WO2025180999A1

WO2025180999A1 - Electrochemical oxidative decarboxylation of alpha-amino acids

Info

Publication number: WO2025180999A1
Application number: PCT/EP2025/054838
Authority: WO
Inventors: Guixian Hu; Paul Hanselmann; David KOEPFLER; Oliver KAPPE; David CANTILLO
Original assignee: Lonza AG
Current assignee: Lonza AG
Priority date: 2024-02-26
Filing date: 2025-02-24
Publication date: 2025-09-04
Anticipated expiration: 2026-08-26

Abstract

The present invention relates to a method for electrochemical oxidative decarboxylation of Fmoc-protected alpha-amino acids in a solvent mixture comprising an oxidizable solvent in the presence of a base.

Description

TITLE OF THE INVENTION

ELECTROCHEMICAL OXIDATIVE DECARBOXYLATION

OF ALPHA-AMINO ACIDS

FIELD OF THE INVENTION

The present invention relates to a method for electrochemical oxidative decarboxylation of alpha-amino acids in a solvent mixture comprising an oxidizable solvent in the presence of a base.

BACKGROUND OF THE INVENTION

US 2020/385422 Al discloses in Example 3 the oxidative acetoxylation of Fmoc-Gly-Gly- OH with AcOH in the presence of Pb(OAc)4 in THF with a yield of 91.3%.

Lead's toxicity is known since the ancient Greeks and Romans, it is a neurotoxin that accumulates in soft tissues and bones; it damages the nervous system and interferes with the function of biological enzymes, causing neurological disorders ranging from behavioral problems to brain damage, and also affects general health, cardiovascular, and renal systems (en.wikipedia.org) .

US 4426377 A discloses in Column 34 under Description 2 the electrochemical decarboxylative acetoxylation of Z-Gyl-Gly (benzyloxycarbonylglycylglycine) in a solution of anhydrous sodium acetate in glacial acetic acid. The mixture was electrolysed with platinum foil electrodes using a current of 200 to 250 mA for 5 hours providing N- acetoxymethyl-2-(benzyloxycarbonylamino)-acetamide according to Scheme 3. Isolated yield was 44%.

Scheme 3 Seebach et al. in Helv. Chim. Acta, 1989, 72, 401-425, diclose the electrochemical oxidative decarboxylation and acetoxylation or methoxylation of a number of benzyloxycarbonyl (Z)- and tert-butyloxycarbonyl (Boc)-protected dipeptides:

5a Z-Ala-Val MeOH - 2.3 F/mol - Et₃N 5b (R = Me) 98% (*)

5a Z-Ala-Val AcOH - 2.3 F/mol - Et₃N 5b' (R = Ac) 67% (*)

12a Z-Leu-Gly MeOH - 7.0 F/mol - EtN(i-Pr)₂ 12b (R = Me) 60%

12a Z-Leu-Gly AcOH - 1.5 F/mol - EtN(i-Pr)₂ 12b' (R = Ac) 48%

13a Boc-Ala-Sar MeOH - 2.5 F/mol - Et₃N 13b (R = Me) 58%

13a Boc-Ala-Sar AcOH - 3.2 F/mol - Et₃N 13b' (R = Ac) 69%

(*) ca. (1 :l)-mixture of two diastereoisomeric products

There was a need for a method of converting the C-terminal carboxylic acid residue of amino acids or peptides into an acetate or alkylate with high conversion, selectivity and yield but without the need of involving lead in the method.

Today, a relevant amino-protecting group used in solid phase peptide synthesis is the fluorenylmethyloxycarbonyl protecting group (Fmoc). It has been found by the inventors of present invention, that electrochemical oxidative decarboxylations as described in the prior art cannot be used for Fmoc-protected amino acids and peptides, because the fhiorenyl-moiety of Fmoc is co-oxidized. So there was a need for providing a method for electrochemical oxidative decarboxylation of Fmoc-protected alpha-amino acids and peptides.

It was found by the inventors of the present invention, that the use of certain specific solvent mixtures makes electrochemical oxidative decarboxylations of Fmoc-protected amino acids and peptides accessible, at good conversion, selectivity and yield.

SUMMARY OF THE INVENTION

Subject of the invention is a method for preparation of compound of formula (I) by an electrochemical oxidative decarboxylation reaction of compound of formula (II) in a solvent mixture and in the presence of a base; the electrochemical oxidative decarboxylation reaction takes place by passing an electrical current through a reaction mixture comprising the compound of formula (II), the solvent mixture and the base; the electrical current is provided by a power supply which is connected to two electrodes, an anode and a cathode, which are immersed into the reaction mixture; wherein Y is selected from the group consisting of residue of formula (Yl), residue of formula (Y2), residue of formula (Y3), residue of formula (Y 4), and residue of formula (Y5);

Ria, R2a, R3a, R4a and R5a are identical or different and independently from each other side chains of alpha amino acids and Rib, R2b, R3b, R4b and R5b are identical or different and independently from each other H or CH3; or one or more of Rib, R2b, R3b, R4b and R5b, independently from each other, and the respective N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, together with the respective residue Ria, R2a, R3a, R4a and R5a and the alpha C atom, to which said Ria, R2a, R3a, R4a or R5a is bonded, can also form a three, four, five or six membered nonaromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only one endocyclic heteroatom, except in case of a six membered ring wherein one further endocyclic O atom may be present in said six membered ring; the solvent mixture comprises a solvent R9OH and an oxidizable solvent; the base is selected from the group consisting of R90M, phosphates, carbonates, phosphazenes, tertiary amine of formula R20(R21)(R22)N and DBU;

R9 is selected from the group consisting of acetyl (Ac) and C1-4 alkyl;

M is selected from the group consisting of Na, K, Li, and R1O(R11)(R12)(R13)N;

RIO, R11, R12 and R13 are identical or different and independently from each other selected from the group consisting of H and C_1-4 alkyl;

R20, R21 and R22 are identical or different and independently from each other selected from the group consisting of C1-4 alkyl.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the experimental set up for examples which were done in batch mode.

Figure 2 shows the experimental set up for examples which were done in circular flow mode. Figure 3 shows cyclic voltammograms for solutions 1 to 5, the numerals having the meanings as follows:

1 cyclic voltammograms of Solution 1 with Fmoc-Leu-Gly-OH

1 -2 the peak potential of the COO- oxidation without addition of solvent

1 -3 the peak potential of the ferrocene oxidation

1 -4 the peak potential of the Fmoc oxidation

2 cyclic voltammograms of Solution 2 with Fmoc-Leu-Gly-OH and DMSO 2-1 the peak potential of the DMSO oxidation

2-2 the peak potential of the COO- oxidation in the presence of DMSO

3 cyclic voltammograms of Solution 3 with Fmoc-Leu-Gly-OH and DMA

3-1 the peak potential of the DMA oxidation

3-2 the peak potential of the COO- oxidation in the presence of DMA

4 cyclic voltammograms of Solution 4 with Fmoc-Leu-Gly-OH and NMP

4-1 the peak potential of the NMP oxidation

4-2 the peak potential of the COO- oxidation in the presence of NMP

5 cyclic voltammograms of Solution 5 with Fmoc-Leu-Gly-OH and Sulfolane

5-1 the absence of Sulfolane oxidation

5-2 the peak potential of the COO- oxidation in the presence of sulfolane

Figure 4 shows cyclic voltammograms for solutions 6 to 9, the numerals having the meaning as follows:

6 cyclic voltammograms of Solution 6 (DMSO)

7 cyclic voltammograms of Solution 7 (DMA)

8 cyclic voltammograms of Solution 8 (NMP)

9 cyclic voltammograms of Solution 9 (Sulfolane)

Figure 5 shows cyclic voltammograms for solution 10. Figure 6 shows cyclic voltammograms for solutions 11.

Figure 7 shows the experimental set up for examples which were done in single pass flow mode.

DETAILED DESCRIPTION OF THE INVENTION

The electrochemical oxidative decarboxylation reaction of compound of formula (II) is an electrolysis reaction; for ease of reading the electrochemical oxidative decarboxylation reaction of compound of formula (II) is shortly called "reaction" or "electrolysis" herein. The reaction oxidizes the carboxylic acid group depicted in formula (II), CO2 is thereby formed.

If R9 in formula (I) is Ac then the reaction is an electrochemical oxidative and decarboxylative acetoxylation.

If R9 in formula (I) is C 1.4 alkyl then the reaction is an electrochemical oxidative and decarboxylative alkoxylation. The reaction mixture at the beginning of the electrochemical oxidative decarboxylation reaction of compound of formula (II) comprises compound of formula (II), the solvent mixture and the base.

The terminal N atom, which is displayed to the left in the formulae (Yl), (Y2), (Y3), (Y4) and (Y5), is bonded to the fluorenylmethyloxycarbonyl protecting group (Fmoc) in formula (II) and formula (I), respectively.

When Y is residue of formula (Yl) then compound of formula (II) is a Fmoc protected alpha amino acid. Residue of formula (Yl) together with the COOH residue depicted in formula (II) is also called alpha amino acid residue AA1 which contains Ria and Rib.

When Y is residue of formula (Y2) then compound of formula (II) is a Fmoc protected dipeptide. Residue of formula (Y2) together with the COOH residue depicted in formula (II) is a dipeptidyl residue Y2 of the alpha amino acid residue AA1 and an alpha amino acid residue AA2 which contains R2a and R2b. The alpha amino acid residue AA1 is the C- terminal residue and the alpha amino acid residue AA2 is the N-terminal residue of the dipeptidyl residue Y2.

When Y is residue of formula (Y3) then compound of formula (II) is a Fmoc protected tripeptide. Residue of formula (Y3) together with the COOH residue depicted in formula (II) is a tripeptidyl residue Y3 of the alpha amino acid residue AA1, the alpha amino acid residue AA2 and an alpha amino acid residue AA3 which contains R3a and R3b. The alpha amino acid residue AA1 is the C-terminal residue and the alpha amino acid residue AA3 is the N- terminal residue of the tripeptidyl residue Y3;

When Y is residue of formula (Y 4) then compound of formula (II) is a Fmoc protected tetrapeptide. Residue of formula (Y 4) together with the COOH residue depicted in formula (II) is a tetrapeptidyl residue Y4 of the alpha amino acid residue AA1, the alpha amino acid residue AA2, the alpha amino acid residue AA3 and an alpha amino acid residue AA4 which contains R4a and R4b. The alpha amino acid residue AA1 is the C-terminal residue and the alpha amino acid residue AA4 is the N-terminal residue of the tetrapeptidyl residue Y4; When Y is residue of formula (Y5) then compound of formula (II) is a Fmoc protected pentapeptide. Residue of formula (Y 5) together with the COOH residue depicted in formula (II) is a pentapeptidyl residue Y5 of the alpha amino acid residue AA1, the alpha amino acid residue AA2, the alpha amino acid residue AA3, the alpha amino acid residue AA4 and an alpha amino acid residue AA5 which contains R5a and R5b. The alpha amino acid residue AA1 is the C-terminal residue and the alpha amino acid residue AA5 is the N-terminal residue of the pentapeptidyl residue Y5.

Within the meaning of the invention a side chain of an alpha amino acid, in particular of a natural alpha amino acid, can be H which is for example the case for the natural alpha amino acid Gly.

When Ria is not H then compound of formula (I) is usually a mixture of two enantiomers.

If Y is residue of formula (Yl) then compound of formula (II) is a Fmoc protected alpha amino acid; the Fmoc protecting group is bonded to the alpha amino residue of the alpha amino acid.

If Y is residue of formula (Y2), residue of formula (Y3), residue of formula (Y4) or residue of formula (Y5) then compound of formula (II) is a Fmoc protected di-, tri-, tetra- or pentapeptide of alpha amino acids, respectively; the Fmoc protecting group is bonded to the alpha amino group of the N-terminal alpha amino acid of the respective peptide.

Ria is a side chain of an alpha amino acid and Rib is H or CH3, or

Rib and the N atom, to which Rib is bonded, together with Ria and the alpha C atom, to which Ria is bonded, form a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms against an O atom.

R2a is a side chain of an alpha amino acid and R2b is H or CH3, or

R2b and the N atom, to which R2b is bonded, together with R2a and the alpha C atom, to which R2a is bonded, form a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms against an O atom.

R3a is a side chain of an alpha amino acid and R3b is H or CH3, or

R3b and the N atom, to which R3b is bonded, together with R3a and the alpha C atom, to which R3a is bonded, form a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms against an O atom.

R4a is a side chain of an alpha amino acid and R4b is H or CH3, or

R4b and the N atom, to which R4b is bonded, together with R4a and the alpha C atom, to which R4a is bonded, form a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms against an O atom.

R5a is a side chain of an alpha amino acid and R5b is H or CH3, or

R5b and the N atom, to which R5b is bonded, together with R5a and the alpha C atom, to which R5a is bonded, form a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms against an O atom.

Preferably, when anyone of Ria, R2a, R3a, R4a and R5a is a side chain of an alpha amino acid, then it is a side chain of a natural or non-natural alpha amino acid.

Examples of side chains of alpha amino acids, such as natural or non-natural alpha amino acids, are selected from the group of H, C_1-4 alkyl, phenyl, benzyl, C_1-4 hydroxy alkyl;

H₂NC(O)CH₂*, H₂NC(O)CH₂CH₂*, C_1-4 thiol alkyl, CH3SCH2CH2

^CH3 11107 = 2, 3, 4, 5 with an * in the formulae denoting the C atom where the side chain is bonded and with ** in the formulae denoting where a residue of a side chain is bonded.

If one or more of Rib, R2b, R3b, R4b and R5b, independently from each other, and the respective N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, together with the respective residue Ria, R2a, R3a, R4a and R5a and the alpha C atom, to which said Ria, R2a, R3a, R4a or R5a is bonded, forms a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only one endocyclic heteroatom, except in case of a six membered ring wherein one further endocyclic O atom may be present in said six membered ring, then in case of said six membered ring said one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms, but except for the alpha C atom, against an O atom.

If anyone of Rib, R2b, R3b, R4b and R5b and the respective N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, together with the respective residue Ria, R2a, R3a, R4a or R5a and the alpha C atom, to which said Ria, R2a, R3a, R4a or R5a is bonded, forms a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only one endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms, but except for the alpha C atom, against an O atom, then examples for the respective alpha amino acid residues AA1, AA2, AA3, AA4 or AA5 are selected from the group consisting

More preferably, Ria, R2a, R3a, R4a and R5a are identical or different and independently from each other side chains of natural alpha amino acids and

Rib, R2b, R3b, R4b and R5b are identical or different and independently from each other H or CH3, preferably H; or one or more of Rib, R2b, R3b, R4b and R5b, independently from each other, can also be together with the respective residue R1a, R2a, R3a, R4a and R5a a CH2CH2CH2 chain and forms together with the N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, and the alpha C atom, to which the respective residue Ria, R2a, R3a, R4a and R5a is bonded, a five membered ring.

In case that anyone of Rib, R2b, R3b, R4b and R5b is together with the respective residue Ria, R2a, R3a, R4a and R5a, a CH2CH2CH2 chain and forms together with the N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, and the alpha C atom, to which the respective residue Ria, R2a, R3a, R4a and R5a is bonded, a five membered ring, then the respective alpha amino acid residue AA1, AA2, AA3, AA4 and AA5 is proline (Pro, P, the three- and one-letter code of proline).

More preferably,

Ria is a side chain of a natural alpha amino acid and Rib is H or CH3, preferably H, or

Ria together with Rib are CH2CH2CH2 and form together with the N atom, to which Rib is bonded, and the alpha C atom, to which Ria is bonded, a five membered ring.

More preferably,

R2a is a side chain of a natural alpha amino acid and R2b is H or CH3, preferably H, or

R2a together with R2b are CH2CH2CH2 and form together with the N atom, to which R2b is bonded, and the alpha C atom, to which R2a is bonded, a five membered ring.

More preferably,

R3a is a side chain of a natural alpha amino acid and R3b is H or CH3, preferably H, or R3a together with R3b are CH2CH2CH2 and form together with the N atom, to which R3b is bonded, and the alpha C atom, to which R3a is bonded, a five membered ring.

More preferably,

R4a is a side chain of a natural alpha amino acid and R4b is H or CH3, preferably H, or

R4a together with R4b are CH2CH2CH2 and form together with the N atom, to which R4b is bonded, and the alpha C atom, to which R4a is bonded, a five membered ring.

More preferably,

R5a is a side chain of a natural alpha amino acid and R5b is H or CH3, preferably H, or

R5a together with R5b are CH2CH2CH2 and form together with the N atom, to which R5b is bonded, and the alpha C atom, to which R5a is bonded, a five membered ring.

More preferably,

Ria together with Rib are CH2CH2CH2 and form together with the N atom, to which Rib is bonded, and the alpha C atom, to which Ria is bonded, a five membered ring;

R2a together with R2b are CH2CH2CH2 and form together with the N atom, to which R2b is bonded, and the alpha C atom, to which R2a is bonded, a five membered ring;

R3a is a side chain of a natural alpha amino acid and R3b is H or CH3, preferably H, or

R3a together with R3b are CH2CH2CH2 and form together with the N atom, to which R3b is bonded, and the alpha C atom, to which R3a is bonded, a five membered ring;

R4a together with R4b are CH2CH2CH2 and form together with the N atom, to which R4b is bonded, and the alpha C atom, to which R4a is bonded, a five membered ring;

Examples of side chains of natural alpha amino acids, that is examples for anyone of Ria, R2a, R3a, R4a and R5a, are selected from the group H (Gly, G), H3C (Ala, A), (H3C)₂CH (Vai, V), CH₃CH₂CH(CH3) (fie, I), (CH3)₂CHCH₂ (Leu, L), C₆H₅CH₂ (Phe, F), HOCH₂ (Ser, S), CH3CHOH (Thr, T), H₂NC(O)CH₂ (Asn, N), H₂NC(O)CH₂CH₂ (Gin, Q), HSCH₂ (Cys,

(Lys, K), and H2NC(NH)NH(CH2)2CH2 (Arg, R); in brackets are the three- and one-letter code of the respective naturally occurring alpha amino acid.

In one embodiment Y is residue of formula (Yl), residue of formula (Y2), residue of formula (Y3) or residue of formula (Y 4).

In one embodiment Y is residue of formula (Yl), residue of formula (Y2) or residue of formula (Y3).

In one embodiment Y is residue of formula (Yl) or residue of formula (Y2).

In one embodiment Y is residue of formula (Yl).

In one embodiment Y is residue of formula (Y2).

In one embodiment Y is residue of formula (Y3).

In one embodiment Y is residue of formula (Y4).

In one embodiment, Y is residue of formula (Y2) and R9 is acetyl.

If Y is residue of formula (Y2) with both Ria and R2a being H, then compound of formula (II) is Fmoc-Gly-Gly-OH.

In one embodiment, Y is residue of formula (Y2) with both Ria and R2a being H; this means that compound of formula (II) is Fmoc-Gly-Gly-OH and compound of formula (I) is Fmoc- Gly-NH-CH2-OR9, that is compound of formula (I-GR9).

Fmoc

Fmoc-Gly-Gly-OH (I-GR9) In one embodiment, Y is residue of formula (Y2) with both Ria and R2a being H and R9 is acetyl, this means that compound of formula (II) is Fmoc-Gly-Gly-OH and compound of formula (I) is Fmoc-Gly-NH-CHi-OAc, that is compound of formula (1-GAc).

(1-GAc)

In another embodiment, Y is residue of formula (Y2),

Ria and R2a are H,

Rib and R2b are H, and

R9 is acetyl (Ac).

An embodiment of the invention is a method for preparation of compound of formula (I) by an electrochemical oxidative decarboxylation reaction of compound of formula (II) in a solvent mixture and in the presence of a base; wherein Y is selected from the group consisting of residue of formula (Yl), residue of formula (Y2), residue of formula (Y3), residue of formula (Y4), and residue of formula (Y5);

Ria, R2a, R3a, R4a and R5a are identical or different and independently from each other side chains of alpha amino acids and Rib, R2b, R3b, R4b and R5b are identical or different and independently from each other H or CH3; or one or more of Rib, R2b, R3b, R4b and R5b, independently from each other, and the respective N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, together with the respective residue Ria, R2a, R3a, R4a and R5a and the alpha C atom, to which said Ria, R2a, R3a, R4a or R5a is bonded, can also form a three, four, five or six membered nonaromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only one endocyclic heteroatom, except in case of a six membered ring wherein one further endocyclic O atom may be present in said six membered ring; the solvent mixture comprises a solvent R9OH and an oxidizable solvent; the base is selected from the group consisting of R9OM, phosphates, carbonates, phosphazenes, tertiary amine of formula R20(R21)(R22)N and DBU; R9 is selected from the group consisting of acetyl (Ac) and Ci-4 alkyl;

M is selected from the group consisting of Na, K, Li, and R10(Rl 1)(R12)(R13)N;

R20, R21 and R22 are identical or different and independently from each other selected from the group consisting of C_1-4 alkyl; with the proviso that the compound of formula (I) is not Fmoc-Gly-NH-CFh-OAc.

Another embodiment of the invention is a method for preparation of compound of formula (I) by an electrochemical oxidative decarboxylation reaction of compound of formula (II) in a solvent mixture and in the presence of a base; wherein Y is residue of formula (Y2);

Ria and R2a are H and

Rib and R2b are H; the solvent mixture comprises a solvent R9OH and an oxidizable solvent; the base is selected from the group consisting of R90M;

R9 is acetyl (Ac);

M is selected from the group consisting of Na, K, Li, and R10(Rl 1)(R12)(R13)N;

RIO, R11, R12 and R13 are identical or different and independently from each other selected from the group consisting of H and C_1-4 alkyl.

Preferably, any functional residue contained in Y, which could react under the electrolysis reaction conditions, is protected. Such protection is done using a protecting group known to the skilled person. Such protection serves the purpose to render such functional residue inert under reaction conditions. Suitable protecting groups of functional residues contained in Y can for example be:

• if said functional residue is a hydroxy residue (OH): Fmoc, Boc, benzoyl, Cbz, C(O)- Ci-io alkyl, TBDMS (tert-Butyldimethylsilyl), PNB (p-Nitrobenzyl), ONB (o- Nitrobenzyl), Bn (Benzyl), Al (Allyl), or tBu (tert-Butyl), preferably TBDMS (tert- Butyldimethylsilyl), PNB (p-Nitrobenzyl), ONB (o-Nitrobenzyl), Bn (Benzyl), Al (Allyl), or tBu (tert-Butyl), more preferably TBDMS (tert-Butyldimethylsilyl), PNB (p-Nitrobenzyl), ONB (o-Nitrobenzyl), Bn (Benzyl), or Al (Allyl); • if said functional residue is an amino residue (NH2): Fmoc, Boc, benzoyl, Cbz, C(O)- Ci-io alkyl, Alloc (Allyloxycarbonyl), or Cbz (Z), preferably Fmoc, Boc, Alloc (Allyloxycarbonyl), or Cbz (Z);

• if said functional residue is a thiol residue (SH): Fmoc, Boc, benzoyl, Cbz, C(0)-Ci-io alkyl, Meb (p-Methylbenzyl), Acm (Acetamidomethyl), or Trt, preferably Meb (p- Methylbenzyl), Acm (Acetamidomethyl), or Trt, more preferably Meb (p- Methylbenzyl), or Acm (Acetamidomethyl);

• if said functional residue is a guanidine residue (NHC(NH)NH2, such as in Arg): Boc, 4-methoxy-2,6-dimethylbenzenesulfonyl (Mds), 4-methoxy-2,3,6-trimethylsulfonyl (Mtr), 2,2,5,7,8-pentamethylchroman-6-sulfonyl (Pmc), trimethoxybenzenesulfonyl (Mtb), 3,4-ethylenedioxythiophene-2-sulfonyl (EDOT-2-sulfonyl), 1,2- dimethylindole-3 -sulfonyl (MIS), or Pbf (2,2,4,6,7-Pentamethyl-2,3- dihydrobenzofuran-5 -sulfonyl) ;

• if said functional residue is an amide residue (C(O)NH2, such as in Asn or Gin): Xan (9-Xanthenyl), or Trt (Trityl);

• if said functional residue is an imidazole residue (such as in His): Tos (Tosyl), Bom (Benzyloxymethyl), Trt, or Boc;

• if said functional residue is an carboxylic acid residue (such as in Asp or Glu): tBu, Trt, Dmb (2,4-Dimethoxybenzyl), Fm (9-Fluorenylmethyl), or Bn;

• if said functional residue is an indole residue (such as in Trp): For (Formyl), or Boc, preferably Boc;

• if said functional residue is a methylthio residue (SCH3, such as in Met): it can be oxidized to the sulfoxide before or during the electrochemical reaction, and reduced after the electrochemical reaction.

Examples of side chains of natural alpha amino acids, which could react under the electrolysis reaction conditions, are easily oxidizable side chains such as unprotected side chain of Lys or unprotected side chain of Met. So preferably, Ria, R2a, R3a, R4a and R5a are not unprotected side chain of Lys and not unprotected side chain of Met.

In another embodiment, Y is not residue of formula (Y2) with both Ria and R2a being H and R9 is not acetyl, this means that compound of formula (II) is not Fmoc-Gly-Gly-OH and compound of formula (I) is not compound of formula (1-GAc). In another embodiment, Y is not residue of formula (Y2) with both Ria and R2a being H, this means that compound of formula (II) is not Fmoc-Gly-Gly-OH and compound of formula (I) is not compound of formula (I-GR9).

In another embodiment, Y is not residue of formula (Y2) with both Ria and R2a being H when R9 is acetyl.

In another embodiment, R9 is not acetyl when Y is residue of formula (Y2) with both Ria and R2a being H.

In another embodiment, Y is not residue of formula (Y2) and R9 is not acetyl.

In another embodiment, Y is not residue of formula (Y2) when R9 is acetyl.

In another embodiment, R9 is not acetyl when Y is residue of formula (Y2).

In another embodiment, Y is not residue of formula (Y2).

In another embodiment, the compound of formula (I) is not compound of formula (I-GR9).

In another embodiment, the compound of formula (I) is not compound of formula (1-GAc).

In another embodiment, the compound of formula (I) is not compound of formula (1-GAc) when the compound of formula (II) is Fmoc-Gly-Gly-OH.

In another embodiment, the compound of formula (II) is not Fmoc-Gly-Gly-OH and R9 is not acetyl.

In another embodiment, the compound of formula (I) is not compound of formula (I-GR9) and the compound of formula (II) is not Fmoc-Gly-Gly-OH.

In another embodiment, the compound of formula (I) is not compound of formula (1-GAc) and the compound of formula (II) is not Fmoc-Gly-Gly-OH.

In another embodiment, the compound of formula (I) is not Fmoc-Gly-NH-CH2-OAc.

The cyclovoltammogram of the oxidation of a compound provides three types of potentials of oxidation characterizing the compound: • the onset potential of oxidation, herein also shortly called onset potential, that's the potential where the oxidation of the compound begins to take place, an electrical current starts to flow;

• the peak potential of oxidation, herein also shortly called peak potential, that's the potential at the maximum of the electrical current;

• the half peak potential of oxidation, herein also shortly called half peak potential, that's the potential where half of the maximum of the electrical current flows.

For ease of reading the Fmoc group depicted in formula (II) is herein shortly called the Fmoc group, likewise the carboxylic acid group depicted in formula (II) is herein shortly called the carboxylic acid group.

The oxidizable solvent is different from the solvent R9OH.

The oxidizable solvent and its amount in the reaction mixture is chosen such that its half peak potential together with the amount of the oxidizable solvent at a predetermined amount of compound of formula (II) and a predetermined amount of the base in the reaction mixture provides for an oxidation of the oxidizable solvent to take place preferentially over an oxidation of the Fmoc group, but not over an oxidation of the carboxylic acid group; said Fmoc group and said carboxylic acid group are the Fmoc group depicted in formula (II) and the carboxylic acid group depicted in formula (II) respectively.

If the amount of the oxidizable solvent in the reaction mixture is too high and /or the half peak potential of the oxidizable solvent is too low, then the carboxylic acid group is no longer oxidized; if the amount of the oxidizable solvent in the reaction mixture is too low or its half peak potential is too high, then the Fmoc group is oxidized.

Preferably, the oxidizable solvent and its amount in the reaction mixture at a predetermined amount of compound of formula (II) and a predetermined amount of the base in the reaction mixture is chosen such that the electrical current density at the half peak potential of the Fmoc group due to the oxidation of the oxidizable solvent in a reaction mixture without compound of formula (II) is higher than the electrical current density at the half peak potential of the Fmoc group due to the oxidation of said Fmoc group in a reaction mixture without the oxidizable solvent; said Fmoc group is the Fmoc group depicted in formula (II). The amount of compound of formula (II) in the reaction mixture can be predetermined e.g. based on solubility considerations.

In another embodiment, the oxidizable solvent is chosen such that the half peak potential of the oxidizable solvent is not higher than 2 times, preferably 1.8 times, more preferably 1.6 times, even more preferably 1.4 times, especially 1.3 times, the half peak potential of the Fmoc group depicted in formula (II); and/or the oxidizable solvent is chosen such that the half peak potential of the oxidizable solvent is about equal to or higher than the half peak potential of the carboxylic acid group; and/or the molar amount of the oxidizable solvent is at least about equal to or greater than the molar amount of compound of formula (II) in the reaction mixture; said Fmoc group and said carboxylic acid group are the Fmoc group depicted in formula (II) and the carboxylic acid group depicted in formula (II) respectively.

The half peak potential may be quantified and measured versus ferrocene as reference, and with ferrocene as reference typically the half peak potential of the carboxylic acid group is from about 1.2 to about 1.3 V, in particular from about 1.24 to about 1.30 V, whereas the half peak potential of oxidation of the Fmoc group is about 1.4 V; so versus ferrocene as reference a half peak potential of the oxidizable solvent can be from about 1.2 V to about 2.8 V, preferably from about 1.2 V to about 2.5 V, more preferably from about 1.2 V to about 2.2 V, even more preferably from about 1.2 V to about 2.0 V, especially from about 1.2 V to about 1.8 V; said Fmoc group and said carboxylic acid group are the Fmoc group depicted in formula (II) and the carboxylic acid group depicted in formula (II) respectively.

Preferably, the oxidizable solvent has also a good solubilizing power to dissolve the substrate and the product.

Preferably, the oxidizable solvent is selected from the group consisting of DMA, NMP, DMSO, DMF, acetone, THF, propylene carbonate, dimethyl carbonate, y-valerolactone and NBP, and mixtures thereof; more preferably, the oxidizable solvent is selected from the group consisting of DMA, NMP, DMSO, DMF, acetone, THF, propylene carbonate, and mixtures thereof; even more preferably, the oxidizable solvent is selected from the group consisting of DMA, NMP, DMSO, and mixtures thereof; especially, the oxidizable solvent is DMA, NMP or DMSO; more especially, the oxidizable solvent is NMP or DMA.

A phosphate as base can be any combination of any metal cation with phosphate, or phosphate as base can be any combination of any tetraalkylammonium cation with phosphate. Preferably, the metal cation is lithium, sodium, potassium, magnesium, or calcium.

Examples for combinations of a metal cation with phosphate are lithium phosphate, sodium phosphate, potassium phosphate, magnesium phosphate, and calcium phosphate.

Preferably, the tetraalkylammonium is R30(R31)(R32)(R33)(R34)N⁺ with R30, R31, R32 and R33 being identical or different and independently from each other selected from the group consisting of C_1-4 alkyl; more preferably, R30, R31, R32 and R33 are identical or different and independently from each other selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, tert-butyl; even more preferably, R30 and R31 are identical, R32 and R33 are identical or different and are identical with or different from R30 and R31 , and R30, R31 , R32 and R33 are selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl; especially, R30, R31 and R32 are identical, R33 is identical with or different from R30, R31 and R32, and R30, R31, R32 and R33 are selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl; examples for combinations of a tetraalkylammonium cation with phosphate are tetramethylammonium phosphate, tetraethylammonium phosphate, tetrabutylammonium phosphate, and trimethylethylammonium phosphate.

A carbonate as base can be any combination of any metal cation with carbonate, or carbonate as base can be any combination of any tetraalkylammonium cation with carbonate.

Preferably, the metal cation can be lithium, sodium, potassium, magnesium, or calcium.

Examples for combinations of a metal cation with carbonate are lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, and calcium carbonate.

Preferably, the tetraalkylammonium can be R30(R31)(R32)(R33)(R34)N⁺ with R30, R31, R32 and R33 being identical or different and independently from each other selected from the group consisting of C1-4 alkyl; more preferably, R30, R31, R32 and R33 are identical or different and independently from each other selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, tert-butyl; even more preferably, R30 and R31 are identical, R32 and R33 are identical or different and are identical with or different from R30 and R31, and R30, R31, R32 and R33 are selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl; especially, R30, R31 and R32 are identical, R33 is identical with or different from R30, R31 and R32, and R30, R31, R32 and R33 are selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl; examples for combinations of a tetraalkylammonium cation with carbonate are tetramethylammonium carbonate, tetraethylammonium carbonate, tetrabutylammonium carbonate, and trimethylethylammonium carbonate.

A phosphazene as base can be a compound of anyone of the formulae 101 to 126 and 131 to 133;

101

further examples for phospazene as base are EtPi(tmg), tBu-Pi(tmg), Pi(tmg), and tBu- Pi(tmg)2(NEt2); further examples for Pi phosphazenes are HPi(pyrr), HPi(dma), EtPi(pyrr), 2,5-C1₂- C6H3P1( pyrr), 2-C1-C6H4P2(pyrr), 4-NMe2-C₆H₄Pi(pyrr), 4-OMe-C6H4Pi(pyrr), PhPi(pyrr), 4-Br-C₆H₄Pi(pyrr), 2-NO₂-4-C₁-C₆H₃Pi(pyrr), 2-Cl-C₆H₄Pi(pyrr), 4-CF₃- C₆H4Pi(pyrr), 4-NO2-C₆H₄Pi(pyrr), 2,5-C12-C6H₃Pi(pyrr), and 2,6-C1₂-C₆H₃Pi(pyrr); further example for P2 phosphazenes is PhP2(pyrr); further example for P₃ phosphazenes is EtP₃(dma); further example for P₄ phosphazenes is 4-MeO-C6H₄P₄(pyrr). "dma" means dimethylamino residue; "pyrr" means pyrrolidinyl residue, "tmg" means tetramethylguanidino residue. Compound of formula 131 can be abbreviated with PhPl(tmg), compound of formula 132 can be abbreviated with PhPl(tmg)2(dma), and compound of formula 133 can be abbreviated with PhPl(tmg)(dma)2. Compound of formula 125 can be abbreviated with BEMP, 2 -tert-butylimino-2-diethylamino- 1,3 -dimethylperhydro- 1, 3 ,2-diaza- phosphorane. Compound of formula 126 can be abbreviated with HMPN, 1,8- Bis(hexamethyltriaminophosphazenyl) naphthalene.

Preferably, the base is selected from the group consisting of R9OM, tertiary amine of formula R20(R21)(R22)N and DBU, more preferably the base is R9OM.

Preferably, R9 is selected from the group consisting of acetyl (Ac), methyl (Me), ethyl (Et), n- propyl (n-Pr), iso-propyl (i-Pr), n-butyl (n-Bu), iso-butyl (i-Bu), and tert-butyl (tert-Bu).

In one embodiment, R9 is Ac.

In one embodiment, R9 is not Ac.

In one embodiment, R9 is C_1-4 alkyl.

In one embodiment, R9 is not C_1-4 alkyl.

In another embodiment, R9 is C_1-4 alkyl, in particular Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, or tert- Bu, preferably Me, Et, n-Pr or i-Pr; more preferably Me or Et, even more preferably Me. In another embodiment, R9 is C_1-4 alkyl, in particular Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, or tert- Bu, preferably Me, Et, n-Pr or i-Pr; more preferably Me or Et, even more preferably Me; and the base is R90M and M is selected from the group consisting of Na, K, Li.

Preferably, M is selected from the group consisting of Na, K, Li, and R10(Rl 1)(R12)(R13)N, RIO, R11, R12 and R13 are identical or different and independently from each other selected from the group consisting of is H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tertbutyl; more preferably, M is selected from the group consisting of Li, Na, K, NH4, (Et)₄N and (Bu)₄N; even more preferably, M is K or Na.

Preferably, R20, R21 and R22 are identical or different and independently from each other selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl; more preferably, R20 and R21 are identical, R22 is different from R20 and R21, and R20, R21 and R22 are selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, tert-butyl; even more preferably, the tertiary amine is EtsN.

In one embodiment, the solvent mixture consists of the solvent and the oxidizable solvent.

In another embodiment, the reaction mixture at the beginning of the reaction consists of compound of formula (II), the solvent mixture and the base, preferably wherein the solvent mixture consists of the solvent and the oxidizable solvent.

Preferably, R9 is Ac, so that the solvent mixture comprises AcOH as solvent and the oxidizable solvent; more preferably, R9 is Ac, so that the solvent mixture comprises AcOH as solvent and the oxidizable solvent, and the base is KOAc or NaOAc; even more preferably, the solvent mixture consists of AcOH as solvent and the oxidizable solvent, wherein the oxidizable solvent is selected from the group consisting of DMA, NMP and DMSO, especially the oxidizable solvent is selected from the group consisting of DMA and NMP.

For the reaction a power supply and two electrodes, an anode and a cathode, are provided. The electrodes may be provided as parts which are separate from a container wherein the reaction takes place, in this case the electrodes fit into the container; or the electrodes may be provided as parts of a container wherein the reaction takes place. The electrodes are immersed into the reaction mixture. The electrodes are connected to a power source. One of the electrodes is connected to the (+) side of the power source and serves as the anode. The other electrode is connected to the (-) side of the power source and serves as the cathode. An electric potential is applied by the power source between the electrodes. The reaction is done with the two electrodes by applying to the reaction mixture an electrical current flowing between the electrodes through the reaction mixture. The electric current is caused to flow by the electric potential that is applied between the electrodes by the power supply. The electrical current is also simply called current herein for ease of reading. The current flows from the cathode through the reaction mixture to the anode. The reaction can be done in batch mode, which can also be called batch reaction mode, or in flow mode, which can also be called flow reaction mode. Flow mode may for example be circular flow mode or single pass flow mode. In one embodiment, the reaction is done in batch mode. In another embodiment, the reaction is done in circular flow mode. In another embodiment, the reaction is done in single pass flow mode.

In batch mode the reaction mixture is contained in a container, such as a flask or a reaction vessel, depending on the volume of reaction mixture. The electrodes are immersed in the reaction mixture.

Circular flow mode means that the reaction mixture is contained in a reservoir, which can for example be a flask or a reaction vessel, depending on the volume of reaction mixture, and the reaction mixture is continuously pumped through a flow electrolysis cell and back into the reservoir. The flow electrolysis cell contains the electrodes. The reaction mixture passes between the electrodes when being pumped through the flow electrolysis cell.

Single pass flow mode means that the reaction mixture is contained in a first reservoir, which can for example be a flask or a reaction vessel, depending on the volume of reaction mixture, and the reaction mixture is continuously pumped through a flow electrolysis cell into a second reservoir. The flow electrolysis cell contains the electrodes. The reaction mixture passes between the electrodes when being pumped through the flow electrolysis cell.

The anode, also called electrode (+), is the electrode into which electrons flow from the reaction mixture and at which an oxidation takes place. Preferably, the anode is a Pt electrode or a carbon electrode; preferably carbon electrode.

Carbon electrode is preferably an impervious graphite electrode or a glassy carbon electrode, more preferably impervious graphite electrode.

In a particular embodiment, the anode is an impervious graphite electrode.

In a particular embodiment, the anode is made of impervious graphite.

The cathode, also called electrode (-), is the electrode from which electrons flow into the reaction mixture and at which a reduction takes place.

Preferably, the cathode is a nickel electrode, a stainless steel electrode or a Pt electrode, more preferably a nickel electrode or a stainless steel electrode, even more preferably a nickel electrode.

In a particular embodiment, the cathode is a nickel electrode. In a particular embodiment, the cathode is made of nickel.

The anode and the cathode face each other when immersed in the reaction mixture; the size of the area of the surface of the anode and of the cathode that face each other is one of the factors that determines the size of the current that flows between the electrodes through the reaction mixture. Between the anode and the cathode there is a gap. In a particular embodiment, the cathode and the anode each have a flat surface facing each other and running parallel to each other. Preferably each flat surface of the cathode and of the anode that face each other has the same size, that means the areas of the surfaces facing each other is the same. Preferably, the size of the gap, also called gap size, that is the distance between the two electrodes, is constant over the flat areas of the electrodes facing each other and running parallel to each other.

Preferably, the gap size is of from 0.01 to 20 mm, preferably from 0.01 to 15 mm, more preferably from 0.01 to 10 mm, even more preferably from 0.05 to 7.5 mm, especially from 0.05 to 6 mm; other embodiments of the gap size, for example for flow mode, within these ranges are from 0.01 to 1 mm, preferably from 0.05 to 1 nun, more preferably from 0.05 to 0.75 mm, even more preferably from 0.05 to 0.5 mm, especially from 0.05 to 0.4 mm, more especially from 0.075 to 0.4 mm, specific values maybe about 0.1 mm and about 0.3 mm; yet other embodiments of the gap size, for example for batch mode, within these ranges are from 0.01 mm to 10 mm, preferably from 0.1 to 7.5 mm, more preferably from 0.5 to 7.5 mm, even more preferably from 1 to 7.5 mm, especially from 2 to 7.5 mm; a specific value may be about 5 mm.

The concentration of compound of formula (II) in the solvent mixture depends on the solubility of compound of formula (II) and of compound of formula (I) in the reaction mixture.

In particular the concentration of compound of formula (II) in the solvent mixture is chosen to be such that both compound of formula (II) and compound of formula (I) remain in solution during the reaction.

Preferably, the concentration of compound of formula (II) in the solvent mixture at the beginning of the reaction is from 0.001 to 1 M, more preferably from 0.005 to 1 M, even more preferably from 0.005 to 1 M, especially from 0.0075 to 1 M, more especially from 0.0075 to 0.75 M, even more especially from 0.0075 to 0.5 M.

Particular values of the concentration of compound of formula (II) in the solvent mixture at the beginning of the reaction are about 0.0125 M, about 0.1 M, about 0.2 M or about 0.4 M. More particularly, the concentration of compound of formula (II) at the beginning of the reaction is about 0.0125 M, about 0.1 M or about 0.2 M, even more particularly about 0.0125 M or about 0.1 M.

The volume ratio solvent : oxidizable solvent can be from 1 : 10 to 10 : 1, preferably from 1 : 5 to 1 : 0.5, more preferably from 1 : 4 to 1 : 0.5, even more preferably from 1 : 4 to 1 : 1, especially from 1 : 3 to 1 : 1.5. A particular value of the volume ratio solvent : oxidizable solvent is about 1 : 2.

The upper limit of the amount of the base depends primarily on the solubility of the base in the reaction mixture, preferably the concentration of the base is chosen to be such that the base remains in solution during the reaction. Preferably the amount of the base is from 1 to 20 equiv, more preferably from 1.5 to 19 equiv, even more preferably from 1.75 to 18 equiv, especially from 1.75 to 17.5 equiv; the equiv being molar equivalents of the molar amount of compound of formula (II); in a particular embodiment, the amount of the base is about 2 equiv or about 3 equiv or about 4 equiv or about 16 equiv, the equiv being molar equivalents based on the molar amount of compound of formula (II).

The amount of the base determines also the conductivity of the reaction mixture; obviously the conductivity needs to be sufficient; whereas a high amount of the base relative to the amount of substrate usually does not have a detrimental effect; therefore, especially in case of decreasing solubility of the substrate and the resulting lower possible concentration in the reaction mixture it is advisable to have an increased amount of the base relative to the mount of substrate.

Preferably, the current density during the reaction is from 0.01 to 100 mA/cm², more preferably from 0.01 to 50 mA/cm², even more preferably from 0.01 to 20 mA/cm², especially from 0.05 to 17.5 mA/cm², more especially from 0.075 to 15 mA/cm², even more especially from 0.075 to 12.5 mA/cm².

In batch reaction mode the current density during the reaction is preferably from 0.1 to 100 mA/cm², more preferably from 0.1 to 15 mA/cm², even more preferably from 0.5 to 12.5 mA/cm², especially from 0.75 to 10 mA/cm², more especially from 1 to 9 mA/cm². Particularly, the current density for batch reaction mode is about 1.3 mA/cm², about 3 mA/cm² or about 6.7 mA/cm².

In flow mode the current density during the reaction is preferably from 0.01 to 50 mA/cm², more preferably from 0.01 to 20 mA/cm², even more preferably from 0.05 to 17.5 mA/cm², especially from 0.1 to 15 mA/cm², more especially from 0.25 to 12.5 mA/cm². Particularly, the current density for circular flow mode is about 0.8 mA/cm², 3.1 mA/cm², about 4.7 mA/cm², about 6.25 mA/cm² or about 9.4 mA/cm².

In circular flow mode the current density during the reaction is usually higher than in single pass flow mode:

In circular flow mode the current density during the reaction is preferably from 0.5 to 50 mA/cm², more preferably from 1 to 20 mA/cm², even more preferably from 2 to 17.5 mA/cm², especially from 2 to 15 mA/cm², more especially from 2 to 12.5 mA/cm². Particularly, the current density for circular flow mode is about 3.1 mA/cm², about 4.7 mA/cm², about 6.25 mA/cm² or about 9.4 mA/cm².

In single pass flow mode the current density during the reaction is preferably from 0.01 to 5 mA/cm², more preferably from 0.05 to 4 mA/cm², even more preferably from 0.1 to 3 mA/cm², especially from 0.25 to 2 mA/cm². Particularly, the current density for circular flow mode is about 0.8 mA/cm².

Preferably, the reaction is done until a current of from 2 F equiv to 20 F equiv, more preferably of from 2 F equiv to 17.5 F equiv, even more preferably from 2 F equiv to 16 F equiv, especially from 2 F equiv to 15 F equiv, more especially from 2.5 F equiv to 14 F equiv, has passed through the reaction mixture, wherein F equiv means an amount of charge equivalent to the molar amount of compound of formula (II) has passed through the reaction mixture.

Particularly, about 3 F equiv, about 3.5 F equiv, about 4 F equiv or about 12 F equiv have passed through the reaction mixture per mole of compound of formula (II), wherein F equiv means an amount of charge equivalent to the molar amount of compound of formula (II) has passed through the reaction mixture.

Especially when the reaction becomes slower which may happen with decreasing concentration of the substrate and/or increasing molecular weight and/or increasing size of substrate, the reaction time needs to be prolonged in order to attain as much conversion is possible; increase of reaction time essentially means increase of the total current based on the molecular equivalents of the substrate that has passed through the reaction mixture.

Preferably, the reaction is done at a temperature of from 0 to 50 °C, more preferably of from 10 to 50 °C, even more preferably of from 15 to 50 °C.

In flow mode, preferably the flow rate is from 0.001 to 15 ml/min, more preferably from 0.0025 to 12.5 ml/min, even more preferably from 0.005 to 10 ml/min. In a particular embodiment, the flow rate is about 0.01 ml/min, about 1.25 ml/min, about 2.5 ml/min, about 5 ml/min or about 7.5 ml/min.

In circular flow mode the flow rate is usually higher than in single pass flow mode:

In circular flow mode, preferably the flow rate is from 0.5 to 15 ml/min, more preferably from 0.75 to 12.5 ml/min, even more preferably from 1 to 10 ml/min. In a particular embodiment, the flow rate is about 1.25 ml/min, about 2.5 ml/min, about 5 ml/min and about 7.5 ml/min.

In single pass flow mode, preferably the flow rate is from 0.001 to 0.1 ml/min, more preferably from 0.0025 to 0.075 ml/min, even more preferably from 0.005 to 0.05 ml/min. In a particular embodiment, the flow rate is about 0.01 ml/min.

When the gap size is 0.1 mm, then the flow velocity is preferably from 0.0001 to 0.626 m/s, more preferably from 0.0002 to 0.522 m/s, even more preferably from 0.00025 to 0.417 m/s. Particularly values are about 0.0004 m/s, about 0.104 m/s, about 0.208 m/s or about 0.313 m/s.

In circular flow mode the flow velocity during the reaction is usually higher than in single pass flow mode:

When the gap size is 0.1 mm, then in circular flow mode the flow velocity is preferably from 0.0208 to 0.626 m/s, more preferably from 0.0313 to 0.522 m/s, even more preferably from 0.0417 to 0.417 m/s. Particularly values are about 0.104 m/s, about 0.208 m/s or about 0.313 m/s.

When the gap size is 0.1 mm, then in single pass flow mode the flow velocity is preferably from 0.0001 to 0.005 m/s, more preferably from 0.0002 to 0.0025 m/s, even more preferably from 0.00025 to 0.001 m/s. A particularly value is about 0.0004 m/s.

When the gap size is 0.3 mm, which is a particular embodiment in circular flow mode, then the flow velocity is preferably from 0.0069 to 0.209 m/s, more preferably from 0.0104 to 0.174 m/s, even more preferably from 0.0139 to 0.139 m/s. Particular values are about 0.035 m/s, about 0.069 m/s or about 0.104 m/s.

The flow velocity in m/s per mm of gap size between the two electrodes is preferably from 0.001 to 6.26 m/[s * mm], more preferably from 0.002 to 5.22 m/[s * nun], even more preferably from 0.0025 to 4.17 m/[s * mm]

In circular flow mode the flow velocity during the reaction in m/s per mm of gap size between the two electrodes is usually higher than in single pass flow mode:

In a particular embodiment in circular flow mode, the flow velocity in m/s per mm of gap size between the two electrodes is preferably from 0.208 to 6.26 m/[s * mm], more preferably from 0.313 to 5.22 m/[s * mm], even more preferably from 0.417 to 4.17 m/[s * mm]. Particular values are about 1.04 m/[s * mm], about 2.08 m/[s * mm] or about 3.13 m/[s * mm].

In a particular embodiment in single pass flow mode, the flow velocity in m/s per mm of gap size between the two electrodes is preferably from 0.001 to 0.05 m/[s * mm], more preferably from 0.002 to 0.025 m/[s * mm], even more preferably from 0.0025 to 0.01 m/[s * mm], A particular value is about 0.004 m/[s * mm].

In a particular batch reaction mode embodiment,

• the anode is a graphite electrode, preferably an impervious graphite electrode;

• the cathode is a nickel electrode or a stainless steel electrode, preferably a nickel electrode;

• the gap size, that is the distance, between the cathode and the anode, is from 0.01 mm to 10 mm; preferably from 0.1 to 7.5 mm, more preferably from 0.5 to 7.5 mm, even more preferably from 1 to 7.5 mm, especially from 2 to 7.5 mm; a particular value is about 5 mm;

• the concentration of compound of formula (II) at the beginning of the reaction is from 0.001 to 1 M, more preferably from 0.005 to 1 M, even more preferably from 0.005 to 1 M, especially from 0.0075 to 1 M, more especially from 0.0075 to 0.75 M, even more especially from 0.0075 to 0.5 M; particularly about 0.0125 M, about 0.1 M or about 0.2 M, even more particularly about 0.0125 M or about 0.2 M;

• the volume ratio solvent : oxidizable solvent is from 1 : 10 to 10 : 1; preferably from 1 : 3 to 1 : 1.5; particularly about 1 : 2; • the upper limit of the amount of the base depends primarily on the solubility of the base in the reaction mixture, preferably the concentration of the base is chosen to be such that the base remains in solution during the reaction. Preferably the amount of the base is from 1 to 20 equiv, more preferably from 1.5 to 19 equiv, even more preferably from 1.75 to 18 equiv, especially from 1.75 to 17.5 equiv; in a particular embodiment, the amount of the base is about 2 equiv or about 3 equiv or about 4 equiv or about 16 equiv, the equiv being molar equivalents based on the molar amount of compound of formula (II);

• the current density during the reaction is preferably from 0.1 mA/cm² to 100 mA/cm², more preferably from 0.1 to 15 mA/cm², even more preferably from 0.5 to 12.5 mA/cm², especially from 0.75 to 10 mA/cm², more especially from 1 to 9 mA/cm². Particularly, the current density for batch reaction mode is about 1.3 mA/cm², about 3 mA/cm² or about 6.7 mA/cm²;

• preferably, the reaction is done until a current of from 2 F equiv to 20 F equiv, more preferably of from 2 F equiv to 17.5 F equiv, even more preferably from 2 F equiv to 16 F equiv, especially from 2 F equiv to 15 F equiv, more especially from 2.5 F equiv to 14 F equiv, has passed through the reaction mixture, particularly about 3 F equiv, about 3.5 F equiv, about 4 F equiv or about 12 F equiv; wherein F equiv means an amount of charge equivalent to the molar amount of compound of formula (II) has passed through the reaction mixture; and/or

• the reaction is conducted at a temperature of 15 to 50 °C, preferably at room temperature (RT).

In a particular circular flow mode embodiment,

• the gap size, that is the distance, between the cathode and the anode, is from 0.01 mm to 1 mm; preferably 0.075 mm to 0.35 mm; particularly about 0.1 mm or about 0.3 mm;

• the volume ratio solvent : oxidizable solvent is from 1 : 10 to 10 : 1; preferably from 1 : 3 to 1 : 1.5; particularly about 1 : 2;

• the upper limit of the amount of the base depends primarily on the solubility of the base in the reaction mixture, preferably the concentration of the base is chosen to be such that the base remains in solution during the reaction. Preferably the amount of the base is from 1 to 20 equiv, more preferably from 1.5 to 19 equiv, even more preferably from 1.75 to 18 equiv, especially from 1.75 to 17.5 equiv; in a particular embodiment, the amount of the base is about 2 equiv or about 3 equiv or about 4 equiv or about 16 equiv, the equiv being molar equivalents based on the molar amount of compound of formula (II);

• preferably the flow rate is from 0.5 to 15 ml/min, more preferably from 0.75 to 12.5 ml/min, even more preferably from 1 to 10 ml/min. In a particular embodiment, the flow rate is about 1.25 ml/min, about 2.5 ml/min, about 5 ml/min and about 7.5 ml/min;

• the current density during the reaction is preferably from 0.5 to 50 mA/cm², more preferably from 1 to 20 mA/cm², even more preferably from 2 to 17.5 mA/cm², especially from 2 to 15 mA/cm², more especially from 2 to 12.5 mA/cm². Particularly, the current density for circular flow mode is about 3.1 mA/cm², about 4.7 mA/cm², about 6.25 mA/cm² or about 9.4 mA/cm²;

In a particular single pass flow mode embodiment,

• the anode is a graphite electrode, preferably an impervious graphite electrode; • the cathode is a nickel electrode or a stainless steel electrode, preferably a nickel electrode;

• the gap size, that is the distance, between the cathode and the anode, is from 0.01 mm to 1 mm; preferably 0.075 mm to 0.35 mm; particularly about 0.1 mm;

• preferably the flow rate is from 0.001 to 0.1 ml/min, more preferably from 0.0025 to 0.075 ml/min, even more preferably from 0.005 to 0.05 ml/min. In a particular embodiment, the flow rate is about 0.01 ml/min;

• In single pass flow mode the current density during the reaction is preferably from 0.01 to 5 mA/cm², more preferably from 0.05 to 4 mA/cm², even more preferably from 0.1 to 3 mA/cm², especially from 0.25 to 2 mA/cm². Particularly, the current density for circular flow mode is about 0.8 mA/cm².

• preferably, the reaction is done until a current of from 2 F equiv to 20 F equiv, more preferably of from 2 F equiv to 17.5 F equiv, even more preferably from 2 F equiv to 16 F equiv, especially from 2 F equiv to 15 F equiv, more especially from 2.5 F equiv to 14 F equiv, has passed through the reaction mixture, particularly about 3 F equiv, about 3.5 F equiv, about 4 F equiv or about 12 F equiv; wherein F equiv means an amount of charge equivalent to the molar amount of compound of formula (II) has passed through the reaction mixture; and/or the reaction is conducted at a temperature of 15 to 50 °C, preferably at room temperature (RT).

Abbreviations, Definitions, Materials, Sources used and disclosed in this specification

Electrode area For batch reaction mode, the reaction mixture was exposed to two flat platelike electrodes, each with an electrode area of 1.5 cm².

With an electrode area of 1.5 cm² particular current densities were:

2 mA: 1.3 mA/cm²

5 mA: 3 mA/cm²

10 mA: 6.7 mA/cm²

For circular flow mode, the reaction mixture was exposed to two flat platelike electrodes, each with an electrode area of 6.4 cm².

With an electrode area of 6.4 cm² particular current densities were:

20 mA: 3.1 mA/cm²

30 mA: 4.7 mA/cm²

40 mA: 6.25 mA/cm²

50 mA: 7.8 mA/cm² 60 mA: 9.4 mA/cm²

For single pass flow mode, the reaction mixture was exposed to two flat plate-like electrodes, each with an electrode area of 6.4 cm².

With an electrode area of 6.4 cm² particular current densities were:

5 mA: 0.8 mA/cm²

IG electrode Impervious graphite electrode, also called only IG herein: Graphite-resin impervious bipolar plates of FC-GR grade by Graphtek LLC, IL 60089, USA were sourced from graphitestore.com, Northbrook, IL 60062, USA. The impervious graphite (IG) plates were molded and of resin- filled grade that combines best properties of graphite such as high electrical conductivity, high thermal conductivity, chemical resistance and easy machining with low permeability of molded composites.

For batch reaction mode, these graphite-resin impervious bipolar plates were cut to the size similar to the size of standard IKA electrodes for the IKA-ElectraSyn™ device (IKA®-Werke GmbH & CO. KG, 79219 Staufen, DE).

For circular flow mode or single pass flow mode, these graphite-resin impervious bipolar plates were cut to a plate of the size of50 x 50 x 6 mm. Impervious graphite electrodes were polished with a 3000 grit whetstone before each experiment.

Nickel electrode For batch reaction mode, a standard nickel electrode for the IKA- ElectraSyn™ device was used, also called Ni electrode or only Ni herein.

For circular flow mode or single pass flow mode, a nickel foil was used (Goodfellow GmbH, 20099 Hamburg, DE; Product Code: NI00-FL-000169; Thickness: 0.3 mm; Dimension: 50 x 50 mm; Temper: As rolled; https ://www. goodfellow, com/ de/ en-gb/displayitemdetails/p/niOO-fl- 000169/nickel-foil).

SS electrode Stainless Steel electrode, also called only SS herein: For batch reaction mode, a standard stainless steel electrode for the IKA- ElectraSyn™ device was used

ACN acetonitrile

Ac acetyl about the term “about” used in connection with a numerical value indicates that the actual value can be within a range of ± 20% of the specified numerical value, preferably within a range of ± 10% of the specified numerical value, more preferably within a range of ± 5% of the specified numerical value. The term “about” encompasses all values within a range of ± 20%, preferably ± 10%, more preferably ± 5%, of the specified numerical value. amino acid the term amino acid refers to organic amino acids, that is organic compounds having an amino group and a carboxylic acid group ; alpha amino acids refers to such organic amino acids wherein the amino group and the carboxylic acid group are covalently bonded to the same C atom as, that is to the alpha C atom according to the common nomenclature of organic chemistry

Boc tert-Butyloxycarbonyl protecting group i-Bu iso-butyl n-Bu n-butyl tert-Bu tert-butyl

CEx Comparative Example current density the size of the current divided by the size of the area of the surface of the electrodes facing each other in the reaction mixture

DBU l,8-Diazabicyclo[5.4.0]undec-7-ene, CAS 6674-22-2

DMA dimethylacetamide

DMF Dimethylformamide

HPLC high performance liquid chromatography equiv molar equivalent

Et ethyl

Ex Example

F Faraday constant:

F = elementary charge e x Avogadro constant NA = 1 .602176634x10"¹⁹ C x 6.02214076 xlO²³ mol^-1 = 9.64853321233100184xl0⁴ C-moE¹

The amount of charge, that passes through a reaction mixture, divided by the Faraday constant equals the molar amount of substrate that has been electrolyzed.

F equiv is used herein to denote the amount of charge equivalent to the molar amount of substrate, that is of compound of formula (II), that has passed through the reaction mixture, so for example 1 F equiv, 2 F equiv or 4 F equiv means that the 1-fold, 2-fold or 4-fold amount of charge equivalent to the molar amount of compound of formula (II) has passed through the reaction mixture.

For a reduction reaction involving two electrons per molecule of substrate 2 F equiv are the stoichiometric charge equivalent to the molar amount of substrate.

Fc ferrocene Flow Velocity The following flow rates correspond to the following flow velocities in circular flow mode or single pass flow mode as shown in Table 9: Fmoc 9-Fluorenylmethyloxycarbonyl protecting group of formula (Fmoc) for the protection of amino residues, the arrow in formula (Fmoc) show the carbonyl C atom which is covalently bonded to the amino residue which is to be protected

Fmoc substrates Source and purity are shown in Table 10

Me methyl

MS molecular sieve with a pore size of 3 A

NBP N-butyl-2-pyrrolidinone, IUPAC 1 -butylpyrrolidin-2-one, sometimes also called N-butyl pyrrolidone, CAS 3470-98-2

NMP N-methyl-2 -pyrrolidone, N-methyl-2-pyrrolidinone, IUPAC 1- methylpyrrolidin-2-one, CAS 872-50-4 i-Pr iso-propyl n-Pr n-propyl rpm rounds per minute

RT room temperature

Sar sarcosine

TEATFB Tetraethylammoniumtetrafluoroborate, Thermo Scientific Al 0211, 99% vs versus Z Benzyloxycarbonyl, also abbreviated with Cbz EXAMPLES

Methods

Experimental set up for Batch Mode and for Flow Mode

Examples in batch reaction mode were conducted with an experimental set up as shown in Figure 1.

Examples in circular flow mode were conducted with an experimental set up as shown in Figure 2.

Examples in single pass flow mode were conducted with an experimental set up as shown in Figure 7.

HPLC analysis method

HPLC analysis was done on a C18 reversed-phase analytical column (150 x 4.6 mm, particle size 5 pm) at 37 °C by using mobile phases A (water/acetonitrile 90: 10 (v/v) + 0.1 vol-% TFA) and B (acetonitrile + 0.1 vol-% TFA), the vol-% of TFA are based on volume of water/acetonitrile 90:10 (v/v) in case of mobile phae A and on volume of acetonitrile in case of mobile phase B, respectively, at a flow rate of 1.5 mL/min. The following gradient was applied: linear in-crease from 30 vol-% solution B to 100 vol-% B in 10 min, switching back in ca. a second to 30 vol-% B and holding 30 vol-% B for 3.5 min.

For HPLC analysis 10 pl of the reaction mixture in an IKA-ElectraSyn™ vial were taken up with a Hamilton syringe and diluted with 1 ml ACN in a HPLC vial with a volume of 1.5 ml, which is then capped and placed in an HPLC sampler for analysis, injection volume is 1 pl. A complete solution should be obtained which is then injected into the HPLC column. If an insoluble white residue forms inside the HPLC vial then it can be dissolved by adding 2 to 3 drops of methanol to the mixture in the HPLC vial.

For HPLC analysis after workup 1 to 2 mg product obtained from the work up are dissolved in 1 ml ACN in a HPLC vial and 1 pl is injected for HPLC analysis.

Conversion and selectivity were calculated from the HPLC peak area-% of product peak, substrate peak and by-product peaks.

Example 1 - Fmoc-LG-OH - Batch Mode

The reaction is shown in Scheme 2:

0.3 mmol (123.1 mg, 0.1 M) Fmoc-Leu-Gly-OH and 1.2 mmol (119 mg, 0.4 M) potassium acetate (dried at 100 °C in a vacuum oven over night) were charged into a 5 ml IKA- ElectrasSyn™ vial. The vial was equipped with a stir bar and 1 ml acetic acid (dry, stored under argon atmosphere with MS) and 2 ml DMA (dry, in septum flask under argon atmosphere) are added to the vial. This solvent addition was also used to flush remaining powder residues to the bottom of the vial. The vial was placed onto a stirring plate and the mixture was stirred at 900 rpm until a solution was obtained, which was after 1 to 2 min. Then the vial was equipped with an impervious graphite electrode and a nickel electrode, the gap size between the electrodes was 5 mm, an electrode area of 1.5 cm² was submersed in the reaction mixture. The vial was closed by screwing the cap on. The IG was connected to the red (+) side of the power source and served as anode and the nickel electrode was connected to the black (-) side of the power source and served as cathode. The power source was set to a current of 2 mA providing a current density of 1.3 mA/cm². After applying the current small bubbles appeared on both electrodes, the starting voltage on the power source read between 2 and 3 V. The reaction mixture was stirred at 900 rpm at RT for 16 h and 4 min, after this reaction time a charge of 4 F equiv had been reached which was the target At the end of the reaction the reaction mixture had a very faint yellow color. The HPLC chromatogram showed that no by-product had formed.

Results are given in Table 1.

Workup:

After the reaction the reaction mixture was diluted with 20 ml Ethylacetate. The organic phase was then extracted 5 times with 20 ml wt% aqueous sodium citrate solution. The organic phase was dried with Na2SO 4 and the solvent was stripped with a rotavap. The product (the acetate) was obtained as a solid, an off-white powder.

Isolated yield is given in Table 1. This workup procedure was applied also to any other example where an isolated yield is stated. The acetyl residue of some of the products hydrolyzed, this is indicated in Table 1. Scheme 1 elucidates the hydrolysis of the acetate to the alcohol for Fmoc-Pro-Pro-OH of Example 6. Essentially all acetates hydrolyzed which did contain an amino acid other than Gly or Ala at the C-terminus. Hydrolysis can be detected by the occurrence of multiple peaks (e.g. alcohol) in HPLC analysis after workup.

Yield was in any case determined without respect to hydrolysis, simply based on the amount of off-white product obtained from the workup.

Example 2- Fmoc-LG-OH - Circular Flow Mode

0.6 mmol (225.2 mg, 0.1 M) Fmoc-Leu-Gly-OH and 2.4 mmol (238 mg, 0.4 M) potassium acetate (dried at 100 °C in a vacuum oven over night) were charged into a 20 ml capped glass vial. The vial was equipped with a stir bar and 2 ml acetic acid (dry, stored under argon atmosphere with MS) and 4 ml DMA (dry, in septum flask under argon atmosphere) were added to the vial; substrate concentration was 0.1 M. The solvent addition was also used to flush powder residues sticking to the walls of the vials down to the bottom of the vial. The vial was placed onto a stirring plate and the mixture was stirred at 900 rpm until a complete solution was obtained, which was after 1 to 2 min. The reaction mixture was circulated from the vial that acted as a reservoir through an flow electrolysis cell, equipped with an impervious graphite electrode and a nickel electrode, the graphite electrode was connected to the red (+) side of the power source and served as anode and the nickel electrode was connected to the black (-) side of the power source and served as cathode, and back into the vial with a Vapourtec SF-10 Pump (Vapourtec Ltd, Bury St Edmunds, UK; flow rate 5 ml/min). The set up of the flow electrolysis cell is the undivided set up as published in W. Jud et al., CHEMISTRY METHODS (2021) 1:36-41, called Jud herein; the interelectrode gap was realized with the flow channel design as shown in Figure 1 of Jud and also as shown in the top left of Figure 2 of Jud marked with the blue point, with the difference that the gap size was not 0.3 mm as disclosed in Jud but 0.1 mm, thereby the volume decreased form the 190 pl as reported in Jud to ca. 69 pl; the electrode surface contact area remained unchanged with 6.4 cm² as well as channel width of 0.400 cm and total length of channel of 17.3 cm. Further details of the flow electrolysis cell are given in the supporting information of Jud and in Figure 2. The flow electrolysis cell had a aluminum casing which was placed on a heating plate set to 60 °C, the temperature of the aluminum casing of the flow electrolysis cell was measured with an infrared camera to be between 35 °C at the top and 40 °C and the bottom, which was in contact with the heating plate. It can be assumed that the electrodes in the flow electrolysis cell and therefore the solution in the flow channel in the flow electrolysis cell had a similar temperature as the aluminum casing of the flow electrolysis cell The reaction mixture was then electrolyzed at a constant current of 40 mA, providing a current density of 6.25 mA/cm², for 1 h 36 min (4 F equiv), while the mixture in the reservoir is stirred at ca. 300 rpm. Thereafter the reaction mixture had a very faint yellow color. The HPLC chromatogram showed that no by-product had formed.

Conversion: 97% Selectivity: 95%

Work up was down similar to Example 1 providing an yield after work-up of 83%.

Examples 3 to 10 and 60 to 63 and 100 to 104 - Different Substrates - Batch Mode

Examples listed in the Table 1, such as 3 to 10 and 60 to 63 and 100 to 104, were done according to Example 1 with different substrates as stated in the Table 1.

The acetyl residue hydrolysed during work up except for Ria being H or CH3, that is except for Gly and Ala as C-terminal amino acids. This hydrolysis is shown for the Fmoc-Pro-Pro- OH of Example 7 as a representative example in Scheme 1.

(1 *) Product was obtained as isomeric mixture.

(2*) Product and substrate peak are almost on top of each other so conversion and selectivity are approximations

(3*) The substrate in the respective column of Table 1 is Fmoc-R-OH, with R being the amino acids or peptides shown in this column of Table 1

(4*) Example 9A and Example 9B: Example 9A was done according to Example 1, that is the electrolysis was carried out until a target current flow of 4 F equiv had been reached, whereas Example 9B was also done according to Example 1 with the difference that the electrolysis was done until a target a current flow of 8 F equiv had been reached and the respective conversion and selectivity are given in the brackets. The yield of 9A was not determined (denoted with nd in the respective row of the table).

(5*) product of Example 10 precipitated, whereas the products of the Examples 1 to Examples 9A/9B stayed in solution.

(6*) Since the product of Example 10, Fmoc-Gly-Gly-NH-CFh-OAc, precipitated already during the reaction the workup was done differently: The reaction mixture was filtered, the press cake was washed with ethyl acetate and dried by in vacuum.

(7*) Amount of substrate was 0.075 mmol (0.025 M), with the other parameters of Example 1 unchanged, i.e. 1.2 mmol potassium acetate resulting in [GGGG] : [CH3COOK] = 1 : 16, 1 ml acetic acid, 2 ml DMA, constant current of 2 mA, providing a current density of 1.3 mA/cm², but the reaction was done for 48 h 12 min (12 F equiv). Scheme 1 decarboxylation

Fmoc-Pro-Pro-OH

Example 11 - Cyclovoltammetry 50 ml of a Stock Solution were prepared containing 1 mmol Fmoc-Leu-Gly-OH (20 mM, 410 mg) and 5 mmol Tetraethylammoniumtetrafluoroborate (TEATFB) (100 mM, 1937 mg) as electrolyte in acetonitrile. The Stock Solution was divided into 5 portions of 10 ml each, the first portion was left untreated and 1 mmol of solvent was added to portions 2, 3, 4 and 5 to obtain the following five solutions 1 to 5:

Solutions 1 to 5

1 10 ml of Acetonitrile, 20 mM Fmoc-Leu-Gly-OH (1 mmol), 100 mM TEATFB (5 mmol)

2 Solution 1 and 1 mmol DMSO added (resulting in ca. 20 mM DMSO)

3 Solution 1 and 1 mmol DMA added (resulting in ca. 20 mM DMA) 4 Solution 1 and 1 mmol NMP added (resulting in ca. 20 mM NMP)

5 Solution 1 and 1 mmol Sulfolane added (resulting in ca. 20 mM Sulfolane) For comparison, four solutions 6 to 10 similar to solutions 2 to 5 but without the substrate were prepared to just observe the oxidation of the solvent:

Solutions 6 to 9

6. 10 ml of Acetonitrile, 100 mM TEATFB (5 mmol) and 1 mmol DMSO added (resulting in ca. 20 mM DMSO)

7. 10 ml of Acetonitrile, 100 mM TEATFB, (5 mmol) and 1 mmol DMA added (resulting in ca. 20 mM DMA)

8. 10 ml of Acetonitrile, 100 mM TEATFB, (5 mmol) and 1 mmol NMP added (resulting in ca. 20 mM NMP)

9. 10 ml of Acetonitrile, 100 mM TEATFB, (5 mmol) and 1 mmol Sulfolane added (resulting in ca. 20 mM Sulfolane)

For estimation of the half peak potential of the Fmoc oxidation a solution 10 was prepared:

10. Identical with Solution 1 (for measurement without addition of Ferrocene Standard Solution).

For estimation of the half peak potential of the Fmoc oxidation a solution 11 was prepared:

11. 10 ml of Acetonitrile, 100 mM TEATFB, (5 mmol) and 1 mmol Fmoc-Cl added (resulting in ca. 20 mM Fmoc-Cl)

Cyclic voltammograms of Solutions 1 to 11 were recorded in a glass cell with a Rodeostat open source potentiostat (by Io Rodeo Inc, Pasadena CA, 91107, USA). A glassy carbon disk (2 mm diameter rod with PTFE shroud) was used as the working electrode and a platinum wire as the counter electrode. A silver wire was utilized as quasi reference electrode, using ferrocene as reference. For this, 0.4 ml of a Ferrocene Standard Solution (50 mM Ferrocene in ACN) were added to 2 ml of Solution 1 prior to measurement, whereas Solution 10 was measured without addition of ferrocene.

Figure 3 shows

• cyclic voltammograms for solutions 1 to 5: Solution 1 (1) with Fmoc-Leu-Gly-OH Solution 2 (2) with Fmoc-Leu-Gly-OH and DMSO Solution 3 (3) with Fmoc-Leu-Gly-OH and DMA

Solution 4 (4) with Fmoc-Leu-Gly-OH and NMP

Solution 5 (5) with Fmoc-Leu-Gly-OH and Sulfolane

• the peak potential of the oxidations:

DMSO oxidation (2-1)

DMA oxidation (3-1)

NMP oxidation (4-1) the absence of Sulfolane oxidation (5-1) ferrocene oxidation (1-3)

Fmoc oxidation (1-4)

• the peak potential of the COO' oxidation:

COO' oxidation (1-2) without addition of oxidizable solvent

COO' oxidation (2-2) in the presence of DMSO

COO' oxidation (3-2) in the presence of DMA

COO' oxidation (4-2) in the presence of NMP

COO' oxidation (5-2) in the presence of sulfolane

Figure 4 shows

• cyclic voltammograms for solutions 6 to 9:

Solution 6 (6) DMSO

Solution 7 (7) DMA

Solution 8 (8) NMP

Solution 9 (9) Sulfolane

Figure 5 shows the cyclic voltammogram for solution 10.

Figure 6 shows the cyclic voltammogram for solution 11.

Scheme 2 illustrates with arrows the C atoms which are oxidized in case of Fmoc oxidation and COO' oxidation. Scheme 2

Table 2 lists the half peak potentials of oxidation vs. ferrocene, which were determined either from the measurements represented in Figure 3, Figure 4, Figure 5 or Figure 6

(*) The half peak potential of the oxidation of ferrocene was set to zero for reference purpose.

(**) The half peak potential of the oxidation of Fmoc was estimated from a cyclovoltammogram of Fmoc-Cl of solution 11 shown in Figure 6. (***) The half peak potential of the oxidation of COO- was estimated from a cyclovoltammogram of Fmoc-Leu-Gly-OH of solution 10 shown in Figure 5

Example 12 to 24 Examples 12 to 24 were done according to example 1 (which was done with Fmoc-Leu-Gly- OH as substrate) with the differences as shown in Table 3.

So the experiments were done with IG as anode and at RT, (-) means cathode.

(nc) no change against Example 1

Comparative Examples 1 to 6

Comparative Examples 1 to 6 were done according to example 1 (which was done with Fmoc-Leu-Gly-OH as substrate, abbreviated with LG in Table 4) with the differences as shown in Table 4.

So the experiments were done with IG as anode, (-) means cathode.

(*) The solvent tBuOH in Comparative Example 6 rendered the solution non-conductive, so no current flowed and now yield was obtained.

Comparative Example 1 in comparison to Example 16 illustrates that in absence of the oxidizable solvent selectivity is considerably lower.

Comparative Example 2 in comparison to Example 24 illustrates that the absence of the oxidizable solvent lowers conversion and in particular selectivity.

Comparative Example 3 in comparison to Example 25 illustrates that the absence of the oxidizable solvent lowers conversion and in particular selectivity.

Comparative Example 4 in comparison to Example 27 illustrates that the exchange of the oxidizable solvent DMSO against sulfolane lowers conversion and selectivity.

Comparative Example 5 in comparison to Example 16 illustrates that in absence of AcOH, that is in absence of a solvent, selectivity is considerably lower.

Example 30 to 42

Examples 30 to 42 were done according to example 1 with the first difference that Fmoc-Gly- Gly-OH was the substrate and not Fmoc-Leu-Gly-OH, and with the further differences as shown in Table 5.

So all experiments were done with IG as anode, Ni as cathode, 4 F equiv.

(nc) no change against Example 1

(*) isolated yield of Example 30 was 89 %, of Example 31 85%

Example 50 - Fmoc-GG-OH - Circular Flow Mode Example 50 was done as follows: 0.6 mmol (225.2 mg, 0.1 M) Fmoc-Gly-Gly-OH and 2.4 mmol (238 mg, 0.4 M) potassium acetate (dried at 100 °C in a vacuum oven over night) were charged into a 20 ml capped glass vial. The vial was equipped with a stir bar and 2 ml acetic acid (dry, stored under argon atmosphere with MS) and 4 ml DMA (dry, in septum flask under argon atmosphere) were added to the vial, substrate concentration was 0.1 M. The solvent addition was also used to flush powder residues sticking to the walls of the vials down to the bottom of the vial. The vial was placed onto a stirring plate and the mixture was stirred at 900 rpm until a complete solution was obtained, which was after 1 to 2 min. The reaction mixture was circulated from the vial that acted as a reservoir through an flow electrolysis cell, equipped with an impervious graphite electrode and a nickel electrode, the graphite electrode was connected to the red (+) side of the power source and served as anode and the nickel electrode was connected to the black (-) side of the power source and served as cathode, and back into the vial with a Vapourtec SF-10 Pump (flow rate 5 ml/min). The set up of the flow electrolysis cell is the undivided set up as published in W. Jud et al., CHEMISTRY METHODS (2021) 1:36-41, called Jud herein; the interelectrode gap was realized with the flow channel design as shown in Figure 1 of Jud and also as shown in the top left of Figure 2 of Jud marked with the blue point, the gap size was 0.3 mm and the volume was 190 p.1 as reported in Jud; the electrode surface contact area remained unchanged with 6.4 cm² as well as channel width of 0.400 cm and total length of channel of 17.3 cm. Further details of the flow electrolysis cell are given in the supporting information of Jud and in Figure 2. The flow electrolysis cell had an aluminum casing which was placed on a heating plate set to 60 °C, the temperature of the aluminum casing of the flow electrolysis cell was measured with an infrared camera to be between 35 °C at the top and 40 °C and the bottom which as in contact with the heating plate. It can be assumed that the electrodes in the flow electrolysis cell and therefore the solution in the flow channel in the flow electrolysis cell had a similar temperature as the aluminum casing of the flow electrolysis cell The reaction mixture was then electrolyzed at a constant current of 40 mA, providing a current density of 6.25 mA/cm², for 1 h 36 min (4 F equiv), while the mixture in the reservoir is stirred at ca. 300 rpm. Thereafter the reaction mixture had a very faint yellow color. The HPLC chromatogram showed that no by-product had formed. Conversion: 99% Selectivity: 97%

Example 51 to 58 - Circular Flow Mode Example 51 to 58 were done according to Example 50, so with Fmoc-Gly-Gly-OH as substrate, and Table 6 shows the parameters which were changed against Example 50.

(nc) no change against Example 1 (*) The amount and concentration of KOAc was the same as in Example 50, that is 0.4 M.

Comparative Example 10 - Acetoxylation with Pb(OAc)4

Acetoxylation with Pb(OAc)4 was done following Example 3 of US 2020/385422 Al: 0.188 mmol of substrate and 0.3 mmol Pb(OAc)4 were dissolved in a 4 ml Vial in 1 ml of dried THF and 0.2 ml of dried glacial AcOH. The Vial was closed and the reaction mixture was stirred for 1.5 h at 61 °C under reflux conditions. Then the reaction mixture was cooled to RT. Part of the product precipitates during the reaction. 10 pl of the supernatant liquid was diluted in 1 ml of ACN and analyzed with HPLC.

Results are given in Table 7.

The products from Comparative Examples 10 to 24 were used to identify the respective product peak in the HPLC analysis of the products of the inventive examples described herein.

The conversion and the selectivity of the electrochemical oxidative acetoxylation of present invention was found to be comparable to the oxidative acetoxylation in the presence of Pb(OAc)₄.

Example 70 to 75 - Circular Flow Mode

Example 71 to 75 were done according to Example 2, so with Fmoc-Leu-Gly-OH as substrate, and Table 8 shows the parameters which were changed against Example 2.

(nc) no change against Example 1

Example 80 - Single Pass Flow Mode

In a 25 ml septum bottle 2.5 mmol Fmoc-Leu-Gly-OH (0.1 M) and 10 mmol CH3COOK (0.4 M) (dried at 100 °C in a vacuum oven over night) were dissolved in 8.33 ml dry glacial acetic acid and 16.66 ml dry DMA. The septum bottle was attached to an syringe pump (Syrris Asia, UK) with a set flow rate of 10 pl/min. The reaction mixture was pumped through an flow electrolysis cell with nickel cathode and impervious graphite anode with a current of 5 mA.

The flow electrolysis cell is the same as the one described in Example 2 (gap size 0.1 mm and volume 69 microliter) and weas placed as described on a heating plate; the temperature thereby was as described in Example 2. At this flowrate and current, a current density of 0.8 mA/cm² was provided and a charge of 3 F equiv was applied to the solution passing through the flow electrolysis cell. After passing through the flow electrolysis cell, the reaction mixture was collected in vials that were changed after 1.5 h, 17 h, 24 h, 25 h and 40 h, when the collection was finished. The yield of the fractions was determined via HPLC and the fractions up to 17 h were combined and purified via dissolution in 60 ml EtOAc and extracting 5 times with 40 ml of 20 wt% aqueous sodium citrate solution, followed by drying the organic phase with sodium sulfate and stripping the solvent in vacuo. The isolated amount of the acetate after 17 h was 336 mg which corresponds to a yield of 78 %.

Example 90 Scheme 4

The reaction according to Scheme 4 was done in analogy to Example 1 with the differences: 0.3 mmol Fmoc-Gly-Gly-OH where charged in a 5 ml IKA ElectrasSyn™ vial and dissolved in 3 ml MeOH. 0.125 equiv MeONa where added and the reaction mixture was electrolyzed at 2 mA with IG as anode, Ni as cathode, 4 F equiv (so no potassium acetate, acetic acid and DMA were used).

The HPLC chromatogram showed:

• Conversion: 94% • Selectivity: 89%

Isolated Yield: 79 mg (78 %)

Claims

1. A method for preparation of compound of formula (I) by an electrochemical oxidative decarboxylation reaction of compound of formula (II) in a solvent mixture and in the presence of a base; the electrochemical oxidative decarboxylation reaction takes place by passing an electrical current through a reaction mixture comprising the compound of formula (II), the solvent mixture and the base; the electrical current is provided by a power supply which is connected to two electrodes, an anode and a cathode, which are immersed into the reaction mixture; wherein Y is selected from the group consisting of residue of formula (Yl), residue of formula (Y2), residue of formula (Y3), residue of formula (Y4), and residue of formula (Y5);

Ria, R2a, R3a, R4a and R5a are identical or different and independently from each other side chains of alpha amino acids and Rib, R2b, R3b, R4b and R5b are identical or different and independently from each other H or CH3; or one or more of Rib, R2b, R3b, R4b and R5b, independently from each other, and the respective N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, together with the respective residue Ria, R2a, R3a, R4a and R5a and the alpha C atom, to which said Ria, R2a, R3a, R4a or R5a is bonded, can also form a three, four, five or six membered nonaromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only one endocyclic heteroatom, except in case of a six membered ring wherein one further endocyclic O atom may be present in said six membered ring; the solvent mixture comprises a solvent R90H and an oxidizable solvent; the base is selected from the group consisting of R90M, phosphates, carbonates, phosphazenes, tertiary amine of formula R20(R21)(R22)N and DBU;

R9 is selected from the group consisting of acetyl (Ac) and C1-4 alkyl;

M is selected from the group consisting of Na, K, Li, and R1O(R11)(R12)(R13)N;

RIO, R11, R12 and R13 are identical or different and independently from each other selected from the group consisting of H and C1-4 alkyl;

R20, R21 and R22 are identical or different and independently from each other selected from the group consisting of C 1-4 alkyl.

2. The method according to claim 1, wherein when anyone of Ria, R2a, R3a, R4a and R5a is a side chain of an alpha amino acid, then it is a side chain of a natural or non-natural alpha amino acid.

3. The method according to claim 1 or 2, wherein side chains of alpha amino acids are selected from the group of H, C1-4 alkyl, phenyl, benzyl, C1.4 hydroxy alkyl, H₂NC(O)CH₂*, H₂NC(O)CH2CH2*, C1-4 thiol alkyl, CH3SCH2CH2*,

with an * in the drawn formulae denoting the C atom where the side chain is bonded and with ** in the drawn formulae denoting where a residue of a side chain is bonded.

4. The method according to one or more of claims 1 to 3, wherein residue of formula (Yl) together with the COOH residue depicted in formula (II) is called alpha amino acid residue AA1 which contains Ria and Rib; residue of formula (Y2) together with the COOH residue depicted in formula (II) is a dipeptidyl residue Y2 of the alpha amino acid residue AA1 and an alpha amino acid residue AA2 which contains R2a and R2b; residue of formula (Y3) together with the COOH residue depicted in formula (II) is a tripeptidyl residue Y3 of the alpha amino acid residue AA1, the alpha amino acid residue AA2 and an alpha amino acid residue AA3 which contains R3a and R3b; residue of formula (Y4) together with the COOH residue depicted in formula (II) is a tetrapeptidyl residue Y4 of the alpha amino acid residue AA1, the alpha amino acid residue AA2, the alpha amino acid residue AA3 and an alpha amino acid residue AA4 which contains R4a and R4b; residue of formula (Y5) together with the COOH residue depicted in formula (II) is a pentapeptidyl residue Y5 of the alpha amino acid residue AA1, the alpha amino acid residue AA2, the alpha amino acid residue AA3, the alpha amino acid residue AA4 and an alpha amino acid residue AA5 which contains R5a and R5b; if anyone of Rib, R2b, R3b, R4b and R5b and the respective N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, together with the respective residue Ria, R2a, R3a, R4a or R5a and the alpha C atom, to which said Ria, R2a, R3a, R4a or R5a is bonded, forms a three, four, five or six membered non-aromatic, saturated or unsaturated heterocyclic ring, with said N atom being the only one endocyclic heteroatom, except in case of a six membered ring wherein one further heteroatom may be present in said heterocyclic ring by an exchange of one of the endocyclic C atoms, except for the alpha C atom, against an O atom, then the respective alpha amino acid residues AA1, AA2, AA3, AA4 or AA5 are

5. The method according to one or more of claims 1 to 4, wherein

Ria, R2a, R3a, R4a and R5a are identical or different and independently from each other side chains of natural alpha amino acids and

Rib, R2b, R3b, R4b and R5b are identical or different and independently from each other H or CH3, preferably H; or one or more of Rib, R2b, R3b, R4b and R5b, independently from each other, can also be together with the respective residue Ria, R2a, R3a, R4a and R5a a CH2CH2CH2 chain and forms together with the N atom, to which said Rib, R2b, R3b, R4b or R5b is bonded, and the alpha C atom, to which the respective residue Ria, R2a, R3a, R4a and R5a is bonded, a five membered ring.

6. The method according to one or more of claims 2 to 5, wherein side chains of natural alpha amino acids are selected from the group H (Gly, G), H3C (Ala, A), (H3Q2CH (Vai, V), CH₃CH₂CH(CH3) (lie, I), (CH₃)2CHCH₂ (Leu, L), C₆H₅CH₂ (Phe, F), HOCH2 (Ser, S), CH3CHOH (Thr, T), H₂NC(O)CH₂ (Asn, N), H₂NC(O)CH₂CH₂ (Gin, Q), H₂N(CH₂)3CH₂ (Lys, K), and H₂NC(NH)NH(CH₂)2CH2 (Arg, R); in brackets are the three- and one-letter code of the respective naturally occurring alpha amino acid.

7. The method according to one or more of claims 1 to 6, wherein

Y is residue of formula (Yl), residue of formula (Y2), residue of formula (Y3) or residue of formula (Y4); or

Y is residue of formula (Yl), residue of formula (Y2) or residue of formula (Y3); or

Y is residue of formula (Yl) or residue of formula (Y2).

8. The method according to one or more of claims 1 to 7; wherein

Y is residue of formula (Y2),

Ria and R2a are H,

Rib and R2b are H, and

R9 is acetyl (Ac).

9. The method according to one or more of claims 1 to 8; wherein the oxidizable solvent is different from the solvent R90H.

10. The method according to one or more of claims 1 to 9; wherein the oxidizable solvent and its amount in the reaction mixture is chosen such that its half peak potential together with the amount of the oxidizable solvent at a predetermined amount of compound of formula (II) and a predetermined amount of the base in the reaction mixture provides for an oxidation of the oxidizable solvent to take place preferentially over an oxidation of the Fmoc group, but not over an oxidation of the carboxylic acid group; preferably, the oxidizable solvent and its amount in the reaction mixture at a predetermined amount of compound of formula (II) and a predetermined amount of the base in the reaction mixture is chosen such that the electrical current density at the half peak potential of the Fmoc group due to the oxidation of the oxidizable solvent in a reaction mixture without compound of formula (II) is higher than the electrical current density at the half peak potential of the Fmoc group due to the oxidation of said Fmoc group in a reaction mixture without the oxidizable solvent; said Fmoc group and said carboxylic acid group are the Fmoc group depicted in formula (II) and the carboxylic acid group depicted in formula (II) respectively.

11. The method according to one or more of claims 1 to 10; wherein the oxidizable solvent is chosen such that the half peak potential of the oxidizable solvent is not higher than 2 times, preferably 1.8 times, more preferably 1.6 times, even more preferably 1.4 times, especially 1.3 times, the half peak potential of the Fmoc group depicted in formula (II); and/or the oxidizable solvent is chosen such that the half peak potential of the oxidizable solvent is about equal to or higher than the half peak potential of the carboxylic acid group; and/or the molar amount of the oxidizable solvent is at least about equal to or greater than the molar amount of compound of formula (II) in the reaction mixture.

12. The method according to one or more of claims 1 to 11 ; wherein the oxidizable solvent is selected from the group consisting of DMA, NMP, DMSO, DMF, acetone, THF, propylene carbonate, dimethyl carbonate, y-valerolactone and NBP, and mixtures thereof; preferably, the oxidizable solvent is selected from the group consisting of DMA, NMP, DMSO, DMF, acetone, THF, propylene carbonate, and mixtures thereof; more preferably, the oxidizable solvent is selected from the group consisting of DMA, NMP, DMSO, and mixtures thereof; even more preferably, the oxidizable solvent is DMA, NMP or DMSO; especially, the oxidizable solvent is NMP or DMA.

13. The method according to one or more of claims 1 to 12; wherein the base is selected from the group consisting of R9OM, tertiary amine of formula R20(R21)(R22)N and DBU, preferably the base is R9OM.

14. The method according to one or more of claims 1 to 13; wherein R9 is Ac.

15. The method according to one or more of claims 1 to 14; wherein

R9 is Ci-4 alkyl, in particular Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, or tert-Bu, preferably Me, Et, n-Pr or i-Pr; more preferably Me or Et, even more preferably Me.

16. The method according to one or more of claims 1 to 15; wherein

M is selected from the group consisting of Li, Na, K, NH4, (EfyN and (Bu)4N; preferably, M is K or Na.

17. The method according to one or more of claims 1 to 16; wherein the anode is a Pt electrode or a carbon electrode; preferably carbon electrode; carbon electrode is preferably an impervious graphite electrode or a glassy carbon electrode, more preferably impervious graphite electrode; in a particular embodiment, the anode is an impervious graphite electrode.

18. The method according to one or more of claims 1 to 17; wherein the cathode is a nickel electrode, a stainless steel electrode or a Pt electrode, preferably a nickel electrode or a stainless steel electrode, more preferably a nickel electrode.

19. The method according to one or more of claims 1 to 18; wherein the anode and the cathode face each other, between the anode and the cathode there is a gap, the gap size is of from 0.01 to 20 mm, preferably from 0.01 to 15 mm, more preferably from 0.01 to 10 mm, even more preferably from 0.05 to 7.5 mm, especially from 0.05 to 6 mm.

20. The method according to one or more of claims 1 to 19; wherein the concentration of compound of formula (II) in the solvent mixture is chosen to be such that both compound of formula (II) and compound of formula (I) remain in solution during the reaction; preferably, the concentration of compound of formula (II) in the solvent mixture at the beginning of the reaction is from 0.001 to 1 M, more preferably from 0.005 to 1 M, even more preferably from 0.005 to 1 M, especially from 0.0075 to 1 M, more especially from 0.0075 to 0.75 M, even more especially from 0.0075 to 0.5 M.

21. The method according to one or more of claims 1 to 20; wherein the volume ratio solvent : oxidizable solvent is from 1 : 10 to 10 : 1, preferably from 1 : 5 to 1 : 0.5, more preferably from 1 : 4 to 1 : 0.5, even more preferably from 1 : 4 to 1 : 1, especially from 1 : 3 to 1 : 1.5.

22. The method according to one or more of claims 1 to 21; wherein the concentration of the base is chosen to be such that the base remains in solution during the reaction; preferably the amount of the base is from 1 to 20 equiv, more preferably from 1.5 to 19 equiv, even more preferably from 1.75 to 18 equiv, especially from 1.75 to 17.5 equiv; the equiv being molar equivalents of the molar amount of compound of formula (II).

23. The method according to one or more of claims 1 to 22; wherein the current density during the reaction is from 0.01 to 100 mA/cm², preferably from 0.01 to 50 mA/cm², more preferably from 0.01 to 20 mA/cm², even more preferably from 0.05 to 17.5 mA/cm², especially from 0.075 to 15 mA/cm², more especially from 0.075 to 12.5 mA/cm².

24. The method according to one or more of claims 1 to 23; wherein the reaction is done until a current of from 2 F equiv to 20 F equiv, preferably of from 2 F equiv to 17.5 F equiv, more preferably from 2 F equiv to 16 F equiv, even more preferably from 2 F equiv to 15 F equiv, especially from 2.5 F equiv to 14 F equiv, has passed through the reaction mixture, wherein F equiv means an amount of charge equivalent to the molar amount of compound of formula (II) has passed through the reaction mixture.