WO2014095657A1

WO2014095657A1 - Synthesis of a viral protease inhibitor intermediate

Info

Publication number: WO2014095657A1
Application number: PCT/EP2013/076594
Authority: WO
Inventors: Sergio Riva; Pietro Allegrini; Emanuele Attolino; Renzo GRAZIOSI
Original assignee: Dipharma Francis SRL
Current assignee: Dipharma Francis SRL
Priority date: 2012-12-20
Filing date: 2013-12-13
Publication date: 2014-06-26
Anticipated expiration: 2015-06-20
Also published as: ITMI20122205A1

Abstract

Synthesis of an intermediate useful in the synthesis of a viral protease inhibitor, and its use in the preparation of said inhibitor.

Description

SYNTHESIS OF A VIRAL PROTEASE INHIBITOR INTERMEDIATE

FIELD OF INVENTION

The present invention relates to a process for the preparation of an intermediate useful in the preparation of a viral protease inhibitor.

PRIOR ART

(IS, 3a , 6aS)-2-[(2S)-2-[[(2S)-2-cyclohexyl-2-[(2-pyrazinylcarbonyl) amino]acetyl]amino-3,3-dimethylbutanoyl]-N-[(l S)- 1 -[(cyclopropylamino) (oxo)acetyl]butyl]-3,3a,4,5,6,6a-hexahydro-lH-cyclopenta[c]pyrrole-3- carboxamide of formula (I), also known as telaprevir, is a potent viral protease inhibitor use

The preparation of telaprevir, reported in US 7820671 , involves the assembly of 6 different structural units to create 5 amide bonds (Scheme 1), and subsequent oxidation of the alcoholic hydroxyl group of the compound of formula E. The compounds of formulas A and F are pyrazine-carboxylic acid and cyclopropylamine respectively.

B C D E F

Scheme 1 The compounds of formulas B, C, D and E are four amino acids, all with the (S) configuration.

The compounds of formulas B and C are (S)-cyclohexylglycine and (S)-tert-leucine respectively, namely simple amino acids available on the market, while the compounds of formula D and E are two synthetic amino acids with a more complex structure.

The preparation of the structural units of formulas D and E is reported in US 7,820,671.

The preparation of the amino acid derived from norvaline of formula E has long been reported in the literature and can be effected by known methods; however, the preparation of the key aminoacid of formula D is rather complex, and uses expensive reagents which are often not commercially available, especially if the expensive chiral phase-transfer catalysts (chiral PTC) are used.

More recently, the synthesis of the protected bicyclic amino acid of formula D was reported in US 7,820,671 and US 2010/292219 by radical deoxygenation reaction of a xanthate or thiocarbonate intermediate of formula (D2) synthesisable from the known alcohol of formula (Dl), the synthesis of which is reported in the patents cited above (Schem 2).

X= SMe, OPh

P= Cbz, Boc

Scheme 2

These processes use particularly toxic, hazardous or expensive compounds, which prevents their use on an industrial scale. US 7820671 also reports the elimination of the alcohol functional group of Dl after conversion of said functional group to a good leaving group, in that specific case a trifluorome thane sulphonate of formula D3, to obtain an olefin of formula D4. The elimination reaction leads to the production of olefin D4 with low yields (30%), and requires its purification from impurities by silica-gel chromatography, which is of limited applicability on a large scale (Scheme 3). H(

D1 D3 D4

Scheme 3

Moreover, the compound D thus prepared is a racemate. The subsequent preparation of optically active compound D, having the (S) configuration on the CI carbon atom of the amino acid, which is necessary for the preparation of telaprevir of formula (I), takes place by resolution via diastereomeric salts, obtained from the carboxylic acid of compound D with a chiral amine. There is consequently a need for a more advantageous alternative method of preparing structural unit D, and therefore telaprevir.

SUMMARY OF THE INVENTION

The invention provides an enantio selective enzymatic hydrolysis process for the preparation of a compound of formula (II), as a single enantiomer, or a salt thereof,

wherein Y, Y', Z and are as defined herein, and its use as starting material in the synthesis of telaprevir.

DETAILED DESCRIPTION OF THE INVENTION

The subject of the invention is a process for the preparation of a compound of formula (II), as a single enantiomer, or a salt thereof,

z

(Π)

wherein R is a free or esterified carboxyl group; Z is H or an amino- protecting group; and each of Y and Y' is H, or one of Y and Y' is H and the other is independently hydroxy, halogen, or an -OSO₂Rl group wherein Rl is an optionally substituted Ci-C₄ alkyl group, or an optionally substituted phenyl ring, or Y and Y', taken together with the carbon atom to which they are bonded, form a carbonyl group (C=O); comprising enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of a racemic compound of formula (II) or a salt thereof, wherein R, being as defined above, is an esterified carboxyl group, in a solvent mixture; and, if applicable, the conversion of a single enantiomer of a compound of formula (II) to another compound of formula (II) having the same configuration, or a salt thereof.

A salt of a compound of formula (II) is typically a pharmaceutically acceptable salt.

Y as halogen is, for example, chlorine, bromine or iodine, typically chlorine or bromine.

A group R, as esterified carboxyl group, is, for example a -COORa group, wherein Ra is an optionally substituted Q-C^ alkyl group, an optionally substituted C3-C₁₀ cycloalkyl group, or an optionally substituted aryl or heteroaryl group. A Ci-C₄ alkyl group, which can be straight or branched, such as methyl, ethyl, isopropyl or tert-butyl, and can be substituted, for example, by one to three halogen atoms, is preferably CH₃ or CF₃.

A C1-C12 alkyl group, which can be straight or branched, is, for example, a Ci-C₄ alkyl group, typically methyl, ethyl, isopropyl or tert-butyl, optionally substituted by one to three substituents selected, for example, from phenyl, hydroxy and halogen, typically fluorine, and preferably tert-butyl.

A C₃-Cio cycloalkyl group, which can be optionally substituted, for example, by one to three halogen atoms, typically fluorine, is preferably a C₃-C₇ cycloalkyl group, in particular cyclopropyl or cyclohexyl.

An aryl group is, for example, phenyl or naphthyl, preferably phenyl, optionally substituted by one or more substituents, typically one to three, selected, for example, from hydroxy and halogen, in particular fluorine or chlorine.

l as optionally substituted phenyl is, for example, replaced by one or more, typically one or two, substituents selected independently from halogen, such as bromine, Ci-C₄ alkyl and nitro; preferably selected independently from methyl, nitro and bromine.

A heteroaryl group, which can be heteromonocyclic or heterobicyclic, unsaturated, saturated or partly unsaturated, contains one to three heteroatoms independently selected from oxygen, nitrogen and sulphur, and optionally substituted, for example, by one to three atoms of halogen, typically fluorine, is preferably pyridyl or imidazolyl.

An amino protecting group can be, for example, a protecting group known from peptide chemistry, preferably tert-butyloxycarbonyl (Boc) or benzyloxycarbonyl (Cbz).

The enantioselective enzymatic hydrolysis reaction can be effected by an enzyme belonging to the protease or lipase group. Said enzyme can derive from various sources such as bacteria, fungi, animals or plants. A lipase or a protease, according to the invention, preferably belongs to the class of those active at a pH of between about 6 and 9.

The enantiomers of a compound of formula (II), or a salt thereof, can preferably be separated with a lipase. In particular, it has surprisingly been found that enantioselective enzymatic hydrolysis of a compound of formula (II) can be advantageously effected with high yields and chemical and stereochemical purity, evaluated by HPLC, using a lipase deriving from Aspergillus niger.

A solvent mixture is formed, for example, by a solution comprising an aqueous buffer at a pH of between about 6.0 and 9.0, preferably around a pH of about 7.5, and optionally an organic co-solvent, miscible or immiscible with the buffer.

A solution of an aqueous buffer may, for example, be a known phosphate buffer, ammonium bicarbonate, ethanolamine/HCl or borate; the reaction is preferably carried out in phosphate buffer.

An organic co-solvent may, for example, be a polar aprotic solvent such as dime thy lformamide, dimethylacetamide, acetonitrile or dimethyl sulphoxide; a ketone, such as acetone or methyl isobutyl ketone; an ether, such as tetrahydrofuran or dioxane; or an apolar aprotic solvent, preferably an apolar aprotic solvent such as toluene.

According to a further aspect of the invention, a racemic compound of formula (II), wherein the two substituents Y and Y', taken together with the carbon atom to which they are bonded, constitute a carbonyl group (C=O), as starting material, before undergoing the enzymatic resolution process according to the invention, can be converted to another racemic compound of formula (II) wherein each of Y and Y' is H.

Said conversion can be carried out by a deoxygenation reaction. The deoxygenation reaction of a compound of formula (II) can be effected, for example, by Clemmensen reduction in the presence of an amalgam of zinc and hydrochloric acid, or Wolff-Kishner reaction with hydrazine and a hydroxide or a C₁-C5 alkoxide of an alkali metal, preferably potassium hydroxide or potassium tert-butoxide.

The reaction can be conducted in the presence of a solvent, which in the case of the Wolff-Kishner reaction can be a C₁-C5 alkanol, such as butanol, or a high-boiling ether, such as propylene glycol.

According to a preferred aspect of the invention, a racemic compound of formula (II), or a salt thereof, wherein Z is H, and is an esterified carboxyl group -COOC₁-C₁₂ alkyl, wherein the alkyl group is optionally substituted, can be used as starting material to be subjected to enantioselective enzymatic hydrolysis.

The concentration of the racemic substrate, namely the racemic mixture of a compound of formula (II), or a salt thereof, in the solvent mixture, comprising a solution of a buffer and optionally an organic co-solvent, can range between about 5% (g/g) and 50% (g/g), preferably between about 5% (g/g) and 20% (g/g).

The reaction clearly does not involve highly diluted operating conditions, as commonly occurs with enzymatic systems. This result, on an industrial scale, allows the reaction to be conducted in reactors of the size conventionally used for organic synthesis.

The reaction can be conducted at a temperature of between about 15 and 60°C, preferably between about 20 and 40°C, and more preferably at about 25°C. The reaction times depend on the reaction temperature and the type of enzyme used.

Typically, the enzyme is left to react until about 50% conversion of the starting racemate is detected by HPLC. If the reaction is conducted in the presence of an automatic titrator (pH-stat), the endpoint of the reaction can be set, for example, at pH 7, and the reaction mixture left under stirring until the titrator no longer corrects the pH of the mixture. According to the preferred operating conditions indicated above, enzymatic hydrolysis is normally complete in about 10-30 hours.

In this way, one of the two enantiomers of a compound of formula (II), which is not a substrate for the enzyme, remains unchanged as ester, while the other enantiomer, which is a substrate for the enzyme, is hydrolysed to obtain a compound of formula (II) wherein R is a free carboxyl group, and consequently as free carboxylic acid.

The non-hydrolysed enantiomer of an optically pure compound of formula (II), which is consequently still present as carboxylic ester in the end-of-reaction mixture, can be isolated from the reaction mixture by extraction with an organic solvent immiscible with water, such as ethyl acetate, optionally after removal of protecting group Z of the functional amino group, if any.

Conversely, when the organic phase has been separated from the aqueous phase, the hydrolysed enantiomer wherein R is free carboxyl group, which is consequently present as carboxylate in the end-of-reaction mixture, can be recovered by acidifying the saline end-of-reaction mixture to a pH of about 4-5 by adding an acid, such as hydrochloric acid, and extracting with a solvent immiscible with water, such as ethyl acetate.

By concentrating the organic solution, the enantiomer of a compound of formula (II), wherein R is a free carboxyl group, namely as free carboxylic acid is obtained. Alternatively, the extraction in organic phase of the enantiomer of a compound of formula (II), wherein R is a free carboxyl group, can also be effected after its conversion to a compound of formula (II) wherein R is an esterified carboxyl group, and/or, optionally, after protecting the functional amino group with a protecting group Z.

Both enantiomers of a compound of formula (II) are thus obtained, with excellent yields, typically between about 40 and about 50%, starting from the racemate of formula (II), and a chemical purity evaluated by HPLC as equal to or greater than 95%, preferably equal to or greater than 98%. The enantiomeric purity of the isolated enantiomers of formula (II), calculated by HPLC on chiral stationary phase, is expressed in terms of enantiomeric ratio, and is typically equal to or greater than 98:2, and preferably equal to or greater than 99: 1.

If applicable, a single enantiomer of a thus obtained compound of formula

(II), can be converted to another enantiomeric compound of formula (II) having the same configuration, according to known methods.

For example, an enantiomer of a compound of formula (II), wherein is a free carboxyl group, can be converted to a salt thereof by reaction with an organic or inorganic base, preferably a tertiary amine, in a solvent, according to known methods.

An enantiomer of formula (II), or a salt thereof, wherein R is a free carboxyl group, can be converted to another compound of formula (II), wherein R is an esterified carboxyl group, or vice versa, according to known methods.

An enantiomer of formula (II), or a salt thereof, wherein Z is H, can be converted to another compound of formula (II) wherein Z is an amino protecting group and vice versa, according to known methods.

An enantiomer of a compound of formula (II), or a salt thereof, wherein the two substituents Y and Y', taken together with the carbon atom to which they are bonded, constitute a carbonyl group (C=O), can be converted to another compound of formula (II) having the same configuration, wherein each of Y and Y' is H, according to known methods, such as the deoxygenation method described above.

An enantiomer of a compound of formula (II), or a salt thereof, wherein the two substituents Y and Y', taken together with the carbon atom to which they are bonded, constitute a carbonyl group (C=O), can be converted to another compound of formula (II) having the same configuration at the carbon atom indicated by asterisk *, wherein one of Y and Y' is H and the other is hydroxy, halogen or an -OSO₂-RI group, wherein Rl is as defined above, according to known methods. A further subject of the invention is a process for the preparation of a compound of formula (II), as a single enantiomer, wherein each of Y and Y' is H, or a salt thereof, comprising:

enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of a racemic compound of formula (II), wherein Y and Y', taken together with the carbon atom to which they are bonded, form a carbonyl group (C=O), and is an esterified carboxyl group, in a solvent mixture, and the conversion of the single enantiomer thus obtained to another compound of formula (II) having the same configuration at the stereocentre indicated by asterisk * in the 1 position, wherein each of Y and Y' is H; or

the conversion of a racemic compound of formula (II), wherein R is an esterified carboxyl group, and Y and Y', taken together with the carbon atom to which they are bonded, form a carbonyl group (C=O), to another racemic compound of formula (II), wherein each of Y and Y' is H, and enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of said racemic compound of formula (II); and, if applicable, the conversion of a thus obtained compound of formula (II), as single enantiomer, wherein each of Y and Y' is H, to another enantiomeric compound of formula (II) having the same configuration at the stereocentre indicated by asterisk * in the 1 position, or a salt thereof.

The invention also relates to the use of a compound of formula (II), wherein each of Y and Y' is H and the stereocentre indicated by asterisk * in the 1 position has the (S) configuration, or a salt thereof, as starting material in a method for the preparation of telaprevir of formula (I) or a salt thereof.

A further subject of the invention is a process for the preparation of a compound of formula (I), or a pharmaceutically acceptable salt thereof, comprising the use as starting material of a compound of formula (II), wherein each of Y and Y' is H, and the stereocentre, indicated by asterisk * in the 1 position, has the (S) absolute configuration, or a salt thereof, obtained by the enzymatic resolution process described herein.

A compound of formula (Ha)

z (Ila)

can preferably be used as starting material, wherein each of Y and Y' is H, Z is H, and 2 is an esterified carboxyl group, in particular a -COORb group, wherein Rb is an optionally substituted C C₁₂ alkyl group, and wherein the stereocentre, indicated by asterisk * in the 1 position, has the (S) absolute configuration.

As a compound of formula (II), thus obtained, possesses a high degree of enantiomeric purity, when used in a process for the preparation of telaprevir of formula (I), telaprevir with a surprisingly high chemical and enantiomeric purity is obtained.

A compound of formula (II), wherein the two substituents Y and Y', taken together with the carbon atom to which they are bonded, form a carbonyl group (C=O) in racemic form, can be prepared by known methods, for example according to the process described in J. Org. Chem., 1994, 59, 2773-2778.

The following examples illustrate the invention.

Example 1: Enantioselective enzymatic hydrolysis of 4-oxo-hexahydro- cyclopenta[c]pyrrole-l-carboxylic acid ethyl ester (II)

The hydrochloride salt of 4-oxo-hexahydro-cyclopenta[c] pyrrole- 1- carboxylic acid ethyl ester (100 mg, 0.57 mmol) is dissolved in 5 ml of 10 mM phosphate buffer at pH 7.0. The resulting solution is treated with 500 mg of lipase obtained from Aspergillus niger. The solution is placed under stirring at 130 rpm at the temperature of 30°C. The reaction is stopped after 24 hours and treated with 2M NaOH (0.32 ml), ethyl acetate (8 ml) and di-tert-butyl dicarbonate (92 mg). The biphasic mixture is maintained under stirring at a temperature of about 20°C; the two phases are then separated, and the organic phase is dried on Na₂SO₄, filtered and concentrated at low pressure. Optically active 4-oxo-hexahydro- cyclopenta[c]pyrrole-l,2-dicarboxylic acid 2-tertbutyl ester ethyl ester (II) is obtained, with an enantiomeric purity exceeding 98:2 evaluated by HPLC on chiral stationary phase.

Example 2: Enantioselective enzymatic hydrolysis of hexahydro- cyclopenta[c]pyrrole-l-carboxylic acid ethyl ester (II)

The hydrochloride salt of hexahydro-cyclopenta[c] pyrrole- 1 -carboxylic acid ethyl ester (100 mg, 0.62 mmol) is dissolved in 5 ml of 10 mM phosphate buffer at pH 7.0. The resulting solution is treated with 500 mg of lipase obtained from Aspergillus niger. The solution is placed under stirring at 130 rpm at the temperature of 30°C. The reaction is stopped after 24 hours and treated with 2M NaOH (0.35 ml), ethyl acetate (8 ml) and di-tert-butyl dicarbonate (100 mg). The biphasic mixture is maintained under stirring at a temperature of about 20°C; the two phases are then separated, and the organic phase is dried on Na₂SO₄, filtered and concentrated at low pressure. Optically active hexahydro- cyclopenta[c]pyrrole-l,2-dicarboxylic acid 2-tertbutyl ester ethyl ester (II) is obtained, with an enantiomeric purity exceeding 98:2 evaluated by HPLC on chiral stationary phase.

^ NMR (CDC1₃, T = 50°C): 4.17 (q, 2 H), 4.03 (m, 1 H), 3.68 (m, 1 H), 3.28 (m, 1 H), 2.62 (m, 2 H), 1.94 (m, 1 H); 1.9-1.7 (m, 2 H), 1.68-1.5 (m, 2 H), 1.48 (m, 1 H), 1.43 (s, 9 H), 1.28 (t, 3 H).

GC/MS (m/z): 281, 210, 182, 154, 1 10.

Claims

1. A process for preparing a compound of formula (II), as a single enantiomer, or a salt thereof,

wherein is a free or esterified carboxyl group; Z is H or an amino protecting group; and each of Y and Y' is H, or one of Y and Y' is H and the other is independently hydroxy, halogen, or an -O-SO₂-Rl group, wherein Rl is an optionally substituted Ci-C₄ alkyl group, or an optionally substituted phenyl ring; or Y and Y', taken together with the carbon atom to which they are linked, form a carbonyl (C=O) group; comprising the enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of a racemic compound of formula (II) or a salt thereof, wherein R, being as defined above, is an esterified carboxyl group, in a solvent mixture; and, if applicable, the conversion of a single enantiomer of a compound of formula (II) to another compound of formula (II) having the same configuration at the stereocentre identified by the asterisk *, or a salt thereof.

2. A process according to claim 1 , wherein the enantioselective enzymatic hydrolysis is carried out by an enzyme belonging to the protease or lipase group belonging to the class of those active at a pH ranging from 6 to 9.

3. A process according to claim 2, wherein the enzyme is a lipase.

4. A process according to claim 3, wherein the lipase is a lipase deriving from Aspergillus niger.

5. A process according to claim 1 or 2, wherein the solvent mixture is a solution comprising an aqueous buffer at a pH ranging between about 6.0 and 9.0, more preferably at a pH of about 7.5; and optionally an organic co-solvent, miscible or immiscible with the buffer.

6. A process according to claim 5, wherein the aqueous buffer solution is a phosphate buffer, ammonium bicarbonate, ethanolamine/HCl or borate.

7. A process according to claim 6, wherein the aqueous buffer is a phosphate buffer.

8. A process according to claim 5, wherein an organic co-solvent is a polar aprotic solvent; a ketone; an ether; an apolar aprotic solvent.

9. A process according to claim 8, wherein the co-solvent is an apolar aprotic solvent, such as toluene.

10. A process according to the previous claims, wherein the concentration of a racemic compound of formula (II), or a salt thereof, in the solvent mixture ranges between 5% and 50%, preferably between 5% and 20%.

1 1. A process according to claims 1- 10, wherein the reaction is carried out at a temperature ranging between 15 and 60°C, preferably between 20 and 40°C, more preferably at about 25°C.

12. A process according to claim 1 , comprising converting an enantiomer of a compound of formula (II), or a salt thereof, wherein substituents Y and Y', taken together with the carbon atom to which they are linked, form a carbonyl (C=O) group, to another compound of formula (II), wherein each of Y and Y' is H.

13. A process according to each of claims 1 to 1 1 , wherein a compound of formula (II), as a single enantiomer, wherein each of Y and Y' is H, or a salt thereof, is obtained by a process comprising:

enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of a racemic compound of formula (II), wherein Z is as defined in claim 1 , Y and Y', taken together with the carbon atom to which they are linked, form a carbonyl (C=O) group, and is an esterified carboxyl group, in a solvent mixture; and the conversion of the single enantiomer, thus obtained, to another compound of formula (II) wherein each of Y and Y' is H; or

the conversion of a racemic compound of formula (II) wherein R is an esterified carboxyl group, and Y and Y', taken together with the carbon atom to which they are linked, form a carbonyl (C=O) group, to another racemic compound of formula (II) wherein each of Y and Y' is H; and the enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of said racemic compound of formula (II).

14. A process according to the previous claims, further comprising utilising a compound of formula (II), wherein each of Y and Y' is H and the stereocentre identified by the asterisk * in the 1 position has the (S) configuration, or a salt thereof, in a method for preparing telaprevir of formula (I), or a salt there

15. A process for preparing a compound of formula (I), or a pharmaceuticall acceptable salt thereof,

(I)

comprising utilising, as starting material, a compound of formula (Ha),

(Ila)

wherein each of Y and Y' is H, Z is hydrogen, and R2 is an esterified carboxy group, in particular a -COORb group, wherein Rb is an optionally substituted C C₁₂ alkyl group, and the stereocentre identified by the asterisk * in the 1 position has the (S) configuration, or a salt thereof, as obtained by the process of claim 1.