. In embodiments of the present invention, the Vitamin D molecule is for mass spectrometric determination of an analyte. Preferably, such a Vitamin D molecule is obtainable, in particular obtained, by using a compound labelled with a stable isotope. Preferably, at least one hydrogen atom of the compound is deuterium or at least one carbon atom of the compound is ¹³C. More preferably, at least one, preferably 2, 3, 4 or 5, hydrogen atoms of the side chain starting at C^a of the compound according to formula (I) are deuterium or at least one, preferably all, carbon atoms of the side chain starting at C^a of the compound according to formula (I) are ¹³C. More preferably, all carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C. In embodiments of the present invention, the Vitamin D molecule is for use as an internal standard for mass spectrometric determination of an analyte. In embodiments of the present invention, the Vitamin D molecule is for use in calibrating an assay, in particular mass spectrometry assay, for determination of an analyte. In embodiments of the present invention, the analyte is selected from the group consisting of 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25- dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25- dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi-1,25- dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25- dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3-epi- 1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. In embodiments of the present invention, the analyte is 25-hydroxyvitamin D2, 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi- 1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25- dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25- dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3. Preferably, the analyte is 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25- dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3 or (24R)-24,25-dihydroxyvitamin D3. In a sixth aspect, the present invention relates to a method of producing a compound according to the third aspect comprising the steps of (A) providing a compound according to formula (VII)

wherein R1 is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X1* is H, OH or not present, X2* is H, OH or not present, X3* is H, OH or not present, X4* is methyl, ethyl, H or not present; wherein X5* comprises an alkyl moiety, which is optionally substituted with a hydroxyl group; wherein Y is C^b or O, wherein X1* is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X5* is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein at least one hydroxyl group is connected to any one of C^b, C^c or C^d, or is comprised in X5*; and at least one second silyl ether group triflate, wherein the second silyl ether group is selected from a group of silyl groups being different to the first silyl ether group; (B) substituting the hydrogen of each hydroxyl group of the compound according to formula (VII) with a silyl group to obtain a compound according to the third aspect. All embodiments mentioned for the first and/or second and/or third and/or fourth and/or fifth aspect of the invention apply for the sixth aspect of the invention and vice versa. In embodiments of the present invention, X₁* is H, OH or not present, X₂* is H, OH or not present, X₃* is H, OH or not present, X₄* is methyl, H or not present. In embodiments of the present invention, X₁* is H or not present. In embodiments of the present invention, X₁* is H. In embodiments of the present invention, X₁* is not present. In embodiments of the present invention, X₂* is H or OH. In embodiments of the present invention, X₂* is not present. In embodiments of the present invention, X₂* is H. In embodiments of the present invention, X₂* is OH. In embodiments of the present invention, X₃* is H or OH. In embodiments of the present invention, X₃* is OH. In embodiments of the present invention, X₃* is H. In embodiments of the present invention, X₄* is methyl, H or not present. In embodiments of the present invention, X₄* is methyl or H. In embodiments of the present invention, X₄* is H. In embodiments of the present invention, X₄* is methyl. In embodiments of the present invention, X₄* is ethyl. In embodiments of the present invention, the alkyl moiety of X₅* is a linear alkyl, branched alkyl or cycloalkyl moiety. In embodiments of the present invention, the alkyl moiety of X₅* is a substituted or unsubstituted alkyl moiety. Preferably, the alkyl moiety is substituted with an alkyl, heteroalkyl, alkenyl, heteroalkenyl, aryl, acyl, alkoxy, acyloxy, aryloxy, cycloalkyl or heterocycloalkyl moiety. In embodiments of the present invention, X₅* is cyclopropyl, 1-methylethyl, 1-hydroxyl-1-methylethyl, 1-hydroxyl-1-trichloromethyl-2,2,2-trichloroethyl or 2-hydroxyl-2-ethylbutyl. Preferably, X₅* is 1-hydroxyl-1-methylethyl. In embodiments of the present invention, the bond between C^a and C^b is a single, double or triple bond. Preferably, the bond between C^a and C^b is a single or double bond. Most preferably, the bond between C^a and C^b is a single bond. In embodiments of the present invention, the bond between C^b and C^c is a single, double or triple bond. Preferably, the bond between C^b and C^c is a single or double bond. In particular, the bond between C^b and C^c is a single bond. In particular, the bond between C^b and C^c is a double bond. In embodiments of the present invention, the bond between C^c and C^d is a single, double or triple bond. Preferably, bond between C^c and C^d is a single or double bond. Most preferably, the bond between C^c and C^d is a single bond. In embodiments of the present invention, the bond between C^d and X₅* is a single or double bond. Preferably, the bond between C^d and X₅* is a single bond. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is H or not present, X₂* is H, OH or not present, X₃* is H or OH, X₄* is methyl or H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single or double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is H, X₂* is H or OH, X₃* is H or OH, X₄* is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is H, X₂* is H, X₃* is OH, X₄* is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. Preferably, the stereocenter of C^d is in the R or S configuration, more preferably in the R configuration. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is H, X₂* is OH, X₃* is H, X₄* is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is H, X₂* is H, X₃* is H, X₄* is H; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is not present, X₂* is not present, X₃* is OH, X₄* is methyl; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is not present, X₂* is not present, X₃* is OH, X₄* is methyl; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. Preferably, the stereocenter of C^d is in the R or S configuration, more preferably in the R configuration. In embodiments of the present invention, X₅* is 1-hydroxyl-1-methylethyl; X₁* is not present, X₂* is not present, X₃* is H, X₄* is methyl; Y is C^b; and the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. Preferably, the stereocenter of C^d is in the R or S configuration, more preferably in the R configuration. In embodiments of the present invention, R₁ is OTES. In embodiments of the present invention, the compound is labelled with a stable isotope. In embodiments of the present invention, at least one hydrogen atom of the compound according to formula (VII) is deuterium. Preferably, at least one, preferably 2, 3, 4 or 5, hydrogen atoms of the side chain starting at C^a of the compound according to formula (VII) are deuterium. In embodiments of the present invention, at least one carbon atom of the compound according to formula (VII) is ¹³C. In embodiments of the present invention, at least one, preferably all, carbon atoms of the side chain starting at C^a of the compound according to formula (VII) is ¹³C. Preferably, at least three carbon atoms are ¹³C. Alternatively, one, two, three, four, five or six carbon atoms are ¹³C. In embodiments of the present invention, at least one carbon atom of the side chain starting at C^c of the compound according to formula (VII) is ¹³C. Preferably, at least three carbon atoms are ¹³C. Alternatively, one, two, three, four, five or six carbon atoms are ¹³C. In embodiments of the present invention, all carbon atoms of the side chain starting at C^c of the compound according to formula (VII) are ¹³C. Preferably, at least three carbon atoms are ¹³C. Alternatively, one, two, three, four, five or six carbon atoms are ¹³C. In embodiments of the present invention, C^c, C^d, all carbon atoms of X₅* and, in case X₄* is an alkyl, all carbon atoms of X₄* are ¹³C. In embodiments of the present invention, C^d, all carbon atoms of X₅* and, in case X₄* is an alkyl, all carbon atoms of X₄* are ¹³C. In embodiments of the present invention, C^c, C^d and all carbon atoms of X₅* are ¹³C. In embodiments of the present invention, C^d and all carbon atoms of X₅* are ¹³C. In embodiments of the present invention, the compound of formula (VII) is not labelled with a stable isotope. In embodiments of the present invention, the silyl group in the silyl ether triflate is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di- t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl. In embodiments of the present invention, the silyl group in the silyl ether triflate is TBDMS, TBDPS or TIPS. In embodiments of the present invention, the silyl group in the silyl ether triflate is TBDMS. In embodiments of the present invention, the compound according to formula (VII) is

In embodiments of the present invention, wherein the compound according to formula (VII) is

wherein all carbon atoms of the side chain starting at C^c of the compound according to formula (VII) are ¹³C or wherein all carbon atoms of the side chain starting at C^d of the compound according to formula (VII) are ¹³C. In embodiments of the present invention, wherein the compound according to formula (VII) is

wherein all carbon atoms of the side chain starting at C^c of the compound according to formula (VII) are ¹³C or wherein all carbon atoms of the side chain starting at C^d of the compound according to formula (VII) are ¹³C. In further embodiments, the present invention relates to the following aspects: 1. A method of selectively deprotecting a transhydrindane-skeleton-based compound comprising at least two different silyl ether groups comprising the steps of: (a) providing a compound comprising at least one first silyl ether group and at least one second silyl ether group, wherein the at least one first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS, and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group; (b) selectively deprotecting the transhydrindane-skeleton-based compound wherein the TES or TMS group of the at least one first silyl ether group is replaced with hydrogen to obtain a hydroxyl group and wherein the silyl group of the at least one second silyl ether group is not replaced, in particular with hydrogen. 2. The method of aspect 1, wherein the transhydrindane-skeleton-based compound comprising at least two different silyl ether groups is a compound according to formula (I):

wherein R₁ is a first silyl ether group; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; and wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and step (b) is a step of selectively deprotecting the silyl ether group of R₁ by replacing the TES or TMS group with hydrogen to obtain a hydroxyl group, while the second silyl ether group of X₅, R₂, R₃ and/or R₄ is not replaced, in particular with hydrogen. 3. The method of aspect 1 or 2, wherein the silyl group of the at least one second silyl ether group is selected from the group consisting of TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t- butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl and Tris(trimethylsilyl)silyl. 4. A method of synthesizing a Vitamin D molecule comprising the steps of: (a) providing a transhydrindane-skeleton-based compound according to formula (I)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group ; and an A-ring fragment; (b) selectively deprotecting the compound according to formula (I), wherein the TES or TMS group of the first silyl ether group is replaced with hydrogen to obtain a hydroxyl group, while the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is not replaced, in particular with hydrogen; and (c) reacting the A-ring fragment with the oxygen of R₁ of a derivative of the compound of formula (I) obtained in step (b) to obtain a Vitamin D molecule. 5. The method according to aspect 4, wherein the Vitamin D molecule is selected from the group consisting of 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25- dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi-1,25- dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25- dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3-epi- 1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. 6. The method according to aspect 4 or 5, wherein the Vitamin D molecule is 25- hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)- 24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi- 1,25-dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3, in particular 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2,(24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25- dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3 or (24R)-24,25-dihydroxyvitamin D3. 7. The method according to any one of aspects 2 to 6, wherein X₁ is H, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, H or not present. 8. The method according to any one of aspects 2 to 7, wherein the alkyl moiety of X₅ is a linear alkyl, branched alkyl or cycloalkyl moiety. 9. The method according to any one of aspects 2 to 8, wherein the alkyl moiety of X₅ is a substituted or unsubstituted alkyl moiety. 10. The method according to aspect 9, wherein the alkyl moiety is substituted with an alkyl, heteroalkyl, alkenyl, heteroalkenyl, aryl, acyl, alkoxy, acyloxy, aryloxy, cycloalkyl or heterocycloalkyl moiety. 11. The method according to any one of aspects 2 to 10, wherein X₅ is cyclopropyl, 1-methylethyl, 1-hydroxyl-1-methylethyl, 1-hydroxyl-1-trichloromethyl-2,2,2- trichloroethyl, or 2-hydroxyl-2-ethylbutyl, wherein in case a hydroxyl group is present in X₅, the hydrogen is replaced by a silyl group forming a second silyl ether group. 12. The method according to any one of aspects 2 to 11, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; whereinX₁ is H or not present, X₂ is H, R₃ or not present, wherein R₃ is a second silyl ether group, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is methyl or H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single or double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 13. The method according to any one of aspects 2 to 12, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is H or R₃, wherein R₃ is a second silyl ether group, X₃ is H or R₄, wherein R₄ is a second silyl ether group , X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 14. The method according to any one of aspects 2 to 13, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is R₃, wherein R₃ is a second silyl ether group, X₃ is H, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 15. The method according to any one of aspects 2 to 13, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is H, X₃ is R₄, wherein R₄ is a second silyl ether group, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 16. The method according to aspect 15, wherein the stereocenter of C^d is in the R or S configuration, preferably in the R configuration. 17. The method according to any one of aspects 2 to 13, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is H, X₃ is H, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 18. The method according to any one of aspects 2 to 12, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is not present, X₂ is not present, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 19. The method according to any one of aspects 2-12 or 18, wherein X₅ is 1- hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is not present, X₂ is not present, X₃ is R₄, wherein R₄ is a second silyl ether group, X₄ is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 20. The method according to any one of aspects 2-12 or 18, wherein X₅ is 1-hydroxyl-1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is not present, X₂ is not present, X₃ is H, X₄ is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 21. The method according to aspect 19 or 20, wherein the stereocenter of C^d is in the R or S configuration, preferably in the R configuration. 22. The method according to any one of aspects 2 to 21, wherein R₁ is OTES. 23. The method according to any one of aspects 1 to 22, wherein the compound comprises exactly one first silyl ether group. 24. The method according to any one of aspects 1 to 23, wherein the compound comprises exactly one, two or three second silyl ether groups. 25. The method according to any one of aspects 1 to 24, wherein the compound comprises exactly one first silyl ether group and exactly one second silyl ether groups. 26. The method according to any one of aspects 1 to 24, wherein the compound comprises exactly one first silyl ether group and exactly two second silyl ether groups. 27. The method according to any one of aspects 1 to 26, wherein the compound is labelled with a stable isotope. 28. The method according to any one of aspects 1 to 27, wherein at least one hydrogen atom of the compound is deuterium. 29. The method according to any one of aspects 2 to 28, wherein at least one, preferably 2, 3, 4 or 5, hydrogen atoms of the side chain starting at C^a of the compound according to formula (I) are deuterium. 30. The method according to any one of aspects 1 to 29, wherein at least one carbon atom of the compound is ¹³C. 31. The method according to any one of aspects 2 to 30, wherein at least one, preferably all, carbon atoms of the side chain starting at C^a of the compound according to formula (I) are ¹³C. 32. The method according to any one of aspects 2 to 31, wherein all carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C. 33. The method according to aspect 31 or 32, wherein at least three carbon atoms are ¹³C. 34. The method according to aspect 31 or 32, wherein one, two, three, four, five or six carbon atoms are ¹³C. 35. The method according to any one of aspects 2 to 30, wherein C^c, C^d, all carbon atoms of X₅ and, in case X₄ is an alkyl, all carbon atoms of X₄ are ¹³C. 36. The method according to any one of aspects 2 to 30, wherein C^d, all carbon atoms of X₅ and, in case X₄ is an alkyl, all carbon atoms of X₄ are ¹³C. 37. The method according to any one of aspects 2 to 30, wherein C^c, C^d and all carbon atoms of X₅ are ¹³C. 38. The method according to any one of aspects 2 to 30, wherein C^d and all carbon atoms of X₅ are ¹³C. 39. The method according to any one of aspects 2 to 26, wherein the compound is not labelled with a stable isotope. 40. The method according to any one of aspects 1 to 39, wherein the silyl group of the at least one second silyl ether group is not TES or TMS. 41. The method according to any one of aspects 1 to 40, wherein the silyl group of the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is not TES or TMS. 42. The method according to any one of aspects 2 to 41, wherein the silyl group in the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t- butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TBDMS, TBDPS or TIPS, preferably TBDMS. 43. The method according to aspect 42, wherein the silyl groups in the second silyl ether groups of X₅, R₂, R₃ and/or R₄, in particular X₅, R₂, R₃ and R₄, are the same or different, preferably the same. 44. The method of any one of aspects 1 to 43, wherein in step (b) the selective deprotection is carried out by reacting the compound in an acidic solution. 45. The method of aspect 44, wherein the acidic solution has a pH of 7.0 or less, 6.0 or less, 5.0 or less, 4.0 or less, preferably 4.0 to 3.5. 46. The method of any one of aspects 1 to 45, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an acid selected from the group consisting of acetic acid (AcOH), H₂SiF₆, citric acid, trifluoro acetic acid (TFA), HClO₄, HCl, HBr, HI, HF, HF*pyridine, HF*NEt₃, HF*urea, p-toluenesulfonic acid (pTSA), pyridinium p- toluenesulfonate (PPTS), camphorsulfonic acid (CSA), formic acid, H₂SO₄, trichloroacetic acid, CH₃SO₃H, CF₃SO₃H, H₃PO₄, Sc(OTf)₃, CeCl₃, CuCl₂, FeCl₃, Al₂O₃, and combinations thereof. 47. The method of any one of aspects 1 to 46, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an organic solvent selected from the group consisting of tetrahydrofuran (THF), 1,4-dioxane, diethylether, dimethoxyethane (DME), methyl tert-butyl ether (MTBE), 2-methyl-tetrahydrofuran, toluene, benzene, ethanol, isopropyl alcohol, 1-butanol, 1-octanol, methanol, n-hexane, n-pentane, n-heptane, diglyme, dichloromethane, 1,2-dichloroethane, acetonitrile, cyclohexane, pyridine, di-isopropyl ether and combinations thereof. 48. The method of any one of aspects 1 to 46, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising tetrahydrofuran (THF), 1,4-dioxane or methyl-tetrahydrofuran. 49. The method of any one of aspects 1 to 48, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising water. 50. The method of aspect 49, wherein the ratio between acid, organic solvent and water in the solution is 1-100:1-100:1, 1-50:1-50:1, 1-20:1-20:1, 1-10:1-10:1, 1-6:1- 4:1, preferably 2-6:2-4:1 (v(acid):v(organic solvent):v(water)). 51. The method according to any one of aspects 1 to 50, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising acetic acid, tetrahydrofuran and water. 52. The method according to aspect 51, wherein the ratio between acetic acid, tetrahydrofuran and water in the solution is 1-100:1-100:1, 1-50:1-50:1, 1-20:1-20:1, 1-10:1-10:1, 1-6:1-4:1, preferably 2-6:2-4:1 (v(AcOH):v(THF):v(water)). 53. The method according to aspect 51 or 52, wherein the ratio between acetic acid, tetrahydrofuran and water in the solution is 6:4:1 (v(AcOH):v(THF):v(water)). 54. The method according to any one of aspects 44 to 53, wherein in step (b) the solution comprising the compound is heated. 55. The method according to any one of aspects 44 to 54, wherein the solution comprising the compound has a temperature of 0 to 100 °C, 0 to 70 °C, 10 to 70 °C, 20 to 70 °C, 30 to 60 °C, preferably 45 to 55 °C. 56. The method according to any one of aspects 44 to 55, wherein in step (b) the solution comprising the compound is reacted for at least 30 min, 1 h, 5 h, 10 h, preferably for 30 min to 7 days, 30 min to 24 h, 10 h to 24 h. 57. The method according to any one of aspects 44 to 56, wherein step (b) is performed at 0.5 to 10 bar, 0.5 to 5 bar, 0.5 to 2.5 bar, 0.9 to 1.2 bar (absolute pressure). 58. The method according to any one of aspects 44 to 57, wherein step (b) is performed at atmospheric pressure.

59. The method according to any one of aspects 4 to 58, wherein in step (c) the A- ring fragment is reacted to obtain a compound according to formula (II)

wherein X5 comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein X1 is H, OH, R2 or not present, X2 is H, OH, R3 or not present, X3 is H, OH, R4 or not present, X4 is methyl, ethyl, H or not present, wherein R2, R3 and R4 are a second silyl ether group; wherein Y is C^b or O, wherein X1 is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X5 is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X5; and wherein R₅ is a third silyl ether group, R₆ is a third silyl ether group or H, R₇ is a third silyl ether group or H and R₈ is a methylene group or H. 60. The method according to aspect 59, wherein in a further step (d) any silyl groups on the compound according to formula (II) are replaced with hydrogen or 1- hydroxylpropyl, preferably hydrogen. 61. The method according to any one of aspects 4 to 60, wherein step (c) comprises a step of oxidizing the hydroxyl-group at the cyclohexyl-ring of the transhydrindane skeleton of the compound of formula (I) to form a keto-group to obtain a compound according to formula (III)

62. The method according to any one of aspects 59 to 61, wherein the A-ring fragment provided in step (a) is a compound according to formula (IV)

wherein R₅ is a third silyl ether group, R₆ is a third silyl ether group or H, R₇ is a third silyl ether group or H and R₈ is a methylene group or H. 63. The method according to any one of aspects 59 to 62, wherein the silyl group in the third silyl ether group of R₅, R₆ and/or R₇, in particular R₅, R₆ and R₇, is TES, TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di- t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TES, TBDPS or TIPS or TBDMS, preferably TES or TBDMS, preferably TBDMS. 64. The method according to any one of aspects 59 to 63, wherein the third silyl ether groups of R₅, R₆ and/or R₇, in particular R₅, R₆ and R₇, are the same or different, preferably the same. 65. The method according to any one of aspects 59 to 64, wherein the silyl group in the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇, in particular X₅, R₂, R₃, R₄, R₅, R₆ and R₇, is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di- t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TBDMS, TBDPS or TIPS, preferably TBDMS. 66. The method according to any one of aspects 59 to 65, wherein the silyl ether groups of X₅, R₂, R₃, R₄, R₅, R₆ and/or R₇, in particular X₅, R₂, R₃, R₄, R₅, R₆ and R₇, are the same or different, preferably the same. 67. The method according to any one of aspects 59 to 66, wherein R₆ is H and R₈ is a methylene group. 68. The method according to any one of aspects 59 to 67, wherein R₅ is a third silyl ether group, R₆ is H, R₇ is a third silyl ether group and R₈ is a methylene group. 69. The method according to any one of aspects 59 to 67, wherein R₅ is a third silyl ether group, R₆ is H, R₇ is H and R₈ is a methylene group. 70. The method according to any one of aspects 61 to 69, wherein step (c) comprises a step of: (i) reacting a compound according to formula (III) with a compound according to formula (IV) to obtain a compound according to formula (II). 71. The method according to any one of aspects 61 to 69, wherein step (c) comprises the steps of: (i) substituting the keto-group with a methyl-halide-group to obtain a compound according to formula (VI)

wherein X₆ is Br, F, Cl or I, preferably X₆ is Br or F, most preferably X₆ is Br; (ii) reacting a compound according to formula (VI) with a compound according to formula (V) to obtain a compound according to formula (II).

72. The method according to any one of aspects 59 to 71, wherein the compound according to formula (II) is

73. The method according to any one of aspects 61 to 72, wherein the compound according to formula (III) is

74. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

75. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (I) are ¹³C . 76. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

. 77. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (I) are ¹³C . 78. The method according to any one of aspects 2 to 74, wherein the compound according to formula (I) is

79. The method according to any one of aspects 2 to 75, wherein the compound according to formula (I) is

80. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

81. The method according to any one of aspects 2-73 or 80, wherein the compound according to formula (I) is

82. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

83. The method according to any one of aspects 2-73 or 82, wherein the compound according to formula (I) is

84. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

85. The method according to any one of aspects 2-73 or 84, wherein the compound according to formula (I) is

86. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

87. The method according to any one of aspects 2-73 or 86, wherein the compound according to formula (I) is

wherein C^d, the methyl group attached to C^d and the carbon atoms of the 1-oxy-1- methylethyl residue are ¹³C.

88. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

89. The method according to any one of aspects 2-73 or 88, wherein the compound according to formula (I) is

90. The method according to any one of aspects 2 to 73, wherein the compound according to formula (I) is

91. The method according to any one of aspects 2-73 or 90, wherein the compound according to formula (I) is

92. The method according to any one of aspects 62 to 91, wherein the compound according to formula (IV) is

or the compound according to formula (V) is

93. The method according to any one of aspects 71 to 92, wherein the compound according to formula (VI) is

94. A compound according to formula (I)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group. 95. The compound of aspect 94, wherein the compound is for use in the synthesis of a Vitamin D molecule. 96. The compound of aspect 95, wherein the Vitamin D molecule is selected from the group consisting of 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25- dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25- dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi-1,25- dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25- dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3-epi- 1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. 97. The compound of aspect 95 or 96, wherein the Vitamin D molecule is 25- hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)- 24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi- 1,25-dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3, in particular 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25- dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3 or (24R)-24,25-dihydroxyvitamin D3. 98. The compound of any one of aspects 94 to 97, wherein X₁ is H, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, H or not present. 99. The compound of any one of aspects 94 to 98, wherein the alkyl moiety of X₅ is a linear alkyl, branched alkyl or cycloalkyl moiety. 100. The compound of any one of aspects 94 to 99, wherein the alkyl moiety of X₅ is a substituted or unsubstituted alkyl moiety. 101. The compound of aspect 100, wherein the alkyl moiety is substituted with an alkyl, heteroalkyl, aryl, acyl, alkoxy, acyloxy, aryloxy, cycloalkyl or heterocycloalkyl moiety. 102. The compound of any one of aspects 94 to 101, wherein X₅ is cyclopropyl, 1-methylethyl, 1-hydroxyl-1-methylethyl, 1-hydroxyl-1-trichloromethyl-2,2,2- trichloroethyl, or 2-hydroxyl-2-ethylbutyl, wherein in case a hydroxyl group is present in X₅, the hydrogen is replaced by a silyl group forming a second silyl ether group. 103. The compound of any one of aspects 94 to 102, wherein X₅ is 1-hydroxyl-1- methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H or not present, X₂ is H, R₃ or not present, wherein R₃ is a second silyl ether group, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is methyl or H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single or double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 104. The compound of any one of aspects 94 to 103, wherein X₅ is 1-hydroxyl-1- methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is H or R₃, wherein R₃ is a second silyl ether group, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 105. The compound of any one of aspects 94 to 104, wherein X₅ is 1-hydroxyl-1- methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is R₃, wherein R₃ is a second silyl ether group, X₃ is H, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 106. The compound of any one of aspects 94 to 104, wherein X₅ is 1-hydroxyl-1- methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is H, X₃ is R₄, wherein R₄ is a second silyl ether group, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 107. The compound of aspect 106, wherein the stereocenter of C^d is in the R or S configuration, preferably in the R configuration. 108. The compound of any one of aspects 94 to 107, wherein X₅ is 1-hydroxyl-1- methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is H, X₂ is H, X₃ is H, X₄ is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 109. The compound of any one of aspects 94 to 103, wherein X₅ is 1-hydroxyl-1- methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is not present, X₂ is not present, X₃ is H or R₄, wherein R₄ is a second silyl ether group, X₄ is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 110. The compound of any one of aspects 94-103 or 109, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is not present, X₂ is not present, X₃ is R₄, wherein R₄ is a second silyl ether group, X₄ is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 111. The compound of any one of aspects 94-103 or 109, wherein X₅ is 1-hydroxyl- 1-methylethyl, wherein the hydrogen of the hydroxyl group is replaced by a silyl group forming a second silyl ether group; wherein X₁ is not present, X₂ is not present, X₃ is H, X₄ is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅ is a single bond. 112. The compound of aspect 110 or 111, wherein the stereocenter of C^d is in the R or S configuration, preferably in the R configuration. 113. The compound of any one of aspects 94 to 112, wherein R₁ is OTES. 114. The compound of any one of aspects 94 to 113, wherein the compound comprises exactly one first silyl ether group. 115. The compound of any one of aspects 94 to 114, wherein the compound comprises exactly one, two or three second silyl ether groups. 116. The compound of any one of aspects 94 to 115, wherein the compound comprises exactly one first silyl ether group and exactly one second silyl ether group. 117. The compound of any one of aspects 94 to 115, wherein the compound comprises exactly one first silyl ether group and exactly two second silyl ether groups. 118. The compound of any one of aspects 94 to 117, wherein the compound is labelled with a stable isotope. 119. The compound of any one of aspects 94 to 118, wherein at least one hydrogen atom of the compound is deuterium. 120. The compound of aspect 119, wherein at least one, preferably 2, 3, 4 or 5, hydrogen atoms of the side chain starting at C^a of the compound according to formula (I) are deuterium. 121. The compound of any one of aspects 94 to 120, wherein at least one carbon atom of the compound is ¹³C. 122. The compound of aspect 121, wherein at least one, preferably all, carbon atoms of the side chain starting at C^a of the compound according to formula (I) are ¹³C. 123. The compound of aspect 121, wherein all carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C. 124. The compound of aspect 122 or 123, wherein at least three carbon atoms are ¹³C. 125. The compound of aspect 122 or 123, wherein one, two, three, four, five or six carbon atoms are ¹³C. 126. The compound of any one of aspects 94 to 121, wherein C^c, C^d, all carbon atoms of X₅ and, in case X₄ is an alkyl, all carbon atoms of X₄ are ¹³C. 127. The compound of any one of aspects 94 to 121, wherein C^d, all carbon atoms of X₅ and, in case X₄ is an alkyl, all carbon atoms of X₄ are ¹³C. 128. The compound of any one of aspects 94 to 121, wherein C^c, C^d and all carbon atoms of X₅ are ¹³C. 129. The compound of to any one of aspects 94 to 121, wherein C^d and all carbon atoms of X₅ are ¹³C. 130. The compound of any one of aspects 94 to 117, wherein the compound is not labelled with a stable isotope. 131. The compound of any one of aspects 94 to 130, wherein the silyl group of the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is not TES or TMS. 132. The compound of any one of aspects 94 to 131, wherein the silyl group in the second silyl ether group X₅ is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TBDMS, TBDPS or TIPS, preferably TBDMS. 133. The compound of any one of aspects 94 to 132, wherein the silyl group in the second silyl ether groups of R₂, R₃ and/or R₄ is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t- butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TBDMS, TBDPS or TIPS, preferably TBDMS. 134. The compound of any one of aspects 94 to 133, wherein the second silyl ether groups of R₂, R₃ and/or R₄, in particular R₂, R₃ and R₄, are the same or different, preferably the same. 135. The compound of any one of aspects 94 to 134, wherein the second silyl ether groups of X₅, R₂, R₃ and/or R₄, in particular X₅, R₂, R₃ and R₄, are the same or different, preferably the same. 136. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

137. The compound according to any one of aspects 94 to 136, wherein the compound according to formula (I) is

wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (I) are ¹³C . 138. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

. 139. The compound of any one of aspects 94-135 or 138, wherein the compound according to formula (I) is

, wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (I) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (I) are ¹³C . 140. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

141. The compound of any one of aspects 94-135 or 141, wherein the compound according to formula (I) is

wherein C^c, C^d and the carbon atoms of the 1-oxy-1-methylethyl residue are ¹³C. 142. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

143. The compound of any one of aspects 94 to 135 or 142, wherein the compound according to formula (I) is

wherein C^c, C^d and the carbon atoms of the 1-oxy-1-methylethyl residue are ¹³C. 144. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

145. The compound of any one of aspects 94-135 or 144, wherein the compound according to formula (I) is

wherein C^c, C^d and the carbon atoms of the 1-oxy-1-methylethyl residue are ¹³C. 146. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

147. The compound of any one of aspects 94-135 or 146, wherein the compound according to formula (I) is

wherein C^c, C^d and the carbon atoms of the 1-oxy-1-methylethyl residue are ¹³C. 148. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

149. The compound of any one of aspects 94-135 or 148, wherein the compound according to formula (I) is

wherein C^d, the methyl group attached to C^d and the carbon atoms of the 1-oxy-1- methylethyl residue are ¹³C. 150. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

151. The compound of any one of aspects 94-135 or 150, wherein the compound according to formula (I) is

wherein C^d, the methyl group attached to C^d and the carbon atoms of the 1-oxy-1- methylethyl residue are ¹³C. 152. The compound of any one of aspects 94 to 135, wherein the compound according to formula (I) is

153. The compound of any one of aspects 94-135 or 152, wherein the compound according to formula (I) is

wherein C^d, the methyl group attached to C^d and the carbon atoms of the 1-oxy-1- methylethyl residue are ¹³C. 154. Use of a compound according to any one of aspects 94 to 153 for the synthesis of a Vitamin D molecule. 155. The use according to aspect 154, wherein the Vitamin D molecule is selected from the group consisting of 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25- dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi-1,25- dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25- dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3-epi- 1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. 156. The use according to aspect 154 or 155, wherein the Vitamin D molecule is 25- hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)- 24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi- 1,25-dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3, in particular 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25- dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3. 157. A Vitamin D molecule obtainable, in particular obtained, from a method according to any one of aspects 4 to 93. 158. The Vitamin D molecule of aspect 157, wherein the Vitamin D molecule is selected from the group consisting of 25-hydroxyvitamin D2, 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi- 1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25- dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25- dihydroxyvitamin D3, 3-epi-1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. 159. The Vitamin D molecule of aspect 157 or 158, wherein the Vitamin D molecule is 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi- 1,25-dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3, in particular 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25- dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3. 160. The Vitamin D molecule of any one of aspects 157 to 159 for mass spectrometric determination of an analyte. 161. The Vitamin D molecule of any one of aspects 157 to 160, wherein the compound according to formula (I) used is a compound according to any one of aspects 118 to 129, 137, 139, 141, 143, 145, 147, 149, 151 or 153. 162. The Vitamin D molecule of aspect 160 or 161 for use as an internal standard for mass spectrometric determination of an analyte. 163. The Vitamin D molecule of aspect 160 or 161 for use in calibrating an assay, in particular mass spectrometry assay, for determination of an analyte. 164. The Vitamin D molecule of any one of aspects 160 to 163, wherein the analyte is selected from the group consisting of 25-hydroxyvitamin D2, 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25-dihydroxyvitamin D2, 3-epi- 1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25- dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25- dihydroxyvitamin D3, 3-epi-1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof. 165. The Vitamin D molecule of any one of aspects 160 to 163, wherein the analyte is 25-hydroxyvitamin D2, 1,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi- 1,25-dihydroxyvitamin D3 or 3-epi-1,25-dihydroxyvitamin D3, in particular 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25-dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25- dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3 or (24R)-24,25-dihydroxyvitamin D3.

166. A method of producing a compound according to any one of aspects 94 to 153 comprising the steps of (A) providing a compound according to formula (VII)

wherein R1 is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X1* is H, OH or not present, X2* is H, OH or not present, X3* is H, OH or not present, X4* is methyl, ethyl, H or not present; wherein X5* comprises an alkyl moiety, which is optionally substituted with a hydroxyl group; wherein Y is C^b or O, wherein X1* is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X5* is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein at least one hydroxyl group is connected to any one of C^b, C^c and/or C^d, and/or is comprised in X5*; and at least one second silyl ether group triflate, wherein the second silyl ether group is selected from a group of silyl groups being different to the first silyl ether group; (B) substituting the hydrogen of each hydroxyl group of the compound according to formula (VII) with a silyl group to obtain a compound according to any one of aspects 94 to 153. 167. The method according to aspect 166, wherein the alkyl moiety of X₅* is a linear alkyl, branched alkyl or cycloalkyl moiety. 168. The method according to aspect 166 or 167, wherein the alkyl moiety of X₅* is a substituted or unsubstituted alkyl moiety. 169. The method according to aspect 168, wherein the alkyl moiety is substituted with an alkyl, heteroalkyl, aryl, acyl, alkoxy, acyloxy, aryloxy, cycloalkyl or heterocycloalkyl moiety. 170. The method according to any one of aspects 166 to 169, wherein X₅* is cyclopropyl, 1-methylethyl, 1-hydroxyl-1-methylethyl, 1-hydroxyl-1- trichloromethyl-2,2,2-trichloroethyl or 2-hydroxyl-2-ethylbutyl. 171. The method according to any one of aspects 166 to 170, wherein X₅* is 1- hydroxyl-1-methylethyl; wherein X₁* is H or not present, X₂* is H, OH or not present, X₃* is H or OH, X₄* is methyl or H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single or double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 172. The method according to any one of aspects 166 to 171, wherein X₅* is 1- hydroxyl-1-methylethyl; wherein X₁* is H, X₂* is H or OH, X₃* is H or OH, X₄* is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 173. The method according to any one of aspects 166 to 172, wherein X₅* is 1- hydroxyl-1-methylethyl; wherein X₁* is H, X₂* is H, X₃* is OH, X₄* is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 174. The method according to aspect 173, wherein the stereocenter of C^d is in the R or S configuration, preferably in the R configuration. 175. The method according to any one of aspects 166 to 172, wherein X₅* is 1-hydroxyl-1-methylethyl; wherein X₁* is H, X₂* is OH, X₃* is H, X₄* is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 176. The method according to any one of aspects 166 to 172, wherein X₅* is 1-hydroxyl-1-methylethyl; wherein X₁* is H, X₂* is H, X₃* is H, X₄* is H; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a single bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 177. The method according to any one of aspects 166 to 171, wherein X₅* is 1-hydroxyl-1-methylethyl; wherein X₁* is not present, X₂* is not present, X₃* is H or OH, X₄* is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 178. The method according to any one of aspects 166-171 or 177, wherein X₅* is 1- hydroxyl-1-methylethyl; wherein X₁* is not present, X₂* is not present, X₃* is OH, X₄* is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 179. The method according to any one of aspects 166-171 or 177, wherein X₅* is 1- hydroxyl-1-methylethyl; wherein X₁* is not present, X₂* is not present, X₃* is H, X₄* is methyl; wherein Y is C^b; and wherein the bond between C^a and C^b is a single bond, the bond between C^b and C^c is a double bond, the bond between C^c and C^d is a single bond and the bond between C^d and X₅* is a single bond. 180. The method according to aspect 178 or 179, wherein the stereocenter of C^d is in the R or S configuration, preferably in the R configuration. 181. The compound of any one of aspects 166 to 180, wherein R₁ is OTES. 182. The method according to any one of aspects 166 to 181, wherein the compound is labelled with a stable isotope. 183. The method according to any one of aspects 166 to 182, wherein at least one hydrogen atom of the compound according to formula (VII) is deuterium. 184. The method according to any one of aspects 166 to 183, wherein at least one, preferably 2, 3, 4 or 5, hydrogen atoms of the side chain starting at C^a of the compound according to formula (VII) are deuterium. 185. The method according to any one of aspects 166 to 184, wherein at least one carbon atom of the compound according to formula (VII) is ¹³C. 186. The method according to any one of aspects 166 to 185, wherein at least one, preferably all, carbon atoms of the side chain starting at C^a of the compound according to formula (VII) are ¹³C. 187. The method according to aspect 185 or 186, wherein all carbon atoms of the side chain starting at C^c of the compound according to formula (VII) are ¹³C. 188. The method according to aspect 186 or 187, wherein at least three carbon atoms are ¹³C. 189. The method according to aspect 186 or 187, wherein one, two, three, four, five or six carbon atoms are ¹³C. 190. The method according to any one of aspects 166 to 185, wherein C^c, C^d, all carbon atoms of X₅* and, in case X₄* is an alkyl, all carbon atoms of X₄* are ¹³C. 191. The method according to any one of aspects 166 to 185, wherein C^d, all carbon atoms of X₅* and, in case X₄* is an alkyl, all carbon atoms of X₄* are ¹³C. 192. The method according to any one of aspects 166 to 185, wherein C^c, C^d and all carbon atoms of X₅* are ¹³C. 193. The method according to any one of aspects 166 to 185, wherein C^d and all carbon atoms of X₅* are ¹³C. 194. The method according to any one of aspects 166 to 181, wherein the compound is not labelled with a stable isotope. 195. The method according to any one of aspects 166 to 194, wherein the silyl group in the silyl ether triflate is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t-butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl, preferably TBDMS, TBDPS or TIPS, preferably TBDMS. 196. The method according to any one of aspects 166 to 195, wherein the compound according to formula (VII) is

197. The method according to any one of aspects 166-193 or 195-196, wherein the compound according to formula (VII) is

wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (VII) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (VII) are ¹³C . 198. The method according to any one of aspects 166 to 195, wherein the compound according to formula (VII) is

199. The method according to any one of aspects 166-193, 195 or 198, wherein the compound according to formula (VII) is

wherein at least one, preferably all, carbon atoms of the side chain starting at C^c of the compound according to formula (VII) are ¹³C or wherein at least one, preferably all, carbon atoms of the side chain starting at C^d of the compound according to formula (VII) are ¹³C . EXAMPLES The following examples are provided to illustrate, but not to limit the presently claimed invention. Example 1 All reactions were magnetically stirred and, unless otherwise noted, carried out under a positive pressure of argon utilizing standard Schlenk-techniques. Glassware was dried at 650 °C in vacuo prior to use. Liquid reagents and solvents were added by syringes through rubber septa. Solids were added under inert gas counter flow or were dissolved in appropriate solvents. Low temperature reactions were carried out in a Dewar vessel filled with a cooling agent: acetone/dry ice (−78 °C), NaCl/ice (−20 °C) or H₂O/ice (0 °C). Reaction temperatures above room temperature were conducted in a heated oil bath. Yields refer to isolated homogenous and spectroscopically pure materials, if not indicated otherwise. In drawings, a “*” indicates a ¹³C-labelled site. Solvents and Reagents Diethyl ether (Et₂O) was distilled under reduced pressure prior to use. Solvents for extraction, crystallization and flash column chromatography were purchased in LiChrosolv^® hypergrade from Merck KGaA.¹³C-enriched compounds 8’ and ¹³C₃- acetone were purchased from Cambridge Isotope Laboratories. Inhoffen-Lythgoe diol and Wittig reagent were purchased from Merck KGaA. All other reagents and solvents were purchased from chemical suppliers (Sigma-Aldrich/Merck KGaA, Acros Organics, Honeywell/Fluka) and were used as received. n-BuLi in THF was titrated against diphenyl acetic acid (100 mg dissolved in 8.0 mL dry THF) prior to use to determine the exact molarity of the solution. Chromatography Qualitative thin-layer chromatography (TLC) on silica gel 60 F254 TLC plates from Merck KGaA was used to monitor reactions and preparative chromatography. Analytes on the plates were visualised by irradiation with UV-light (245 nm) and/or staining with an appropriate staining solution. The plate was immersed into the staining solution and then heated with a hot-air gun (350 °C). The following staining solutions were applied: p-Anisaldehyde staining solution (3.7 mL para-anisaldehyde, 5.0 mL concentrated aqueous H₂SO₄, 1.5 mL glacial AcOH, 135 mL EtOH) and KMnO₄ staining solution, (3.0 g KMnO₄, 20 g K₂CO₃, 5.0 mL aqueous 5% NaOH, 300 mL H₂O). Experimental flash column chromatography was performed on Geduran^® Si60 60 (40 – 63 μm) silica gel from Merck KGaA. All fractions containing a desired substrate were combined and solvents were removed under reduced pressure followed by drying in high vacuo (10 – 2 mbar) for non volatile substances. NMR spectroscopy NMR spectra were recorded on an Agilent 400-MR DD2400 MHz spectrometer equipped with an OneNMR Probe operating at 400 MHz for proton nuclei and at 101 MHz for carbon nuclei. The chemical shifts δ of the NMR spectra are reported in ppm relative to the shift of the standard TMS. NMR shifts are calibrated to the residual solvent resonances of CDCl₃ (7.26 ppm for ¹H-NMR and 77.16 ppm for ¹³C- NMR). Spectroscopic data is reported as follows: Chemical shift in ppm (multiplicity, coupling constants J in Hz, integration intensity). In the report of spectroscopic data, the multiplicity of signals is abbreviated with s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and mc (centrosymmetric multiplet). In case of combined multiplicities, the multiplicity with the larger coupling constant is stated first. With exception of the multiplets, the reported chemical shifts of the signal corresponds to the center of the resonance range. The ¹J_13CH coupling constants exceed any other J_HH coupling constants and are therefore reported in the cases of centrosymmetric multiplet. Additionally to ¹H and ¹³C-NMR measurements, 2D NMR techniques like homonuclear correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond coherence (HMBC) were used to assign signals. All NMR spectra were analysed using the program MestReNOVA 14.1.1 from Mestrelab Research. High Performance Liquid Chromatography (HPLC) HPLC was carried out using HPLC grade solvents and deionized, ultra-filtered water. All separations were conducted at room temperature. Analytical UV-Vis spectra were recorded on a 1260 Infinity HPLC system from Agilent Technologies Inc. that was computer-controlled through Chromeleon^TM Chromatography Data Systems Software Version 7.2 SR5 Mui (24070) from Thermo Fisher Scientific Inc., using an ACQUITY UPLC, Oligonucleotide 130A, 1.7 μm, BEH C18 column from Waters Corporation, detecting at 265 nm wavelength. Synthesis of the C^a to X₅ side chain moiety Synthesis of triethyl phosphonoacetate (11)

Triethyl phosphite (5.11 mL, 29.7 mmol, 1.1 eq) was added to ethyl 2-bromoacetate (8, 3.00 mL, 27.0 mmol, 1.0 eq) and the reaction mixture was stirred at 130 °C for four hours. Vacuum suction was applied to the reaction tube every hour for five minutes to remove the developing ethyl bromide from the reaction. After cooling to room temperature, the product was purified via distillation at reduced pressure (0.43 – 0.44 mbar, 92 – 96 °C). Triethyl phosphonoacetate (11, 5.57 g, 24.8 mmol, 92 % yield) was obtained as a colourless liquid. Empirical formula: C8H17O5P (224.19 g mol⁻¹, 11). ¹H-NMR (400 MHz, CDCl3): δ = 4.23 – 4.12 (m, 6H), 2.95 (d, J = 21.6 Hz, 2H), 1.34 (t, J = 7.1 Hz, 6H), 1.28 (t, J = 7.1 Hz, 3H) ppm. ¹³C-NMR (101 MHz, CDCl3): δ = 166.0 (d, J = 6.2 Hz), 62.8 (d, J = 6.2 Hz), 61.7, 34.5 (d, J = 134.1 Hz), 16.5 (d, J = 6.3 Hz), 14.2 ppm. Synthesis of triethyl (¹³C2)phosphonoacetate (11’)

Similar to the synthesis of compound 11, triethyl phosphite (5.08 mL, 29.5 mmol, 1.1 eq) was added to ethyl 2-(¹³C₂)bromoacetate (8’, 3.00 mL, 26.9 mmol, 1.0 eq) and further worked up as described above to yield triethyl (¹³C₂)phosphonoacetate (11’, 5.33 g, 23.6 mmol, 88 % yield) as a colourless liquid. Empirical formula: C₆ ¹³C₂H₁₇O₅P (226.18 g mol⁻¹, 11’). ¹H-NMR (400 MHz, CDCl₃): δ = 4.24 – 4.11 (m, 6H), 2.95 (ddd, J = 129.8, 21.5, 7.4 Hz, 2H), 1.34 (t, J = 7.0 Hz, 6H), 1.28 (t, J = 7.2 Hz, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 166.0 (dd, J = 58.7, 6.1 Hz), 62.8 (d, J = 6.3 Hz), 61.7 (d, J = 2.6 Hz), 34.5 (dd, J = 134.3, 58.9 Hz), 16.5 (d, J = 6.2 Hz), 14.2 (d, J = 2.2 Hz) ppm. Synthesis of ethyl-3-methylcrotonate (12)

To a stirred solution of triethyl phosphonoacetate (11, 4.11 g, 18.3 mmol, 1.0 eq.) in dry THF (60 mL) n-BuLi (8.35 mL, 2.13 M solution in hexanes, 17.8 mmol, 0.97 eq.) was slowly added at −78 °C. The reaction mixture was stirred at −78 °C for one hour and at −40 °C for 30 min. Dry acetone (9, 1.80 mL, 23.8 mmol, 1.3 eq.) was added dropwise at −78 °C and stirring was continued at that temperature for 30 min. The reaction mixture was then warmed to room temperature and stirred overnight. After addition of saturated aqueous NH₄Cl (60 mL) the mixture was extracted with diethyl ether (3 × 100 mL). The combined organic layers were washed with brine (60 mL) and dried over Na₂SO₄. The solvents were removed under reduced pressure and the crude product was purified via flash column chromatography (silica, n-pentane:Et₂O = 100:0 to 94:6) to yield ethyl-3-methylcrotonate (12, 1.77 g, 18.3 mmol, 75 % yield) as a colourless liquid. 12 is volatile, which is why solvents must be removed carefully (≥100 mbar, 30 °C). Empirical formula: C₇H₁₂O₂ (128.17 g mol⁻¹, 12). R_f = 0.29 (n-Hex/EtOAc = 6:0.2). ¹H-NMR (400 MHz, CDCl₃): δ = 5.68 – 5.65 (m, 1H), 4.14 (q, J = 7.1 Hz, 2H), 2.16 (d, J = 1.0 Hz, 3H), 1.89 (d, J = 1.1 Hz, 3H), 1.27 (t, J = 7.1 Hz, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 166.9, 156.5, 116.3, 59.6, 27.5, 20.3, 14.5 ppm. Synthesis of ethyl-3-(¹³C₅)methylcrotonate (12’)

Similar to the synthesis of compound 12, n-BuLi (10.3 mL, 2.19 M solution in hexanes, 22.6 mmol, 0.97 eq.) was added to a solution of triethyl (¹³C2)phosphonoacetate (11’, 5.27 g, 23.3 mmol, 1.0 eq) in dry THF (76 mL) at –78 °C. After stirring of the solution at –78 °C for one hour and at –40 °C for 30 min, (¹³C3)acetone (9’, 2.23 mL, 30.3 mmol, 1.3 eq.) was added at –78 °C. The reaction was further worked up as described above to yield ethyl-3- (¹³C5)methylcrotonate (12’, 2.23 g, 16.7 mmol, 72 % yield) as a colourless liquid. 12’ is volatile, which is why solvents should be removed carefully (≥100 mbar, 30 °C). Empirical formula: C2¹³C5H12O2 (133.13 g mol⁻¹, 12’). Rf = 0.29 (n-Hex/EtOAc = 6:0.2). ¹H-NMR (400 MHz, CDCl3): δ = 5.67 (dmc, ¹J13CH = 159.7 Hz, 1H), 4.14 (qd, J = 7.1, 3.0 Hz, 2H), 2.16 (dmc, ¹J13CH = 127.6 Hz, 3H), 1.89 (dmc, ¹J13CH = 126.8, 3H), 1.27 (t, J = 7.2 Hz, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 166.9 (ddt, J = 75.6, 7.5, 1.8 Hz), 156.5 (dtd, J = 72.2, 40.3, 2.2 Hz), 116.2 (ddd, J = 76.1, 72.2, 4.0 Hz), 59.6, 27.9 – 27.2 (m), 20.6 – 19.9 (m), 14.5 ppm. Synthesis of 3-methyl-2-buten-1-ol (10)

DIBAL (10.3 mL, 1.0 M solution in hexanes, 10.3 mmol, 2.2 eq.) was slowly added to a stirred solution of ethyl-3-methylcrotonate (12, 599 mg, 4.67 mmol, 1.0 eq.) in dry diethyl ether (16 mL) at −78 °C. The reaction mixture was stirred at −78 °C for two hours and at −50 °C for 30 min. The reaction was quenched via the addition of saturated aqueous Na/K-tartrate solution (26 mL) and the mixture was allowed to warm to room temperature under vigorous stirring. The layers were separated, and the aqueous phase was extracted with diethyl ether (3 × 30 mL). The combined organic layers were dried over Na₂SO₄, and the solvent was removed under reduced pressure. 3-Methyl-2-buten-1-ol (10, 383 mg, 4.45 mmol, 95 % yield) was obtained as a colourless liquid. 10 is volatile, which is why solvents should be removed carefully (≥100 mbar, 30 °C). Empirical formula: C₅H₁₀O (86.13 g mol⁻¹, 10). R_f = 0.21 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃): δ = 5.44 – 5.37 (m, 1H), 4.12 (d, J = 7.0 Hz, 2H), 1.74 (s, 3H), 1.68 (s, 3H), 1.24 (s, 1H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 136.6, 123.7, 59.5, 25.9, 18.0 ppm. Synthesis of (¹³C₅)3-methyl-2-buten-1-ol (10’)

Similar to the synthesis of 10, DIBAL (36.0 mL, 1.0 M in hexanes, 36.0 mmol, 2.2 eq.) was slowly added to a stirred solution of ethyl-3-(¹³C₅)methylcrotonate (12’, 2.20 g, 16.5 mmol, 1.0 eq.) in dry Et₂O (55 mL) at −78 °C and further worked up as described above to yield (¹³C₅)3-methyl-2-buten-1-ol (10’, 1.41 g, 15.5 mmol, 94 % yield) as a colourless liquid. 10’ is volatile, which is why solvents should be removed carefully (≥100 mbar, 30 °C). Empirical formula: ¹³C₅H₁₀O (91.10 g mol⁻¹, 10’). R_f = 0.21 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃): δ = 5.41 (dm_c, ¹J_13CH = 153.1 Hz, 1H), 4.12 (dm_c, ¹J_13CH = 141.7 Hz, 2H), 1.74 (dq, ¹J_13CH = 125.7 Hz, J = 5.6 Hz, 3H), 1.68 (dq, ¹J_13CH = 125.7 Hz, J = 5.2 Hz, 3H), 1.19 (s, 1H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 136.6 (dtd, J = 72.9, 41.8, 1.6 Hz), 123.7 (dddd, J = 72.8, 47.6, 3.0, 1.6 Hz), 60.0 – 58.9 (m), 26.3 – 25.5 (m), 18.5 – 17.5 (m) ppm. Synthesis of 1-bromo-3-methylbut-2-ene (7)

The reaction was carried out under ambient conditions. HBr (2.40 mL, 48% aqueous solution, 21.0 mmol, 5.2 eq.) was added to a solution of 3-methyl-2- buten-1-ol (10, 350 mg, 4.06 mmol, 1.0 eq.) in CH₂Cl₂ (5.5 mL) at 0 °C. The reaction mixture was stirred at 0 °C for 90 min under the exclusion of light. After addition of saturated aqueous NaHCO₃ (20 mL), the layers were separated, and the aqueous phase was extracted with n-pentane (3 × 15 mL). The combined organic layers were dried over Na₂SO₄, and the solvents were removed by distillation at atmospheric pressure (oil bath temperature 40 – 42 °C). 1-Bromo- 3-methylbut-2-ene (7, 1.34 g, 39.5 wt.% in n-pentane, 3.55 mmol, 88 % yield) was obtained as a colourless liquid. Empirical formula: C₅H₉Br (149.03 g mol⁻¹, 7). R_f = 0.87 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃): δ = 5.53 (tq, J = 8.4, 1.6 Hz, 1H), 4.01 (d, J = 8.4 Hz, 2H), 1.78 (s, 3H), 1.73 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 140.3, 120.9, 29.9, 25.9, 17.7 ppm. Synthesis of 1-bromo-3-(¹³C₅)methylbut-2-ene (7’)

The reaction was carried out under ambient conditions. Similar to the synthesis of 7, HBr (9.21 mL, 48 % aqueous solution, 80.7 mmol, 5.3 eq.) was added to a stirred solution of (¹³C₅)3-methyl-2-buten- 1-ol (10’, 1.39 g, 15.2 mmol, 1.0 eq.) in CH₂Cl₂ (20 mL) at 0 °C. The reaction was further worked up as described above to yield 1-bromo-3-(¹³C₅)methylbut-2-ene (7’, 6.31 g, 34.7 wt.% in n- pentane, 14.2 mmol, 93 % yield) as a colourless liquid. Empirical formula: ¹³C₅H₉Br (153.99 g mol⁻¹, 7’). R_f = 0.87 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃): δ = 5.52 (dm_c, ¹J_13CH = 158.7 Hz, 1H), 4.01 (ddt, ¹J_13CH = 152.8, J = 9.2, 4.8 Hz, 2H), 1.78 (dq, ¹J_13CH = 126.1, J = 5.6 Hz, 3H), 1.73 (dq, ¹J_13CH = 126.0, J = 5.0 Hz, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 140.3 (dtd, J = 73.7, 41.6, 1.4 Hz), 120.9 (dddd, J = 73.5, 47.7, 3.9, 1.5 Hz), 30.3 – 29.5 (m), 26.3 – 25.5 (m), 17.9 – 17.3 (m) ppm. Synthesis of the terpenoid scaffold Synthesis of iodide 13

Inhoffen-Lythgoe diol (2, 300 mg, 1.41 mmol, 1.0 eq.), triphenylphosphine (408 mg, 1.55 mmol, 1.1 eq.) and imidazole (288 mg, 4.24 mmol, 3.0 eq.) were dissolved in dry THF (15 mL) and cooled to −20 °C. A solution of iodine (393 mg, 1.51 mmol, 1.1 eq.) in dry THF (7.5 mL) was added dropwise to the stirred mixture. The reaction was stirred at −20 °C for 15 min and at room temperature for one hour. The reaction was quenched via the addition of a mixture of saturated aqueous NaHCO3 (100 mL) and diluted with Et2O (100 mL). The layers were separated, and the aqueous phase was extracted with diethyl ether (3 × 100 mL). The combined organic layers were consecutively washed with an aqueous solution of Na2S2O3 (10%, 100 mL) and with water (100 mL) and dried over Na2SO4. The solvents were removed under reduced pressure and the crude product was purified via flash column chromatography (silica, n-Hex:Et2O = 92:8 to 80:20) to yield iodide 13 (385 mg, 1.19 mmol, 85 % yield) as a colourless solid. Empirical formula: C13H23IO (322.23 g mol⁻¹, 13) R_f = 0.39 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃): δ = 4.09 (s, 1H), 3.33 (d, J = 9.4 Hz, 1H), 3.18 (dd, J = 9.4, 4.5 Hz, 1H), 1.94 (d, J = 13.0 Hz, 1H), 1.91 – 1.74 (m, 3H), 1.65 – 1.35 (m, 6H), 1.32 – 1.10 (m, 4H), 1.00 (d, J = 5.4 Hz, 3H), 0.98 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 69.4, 56.0, 52.5, 42.0, 40.2, 36.5, 33.7, 26.7, 22.5, 21.5, 20.8, 17.5, 14.5 ppm. Synthesis of sulfone 14

Iodide 13 (375 mg, 1.16 mmol, 1.0 eq.) was dissolved in dry DMSO (5.0 mL) at room temperature. Sodium benzenesulfinate (382 mg, 2.33 mmol, 2.0 eq.) was added in one portion and the reaction mixture was stirred at room temperature for 19 hours. The reaction was then quenched by the addition of saturated aqueous NH₄Cl, saturated aqueous NaCl and H₂O (90 mL, v/v/v = 1:1:1) and diluted with diethyl ether (100 mL). The layers were separated, and the aqueous phase was extracted with diethyl ether (2 × 100 mL). The combined organic layers were dried over Na₂SO₄, and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex:EtOAc = 85:15 to 75:25) to yield sulfone 14 (338 mg, 1.00 mmol, 86 % yield) as a colourless oil. Empirical formula: C₁₉H₂₈O₃S (336.49 g mol⁻¹, 14). R_f = 0.16 (n-Hex/EtOAc = 3:1). ¹H-NMR (400 MHz, CDCl₃): δ = 7.95 – 7.87 (m, 2H), 7.68 – 7.61 (m, 1H), 7.60 – 7.53 (m, 2H), 4.05 (q, J = 3.1 Hz, 1H), 3.14 (dd, J = 14.3, 1.8 Hz, 1H), 2.84 (dd, J = 14.2, 9.5 Hz, 1H), 2.14 – 2.00 (m, 1H), 1.95 (dt, J = 13.0, 2.5 Hz, 1H), 1.86 – 1.67 (m, 3H), 1.57 – 1.48 (m, 1H), 1.47 – 1.36 (m, 3H), 1.30 (ddd, J = 13.6, 7.0, 2.5 Hz, 1H), 1.21 (s, 1H), 1.17 (d, J = 6.7 Hz, 3H), 1.16 – 1.01 (m, 3H), 0.90 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 140.5, 133.7, 129.4, 128.0, 69.2, 62.1, 56.0, 52.7, 42.3, 40.3, 33.7, 32.1, 27.2, 22.5, 20.1, 17.4, 13.5 ppm. Synthesis of sulfone 6

Sulfone 14 (480 mg, 1.43 mmol, 1.0 eq.) was dissolved in dry DMF (5.4 mL) at room temperature and imidazole (583 mg, 8.56 mmol, 6.0 eq.) and TESCl (0.84 mL, 4.99 mmol, 3.5 eq.) were added to the solution. The reaction mixture was stirred at room temperature for 3 hours, then, abs. ethanol (0.33 mL, 5.71 mmol, 4.0 eq.) was added and stirring was continued for another 10 min. The reaction was quenched with saturated aqueous NaHCO3 (100 mL) and diluted with diethyl ether (100 mL). After separation of the layers, the aqueous phase was extracted with diethyl ether (2 × 100 mL). The combined organic layers were dried over Na2SO4, and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n- Hex:EtOAc = 97:03 to 91:09) to yield sulfone 6 (597 mg, 1.32 mmol, 93 % yield) as a colourless solid. Empirical formula: C25H42O3SSi (450.75 g mol⁻¹, 6). Rf = 0.67 (n-Hex/EtOAc = 3:1). ¹H-NMR (400 MHz, CDCl3): δ = 7.91 (dt, J = 3.4, 2.4 Hz, 2H), 7.67 – 7.62 (m, 1H), 7.60 – 7.54 (m, 2H), 3.99 (d, J = 2.5 Hz, 1H), 3.15 (dd, J = 14.2, 1.4 Hz, 1H), 2.83 (dd, J = 14.2, 9.7 Hz, 1H), 2.11 – 1.99 (m, 1H), 1.91 (dt, J = 12.4, 3.0 Hz, 1H), 1.85 – 1.72 (m, 1H), 1.72 – 1.62 (m, 2H), 1.60 – 1.47 (m, 1H), 1.41 – 1.24 (m, 3H), 1.23 – 1.17 (m, 1H), 1.16 (d, J = 6.5 Hz, 3H), 1.13 – 1.01 (m, 3H), 0.93 (t, J = 7.9 Hz, 9H), 0.87 (s, 3H), 0.53 (q, J = 7.8 Hz, 6H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 140.5, 133.6, 129.4, 128.0, 69.3, 62.2, 56.2, 53.2, 42.5, 40.7, 34.6, 32.1, 27.3, 23.0, 20.1, 17.7, 13.5, 7.1, 5.1 ppm. Synthesis of sulfone 15

n-BuLi (50.0 μL, 2.31 M solution in hexanes, 116 μmol, 1.3 eq.) was added to a solution of sulfone 6 (40 mg, 88.7 μmol, 1.0 eq.) in dry THF (1.5 mL) at −40 °C. The mixture was stirred at this temperature for 30 min. Then, a solution of 1- bromo-3-methylbut-2-ene (50.2 mg, 39.5 wt.% in n-pentane, 133 μmol, 1.5 eq., 7) in dry THF (0.6 mL) was added slowly and stirring was continued at −40 °C for 3 hours. The reaction was quenched via the addition of saturated aqueous NH4Cl (20 mL) and diluted with diethyl ether (20 mL). The layers were separated, and the aqueous phase was extracted with diethyl ether (3 × 20 mL). The combined organic layers were dried over Na2SO4, and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex:Et2O = 97:03 to 91:09) to yield sulfone 15 (40 mg, 77.1 μmol, 87 % yield) as a colorless solid. The material was obtained as a mixture of diastereomers (dr = 3:7, quantified via ¹H-NMR), that was employed in the next step without further purification. Empirical formula: C30H50O3SSi (518.87 g mol⁻¹, 15). Rf = 0.45 (n-Hex/EtOAc = 10:1). ¹H-NMR (400 MHz, CDCl3, only major isomer quoted): δ = 7.84 (dt, J = 7.0, 1.4 Hz, 2H), 7.65 – 7.58 (m, 1H), 7.57 – 7.50 (m, 2H), 4.82 (tq, J = 7.1, 1.5 Hz, 1H), 3.99 (q, J = 2.7 Hz, 1H), 3.07 (td, J = 6.4, 1.5 Hz, 1H), 2.64 – 2.15 (m, 3H), 2.13 – 1.86 (m, 2H), 1.86 – 1.71 (m, 1H), 1.54 (s, 3H), 1.51 (s, 3H), 1.71 – 0.99 (m, 9H), 1.08 (d, J = 6.8 Hz, 3H), 0.99 – 0.88 (m, 9H), 0.85 (s, 3H), 0.53 (q, J = 7.8 Hz, 6H) ppm. Characteristic signal for minor isomer: δ = 4.04 (q, J = 2.7 Hz, 1H) ppm. 1³C-NMR (101 MHz, CDCl₃, only major isomer quoted): δ = 140.0, 133.4, 133.3, 129.0, 128.6, 121.5, 69.3, 67.3, 54.6, 53.3, 42.6, 40.9, 34.6, 34.0, 27.4, 25.7, 22.9, 22.8, 17.9, 17.7, 14.8, 13.3, 7.1, 5.1 ppm. Synthesis of (¹³C₅)sulfone 15’

Similar to the synthesis of 15, n-BuLi (664 μL, 2.18 M solution in hexanes, 1.45 mmol, 1.3 eq.) was added to a solution of sulfone 6 (502 mg, 11.1 mmol, 1.0 eq.) in dry THF (11 mL) at −40 °C. The mixture was stirred at this temperature for 30 min, then, 1-bromo-3-(¹³C₅)methylbut-2-ene (717 mg, 34.7 wt.% in n-pentane, 1.62 mmol, 1.5 eq., 7’) was added slowly and stirring was continued at −40 °C for 3 hours. The reaction was further worked up as described above to yield (¹³C₅)sulfone 15’ (535 mg, 1.02 mmol, 92 % yield) as a colourless solid. The material was obtained as a mixture of diastereomers (dr = 3:7, quantified via ¹H-NMR). Empirical formula: C₃₀H₅₀O₃SSi (518.87 g mol⁻¹, 15’). R_f = 0.45 (n-Hex/EtOAc = 10:1). 1H-NMR (400 MHz, CDCl₃, only major isomer quoted): δ = 7.91 – 7.80 (m, 2H), 7.65 – 7.58 (m, 1H), 7.58 – 7.50 (m, 2H), 4.80 (d, ¹J_13CH = 152.8 Hz, 1H), 3.99 (s, 1H), 3.12 – 3.02 (m, 1H), 2.79 – 2.14 (m, 3H), 1.99 (dt, J = 59.8, 10.9 Hz, 2H), 1.86 – 1.72 (m, 1H), 1.71 – 0.99 (m, 15H), 1.08 (d, J = 6.6 Hz, 3H), 0.99 – 0.89 (m, 9H), 0.85 (s, 3H), 0.53 (q, J = 8.1 Hz, 6H) ppm. Characteristic signal for minor isomer: δ = 4.03 (s, 1H) ppm. ¹³C-NMR (101 MHz, CDCl₃, only major isomer quoted): δ = 140.0, 133.4 (dt, J = 74.5, 42.5 Hz), 133.3, 129.1, 128.6, 121.5 (dddd, J = 74.4, 44.1, 3.6, 1.7 Hz), 69.3, 67.4 – 66.8 (m), 54.6, 53.3, 42.6, 40.9, 34.6, 34.0, 27.4, 26.2 – 25.4 (m), 23.2 – 22.4 (m), 18.2 – 17.4 (m), 14.8, 13.3, 7.1, 5.1 ppm. Synthesis of alkene 5 and diene 16

Sodium amalgam (1.17 g, 5 wt.% Na, 2.54 mmol, 13.2 eq.) was added to a solution of sulfone 15 (100 mg, 193 μmol, 1.0 eq.) in dry THF/MeOH (5 mL, v/v = 3:2) and the mixture was stirred at room temperature for 19 hours. The solution was then decanted from the remaining Hg and added into a mixture consisting of saturated aqueous NH4Cl (80 mL) and diethyl ether (80 mL). The layers were separated, and the aqueous phase was extracted with Et2O (3× 80 mL). The remaining Hg was washed with Et2O (3× 10 mL). The combined organic layers were dried over Na2SO4 and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex) to yield a 1:8 mixture (67 mg, 178 μmol, 92% yield, quantified via ¹HNMR) of diene 16 and alkene 5 as a colourless oil. Empirical formula: C24H46OSi (378.72 g mol⁻¹, alkene 5), C24H44OSi (376.70 g mol⁻¹, diene 16). Rf (5 and 16) = 0.89 (n-Hex/EtOAc = 6:0.2). ¹H-NMR (400 MHz, CDCl3, only alkene 5 quoted): δ = 5.09 (tt, J = 7.0, 1.4 Hz, 1H), 4.03 (q, J = 2.8 Hz, 1H), 2.14 – 1.90 (m, 2H), 1.90 – 1.72 (m, 3H), 1.68 (s, 3H), 1.60 (s, 3H), 1.71 – 1.49 (m, 2H), 1.46 – 1.27 (m, 5H), 1.28 – 1.16 (m, 2H), 1.15 – 0.99 (m, 3H), 0.95 (t, J = 7.9 Hz, 9H), 0.95 – 0.87 (m, 3H), 0.90 (s, 3H), 0.55 (q, J = 7.7 Hz, 6H) ppm. Characteristic signals for diene 16: δ = 6.12 (dd, J = 15.0, 10.8 Hz, 1H), 5.75 (d, J = 10.8 Hz, 1H), 5.39 (dd, J = 15.0, 8.7 Hz, 1H) ppm. 1³C-NMR (101 MHz, CDCl₃, only alkene 5 quoted): δ = 131.0, 125.5, 69.6, 56.9, 53.3, 42.3, 41.0, 36.1, 35.3, 34.8, 27.5, 25.9, 24.9, 23.2, 18.7, 17.9, 17.8, 13.7, 7.1, 5.1 ppm. Synthesis of alkene 5’ and diene 16’

Sodium amalgam (6.14 g, 5 wt.% Na, 13.4 mmol, 13.2 eq.) was added to a solution of sulfone 15’ (530 mg, 1.01 mmol, 1.0 eq.) in dry THF/MeOH (26.3 mL, v/v = 3:2) and the mixture was stirred at room temperature for 19 hours. The solution was then decanted from the remaining Hg and added into a mixture consisting of saturated aqueous NH₄Cl (200 mL) and diethyl ether (200 mL). The layers were separated, and the aqueous phase was extracted with Et₂O (3× 100 mL). The remaining Hg was washed with Et₂O (3 × 10 mL). The combined organic layers were dried over Na₂SO₄ and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex) to yield a 1:8 mixture (362 mg, 0.95 mmol, 94 % yield, quantified via ¹HNMR) of diene 16’ and alkene 5’ as a colourless oil. Empirical formula: C₁₉ ¹³C₅H₄₆OSi (383.68 g mol⁻¹, alkene 5’), C₁₉ ¹³C₅H₄₄OSi (381.66 g mol⁻¹, diene 16’). R_f (5’ and 16’) = 0.89 (n-Hex/EtOAc = 6:0.2). Synthesis of diol 18_S

To a solution of a 1:8 mixture of diene 16 and alkene 5 (142 mg, 377 μmol, 1.0 eq.) in water/t-BuOH (5.0 mL, v/v = 1:1), MeSO₂NH₂ (36.0 mg, 377 μmol, 1.0 eq.) and AD-Mix α (575 mg, 0.5 wt.% OsO₄, 11.3 μmol, 0.03 eq.) were added in one portion at 4 °C. The mixture was stirred at this temperature for three days before being quenched via the addition of aqueous Na₂S₂O₃ (10%, 100 mL). The mixture was extracted with EtOAc (5 × 100 mL), the combined organic layers were dried over Na₂SO₄ and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n- Hex:EtOAc = 95:05 to 75:25) to yield a inseparable 1:8 mixture (121 mg, 294 μmol, 78 % yield, quantified via ¹H-NMR) of diol 17_S and diol 18_S as a colourless wax. The obtained 1:8 mixture of diol 17_S and diol 18_S (113 mg, 0.28 mmol, 1.0 eq.) was dissolved in abs. EtOH (9 mL) at room temperature, and Pd/C (59 mg, 10 wt.% Pd, 0.6 mmol, 20 mol%) was added. The inert gas was replaced by hydrogen (1 atm) and the suspension was then stirred vigorously for 17 hours. The mixture was filtered over a pad of Celite^®, washed thoroughly with EtOAc (80 mL) and the solvent was removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex:EtOAc = 88:12 to 78:22) to yield diol 18_S (106 mg, 0.25 mmol, 89 % yield) as a colourless oil. Empirical formula: C₂₄H₄₆O₃Si (410.71 g mol⁻¹, diol 17_S), C₂₄H₄₈O₃Si (412.73 g mol⁻¹, diol 18_S). R_f (18_S) = 0.17 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃, 18_S) δ = 4.03 (d, J = 2.8 Hz, 1H), 3.28 (ddd, J = 10.3, 4.4, 2.0 Hz, 1H), 2.03 (d, J = 4.5 Hz, 1H), 1.98 – 1.89 (m, 2H), 1.88 – 1.62 (m, 4H), 1.62 – 1.51 (m, 2H), 1.48 – 1.19 (m, 6H), 1.21 (s, 3H), 1.16 (s, 3H), 1.19 – 0.97 (m, 4H), 0.94 (t, J = 8.0 Hz, 9H), 0.91 (d, J = 6.1 Hz, 6H), 0.55 (q, J = 7.9 Hz, 6H) ppm. Characteristic signal for diol 17_S: δ = 5.60 (ddd, J = 15.4, 8.5, 1.0 Hz, 1H), 5.40 (dd, J = 15.4, 7.2 Hz, 1H), 3.84 (dd, J = 7.2, 3.0 Hz, 1H) ppm. ¹³C-NMR (101 MHz, CDCl₃, 18_S): δ = 79.8, 73.4, 69.5, 56.8, 53.2, 42.3, 41.0, 35.6, 34.8, 33.3, 28.5, 27.4, 26.7, 23.4, 23.2, 18.9, 17.9, 13.7, 7.1, 5.1 ppm. Synthesis of silyl ether 19_S

Diol 18_S (92 mg, 223 μmol, 1.0 eq.) was dissolved in dry CH2Cl2 (2.5 mL) and 2,6-lutidine (0.16 mL, 1.40 mmol, 6.25 eq.) was added. The solution was cooled to 0 °C and TBSOTf (220 μL, 946 μmol, 4.2 eq.) was added. The mixture was stirred at 0 °C for 1.5 hours and then quenched with saturated aqueous NaHCO3 (80 mL). The mixture was extracted with Et2O (3 × 80 mL), the combined organic layers were washed with saturated aqueous NH4Cl (80 mL) and dried over Na2SO4. The solvents were removed under reduced pressure and the crude product was purified via flash column chromatography (silica, n-Hex) to yield silyl ether 19_S (135 mg, 211 μmol, 96 % yield) as a colourless wax. Empirical formula: C36H76O3Si3 (641.26 g mol−1, silyl ether 19_S). Rf = 0.93 (n-Hex/EtOAc = 9:1). ¹H-NMR (400 MHz, CDCl₃) δ = 4.03 (d, J = 2.4 Hz, 1H), 3.17 (dd, J = 7.6, 2.3 Hz, 1H), 2.01 – 1.89 (m, 2H), 1.88 – 1.73 (m, 2H), 1.70 – 1.51 (m, 3H), 1.43 – 1.16 (m, 7H), 1.18 (s, 3H), 1.10 (s, 3H), 1.14 – 0.98 (m, 3H), 0.95 (t, J = 7.9 Hz, 9H), 0.89 (s, 3H), 0.88 (s, J = 2.9 Hz, 9H), 0.87 (t, J = 2.5 Hz, 3H), 0.85 (s, J = 2.6 Hz, 9H), 0.55 (q, J = 7.9 Hz, 6H), 0.08 (s, J = 2.3 Hz, 3H), 0.07 (s, 3H), 0.06 (s, 3H), 0.03 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 81.4, 76.5, 69.6, 57.1, 53.3, 42.3, 41.0, 36.1, 34.9, 34.2, 29.8, 29.1, 27.7, 26.3, 26.0, 23.2, 18.7, 18.4, 18.3, 17.9, 13.6, 7.1, 5.1, −1.8, −1.8, −3.0, −3.7 ppm. Synthesis of silyl ether 19’

To a solution of a 1:8 mixture of diene 16’ and alkene 5’ (1.10 g, 2.88 mmol, 1.0 eq.) in water/t-BuOH (28.2 mL, v/v = 1:1), MeSO2NH2 (273 mg, 2.88 mmol, 1.0 eq.) and AD-Mix beta (4.38 g, 0.5 wt.% OsO4, 0.09 mmol, 0.03 eq. Os) were added in one portion at 4 °C. The mixture was stirred at this temperature for three days before being quenched via the addition of aqueous Na2S2O3 (10%, 200 mL). The mixture was extracted with EtOAc (5 × 150mL), the combined organic layers were dried over Na2SO4 and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex:EtOAc = 95:05 to 75:25) to yield a inseparable 1:8 mixture (958 mg, 2.30 mmol, 80 % yield, quantified via ¹H-NMR) of diol 17’ and diol 18’ as a colourless wax. The obtained 1:8 mixture of diol 17’ and diol 18’ (940mg, 2.31 mmol, 1.0 eq.) was dissolved in abs. EtOH (75 mL) at room temperature, and Pd/C (487 mg, 10 wt.% Pd, 0.46 mmol, 20 mol%) was added. The inert gas was replaced by hydrogen (1 atm) and the suspension was then stirred vigorously for 17 hours. The mixture was filtered over a pad of Celite^®, washed thoroughly with EtOAc (200 mL) and the solvent was removed under reduced pressure. The crude product was used for further reaction without purification.

Crude diol 18’ (as obtained like described above) was dissolved in dry CH2Cl2 (24.8 mL) and 2,6-lutidine (1.67 mL, 14.3 mmol, 6.25 eq.) was added. The solution was cooled to 0 °C and TBSOTf (2.21 mL, 9.62 mmol, 4.2 eq.) was added. The mixture was stirred at 0 °C for 1.5 hours and then quenched with saturated aqueous NaHCO3 (300 mL). The mixture was extracted with Et2O (3 × 150 mL), the combined organic layers were washed with saturated aqueous NH4Cl (200 mL) and dried over Na2SO4. The solvents were removed under reduced pressure and the crude product was purified via flash column chromatography (silica, n-Hex) to yield silyl ether 19’ (1.40 g, 2.19 mmol, 96 % yield over 2 steps) as a colourless wax. Empirical formula: C31¹³C5H76O3Si3 (646.22 g mol⁻¹, silyl ether 19’). Rf (19’)= 0.93 (n-Hex/EtOAc = 9:1). Synthesis of alcohol 20S

The reaction was conducted under ambient conditions. Silyl ether 19_S (59.0 mg, 92.0 μmol, 1.0 eq.) was dissolved in THF/H₂O (3.8 mL, v/v = 2:1) and acetic acid (3.75 mL). The solution was heated to 50 °C and stirred vigorously at this temperature for 13 hours. The reaction was quenched via the addition of saturated aqueous NaHCO₃ (100 mL) and the resulting mixture was extracted with Et₂O (3 × 100 mL). The combined organic layers were dried over Na₂SO₄ and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex/Et₂O = 195:5 to 185:15) to yield alcohol 20_S (37.8 mg, 71.7 μmol, 78 % yield) as a colourless wax. Empirical formula: C₃₀H₆₂O₃Si₂ (526.99 g mol−1, alcohol 20_S). R_f = 0.23 (n-Hex/EtOAc = 5:0.2). 1H-NMR (400 MHz, CDCl₃) δ = 4.07 (d, J = 2.8 Hz, 1H), 3.18 (dd, J = 7.7, 2.5 Hz, 1H), 2.03 – 1.75 (m, 5H), 1.63 (ddd, J = 17.2, 11.5, 4.1 Hz, 1H), 1.56 – 1.38 (m, 4H), 1.38 – 1.22 (m, 4H), 1.19 (s, 3H), 1.16 – 0.99 (m, 3H), 1.11 (s, 3H), 0.93 (s, 3H), 0.89 (d, J = 9.2 Hz, 3H), 0.89 (s, 9H), 0.85 (s, 9H), 0.91 – 0.83 (m, 1H), 0.08 (s, 3H), 0.07 (s, 3H), 0.07 (s, 3H), 0.04 (s, 3H) ppm. 1³C-NMR (101 MHz, CDCl₃): δ = 81.3, 76.5, 69.6, 56.9, 52.8, 42.0, 40.6, 36.1, 34.1, 33.8, 29.8, 29.1, 27.5, 26.3, 26.0, 23.3, 22.7, 18.7, 18.4, 18.3, 17.6, 13.7, −1.8, −1.9, −3.1, −3.7 ppm. Synthesis of alcohol 20’

The reaction was conducted under ambient conditions. Silyl ether 19’ (1.14 mg, 1.76 mmol, 1.0 eq.) was dissolved in THF/H₂O (22.8 mL, v/v = 2:1) and acetic acid (42.8 mL). The solution was heated to 50 °C and stirred vigorously at this temperature for 13 hours. The reaction was quenched via the addition of saturated aqueous NaHCO₃ (400 mL) and the resulting mixture was extracted with Et₂O (3 × 200 mL). The combined organic layers were dried over Na₂SO₄ and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex/Et₂O = 195:5 to 185:15) to yield alcohol 20’ (831 mg, 1.56 mmol, 89 % yield) as a colourless wax. Empirical formula: C₂₅ ¹³C₅H₆₂O₃Si₂ (531.95 g mol−1, alcohol 20’). R_f = 0.23 (n-Hex/EtOAc = 5:0.2). Synthesis of ketone 4_S

To a solution of alcohol 20_S (53.0 mg, 100 μmol, 1.0 eq.) in dry acetone (3.5 mL), NMO (35.0 mg, 299 μmol, 3.0 eq.) and molecular sieves (4 Å, 1 spatula) were added. The suspension was stirred at room temperature for 10 min, then TPAP (7.10 mg, 20.2 μmol, 0.2 eq.) was added. The reaction mixture was stirred for three hours at room temperature and then filtered over a pad of silica (Et₂O washings). The solvents were removed under reduced pressure, and the crude product was purified via flash column chromatography (silica, n- Hex/EtOAc = 90:10). Ketone 4_S (48.9 mg, 93.1 μmol, 92 % yield) was obtained as a colorless wax. Empirical formula: C₃₀H₆₀O₃Si₂ (524.98 g mol−1, ketone 4_S). R_f = 0.69 (n-Hex/EtOAc = 4:1). ¹H-NMR (400 MHz, CDCl₃) δ = 3.18 (dd, J = 7.7, 2.5 Hz, 1H), 2.43 (dd, J = 11.7, 7.4 Hz, 1H), 2.34 – 2.18 (m, 2H), 2.12 (dt, J = 13.1, 3.8 Hz, 1H), 2.06 – 1.82 (m, 4H), 1.79 – 1.22 (m, 7H), 1.19 (s, 3H), 1.11 (s, 3H), 1.15 – 0.99 (m, 1H), 0.95 (d, J = 6.4 Hz, 3H), 0.98 – 0.89 (m, 1H), 0.89 (s, 9H), 0.85 (s, 9H), 0.64 (s, 3H), 0.09 (s, 3H), 0.07 (s, 3H), 0.06 (s, 3H), 0.04 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 212.3, 81.2, 76.5, 62.2, 56.9, 50.1, 41.1, 39.2, 36.3, 34.1, 29.7, 29.0, 27.9, 26.2, 26.0, 24.2, 23.2, 19.3, 18.9, 18.4, 18.3, 12.6, −1.8, −1.8, −3.1, −3.7 ppm. Synthesis of ketone 4^’

To a solution of alcohol 20_S (820 mg, 1.56 mmol, 1.0 eq.) in dry acetone (50.0 mL), NMO (547 mg, 4.67 mmol, 3.0 eq.) and molecular sieves (4 Å, 3 spatulas) were added. The suspension was stirred at room temperature for 10 min, then TPAP (109 mg, 0.31 mmol, 0.2 eq.) was added. The reaction mixture was stirred for three hours at room temperature and then filtered over a pad of silica (Et2O washings). The solvents were removed under reduced pressure, and the crude product was purified via flash column chromatography (silica, n-Hex/EtOAc = 90:10). Ketone 4^’ (771 mg, 1.47 mmol, 94 % yield) was obtained as a colourless wax. Empirical formula: C25¹³C5H60O3Si2 (529.94 g mol−1, ketone 4^’). Rf = 0.69 (n-Hex/EtOAc = 4:1). Synthesis of triene 21_S

Phosphine oxide 3 (109 mg, 241 μmol, 3.0 eq.) was dissolved in dry THF (7.4 mL) and the resulting solution was cooled to −78 °C. n-BuLi (102 μL, 2.13 M, 217 μmol, 2.7 eq.) was added dropwise, and the reaction mixture was stirred for 15 min at −78 °C and 15 min at 0 °C. After cooling the solution to −78 °C again, a solution of ketone 4_S (42.2 mg, 80.4 μmol, 1.0 eq.) in THF (1.0 mL) was added and the reaction was stirred at this temperature for 45 min. Stirring was continued for one hour at −35 °C bevor the reaction was quenched via the addition of saturated aqueous NH4Cl (0.5 mL). After warming to room temperature, the mixture was added into saturated aqueous NaHCO3 (40 mL) and diluted with Et2O (40 mL). The layers were separated, and the aqueous phase was extracted with Et2O (3 × 40 mL). The combined organic layers were dried over Na2SO4 and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex/Et2O = 100:0 to 95:5). Triene 21_S (59.2 mg, 78.0 μmol, 97 % yield) was obtained as a colourless solid. Empirical formula: C45H86O3Si3 (759.44 g mol−1, triene 21_S). Rf = 0.88 (n-Hex/EtOAc = 6:0.4). ¹H-NMR (400 MHz, CDCl3) δ = 6.16 (d, J = 11.1 Hz, 1H), 6.02 (d, J = 11.2 Hz, 1H), 5.00 (t, J = 2.1 Hz, 1H), 4.78 (dd, J = 2.6, 1.3 Hz, 1H), 3.82 (tt, J = 8.8, 4.0 Hz, 1H), 3.18 (dd, J = 7.6, 2.4 Hz, 1H), 2.87 – 2.78 (m, 1H), 2.45 (ddd, J = 12.9, 4.4, 1.7 Hz, 1H), 2.36 (dt, J = 13.4, 4.7 Hz, 1H), 2.23 (dd, J = 13.0, 9.5 Hz, 1H), 2.09 (ddd, J = 13.5, 11.4, 4.4 Hz, 1H), 2.04 – 1.82 (m, 5H), 1.78 – 1.42 (m, 7H), 1.38 – 1.21 (m, 5H), 1.19 (s, 3H), 1.11 (s, 3H), 1.13 – 0.98 (m, 1H), 0.92 (d, J = 5.8 Hz, 3H), 0.89 (s, 18H), 0.85 (s, 9H), 0.55 (s, 3H), 0.08 (s, 3H), 0.07 (s, 6H), 0.06 (s, 6H), 0.04 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 145.6, 141.7, 136.5, 121.5, 118.0, 112.2, 81.3, 76.5, 70.8, 56.8, 56.5, 47.1, 45.9, 40.8, 37.0, 36.6, 34.3, 33.0, 29.8, 29.2, 29.1, 28.1, 26.3, 26.1, 26.0, 23.7, 23.3, 22.4, 19.0, 18.4, 18.3, 18.3, 12.2, −1.8, −1.9, −3.1, −3.7, −4.4, −4.5 ppm. Synthesis of triene 21^’

Phosphine oxide 3 (970 mg, 2.14 mmol, 3.0 eq.) was dissolved in dry THF (27.5 mL) and the resulting solution was cooled to −78 °C. n-BuLi (771 μL, 2.50 M, 1.93 μmol, 2.7 eq.) was added dropwise, and the reaction mixture was stirred for 15 min at −78 °C and 15 min at 0 °C. After cooling the solution to −78 °C again, a solution of ketone 4^’ (375 mg, 0.71 mmol, 1.0 eq.) in THF (5.00 mL) was added and the reaction was stirred at this temperature for 45 min. Stirring was continued for one hour at −35 °C bevor the reaction was quenched via the addition of saturated aqueous NH₄Cl (1.5 mL). After warming to room temperature, the mixture was added into saturated aqueous NaHCO₃ (200 mL) and diluted with Et₂O (150 mL). The layers were separated, and the aqueous phase was extracted with Et₂O (3 × 50 mL). The combined organic layers were dried over Na₂SO₄ and the solvents were removed under reduced pressure. The crude product was purified via flash column chromatography (silica, n-Hex/Et₂O = 100:0 to 95:5). Triene 21^’ (518 mg, 0.68 mmol, 96 % yield) was obtained as a colourless solid. Empirical formula: C₄₀ ¹³C₅H₈₆O₃Si₃ (764.40 g mol−1, triene 21^’). R_f = 0.88 (n-Hex/EtOAc = 6:0.4). Synthesis of 24(S),25-dihydroxycholecalciferol (1_S)

Triene 21_S (54 mg, 71 μmol, 1.0 eq.) was dissolved in TBAF solution (2.00 mL, 1.00 M solution in THF, 2.00 mmol, 28 eq.) at room temperature and molecular sieves (4 Å, 1 spatula) was added. The suspension was stirred at room temperature for 25 hours. The reaction was quenched with water (40 mL) and then diluted with EtOAc (40 mL). The layers were separated, and the aqueous phase was extracted with Et2O (3 × 40 mL). The combined organic layers were dried over Na2SO4 and the solvents were removed under reduced pressure. The crude product was then purified via flash column chromatography (silica, n- Hex/EtOAc = 75:25 to 65:35). 24(S),25-Dihydroxycholecalciferol (1_S, 26.1 mg, 62.6 μmol, 87 % yield) was obtained as a colourless wax in a 19:1 (mol/mol) mixture with previtamin (22_S). Final separation of vitamin metabolite 1_S from its previtamin was achieved via preparative HPLC (RP C18, H2O/MeCN = 90:10 to 5:95). To prevent re- isomerization, the product containing fractions were quickly concentrated under reduced pressure at low temperature (≤ 22 °C) and dried briefly in vacuo (≤ 22 °C, ≤ 0.01 mbar) under light exclusion. Substantially pure 24,25- dihydroxycholecalciferol 1_S (12.1 mg, 29 mmol, 62 % yield) was obtained as a colourless solid. Empirical formula: C₂₇H₄₄O₃ (416.65 g mol−1, 24(S),25- dihydroxycholecalciferol 1_S). R_f = 0.13 (n-Hex/EtOAc = 3:2). ¹H-NMR (400 MHz, CDCl₃) δ = 6.23 (d, J = 11.2 Hz, 1H), 6.03 (d, J = 11.2 Hz, 1H), 5.05 (d, J = 1.4 Hz, 1H), 4.82 (d, J = 2.5 Hz, 1H), 3.94 (tt, J = 7.5, 3.6 Hz, 1H), 3.28 (d, J = 9.4 Hz, 1H), 2.82 (dd, J = 12.0, 4.1 Hz, 1H), 2.57 (dd, J = 13.0, 3.8 Hz, 1H), 2.40 (ddd, J = 13.2, 7.7, 4.7 Hz, 1H), 2.28 (dd, J = 13.2, 7.6 Hz, 1H), 2.17 (ddd, J = 13.7, 8.6, 4.8 Hz, 1H), 2.11 (s, 1H), 2.06 – 1.81 (m, 5H), 1.81 – 1.36 (m, 10H), 1.31 (dq, J = 10.2, 6.5, 5.0 Hz, 3H), 1.22 (s, 3H), 1.16 (s, 3H), 1.20 – 0.97 (m, 2H), 0.95 (d, J = 6.5 Hz, 3H), 0.54 (s, 3H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 145.2, 142.3, 135.3, 122.6, 117.7, 112.6, 79.7, 73.4, 69.4, 56.6, 56.4, 46.1, 46.0, 40.7, 36.5, 35.3, 33.4, 32.1, 29.2, 28.5, 27.8, 26.7, 23.7, 23.4, 22.4, 19.1, 12.2 ppm. Synthesis of 24(R),25-dihydroxycholecalciferol (1’)

Triene 21’ (188 mg, 0.25 mmol, 1.0 eq.) was dissolved in TBAF solution (6.94 mL, 1.00 M solution in THF, 6.94 mmol, 28 eq.) at room temperature and molecular sieves (4 Å, 2 spatulas) was added. The suspension was stirred at room temperature for 25 hours. The reaction was quenched with water (80 mL) and then diluted with EtOAc (80 mL). The layers were separated, and the aqueous phase was extracted with Et2O (3 × 80 mL). The combined organic layers were dried over Na2SO4 and the solvents were removed under reduced pressure. The crude product was then purified via flash column chromatography (silica, n- Hex/EtOAc = 75:25 to 65:35). 24(R),25-Dihydroxycholecalciferol-¹³C5 (1^’, 93 mg, 0.23 mmol, 89 % yield) was obtained as a colourless wax in a 19:1 (mol/mol) mixture with its previtamin. Final separation of vitamin metabolite 1^’ from its previtamin was achieved via preparative HPLC (RP C18, H2O/MeCN = 90:10 to 5:95). To prevent re- isomerization, the product containing fractions were quickly concentrated under reduced pressure at low temperature (≤ 22 °C) and dried briefly in vacuo (≤ 22 °C, ≤ 0.01 mbar) under light exclusion. Substantially pure 24(R),25- dihydroxycholecalciferol-¹³C₅ (1^’, 85 mg, 0.21 mmol, 81 % yield) was obtained as a colourless solid. Empirical formula: C₂₂ ¹³C₅H₄₄O₃ (421.61 g mol−1, 24(R),25- dihydroxycholecalciferol-¹³C₅, 1^’). R_f = 0.13 (n-Hex/EtOAc = 3:2). ¹H NMR (500 MHz, CDCl₃): δ = 6.24 (d, J = 11.2 Hz, 1H), 6.04 (d, J = 11.2 Hz, 1H), 5.06 (s, 1H), 4.83 (s, 1H), 3.95 (m_C, 1H), 3.34 (d, br, ¹J_13CH = 141.3 Hz, 1H), 2.85 – 2.81 (m, 1H), 2.58 (dd, J = 13.2, 3.3 Hz, 1H), 2.41 (m_C, 1H), 2.30 (dd, J = 13.1, 7.5 Hz, 1H), 2.19 (m_C, 1H), 2.02 – 1.98 (m, 3H), 1.96 – 1.91 (m, 2H), 1.89 (s, br, 1H), 1.73 – 1.66 (m, 3H), 1.55 – 1.40 (m, 6H), 1.36 – 1.25 (m, 8H), 1.02(dq, J = 25.9, 4.4 Hz, 3 H), 0.95 (d, J = 6.9 Hz, 3H), 0.56 (s, 3H) ppm. Example 2 All reactions were magnetically stirred and, unless otherwise noted, carried out under a positive pressure of argon utilizing standard Schlenk-techniques. Glassware was dried at 650 °C in vacuo prior to use. Liquid reagents and solvents were added by syringes through rubber septa. Solids were added under inert gas counter flow or were dissolved in appropriate solvents. Low temperature reactions were carried out in a Dewar vessel filled with a cooling agent: acetone/dry ice (−78 °C), NaCl/ice (−20 °C) or H₂O/ice (0 °C). Reaction temperatures above room temperature were conducted in a heated oil bath. Yields refer to isolated homogenous and spectroscopically pure materials, if not indicated otherwise. In drawings, a “*” indicates a ¹³C-labelled site. Solvents and Reagents Diethyl ether (Et₂O) was distilled under reduced pressure prior to use. Solvents for extraction, crystallization and flash column chromatography were purchased in LiChrosolv^® hypergrade from Merck KGaA. The ¹³C-enriched compounds TMS- acetylene-¹³C₂ and ¹³C₃-acetone were purchased from Cambridge Isotope Laboratories. Inhoffen-Lythgoe diol and Wittig reagent were purchased from Merck KGaA. All other reagents and solvents were purchased from chemical suppliers (Sigma-Aldrich/Merck KGaA, Acros Organics, Honeywell/Fluka) and were used as received. n-BuLi in THF was titrated against diphenyl acetic acid (100 mg dissolved in 8.0 mL dry THF) prior to use to determine the exact molarity of the solution. Chromatography Qualitative thin-layer chromatography (TLC) on silica gel 60 F254 TLC plates from Merck KGaA was used to monitor reactions and preparative chromatography. Analytes on the plates were visualised by irradiation with UV-light (245 nm) and/or staining with an appropriate staining solution. The plate was immersed into the staining solution and then heated with a hot-air gun (350 °C). The following staining solutions were applied: p-Anisaldehyde staining solution (3.7 mL para-anisaldehyde, 5.0 mL concentrated aqueous H₂SO₄, 1.5 mL glacial AcOH, 135 mL EtOH) and KMnO₄ staining solution, (3.0 g KMnO₄, 20 g K₂CO₃, 5.0 mL aqueous 5% NaOH, 300 mL H₂O). Experimental flash column chromatography was performed on Geduran^® Si60 60 (40 – 63 μm) silica gel from Merck KGaA. All fractions containing a desired substrate were combined and solvents were removed under reduced pressure followed by drying in high vacuo (10 – 2 mbar) for non volatile substances. NMR spectroscopy NMR spectra were recorded on an Agilent 400-MR DD2 spectrometer equipped with a One Probe operating at 400 MHz for proton nuclei and at 101 MHz for carbon nuclei, or at a 500 MHz Bruker Avance Neo spectrometer equipped with a Prodigy Probe operating at 500 MHz for proton nuclei and at 125 MHz for carbon nuclei. The chemical shifts δ of the NMR spectra are reported in ppm relative to the shift of the standard TMS. NMR shifts are calibrated to the residual solvent resonances of CDCl₃ (7.26 ppm for ¹H-NMR and 77.16 ppm for ¹³C-NMR). Spectroscopic data is reported as follows: Chemical shift in ppm (multiplicity, coupling constants J in Hz, integration intensity). In the report of spectroscopic data, the multiplicity of signals is abbreviated with s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and mc (centrosymmetric multiplet). In case of combined multiplicities, the multiplicity with the larger coupling constant is stated first. With exception of the multiplets, the reported chemical shifts of the signal corresponds to the center of the resonance range. The ¹J_13CH coupling constants exceed any other J_HH coupling constants and are therefore reported in the cases of centrosymmetric multiplet. Additionally to ¹H and ¹³C-NMR measurements, 2D NMR techniques like homonuclear correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond coherence (HMBC) were used to assign signals. All NMR spectra were analysed using the program MestReNOVA 14.1.1 from Mestrelab Research. High Performance Liquid Chromatography (HPLC) HPLC was carried out using HPLC grade solvents and deionized, ultra-filtered water. All separations were conducted at room temperature. Analytical UV-Vis spectra were recorded on a 1260 Infinity HPLC system from Agilent Technologies Inc. that was computer-controlled through Chromeleon^TM Chromatography Data Systems Software Version 7.2 SR5 Mui (24070) from Thermo Fisher Scientific Inc., using an ACQUITY UPLC, Oligonucleotide 130A, 1.7 μm, BEH C18 column from Waters Corporation, detecting at 265 nm wavelength. Synthesis of 24(R),25-Dihydroxyvitamin D2 Synthesis of Aldol 24

To a stirred solution of LDA (Lithium diisopropylamide) (1.66 M in THF, 6.97 mL, 11.60 mmol, 4.2 eq.) was added dry THF (4.60 mL) at -78 °C under inert gas atmosphere. To this 1.00 molar solution of LDA, butanone 23, which was synthesized from TMS acetylene and acetone according to Angew. Chem. Int. Ed. 2015, 54, 9066 – 9069, Synthesis 2019, 51, 739–746 and J. Mater. Chem., 2011, 21, 4242–4250, (894 mg, 4.13 mmol, 1.5 eq.) in dry THF (7.00 mL) was added slowly over the course of 15 minutes. The resulting mixture was stirred for an hour, before adding aldehyde 22, which was synthesized from Inhoffen- Lythgoe diol 2 (894 mg, 2.75 mmol, 1.0 eq.) in dry THF (6.00 mL) slowly over the course of 20 min. The reaction mixture was stirred at −78 °C for two hours before being quenched by the addition of sat. NH4Cl solution (2.00 mL). After warming to room temperature the reaction was diluted with aq. oxalic acid (100 mL, 5 wt-%) and extracted with diethylether (3 x 100 mL). The combined organic layers were washed with aq. sat. Bicarb solution (100 mL), dried over Na₂SO₄, filtered and concentrated in vacuo. The crude product was purified via flash column chromatography (silica, n-Hex:Et₂O = 100:0 to 90:10) to yield aldol 24 (1.18 g, 2.18 mmol, 79%) as a slightly yellow oil as a diastereomeric mixture on carbon C-22 of undetermined ratio. Empirical formula: C₃₀H₆₀O₄Si₂ (540.98 g mol⁻¹, 24). R_f = 0.40 (n-Hex/EtOAc = 10:1). ¹H-NMR (400 MHz, CDCl₃, mixture of two isomers): δ = 4.15–4.08 (m, 1H), 4.05 (dd, J = 3.1, 2.4 Hz, 1H), 2.93–2.79 (m, 2H), 2.73–2.53 (m, 2H), 2.00–1.74 (m, 3H), 1.73–1.43 (m, 4H), 1.42–1.24 (m, 4H), 1.33 (s, 3H), 1.32 (s, 3H), 1.23– 1.05 (m, 1H), 0.99–0.87 (m, 25H), 0.60–0.50 (m, 6H), 0.16–0.10 (m, 6H) ppm. ¹³C-NMR (101 MHz, CDCl₃, mixture of two isomers): δ = 218.4, 217.6, 80.4, 80.3, 69.5, 69.3, 69.0, 68.9, 54.1, 53.2, 53.1, 53.1, 42.6, 42.1, 42.1, 41.1, 40.9, 40.5, 40.2, 36.7, 34.8, 27.3, 27.2, 27.2, 27.1, 26.9, 26.725.9. 25.9. 23.2, 23.0, 18.3, 18.2, 17.9, 17.813.6, 12.8, 12.4, 7.1, 5.1, 5.1, −2.1, −2.1ppm. Synthesis of Enone 25

Aldol 24 (1.17 g, 2.16 mmol, 1,0 eq.) was dissolved in dry toluene (34.0 mL) and Burgess reagent (see J. Org. Chem. 1970, 35, 2594–2596) (1.28 g, 5.39 mmol, 2.5 eq.) was added in one portion at room temperature under inert gas atmosphere. The reaction was stirred for 20 hours and quenched via addition of aq. NaCl solution (100 mL, 10 wt-%). The mixture was extracted with diethylether (3 x 100 mL), dried over Na₂SO₄, filtered and concentrated in vacuo. The crude product was purified via flash column chromatography (silica, n- Hex:Et₂O = 100:0 to 95:5) to yield enone 25 (1.18 g, 2.18 mmol, 79%) as a colourless oil. Empirical formula: C₃₀H₅₈O₃Si₂ (522.96 g mol⁻¹, 25). R_f = 0.56 (n-Hex/EtOAc = 10:1). ¹H-NMR (400 MHz, CDCl₃): δ = 6.84 (dd, J = 15.6, 8.3 Hz, 1H), 6.68 (d, J = 15.6 Hz, 1H), 4.03 (d, J = 2.5 Hz, 1H), 2.3–2.19 (m, 1H), 1.94 (dt, J = 12.4, 3.1 Hz, 1H), 1.89–1.76 (m, 1H), 1.72–1.48 (m, 3H), 1.40–1.29 (m, 6H), 1.34 (s, 3H), 1.27–1.11 (m, 4H), 1.16 (d, J = 6.7 Hz, 3H), 0.96–0.92 (m, 12H), 0.91 (s, 9H), 0.55 (q, J = 7.9 Hz, 6H), 0.09 (d, J = 1.5 Hz, 6H) ppm. ¹³C-NMR (101 MHz, CDCl₃): δ = 203.2, 153.8, 121.8, 79.2, 69.3, 55.9, 53.0, 42.6, 40.8, 39.7, 34.6, 27.5, 27.2, 27.2, 26.0, 23.2, 23.1, 19.2, 18.2, 17.8, 13.7, 7.1, 5.1, −2.2, −2.2 ppm. Synthesis of Allyl Alcohol 26

Methyllithium (1.43 M in Et2O, 1.60 mL, 2.29 mmol, 3.0 eq.) was diluted with dry THF (21.5 mL) at -78 °C under inert gas atmosphere. Enone 25 (400 mg, 0.77 mmol, 1.0 eq.) in dry THF (2.00 mL) was added over the course of 10 min and the resulting mixture was stirred for two hours at this temperature. After quenching via the addition of sat. aq. NH4Cl solution (100 mL) the reaction was extracted with diethylether (3 x 100 mL), dried over Na2SO4, filtered and concentrated in vacuo. The crude product was purified via flash column chromatography (silica, n- Hex:Et2O = 100:0 to 95:5) to yield allyl alcohol 26 (402 g, 0.75 mmol, 98%) as colourless oil as a mixture of diastereomers. Empirical formula: C₃₁H₆₂O₃Si₂ (539.00 g mol⁻¹, 24). R_f = 0.61 (n-Hex/EtOAc = 10:1). ¹H-NMR (400 MHz, CDCl₃, mixture of diastereomers): δ = 5.60–5.36 (m, 2H), 4.07–4.02 (m, 1H), 2.66 (s, br, OH), 2.60 (s, br, OH), 2.10–2.00 (m, 1H), 1.94 (d, J = 13.1 Hz, 1H), 1.87–1.76 (m, 1H), 1.68–1.49 (m, 4H), 1.38–1.20 (m, 6H), 1.21 (m, 6H), 1.16–1.04 (m, 2H), 1.01 (d, J = 7.1 Hz, 3H), 0.96–0.91 (m, 12H), 0.89 (s, 9H), 0.81 (dd, J = 14.9, 6.8 Hz, 1H), 0.55 (q, J = 7.8 Hz, 6H), 0.12 (m, 6H) ppm. ¹³C-NMR (101 MHz, CDCl₃, mixture of diastereomers): δ = 136.1, 136.0, 130.3, 130.1, 78.8, 78.8, 69.4, 56.5, 53.1, 53.142.1, 40.7, 39.9, 39.7, 34.6, 28.0, 27.9, 27.1, 15.8, 25.3, 25.2, 25.2, 23.0, 22.5, 20.5, 20.3, 18.2, 17.7, 13.7, 9.4, 7.0, 4.9, −2.3, −2.3, −2.3 ppm. Synthesis of Diol 27

=₌ Allyl alcohol 26 (396 mg, 0.74 mmol) was dissolved in THF (11.40 mL) at room temperature. Then, glacial acetic acid (16.90 mL) and dest. Water (2.80 mL) were added slowly. The reaction mixture was stirred for 22 hours at 50 °C (oil bath temperature). After diluting with dest. water (100 mL), the reaction was extracted with diethylether (3 x 100 mL). The combined organic layers were washed with water (150 mL) and then aq. sat. bicarb solution (150 mL). After drying over Na2SO4, the organic layer was filtered and concentrated in vacuo. The crude product was purified via flash column chromatography (silica, n-Hex:Et2O = 95:5 to 86:14) to yield diol 27 (256 mg, 0.61 mmol, 82%) as colourless oil as a mixture of two diastereomers. Empirical formula: C₂₅H₄₈O₃Si (424.74 g mol⁻¹, 27). R_f = 0.28 (n-Hex/EtOAc = 6:1). ¹H-NMR (400 MHz, CDCl₃, mixture of diastereomers): δ = 5.58–5.40 (m, 2H), 4.07 (s, br, 1H), 2.66 (s, br, C-24–OH), 2.60 (s, br, C-24–OH), 2.11–1.96 (m, 2H), 1.91–1.63 (m, 3H), 1.55–1.11 (m, 9H), 1.22–1.20 (m, 9H), 1.01 (d, J = 7.0 Hz, 3H), 0.95 (s, 3H), 0.89 (s, 9H), 0.13–0.12 (m, 6H) ppm. ¹³C-NMR (101 MHz, CDCl₃, mixture of diastereomers): δ = 135.9, 130.8, 130.6, 79.0, 79.0, 69.6, 56.7, 56.5, 52.8, 52.8, 42.0, 40.4, 40.0, 39.8, 33.7, 27.9, 27.9, 26.0, 26.0, 26.0, 25.5, 25.4, 25.3, 25.3, 23.1, 22.9, 22.9, 22.8, 22.7, 22.7, 20.6, 20.5, 18.4, 17.6, 13.9, 12.3, 4.7, −2.14 ppm. Synthesis of Ketone 28

Diol 27 (252 mg, 0.59 mmol, 1.0 eq.) was dissolved in dry acetone (10.0 mL) at room temperature under inert gas atmosphere. To this, molecular sieves (2 spatulas, 4 Å) and NMO (209 mg, 4.13 mmol, 1.5 eq.) were added and the resulting mixture stirred for 10 min. Then, TPAP (41.9 mg, 0.20 mmol, 0.2 eq.) was added in one portion and the reaction was stirred for two hours, before being diluted with diethylether (20 mL) and quickly filtered over silica column (silica, Et₂O = 100%). The solvents were removed in vacuo. The crude product was purified via flash column chromatography (silica, n-Hex:Et₂O = 85:15 to 75:25) to yield ketone 28 (250 mg, 0.59 mmol, quant.) yield) as colourless oil as a mixture of two diastereomers. Empirical formula: C₂₅H₄₆O₃Si (422.73 g mol⁻¹, 27). R_f = 0.32 (n-Hex/EtOAc = 5:1). ¹H-NMR (400 MHz, CDCl₃, mixture of diastereomers): δ = 5.65–5.41 (m, 2H), 2.66 (s, br, C-24–OH), 2.60 (s, br, C-24–OH), 2.46 (dd, J = 13.9, 10.8 Hz, 1H), 2.31–2.18 (m, 1H), 2.11–1.88 (m, 4H), 1.78–1.45 (m, 8H), 1.34–1.26 (m, 2H), 1.22–1.20 (m, 8H), 1.07 (d, J = 6.7 Hz, 3H), 0.89 (s, 9H), 0.66 (s, 3H), 0.13– 0.12 (m, 6H) ppm. Synthesis of TMS ether 29

28 29 C₂₅H₄₆O₃S C₂₈H₅₄O₃Si₂ _{M = 422.73 g/moli M = 494.91 g/mol} Ketone 28 (118 mg, 0.28 mmol, 1.0 eq.) was dissolved in dry THF (1.40 mL) at room temperature under inert gas atmosphere. To this, excess TMS-imidazole (1.60 mL, 11.0 mmol, 39,0 eq.) was added and the resulting mixture stirred for 5 days at room temperature. After diluting with dest. water (50 mL), the reaction was extracted with diethylether (3 x 100 mL). The combined organic layers were washed with water (50 mL) and then dried over Na2SO4. The organic layer was filtered and concentrated in vacuo. The crude product 29 (124 mg, 0.25 mmol, 90%) was obtained as colourless wax as a mixture of two diastereomers and used for further reaction without additional purification. Empirical formula: C28H54O3Si2 (494.91 g mol⁻¹, 29). Rf = 0.42 (n-Hex/EtOAc = 10:1). ¹H-NMR (400 MHz, CDCl3, mixture of diastereomers): δ = 5.64–5.52 (dd, J = 15.7, 7.4 Hz, 1H), 5.38–5.31 (dd, J = 15.7, 8.8 Hz, 1H), 2.46 (dd, J = 19.5, 15.1 Hz, 1H), 2.36–2.15 (m, 2H), 2.15–1.83 (m, 4H), 1.83–1.42 (m, 5H), 1.38–1.21 (m, 1H), 1.28 (s, 3H), 1.17 (m, 3H), 1.10 (s, 3H), 1.07–1.05 (m, 3H), 0.86 (s, 9H), 0.67–0.66 (m, 3H), 0.09–0.05 (m, 15H) ppm. Synthesis of Triene 30

Phosphane oxide 3 (568 mg, 1.26 mmol, 5.0 eq.) was dissolved in dry THF (17.0 mL) at room temperature under inert gas atmosphere and cooled to -78 °C. Then, n- BuLi (0.57 mL, 2.16 M, 1.23 mmol, 4.9 eq.) was added dropwise and the resulting deep red solution was stirred for 30 min at this temperature. Thereafter, ketone 29 (124 mg, 0.25 mmol, 1.0 eq.) in dry THF (6.00 mL) was added slowly over the course of 20 minutes. After stirring for another 30 min at -78 °C, the mixture was warmed to -60 to -45 °C and reacted for another 2 hours. After quenching via the addition of sat. aq. NH₄Cl solution (2.0 mL), the reaction was warmed to room temperature, diluted with aq. sat. bicarb solution (50 mL, 10 wt-%) and then extracted with diethylether (3 x 100 mL). The combined organic layers were dried over Na2SO4, filtered and quickly concentrated in vacuo (≤ 30 °C). The crude product was purified via flash column chromatography (silica, n-Hex:Et₂O = 100:0 to 97:3) to yield triene 30 (156 g, 0.22 mmol, 85%) as colourless wax. Empirical formula: C40H72O3Si2 (657.18 g mol⁻¹, 30). Rf = 0.92 (n-Hex/EtOAc = 10:1). ¹H-NMR (400 MHz, CDCl3, mixture of diastereomers): δ = 6.17 (d, J = 10.9 Hz, 1H), 6.01 (d, J = 11.3 Hz, 1H), 5.53 (dd, J = 15.4, 8.5 Hz, 1H), 5.34 (ddd, J = 15.4, 8.4, 4.0 Hz, 1H), 5.00 (s, 1H), 4.77 (s, 1H), 3.82 (m, 1H), 2.83 (d, J = 12.6 Hz, 1H), 2.45 (dd, J = 12.6, 3.3 Hz, 1H), 2.39–2.31 (m, 1H), 2.24 (dd, J = 12.4, 9.5 Hz, 1H), 2.12–2.03 (m, 2H), 2.01–1.96 (m, 1H), 1.92–1.88 (m, 1H), 1.78– 1.65 (m, 3H), 1.60–1.44 (m, 4H), 1.41–1.31 (m, 2H), 1.28 (s, 3H), 1.17 (m, 3H), 1.11 (s, 3H), 1.05–1.02 (m, 3H), 0.88 (s, 9H), 0.86 (s, 9H), 0.58–0.57 (m, 3H), 0.09–0.05 (m, 21H) ppm. ¹³C-NMR (101 MHz, CDCl₃, mixture of diastereomers): δ = 145.5, 145.5, 141.6, 141.6, 136.5, 134.9, 134.7, 133.0, 121.5, 121.5, 118.0, 118.0, 112.3, 80.4, 80.4, 78.5, 78.5, 70.7, 70.7, 56.6, 56.5, 56.5, 56.5, 47.1, 47.0, 45.9, 40.6, 40.6, 40.4, 36.6, 36.5, 33.032.9, 29.1, 28.5, 28.2, 26.0, 25.6, 25.5 25.2, 25.0, 23.6, 22.5, 22.4, 22.0, 21.2, 20.6, 18.4, 18.3,12.7, 12.4, 12.4, 5.1, 2.8, 2.7, 1.2, −2.0, −2.0, −4.4, −4.5 ppm. Synthesis of 24(R),25-Dihydroxyvitamin D2 (31)

Triene 30 (38.0 mg, 0.05 mmol, 1.0 eq.) was dissolved in a solution of TBAF (1 M in THF, 1.00 mL, 17.3 eq.) at room temperature under inert gas atmosphere. After addition of molecular sieves (1 spatula, 4 Å), the reaction was stirred at this temperature for 23 hours und exclusion of light. The mixture was then diluted with diluted with aq. sat. NaCl solution (20 mL, 10 wt-%) and extracted with diethylether (3 x 20 mL). The combined organic layers were dried over Na₂SO₄, filtered and quickly concentrated in vacuo (≤ 30 °C). Purification of vitamin metabolite 32 was achieved via preparative HPLC (RP Hypersil GOLD^TM C4, H₂O/MeCN = 90:10 to 5:95). To prevent re-isomerization, the product containing fractions were quickly concentrated under reduced pressure at low temperature (≤ 22 °C) and dried briefly in vacuo (≤ 22 °C, ≤ 0.01 mbar) under light exclusion. Substantially pure 24(R),25- dihydroxyvitamine D2 (31, 5.45 mg, 0.01 mmol, 22%) was obtained as a colourless solid. Empirical formula: C28H44O3 (657.18 g mol⁻¹, 31). R_f = 0.21 (n-Hex/EtOAc = 3:2). ¹H-NMR (500 MHz, CDCl₃): δ = 6.23 (d, J = 11.3 Hz, 1H), 6.03 (br, J = 11.3 Hz, 1H), 5.67–5.47 (m, 2H), 5.04 (d, J = 2.2 Hz, 1H), 4.81 (d, J = 2.2 Hz, 1H), 4.01–3.87 (m, 1H), 2.83 (dd, J = 12.0, 3.9 Hz, 1H), 2.57 (dd, J = 13.2, 3.9 Hz, 1H), 2.44–2.35 (m, 1H), 2.28 (dd, J = 13.2, 7.5 Hz, 1H), 2.22–2.06 (m, 2H), 2.03–1.89 (m, 5H), 1.75–1.61 (m, 4H), 1.60–1.30 (m, 6H), 1.29–1.26 (m, 1H), 1.28 (s, 3H), 1.21 (s, 3H), 1.19 (s, 3H), 1.05 (d, J = 6.6 Hz, 3H), 0.57 (s, 3H) ppm. ¹³C-NMR (125 MHz, CDCl₃): δ = 145.2, 142.0, 136.8, 135.4, 131.0, 122.5, 117.8, 112.6, 75.1, 69.3, 56.5, 56.3, 46.1, 45.1, 40.5, 35.3, 32.1, 29.1, 28.1, 25.4, 24.5, 23.7, 22.9, 22.4, 20.9, 12.4 ppm. Synthesis of 24(R),25-Dihydroxyvitamin D2-¹³C₅ (33)

Triene 32 (68.4 mg, 0.09 mmol, 1.0 eq.) was dissolved in a solution of TBAF (1 M in THF, 1.80 mL, 17.3 eq.) at room temperature under inert gas atmosphere. After addition of molecular sieves (1 spatula, 4 Å), the reaction was stirred at this temperature for 23 hours und exclusion of light. The mixture was then diluted with diluted with aq. sat. NaCl solution (40 mL, 10 wt-%) and extracted with diethylether (3 x 40 mL). The combined organic layers were dried over Na₂SO₄, filtered and quickly concentrated in vacuo (≤ 30 °C). Purification of vitamin metabolite 32 was achieved via preparative HPLC (RP Hypersil GOLD^TM C4, H₂O/MeCN = 90:10 to 5:95). To prevent re-isomerization, the product containing fractions were quickly concentrated under reduced pressure at low temperature (≤ 22 °C) and dried briefly in vacuo (≤ 22 °C, ≤ 0.01 mbar) under light exclusion. Substantially pure 24(R),25- dihydroxyvitamine D2-¹³C₅ (33, 10.3 mg, 0.02 mmol, 23%) was obtained as a colourless solid. Empirical formula: ¹³C₅C₁₈H₄₄O₃ (433.67 g mol⁻¹, 33). R_f = 0.21 (n-Hex/EtOAc = 3:2). ¹H-NMR (500 MHz, CDCl₃): δ = 6.23 (d, J = 11.3 Hz, 1H), 6.03 (br, J = 11.3 Hz, 1H), 5.57 (ddd, J = 153.2, 15.6, 4.3 Hz 1H), 5.56 (m, 1H), 5.04 (m, 1H), 4.81 (d, J = 2.2 Hz, 1H), 3.97–3.88 (m, 1H), 2.83 (dd, J = 12.0, 3.9 Hz, 1H), 2.57 (dd, J = 13.2, 3.9 Hz, 1H), 2.42–2.37 (m, 1H), 2.29 (dd, J = 13.2, 7.5 Hz, 1H), 2.20–2.15 (m, 1H), 2.15–2.07 (m, 1H), 2.01–1.90 (m, 5H), 1.74–1.64 (m, 4H), 1.59–1.30 (m, 7.5H), 1.29–1.21 (m, 1H), 1.28 (q, J = 3.5 Hz, 3H), 1.10–1.04 (m, 1.5H), 1.05 (d, J = 6.7 Hz, 3H), 0.57 (s, 3H) ppm. ¹³C-NMR (125 MHz, CDCl₃): δ = 145.2, 142.1, 136.5, 135.4, 133.5, 132.0, 131.2, 130.8, 130.2, 124.3, 122.5, 77.1, 77.0, 76.7, 75.5, 75.2, 74.9, 74.6, 69.3, 56.5, 56.3, 46.1, 46.0, 40.5, 40.5, 35.3, 32.1, 29.1, 28.1, 25.6, 25.3, 24.6, 24.3, 23.7, 23.0, 22.7, 22.4, 20.9, 20.9, 12.4 ppm.

Claims

PATENT CLAIMS 1. A method of selectively deprotecting a transhydrindane-skeleton-based compound comprising at least two different silyl ether groups comprising the steps of: (a) providing a compound comprising at least one first silyl ether group and at least one second silyl ether group, wherein the at least one first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS, and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group; (b) selectively deprotecting the transhydrindane-skeleton-based compound wherein the TES or TMS group of the at least one first silyl ether group is replaced with hydrogen to obtain a hydroxyl group and wherein the silyl group of the at least one second silyl ether group is not replaced.

2. The method of claim 1, wherein the transhydrindane-skeleton-based compound comprising at least two different silyl ether groups is a compound according to formula (I):

wherein R₁ is a first silyl ether group; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; and wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and step (b) is a step of selectively deprotecting the silyl ether group of R₁ by replacing the TES or TMS group with hydrogen to obtain a hydroxyl group, while the second silyl ether group of X₅, R₂, R₃ and/or R₄ is not replaced.

3. A method of synthesizing a Vitamin D molecule comprising the steps of: (a) providing a transhydrindane-skeleton-based compound according to formula (I)

wherein R₁ is a first silyl ether group, wherein the first silyl ether group comprises a silyl group which is selected from the group consisting of TES and TMS; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein the at least one second silyl ether group comprises a silyl group which is selected from a group of silyl groups being different to the first silyl ether group; and an A-ring fragment; (b) selectively deprotecting the compound according to formula (I), wherein the TES or TMS group of the first silyl ether group is replaced with hydrogen to obtain a hydroxyl group, while the second silyl ether group of X₅, R₂, R₃ and/or R₄ is not replaced; and (c) reacting the A-ring fragment with the oxygen of R₁ of a derivative of the compound of formula (I) obtained in step (b) to obtain a Vitamin D molecule.

4. The method according to claim 3, wherein the Vitamin D molecule is selected from the group consisting of 25-hydroxyvitamin D2, 1,25- dihydroxyvitamin D2, 24,25-dihydroxyvitamin D2, (24S)-24,25- dihydroxyvitamin D2, (24R)-24,25-dihydroxyvitamin D2, 1-epi-1,25- dihydroxyvitamin D2, 3-epi-1,25-dihydroxyvitamin D2, 25- hydroxyvitamin D3, 1,25-dihydroxyvitamin D3, 23,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, (24S)-24,25-dihydroxyvitamin D3, (24R)-24,25-dihydroxyvitamin D3, 1-epi-1,25-dihydroxyvitamin D3, 3- epi-1,25-dihydroxyvitamin D3, calcipotriol, tacalcitol, paricalcitol, oxacalcitriol, falecalcitriol, eldecalcitol, inecalcitol, seocalcitol and derivatives thereof.

5. The method according to any one of claims 2 to 4, wherein X₁ is H, R₂ or not present, X₂ is H, R₃ or not present, X₃ is H, R₄ or not present, X₄ is methyl, H or not present.

6. The method according to any one of claims 2 to 5, wherein X₅ is cyclopropyl, 1-methylethyl, 1-hydroxyl-1-methylethyl, 1-hydroxyl-1- trichloromethyl-2,2,2-trichloroethyl, or 2-hydroxyl-2-ethylbutyl, wherein in case a hydroxyl group is present in X₅, the hydrogen is replaced by a silyl group forming a second silyl ether group.

7. The method according to any one of claims 1 to 6, wherein the compound is labelled with a stable isotope.

8. The method according to any one of claims 2 to 7, wherein at least one carbon atoms of the side chain starting at C^a of the compound according to formula (I) are ¹³C.

9. The method according to any one of claims 1 to 8, wherein the silyl group of the at least one second silyl ether group is not TES or TMS.

10. The method according to any one of claims 1 to 9, wherein the silyl group in the second silyl ether groups of X₅, R₂, R₃ and/or R₄ is TBDMS, TBDPS, TIPS, Dimethyltexylsilyl, Diethylisopropylsilyl, 2-Norbonyldimethylsilyl, Di-t-butylisobutylsilyl, Tribenzylsilyl, Triphenylsilyl, Di-t- butylsilylmethyl, Diphenylmethylsilyl or Tris(trimethylsilyl)silyl.

11. The method of any one of claims 1 to 10, wherein in step (b) the selective deprotection is carried out by reacting the compound in an acidic solution.

12. The method of any one of claims 1 to 11, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an acid selected from the group consisting of acetic acid (AcOH), H₂SiF₆, citric acid, trifluoro acetic acid (TFA), HClO₄, HCl, HBr, HI, HF, HF*pyridine, HF*NEt₃, HF*urea, p-toluenesulfonic acid (pTSA), pyridinium p-toluenesulfonate (PPTS), camphorsulfonic acid (CSA), formic acid, H₂SO₄, trichloroacetic acid, CH₃SO₃H, CF₃SO₃H, H₃PO₄, Sc(OTf)₃, CeCl₃, CuCl₂, FeCl₃, Al₂O₃ and combinations thereof.

13. The method of any one of claims 1 to 12, wherein in step (b) the selective deprotection is carried out by reacting the compound according to formula (I) in a solution comprising an organic solvent selected from the group consisting of tetrahydrofuran (THF), 1,4-dioxane, diethylether, dimethoxyethane (DME), methyl tert-butyl ether (MTBE), 2-Methyl- tetrahydrofuran, toluene, benzene, ethanol, isopropyl alcohol, 1-butanol, 1- octanol, methanol, n-hexane, n-pentane, n-heptane, diglyme, dichloromethane, 1,2-dichloroethane, acetonitrile, cyclohexane, pyridine, di-isopropyl ether and combinations thereof.

14. The method according to any one of claims 3 to 13, wherein in step (c) the A-ring fragment is reacted to obtain a compound according to formula (II)

wherein X₅ comprises an alkyl moiety, which optionally comprises a second silyl ether group; wherein X₁ is H, OH, R₂ or not present, X₂ is H, OH, R₃ or not present, X₃ is H, OH, R₄ or not present, X₄ is methyl, ethyl, H or not present, wherein R₂, R₃ and R₄ are a second silyl ether group; wherein Y is C^b or O, wherein X₁ is not present in case Y is O; wherein the bond between C^c and C^d is a single, double or triple bond, the bond between C^d and X₅ is a single or double bond, and in case Y is C^b the bond between C^a and C^b is a single, double or triple bond and the bond between C^b and C^c is a single, double or triple bond; wherein the at least one second silyl ether group is connected to any one of C^b, C^c or C^d, or is comprised in X₅; and wherein R₅ is a third silyl ether group, R₆ is a third silyl ether group or H, R₇ is a third silyl ether group or H and R₈ is a methylene group or H.

15. Use of a Vitamin D molecule obtainable by a method of any one of claims 1 to 14 as an internal standard for mass spectrometric determination of an analyte.