WO2013136265A1

WO2013136265A1 - Synthesis of an intermediate of an antiviral compound

Info

Publication number: WO2013136265A1
Application number: PCT/IB2013/051950
Authority: WO
Inventors: Emanuele Attolino; Alessio BOVE; Enrico BRUNOLDI; Pietro Allegrini
Original assignee: Dipharma Francis S.R.L.
Priority date: 2012-03-13
Filing date: 2013-03-12
Publication date: 2013-09-19

Abstract

Process for the preparation of a cyclopropylamide compound which is useful as a structural unit in a process for the preparation of a viral protease inhibitor.

Description

SYNTHESIS OF AN INTERMEDIATE OF AN ANTIVIRAL

COMPOUND

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

PRIOR ART

(I S, 3aR, 6aS)-2-[(2S)-2-[[(2S)-2-Cyclohexyl-2-[(2-pyrazinylcarbonyl) amino]acetyl]amino-3,3-dimethylbutanoyl]-N-[(l S)-l-[(cyclopropylamino) (oxo)acetyl]butyl]-3,3a,4,5,6,6a-hexahydro- lH-cyclopenta[c]pyrrole-3- carboxy amide of formula (I), also known as telaprevir, is a potent viral protease inhibitor used to treat hepatitis C infections.

The preparation of telaprevir reported in US 7,820,671 involves the assembly of 6 different structural units with the creation of 5 amide bonds, as reported below in Scheme 1

Scheme 1

and subsequent oxidation of the alcoholic hydroxyl of the structural unit of formula E. The structural units of formula A and F are pyrazine-carboxylic acid and cyclopropylamine respectively.

The structural units of formula B, C, D and E are four amino acids, all with the (S) configuration.

The compounds of formula B and C are the commercially available amino acids (S)-cyclohexylglycine and (S)-tert-leucine respectively. The compounds of formula D and E are two synthetic amino acids with a more complex structure.

The preparation of the key amino acid of formula D, although rather complex, has been carried out by various methodologies, such as those described in US 7,820,671 and US 7,776,887.

The preparation of the compound of formula EF is reported, for example, in US 7,776,887, starting from L-norvaline, according to two different synthesis sequences. The first, Scheme 2, known as the cyanohydrin method, involves protecting the amine function of L-norvaline as benzyloxycarbonyl (Cbz) to give (El), which is then converted to the corresponding Weinreb amide (E2) by treatment with methoxymethylamine in the presence of a condensing agent. The amide (E2) is then treated with lithium aluminium hydride or diisobutyl aluminium monohydride to give protected norvalinal (E3). By reacting (E3) with an alkaline cyanide or acetone cyanohydrin, cyanohydrin (E4) is obtained which, after hydrolysis and reprotection, provides the acid (E5). The final cyclopropylamide (EF), ready to be condensed with amino acid (D), is obtained once again by condensation between acid (E5) and cyclopropylamine (F) in the presence of a condensing agent and subsequent deprotection of the amine function, Scheme 2. This latter reaction takes place with the use of a complex, expensive condensing agent and provides the desired product in a very modest yield; moreover, the preparation of the aldehyde (E3) is rather laborious, and uses the expensive methoxymethylamine.

Scheme 2

The second synthesis strategy, Scheme 3, attempts to overcome the obstacle of using cyanide and the low yield of the condensation between the acid (E5) and cyclopropylamine (F), by reacting the key aldehyde (E3) with cyclopropyl isocyanide in the presence of trifluoroacetic acid, and subsequent deprotection, Scheme 3. In this way amide (EF) is obtained in a good yield, but the use of a low molecular weight isocyanide like cyclopropylisocyanide unfortunately involves major safety problems which make its use impractical on an industrial scale.

Scheme 3

There is consequently a need for a more advantageous alternative method of preparing structural unit (EF), so as to improve the process for the preparation of telaprevir. Said novel method should be in particular more industrially scalable, involve the use of cheaper, safer, easier to handle reagents and mild reaction conditions, and at the same time provide high yields of the desired compounds.

BRIEF DESCRIPTION OF FIGURE AND ANALYSIS METHODS

3-(S)-Amino-2-hydroxy-hexanoyl cyclopropylamide in crystalline form, designated here as crystalline form a, having formula (II), as defined here, wherein Y is equal to H, and the stereocentre at the 3 position has (S) absolute configuration, has been characterised by X-ray powder diffraction (XRPD). The X-ray diffraction (XRPD) spectrum was obtained with an Ital- Structures APD-2000 automatic powder diffractometer, under the following operating conditions: Bragg-Brentano geometry, CuKa radiation (λ= 1.54 A), scanning with a 2Θ angle range of 3-40°, with a step size of 0.03° for 1 sec. The detector used is a scintillator.

Figure: XRPD spectrum of 3-(S)-amino-2-hydroxy-hexanoyl cyclopropylamide in crystalline form, designated here as Form β; wherein the main peaks (expressed in 20°) are found at 7.17, 1 1.82, 14.34, 18.03, 18.63, 18.99, 19.98, 20.88, 21.30, 21.69, 22.05 and 29.01.

SUMMARY OF THE INVENTION

The present invention provides a process for the preparation of a compound of formula (II), as a single stereoisomer or a mixture of stereoisomers or a salt thereof,

(II)

wherein Y and the asterisk * are as defined below, and its use in a process for the preparation of telaprevir and the diastereoisomers thereof.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is a process for the preparation of a compound of formula (II) as a single stereoisomer or mixture of stereoisomers, or a salt thereof,

wherein Y is H or an amino protecting group and asterisk * indicates the presence of a stereocentre with configuration (R) or (S) or a racemic mixture thereof;

comprising the reaction of a compound of formula (III) or a salt thereof

(III)

wherein X is an -OR_! group wherein R_t is an optionally substituted Ci-C alkyl group, an optionally substituted aryl group, or an optionally substituted heteroaryi group; or X is an -SR₂ group wherein R₂ is an optionally substituted Ci-C₆ alkyl group, an optionally substituted aryl group, or an optionally substituted heteroaryi group; or X is the reactive residue of a carboxylic acid; and Y and asterisk * are as defined above;

with the cyclopropylamine of formula (IV), or a salt thereof,

and, if the case, the conversion of a compound of formula (II) to another compound of formula (II) or a salt thereof. The stereocentres, indicated by asterisk *, in a compound of formula (II) and a compound of formula (III), can be (R) or (S) or a racemic mixture thereof.

A salt of a compound of formula (II), (III) or (IV) is, for example, a pharmaceutically acceptable salt.

A _\- ^ alkyl group, which may be straight or branched, is preferably a C C₄ alkyl group, preferably methyl, ethyl, isopropyl or tert-butyl, optionally substituted by one or more substituents, typically 1 to 3, independently selected from phenyl and halogen, such as fluorine, chlorine and iodine, preferably fluorine. Said group is more preferably a CrC₄ alkyl group, in particular methyl or ethyl, optionally substituted by phenyl.

An aryl group, which can be partly saturated, is, for example, phenyl or naphthyl, preferably phenyl, optionally substituted by one or more substituents, preferably one to three, typically selected from Ci-C₆ alkyl optionally substituted by one to three halogen atoms, preferably fluorine, chlorine and bromine; C C₆ alkoxy; nitro; cyano; halogen, preferably fluorine, chlorine or bromine.

A heteroaryl group, which can be monocyclic or bicyclic, partly saturated, can contain one to four heteroatoms independently selected from nitrogen, oxygen and sulphur, and can be optionally substituted by one or more substituents, preferably one to three, preferably selected from fluorine, chlorine and bromine; C C₆ alkoxy; nitro; cyano; halogen, preferably fluorine, chlorine or bromine.

Examples of heteroaryl groups are pyrrole, pyridine, benzofuran, thionaphthene, indole, quinoline, isoquinoline, pyrazole, imidazole, oxazole, isoxazole, thiazole, indazole, benzoimidazole, benzoxazole, benzothiazole, pyridazine, pyrimidine, quinazoline, pyrazine, 1,2,3-triazole, 1,2,4-triazole, 1,2,3,4-tetrazole and 1,2,3-benzotriazole, bonded to the oxygen or sulphur atom of the -OR₁ or SR₂ group with one of their nitrogen or carbon atoms.

A group Y as amino protecting group can, for example, be one of the protecting groups of said function known from peptide chemistry; the amino group can therefore be protected, for example, as tert-butyl carbamate (Boc), 9-fluorenylmethyl carbamate, benzyl carbamate (Cbz), acetamide, trifluoroacetamide or p-toluene sulphonamide. The amine function can be protected and deprotected at any stage of the process according to the invention, depending on which is most convenient for the process. The protection is preferably carried out immediately before the reaction, and deprotected immediately after the reaction.

The reactive residue X of a compound of formula (III) is a good leaving group, as known to the prior art. Examples of reactive residues X are, in particular, a halogen atom, preferably chlorine; imidazole; or an -OCORa or -OCOORa group, wherein Ra is a straight or branched C C₆ alkyl group, preferably C_rC₄ alkyl, optionally substituted, for example, by phenyl or halogen, such as chlorine or fluorine.

The reaction between a compound (III) or a salt thereof, and a compound (IV) or a salt thereof, proceeds with complete retention of the absolute configuration of the stereocentres indicated by the asterisk *. The absolute configuration of the stereocentre of a compound (II) or a salt thereof will therefore be the same as compound (III) used in the reaction.

If the case, the reaction can be carried out in the presence of a solvent which can be, for example, a polar aprotic solvent, typically an amide, such as dimethylformamide, dimethylacetamide or N-methylpyrrolidone, preferably dimethylacetamide, acetonitrile or dimethyl sulphoxide; or an acyclic or cyclic ether, such as methyl tert-butyl ether or tetrahydrofuran or dioxane; a chlorinated solvent, such as dichloromethane, dichloroethane, chloroform or chlorobenzene; an apolar aprotic solvent, typically toluene; a polar protic solvent, typically a straight or branched Cj-Cg alkanol, such as a C_rC₅ alkanol; water; a tertiary amine, such as triethylamine or a mixture of two or more, preferably two or three, of said solvents.

The reaction can be carried out at a temperature between about 0°C and the solvent reflux temperature, preferably between about 40°C and about 80°C.

The reaction can optionally be carried out in the presence of a catalyst, such as one based on a transition metal, typically titanium or a lanthanide such as scandium, or a Lewis acid of a metal of the principal groups, typically of group IA, IIA, Ilia, IVA or VA, preferably aluminium.

The reaction can be carried out at atmospheric pressure or in a closed reactor so that the reaction takes place under pressure. In particular, the reaction can be carried out at atmospheric pressure or preferably in a closed reactor so that the reaction takes place under pressure, for example between about 1.5 and 5 bar.

A compound of formula (II) can be converted to another compound of formula (II) according to known methods.

For example, a compound of formula (II), wherein Y is a protective group of the amine function, can be converted to a compound of formula (II) where Y is H, according to methods known to the prior art for the deprotection of said function. Similarly, a compound of formula (II), where Y is H, can be converted to a compound of formula (II), where Y is an amino protecting group according to known methods. Similarly, a compound of formula (II) can be converted to a salt thereof, or vice versa, according to known methods.

The compound of formula (II), as defined above, where Y is H and where the stereocentre indicated by asterisk * at the 3 position has (S) absolute configuration, namely 3-(S)-amino-2-hydroxy-hexanoyl cyclopropylamide, obtained by the process according to the present invention, exists in the solid crystalline state in a form designated here as Form β, which presents an XRPD spectrum wherein the main peaks (expressed in 20°) are found at 7.17, 1 1.82, 14.34, 18.03, 18.63, 18.99, 19.98, 20.88, 21.30, 21.69, 22.05 and 29.01.

A compound of formula (II), as defined above, wherein the stereocentre, indicated by asterisk * at the 3 position, has (S) absolute configuration, in particular in crystalline form β, can be usefully employed in a process for the preparation of telaprevir of formula (I), for example by the synthesis method reported in US 7,820,671.

A further object of the invention is therefore 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 Y is as defined above, and wherein the stereocentre, indicated by asterisk * at the 3 position, has (S) absolute configuration, in particular in crystalline form β, or a salt thereof, obtained by the process according to the present invention.

A compound of formula (III) as defined above, and similarly a compound of formula (Ilia)

where Y is H, and T is OH or X, wherein X is as defined above, can be obtained by a process comprising the reduction of the azide group to a compound of formula (V)

where R₃ is H or a C_rC₆ alkyl group and asterisk * is as defined above, according to known methods, for example by catalytic hydrogenation or treatment with PPh₃ and water; and, if the case, subsequent conversion of a compound of formula (Ilia) to another compound of formula (Ilia) or formula (III), as defined above.

A compound of formula (Ilia), as defined above, where T is X as defined above, can be converted to another compound of formula (Ilia) where

T is OH, or vice versa, according to known methods.

A compound of formula (III), where X is the reactive residue of a carboxylic acid, can be prepared from a compound of formula (Ilia), wherein T is OH, by reaction with an activating agent, according to known methods.

Examples of activating agents are thionyl chloride; an acyl chloride, such as acetyl chloride or pivaloyl chloride; an alkyl chloroformate, such as ethyl or isopropyl chloroformate; or carbonyl diimidazole (CDI).

A compound of formula (III), wherein X is an -OR or -SR₂ group, as defined above, can be obtained, for example from a compound of formula

(Ilia), wherein X is the reactive residue of a carboxylic acid, according to known methods.

Alternatively, a compound of formula (Ilia), as defined above, can be prepared by a process comprising acid or basic hydrolysis of the nitrile function in a compound of formula (VIII)

wherein Y and asterisk * are as defined above, to obtain a compound of formula (Ilia), wherein T is OH, and, if the case, its conversion to another compound of formula (Ilia).

The hydrolysis of the azide group in a compound of formula (VIII) can be carried out under the conditions well-known for nitriles, such as treatment with a strong aqueous mineral acid, preferably concentrated hydrochloric acid, or by treatment with a strong aqueous base, preferably NaOH, optionally in the presence of an aqueous solution of H₂O₂, typically an about 30% solution.

A cyanohydrin of formula (VIII), wherein Y and asterisk * are as defined above, can be prepared, for example, as indicated in the literature previously reported for the preparation of cyanohydrin having formula (E4). In particular, a cyanohydrin of formula (VIII) can be obtained by treating an aldehyde of formula (IX) or an adduct thereof,

wherein Y and asterisk * are as defined above, with an alkali metal cyanide or with acetone cyanohydrin.

An adduct of an aldehyde of formula (IX) is, for example, an adduct thereof with sodium bisulphite.

An aldehyde of formula (IX), as defined above, can be prepared as reported in the literature via Weinreb amide, or preferably by oxidation of a compound of formula (X)

wherein Y and asterisk * are as defined above.

Oxidation can be performed by one of the classic methods reported for oxidation of primary alcohols to aldehydes, for example by oxidation with the TEMPO/NaOCl system, Dess Martin reagent, IBX or the classic Swern oxidation. However, many of these methods involve very basic reaction conditions, and there is a very high risk of racemisation of the aldehyde of formula (IX) if the stereocentre of starting alcohol (X) has a well-defined absolute configuration.

It has now surprisingly been found that the aldehyde of formula (IX), as defined above, can be obtained with high yields and chemical and stereochemical purity by performing the oxidation reaction of a primary alcohol of formula (X), as defined above, by treatment with the pyridine-sulphur trioxide complex (pyridine- SO₃) in the presence of dimethyl sulphoxide and a base, and optionally of a solvent.

A suitable solvent can be, for example, a polar aprotic solvent such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, acetonitrile or dimethyl sulphoxide; or an acyclic or cyclic ether, such as methyl tert-butyl ether, tetrahydrofuran or dioxane; a chlorinated solvent, such as dichloromethane, dichloroethane, chloroform or chlorobenzene; an apolar aprotic solvent, typically toluene; a tertiary amine, such as triethylamine, or a mixture of two or more, preferably two or three, of said solvents.

A base can be organic or inorganic, strong or weak.

An organic base is preferably a tertiary amine, such as triethylamine or diisopropylethylamine, whereas an inorganic base is, for example, an alkali metal carbonate, an alkali metal bicarbonate or an alkali metal hydroxide, for example of sodium or potassium.

The oxidation reaction can be carried out at a temperature between about 0°C and the reflux temperature of the solvent, preferably between about 0°C and 25°C.

The compound of formula (X), as defined above, is commercially available, or can be prepared directly from D or L-norvaline or a racemic mixture thereof according to known methods. A compound of formula (V), wherein R₃ is as defined above, can be prepared by reaction of an epoxy compound of formula (VI),

(VI)

wherein R₃ and asterisk * are as defined above, with an alkali or alkaline-earth metal azide, preferably sodium, and optionally in the presence of a Lewis acid, preferably a lithium or magnesium salt in a solvent, as known, for example, from Tetrahedron 1995, 51, 13409.

A compound of formula (V), wherein R₃ is a C_rC₆ alkyl, can be converted to a compound of formula (V) wherein R₃ is H, or vice versa, according to known methods.

The compound of formula (V) thus obtained presents an absolute configuration at the stereocentre, indicated by asterisk * at the 3 position, opposite to the one present in the precursor epoxide of formula (VI).

A compound of formula (VI), wherein R₃ is H or a Ci-C₆ alkyl group, is commercially available in both racemic and optically active form.

Consequently, by using a compound of formula (VI) in the appropriate optically active form, a compound of formula (II), as defined above, is obtained, wherein the stereocentre, indicated by asterisk * at the 3 position, has (S) absolute configuration, which is useful as an intermediate in the preparation of telaprevir.

Alternatively, a compound of formula (VI), in racemic form, can be prepared by epoxidation reaction of a commercially available compound of formula (VII),

(VII) wherein R₃ is as defined above, for example starting from 2 trans- hexenoic acid.

Said epoxidation reaction can be carried out by known methods, for example using metachloroperbenzoic acid or H₂O₂ in the presence of a metal catalyst based on vanadium or tungsten, or using an inorganic peroxide such as potassium persulphate or oxone®.

A compound of formula (II) or (III), as defined above, wherein the stereocentres indicated by asterisk * in position 2 and 3 are as first defined, when they are obtained from a compound of formula (VII) as described above, is obtained in racemic form in terms of stereochemistry.

Alternatively, a compound of formula (II) or (III), in optically active form, can be obtained, for example, from an optically active epoxide of formula (VI), which is obtainable, for example, by carrying out the epoxidation reaction of the compound of formula (VII) using Sharpless epoxidation conditions in the presence of an organic peroxide and an optically active titanium complex.

Said method is not advantageous from the industrial standpoint, because the chiral metal complex is used in very large amounts, involving high manufacturing costs and laborious aqueous treatments necessary to dispose of the high titanium content used.

Attempts to resolve the compounds of formula (V) and (VI) with the classic crystallisation method using diastereomeric salts, obtained from a compound of formula (V) or (VI) wherein is H, with a chiral amine, have produced very disappointing results, leading to products with enantiomeric excesses too low to be used in the continuation of industrial synthesis.

The inventors of the invention have surprisingly found that a mixture of enantiomers of a compound of formula (V) or a compound of formula (VI), wherein R₃ is a C C₆ alkyl group, can be resolved by enantioselective enzymatic hydrolysis.

A further object of the invention is therefore a method for preparing a compound of formula (III) or (Ilia) which comprises enantioselective enzymatic hydrolysis of the ester function of one of the two enantiomers of a compound of formula (V) or (VI), respectively, wherein R₃ is a Ci-Ce alkyl group, in a solvent mixture.

(V) (VI)

If desired, a compound of formula (V) or formula (VI), thus obtained, can be converted to another compound of formula (V) or formula (VI) by known methods. Similarly, if desired, a compound of formula (III) or formula (Ilia), thus obtained, can be converted to another compound of formula (III) or formula (Ilia) respectively.

Preferably, according to said method, a compound of formula (III) is obtained, wherein Y is hydrogen and X is an -ORj group wherein is as defined above, and the stereocentre, indicated by asterisk * at the 3 position, has (S) absolute configuration, which is useful as intermediate in the preparation of telaprevir.

The enzymatic hydrolysis reaction can be carried out with protease or lipase enzymes. Said enzymes can derive from various sources, such as bacteria, fungi, animals or plants.

The lipases and proteases of the process according to the invention are preferably those active at a pH ranging between about 6 and 9.

In this way, one of the two enantiomers of a compound of formula (V) or formula (VI) respectively, 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 (V) or formula (VI), wherein R₃ is H, and consequently as free carboxylic acid.

The separation between the enantiomers of a compound of formula (V) can preferably be carried out with a lipase, in particular a lipase of the genus Candida, preferably Candida antartica.

A solvent mixture is, for example, formed by a solution comprising an aqueous buffer at a pH of between about 6.0 and 9.0, more preferably around a pH of about 7.5; and possibly 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 dimethylformamide, 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 such as toluene, preferably an apolar aprotic solvent.

The concentration of the racemic substrate, namely the racemic mixture of a compound of formula (V) or formula (VI) in the solvent mixture, comprising a solution of a buffer and optionally an organic co-solvent, can be between about 5% and 50%, preferably between about 5% and 20%. 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 carried out in reactors of the size conventionally used for organic synthesis.

The reaction can be carried out at a temperature 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 carried out in the presence of an automatic titrator (pH-stat), the endpoint of the reaction can be set, for example, at pH 7.5, 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.

The optically pure non-hydrolysed enantiomer of a compound of formula (V) or formula (VI), which is still present as ester in the end-of-reaction mixture, can be isolated from the reaction mixture by simple extraction with an organic solvent. Conversely, when the organic phase has been separated from the aqueous phase at a pH of about 7.5, the hydrolysed enantiomer, which is present as carboxylate in the end-of-reaction mixture, can be recovered by acidifying the end-of-reaction saline mixture to a pH of about 4-5 by adding acid, such as hydrochloric acid, and extracting with a solvent, such as ethyl acetate. By concentrating the organic solution, the enantiomer of a compound of formula (V) or formula (VI) is obtained, wherein R₃ is H, namely as free carboxylic acid.

Both enantiomers of a compound of formula (V) or formula (VI) are thus obtained, with excellent yields, typically between about 40 and about 50%, starting from the racemate of formula (V) or formula (VI), 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 enantiomers of formula (V) or (VI) isolated, calculated by chiral HPLC, 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. The enantiomer of a compound of formula (V) or formula (VI), wherein R₃ is H, namely as free carboxylic acid, 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 a compound of formula (V), wherein the stereocentre indicated by asterisk * at the 3 position has (S) configuration, and an enantiomer of a compound of formula (VI), wherein the stereocentre indicated by asterisk * at the 3 position has (R) configuration, obtained by the enzymatic resolution process described herein, or a salt thereof, can be advantageously used in a process for the preparation of telaprevir of formula (I).

A further purpose of the present invention is therefore 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 (V), wherein the stereocentre indicated by asterisk * at the 3 position has (S) configuration, or an enantiomer of a compound of formula (VI), wherein the stereocentre indicated by asterisk * at the 3 position has (R) configuration, or a salt thereof, obtained by the enzymatic resolution process of described herein.

Moreover, when the desired end product is telaprevir of formula (I), a compound of formula (II), wherein the stereocentre indicated by asterisk * at the 3 position has (S) absolute configuration, can be obtained by the process according to the invention using L-norvaline as starting product.

A compound of formula (II), as defined above, wherein the stereocentre indicated by asterisk * at the 3 position has (R) absolute configuration or is an (R,S) mixture, obtained by the process according to the invention, can be advantageously used in a process for the preparation of diastereoisomers of telaprevir. Said compounds can be usefully employed in analytical chemistry to determine the chemical and stereochemical purity of telaprevir.

The following examples illustrate the invention.

Example 1 - Synthesis of 3-propyloxirane-2-carboxylic acid (VI) Trans-2-hexenoic acid (70 g, 0.61 mol) is dissolved in a mixture of acetone (140 mL) and water (140 mL). Solid NaHCO₃ (205 g, 2.44 mol) is added to this mixture under stirring in portions. A mixture consisting of oxone® (395 g, 1.29 mol), Na₂EDTA dihydrate (0.18 g) and water (1.4 L ) is added in 2 hours. After a further 2 hours the reaction mixture is cooled to 0°C and treated slowly with a 37% HC1 solution to pH 2. The reaction mixture is then extracted with ethyl acetate and the combined organic phases are washed with water and a saturated solution of NaCl. The organic phase is dried on sodium sulphate, filtered and concentrated at low pressure until 3-propyloxirane-2-carboxylic acid (VI) is obtained with a yield of 93%; it is not purified, but used "as is" for the subsequent reactions.

1H-NMR, (CDC1₃) δ: 3.25 (d, 1H, J = 1.2 Hz), 3.19 (dt, 1H, J = 1.2, 6.9

Hz), 1.71-1.41 (m, 4H), 0.98 (t, 3H, J = 7.8 Hz)

Example 2 - Synthesis of 3-azido-2-hydroxyhexanoic acid (V)

NaN₃ (50 g, 0.77 mol) and MgSO₄ (89 g, 0.74 mol) are added to the solution obtained by mixing 3-propyloxirane-2-carboxylic acid (VI) obtained as in Example 1 (92 g, 0.70 mol), methanol (180 mL) and 30% NaOH (80 g, 0.6 mol), and the reaction mixture is maintained under stirring for 18 hours at about 25°C. More NaN₃ (10 g, 0.15 mol) is added, and the reaction mixture is maintained under stirring for a further 8 hours. The basic end-of-reaction mixture is treated with water (500 mL) and 37% HC1 until a solution at pH 5 is obtained; said solution is then extracted with ethyl acetate and the combined organic phases are washed, first with water and finally with a saturated solution of NaCl, and then dried on Na₂SO₄, filtered and concentrated until 3-azido-2-hexanoic acid (V) is obtained as a yellow oil with a yield of 80%. 1H-NMR, (CDCI₃) δ: 4.42 (d, 1H, J = 3.0Hz); 3.64 (dt, 1H, J = 3.0, 9.6Hz); 1.84-1.38 (m, 4H); 0.97 (t, 3H, J =7.2 Hz)

Example 3 - Synthesis of methyl 3-azido-2-hydroxyethanoate (V) A solution of 3-azido-2-hydroxyhexanoic acid of formula (V) (22 g, 12.7 mmol), prepared as in Example 2 in methanol (100 mL), is treated with a 15% methanolic solution of HC1 (1 1 g, 4.6 mmol). The reaction mixture is maintained under stirring for 18 hours at about 25°C, after which water is added, and the resulting mixture is extracted with ethyl acetate. The organic phase is washed first with a saturated solution of NaHCO₃ and then with a saturated solution of NaCl, dried on sodium sulphate and concentrated to residue. Methyl 3-azido-2-hydroxyethanoate (V) is obtained as an oil with a yield of 95%; said oil is not purified, but used "as is" in the subsequent reactions.

Ή-NMR, (CDCI₃) 6: 4.33 (d, 1H, J= 3.3Hz); 3.84 (s, 3H), 3.54 (dt, 1H, J = 3.3, 9.6Hz); 1.75-1.32 (m, 4H); 0.95 (t, 3H, J= 7.2 Hz).

Example 4 - Synthesis of methyl (2S,3S) 3-azido-2- hydroxyethanoate (V)

Methyl 3-azido-2-hydroxyethanoate racemate (3.0 g, 16.0 mmol) obtained as in Example 3 is dissolved in toluene (15 mL), and 45 mL of 500 mM phosphate buffer at pH = 8.4 and 0.8 g of lipase solution from Candida antartica type B (CALB) are added to the solution. The reaction mixture is maintained under stirring at room temperature, and the phases are separated after 24 hours. The aqueous phase is extracted with ethyl acetate, and the combined organic phases are washed with a saturated solution of NaHCO₃ followed by a saturated solution of NaCl, dried on sodium sulphate and concentrated to residue to obtain 1.2 g of methyl (2S,3S) 3-azido-2- hydroxyethanoate (V) as an oil with a yield of 40% and an enantiomeric ratio exceeding 99: 1, evaluated by chiral stationary phase HPLC. Example 5 - Synthesis of methyl (2S, 3S) 3-amino-2- hydroxyethanoate (III)

Methyl (2S,3S) 3-azido-2-hydroxyethanoate (1.2 g, 6.4 mmol), obtained as in Example 4, is dissolved in methanol (12 mL) and treated with 5% Pd/C (0.1 g). The reaction mixture is maintained in hydrogen atmosphere and under stirring at room temperature for 6 hours. The end-of-reaction mixture is filtered through a layer of perlite, which is washed with methanol. The solution of methyl (2S,3S) 3-amino-2-hydroxyethanoate (III) thus obtained is concentrated at low pressure to a volume of about 6 mL, and used "as is" in the next step. An aliquot is concentrated for analysis purposes.

Ή-NMR, (CDC1₃) δ: 4.14 (d, 2H, J = 3.9Hz); 3.77 (s, 3H); 3.08-2.98 (m, 1H); 1.55-1.25 (m, 4 H); 0.91 (t, 3H, J= 7.5Hz).

Example 6 - Synthesis of (2S,3S) 3-amino-2-hydroxy-hexanoyI cyclopropylamide (II)

The methyl (2S,3S) 3-amino-2-hydroxyethanoate solution in MeOH obtained in Example 5 is treated with cyclopropylamine (4.1 g, 72 mmol) and maintained under stirring at 70°C for 18 hours in a hermetically sealed test tube. The end-of-reaction solution is then concentrated at low pressure to give 980 mg of (2S,3S) 3-amino-2-hydroxy-hexanoyl cyclopropylamide (II) as a solid, with a yield of 75% in two steps.

Example 7: Synthesis of (S)-2-benzyloxycarbonylaminopentan-l-ol

(X)

L-Norvaline (160 g, 1.34 mol) is dissolved in a 28.5% methanolic solution of HC1 (278 g) and diluted with methanol (500 ml). The solution is refluxed under inert atmosphere for 6 hours. When the reaction has terminated, the solvents are distilled by coevaporating with toluene. The residue is treated at 50°C with tert-butyl methyl ether (800 ml). The suspension obtained is left under stirring at room temperature and then filtered, and the white solid obtained is washed with tert-butyl methyl ether to obtain 216 g of L-norvaline methyl ester hydrochloride with a yield of 97%.

The methyl ester hydrochloride previously obtained (140 g, 0.838 mol) is added in 10 minutes to a solution of K₂CO₃ (120 g, 0.922 mol) in water (850 mL) until a solution is obtained. Benzyl chloroformate (143.2 g, 0.838 mol) followed by THF (850 mL) is dropped into the solution under slow stirring. The biphasic mixture thus obtained is then maintained under vigorous stirring for 4 hours at room temperature. When the reaction has terminated the aqueous phase is discarded, and the organic phase is first washed with a saturated solution of NaCl (300 mL) and then diluted with 550 mL of THF. Solid NaBH₄ (80 g, 2.09 mol) is added to the solution thus obtained in portions. The reaction mixture is then cooled to about 10°C and treated by slow dripping with methanol (250 ml). The temperature of the reaction mixture is left to rise to about 25°C, and the reaction is complete after about 2h. The end-of-reaction suspension is treated with 37% HC1 (220 mL) to an acid pH; the solvents are then distilled and the aqueous phase obtained is treated with heptane (900 ml). The biphasic mixture is heated to 90°C and the aqueous phase is separated from the organic phase, which is slowly cooled to 25°C. The suspension formed is filtered, and the white solid obtained is washed with heptane and dried under vacuum at room temperature. 144 g of the product of formula (X) is obtained with a yield of 73%.

1H-NMR, (CDC1₃) 6: 7.37-7.32 (m, 5H); 5.09 (s, 2H); 4.85 (si, 1H, NH); 3.70-3.66 (m, 2H); 3.60-3.55 (m, 1H); 1.58-1.36 (m, 4H); 0.92 (t, 3H, J = 6.9Hz)

Example 8: Synthesis of (S)-2-benzyIoxycarbonylamino-pentanal

(IX)

Diisopropylethylamine (18.6 g, 144 mmol) is added to the solution obtained by mixing (S)-2-(benzyloxycarbonyl)-aminopentan-l-ol obtained as in Example 7 (8.5 g, 36 mmol), toluene (21 mL) and dimethyl sulphoxide (60 mL), and the reaction mixture is maintained under stirring in inert atmosphere and cooled to 0°C.

Pyridine-sulphur trioxide complex (23 g, 144 mmol) is added very slowly to the reaction mixture in portions. The reaction terminates normally an hour after the last addition of pyridine- SO₃ complex. The end-of-reaction mixture is then diluted with toluene ( 100 mL), a catalytic amount of NaBr is added, the temperature is maintained at 0°C, and an 1 1 .9% NaCIO solution (36 mmol) saturated with NaHCO₃ is dropped into the mixture. The mixture is left under stirring for 1 h and then acidified with 4M HC1 (40 mL) to an acid pH. The biphasic mixture is treated with toluene and water and the phases are separated. The organic phase is washed first with a saturated solution of NaHCO₃ ( 150 mL), then with a saturated solution of sodium thiosulphate ( 150 mL), and finally with a saturated solution of NaCl ( 100 mL). The organic phase is concentrated, and the oily residue is crushed in heptane until a white solid is obtained. The solid is filtered, washed with heptane and dried under vacuum at room temperature. 6. 1 g of (S)-2-(benzyloxycarbonyl)- aminopentanal (IX) with a yield of 74% is obtained.

'H-NMR, (CDC1₃) δ: 9.59 (s, 1H); 7.37-7.32 (m, 5H); 5.22 (si, 1H, NH) 5. 12 (s, 2H); 4.36-4.34 (m, 1 H); 1.95- 1 .89 (m, 1 H); 1 .63- 1.55 (m, 1H); 1 .45- 1.38 (m, 2H) 0.95 (t, 3H, J=7.2 Hz)

Example 9: Synthesis of (S)-3-(benzyIoxycarbonyI)amino-2- hydroxy-hexane-l-nitrile (VIII)

A solution obtained by dissolving 85% sodium dithionite (45.3 g, 221 mmol) in water (220 mL) is dropped at 0°C into the solution obtained by dissolving in methanol (220 mL) the (S)-2-(benzyloxycarbonyl)- aminopentanal (52 g, 221 mmol) prepared as in Example 8. A white precipitate forms, and the suspension is left in the freezer overnight; the suspension is then diluted with methanol/water (1 : 1, 1 10 ml), and a solution obtained by dissolving sodium cyanide (15.3 g, 309.3 mmol) in water (220 mL) is dropped into it. The reaction mixture is maintained under stirring for two hours at about 25°C, then extracted with ethyl acetate, and the organic phase is washed with water followed by a saturated solution of NaCl, dried on sodium sulphate and concentrated to residue. The compound of formula (VIII) is obtained as oil with a yield of 89%; it is not purified, but used "as is" in the subsequent reactions as a mixture of two diastereoisomers A and B in the ratio of about 1 : 1.

Diagnostic signals of the diastereoisomer designated herein as A

1H-NMR, (CDC1₃) δ: 5.1 1 (s, 2H); 4.03-3.91 (m, 1 H);

Diagnostic signals of the diastereoisomer designated herein as B

1H-NMR, (CDCI₃) δ: 5.09 (s, 2H); 3.83-3.78 (m, 1 H);

Overlapping signals of the two diastereoisomers designated herein as A and B.

1H-NMR, (CDCI₃) δ: 7.37-7.32 (m, 10H); 5.07-5.01 (si, 2H, NH) 4.63-4.57 (m, 2 H); 4.03-3.91 (m, 1 H); 3.83-3.78 (m, 1 H); 1.82 (si, 1H, OH) 1.63-1.30 (m, 8H); 0.95-0.92 (m, 6 H)

Example 10: Synthesis of (S)-3-(benzyloxycarbonyl)amino-2- hydroxy-hexanoic acid (Ilia)

(S)-3-(Benzyloxycarbonyl)amino-2-hydroxy-hexane-l-nitrile (1 1.1 g, 42.4 mmol) obtained as in Example 9 is dissolved in dioxane (60 mL) and treated at room temperature with 37% aqueous HC1 (60 mL). The reaction mixture is maintained under stirring at reflux temperature, and the end-of- reaction mixture is concentrated to residue (8.4 g) after two hours. The product thus obtained is dissolved in a mixture of THF (25 mL) and DMF (25 ml) and treated with N-(benzyloxycarbonyloxy)succinimide (9.95 g, 40 mmol). DIPEA (14 ml, 80 mmol) is dropped in 20 minutes into the suspension obtained under inert atmosphere at room temperature, and the reaction mixture is maintained under stirring for 16 hours. The end-of-reaction mixture is diluted with a saturated solution of NaHCO₃ (100 mL) to a slightly basic pH, and washed with ethyl acetate. The phases are separated and the aqueous phase is adjusted to pH 1 with 37% aqueous HCl and extracted with ethyl acetate. The phases are separated and the organic phase is dried on sodium sulphate, filtered and concentrated. The residue obtained is dissolved in MTBE, and the solution obtained is washed 3 times with water. The organic phase is concentrated to obtain 8 g of compound of formula (Ilia) as a white solid with a yield of 75% as a mixture of the two diastereoisomers A and B in the ratio of about 1 : 1, evaluated by NMR analysis.

Diagnostic signal of the diastereoisomer designated herein as A:

1H-NMR, (CDC1₃) δ: 4.38-4.37 (m, 1 H);

Diagnostic signal of the diastereoisomer designated herein as B:

1H-NMR, (CDCI₃) 6: 4.23-4.22 (m, 1 H);

Overlapping signals of the diastereoisomers designated herein as A and

B:

1H-NMR, (CDCI₃) 6: 7.37-7.32 (m, 10H); 5.12-5.08 (m, 6H); 4.18-3.95 (m, 2 H); 1.68-1.30 (m, 8H); 0.95-0.92 (m, 6 H)

Example 11: Synthesis of methyl 3-(S)-(benzyloxycarbonyl)amino-2- hydroxy-hexanoate (III)

3-(S)-(Benzyloxycarbonyl)amino-2-hydroxy-hexanoic acid (1 g, 3.6 mmol) obtained as in Example 10 is dissolved in THF (7 mL) and DIPEA (1.04 mL, 6 mmol) and treated with iodomethane (0.37 mL, 6 mmol). The reaction mixture is maintained under inert atmosphere and stirring at the temperature of 40°C for 16 hours. The end-of-reaction mixture is diluted with ethyl acetate and washed with water. The organic phase is washed with a saturated solution of NaCl, dried on sodium sulphate, filtered and concentrated. 800 mg of the compound of formula (III) are obtained as a white solid as a mixture of diastereoisomers in ratios of about 1 : 1.

Diagnostic signals of the diastereoisomer designated herein as A:

Ή-NMR, (CDC1₃) δ: 5.1 1 (s, 2H); 4.95 (dl, 1H, NH); 4.38-4.37 (m, 1 H); 3.79 (s, 3H);

Diagnostic signals of the diastereoisomer designated herein as B:

1H-NMR, (CDCI₃) δ: 5.05 (s, 2H); 4.88 (dl, 1H, NH); 4.19-4.17 (m, 1 H); 3.75 (s, 3H);

The overlapping signals of the diastereoisomers designated herein as A and B are shown below:

1H-NMR, (CDCI₃) δ: 7.37-7.32 (m, 10H); 4.15-4.05 (m, 2 H); 1.64- 1.23 (m, 8H); 0.94-0.88 (m, 6 H)

Example 12: Synthesis of methyl 3-(S)-(benzyloxycarbonyl)amino-2- hydroxy-hexanoate (III)

(S)-3-(benzyloxycarbonyl)amino-2-hydroxy-hexane-l-nitrile (850 mg,

3.24 mmol), obtained as in Example 9, is treated with 37% aqueous HC1, and the mixture is refluxed for 3 h. At the end of the reaction, the solution obtained is concentrated to residue by coevaporating with toluene and methanol. The crude product obtained is dissolved in methanol (8 mL), and the solution obtained is maintained under reflux stirring overnight. At the end of the reaction the solution is concentrated and the crude product (500 mg) thus obtained is treated with water (8 mL) and NaHCO₃ (0.7 g, 8.3 mmol). Benzyl chloroformate (0.31 mL, 2.2 mol) is added to the mixture by dripping, and THF (8 mL) is dropped into the mixture, the temperature being maintained at about 10°C. The reaction mixture is maintained under stirring for three hours at room temperature. At the end of the reaction the mixture is diluted with water and the product is extracted with ethyl acetate. The phases are separated and the organic phase is washed with a saturated solution of NaHCO₃, a 1M solution of HC1 and a saturated solution of NaCl. The organic phase is then dried on sodium sulphate, filtered and concentrated. The crude product thus obtained is precipitated by a mixture of heptane and ethyl acetate, to obtain the product of formula (III) as a solid; at NMR analysis, said product appears to consist only of the diastereoisomer designated herein as A.

1H-NMR, (CDC1₃) δ: 7.37-7.32 (m, 5H); 5.1 1 (s, 2H); 4.95 (si, 1H, NH); 4.38-4.37 (m, 1 H); 4.12-4.02 (m, 1 H); 3.79 (s, 3H); 1.60-1.23 (m, 4H); 0.94-0.88 (m, 3H, J 6.9 Hz)

Example 13: Synthesis of 3-(S)-(benzyloxycarbonyI)amino-2- hydroxy-hexanoyl cyclopropylamide (II)

Methyl 3-(S)-(benzyloxycarbonyl)amino-2-hydroxy-hexanoate obtained as in Example 1 1 (1.3 g, 4.4 mmol) is dissolved in methanol (1.5 mL) and treated with cyclopropylamine (4.2 g, 73.5 mmol). The reaction mixture is maintained under stirring at 40°C for 14 days and then concentrated at low pressure. The crude residue thus obtained is diluted with ethyl acetate and washed with water. The phases are separated and the organic phase is dried, filtered and concentrated at low pressure. The product of formula (II) is obtained as a solid as the mixture of two diastereoisomers in the ratio of about 1 : 1.

Diagnostic signals of the diastereoisomer designated herein as A:

1H-NMR, (CDCI₃) 6: 6.84 (bs, 1H, NH); 5.21 (bs, 1H, NH) 4.18-4.17 (m, 1H);

Diagnostic signals of the diastereoisomer designated herein as B:

1H-NMR, (CDCI₃) δ: 6.77 (bs, 1H, NH); 5.42 (bs, 1H, NH); 4.08-4.07

(m, 1H)

Overlapping signals of the diastereoisomers designated herein as A and B: 1H-NMR, (CDCI₃) 6: 7.37-7.32 (m, 10H); 5.09-5.04 (m, 4H); 3.83-3.78 (m, 2 H); 2.77-2.64 (m, 2H); 1.78-1.25 (m, 8H); 0.95-0.89 (m, 6 H); 0.76-0.73 (m, 4H); 0.46-0.41 (m, 4H)

Example 14: Synthesis of 3-(S)-(benzyloxycarbonyl)amino-2- hydroxy-hexanoyl cyclopropylamide (II)

In a test tube for pressurised reactions (sealed tube), methyl

3-(S)-(benzyloxycarbonyl)amino-2-hydroxy-hexanoate (1.9 g, 6.4 mol) obtained as in Example 1 1 is dissolved in methanol (2.8 mL) and treated with cyclopropylamine (6.1 g, 107 mmol). The reaction mixture is maintained under stirring at 70°C and under pressure for 48 h, then cooled to about 25°C and dropped into cold water. A suspension forms and the white solid is filtered, washed with water and dried under vacuum at 30°C. 1.9 g of solid is obtained with a yield of 85%.

Diagnostic signals of the diastereoisomer designated herein as A:

1H-NMR, (CDC1₃) δ: 6.84 (bs, 1H, NH); 5.21 (bs, 1H, NH) 4.18-4.17 (m, 1H);

Diagnostic signals of the diastereoisomer designated herein as B:

1H-NMR, (CDCI₃) δ: 6.77 (bs, 1H, NH); 5.42 (bs, 1H, NH); 4.08-4.07 (m, 1H)

Overlapping signals of the diastereoisomers designated herein as A and B: 1H-NMR, (CDCI3) δ: 7.37-7.32 (m, 10H); 5.09-5.04 (m, 4H); 3.83-3.78

(m, 2 H); 2.77-2.64 (m, 2H); 1.78-1.25 (m, 8H); 0.95-0.89 (m, 6 H); 0.76-0.73 (m, 4H); 0.46-0.41 (m, 4H)

Example 15: Synthesis of 3-(S)-(benzyloxycarbonyl)amino-2- hydroxy-hexanoyl cyclopropylamide (II)

In a test tube for pressurised reactions (sealed tube), methyl

3-(S)-(benzyloxycarbonyl)amino-2-hydroxy-hexanoate (11 g, 0.037 mol) obtained as in Example 12 is dissolved in methanol (16.5 mL) and treated with cyclopropylamine (35.4 g, 0.62 mol). The reaction mixture is maintained under stirring at 70°C and under pressure for 48 h and concentrated until a white solid is obtained, which is crystallised by a mixture of ethyl acetate/heptane. The white solid is filtered, washed with water and dried under vacuum at 30°C. 10 g of solid is obtained with a yield of about 83%. On NMR, the solid appears to consist of only one diastereoisomer.

1H-NMR, (CDC1₃) δ: 7.37-7.32 (m, 5H); 6.84 (si, 1H, NH); 5.19-5.07 (m, 3H); 4.18-4.17 (m, 1H); 3.83-3.78 (m, 1H); 2.77-2.64 (m, 1H); 1.78- 1.25 (m, 4H); 0.95-0.89 (t, 3H, J 6.9 Hz); 0.76-0.73 (m, 2H); 0.46-0.41 (m, 2H)

Example 16: Synthesis of 3-(S)-amino-2-hydroxy-hexanoyI cyclopropylamide (II)

A solution of 3-(S)-(benzyloxycarbonyl)amino-2-hydroxy-hexanoyl cyclopropylamide prepared as in Example 14 (1.6 g, 5 mmol) in methanol (38 mL) is treated with 5% Pd/C (300 mg) and maintained under stirring in a hydrogen atmosphere for 2 days. The suspension is then filtered through perlite, and the solution obtained is concentrated at low pressure. The residue is crystallised by a mixture of ethyl acetate/heptane to obtain 920 mg of solid crystalline product, designated here as crystalline form p.

Diagnostic signals of the diastereoisomer designated herein as A:

'H-NIVIR, CDCI₃ δ: 3.84-3.80 (m, 1 H); 3.09-2.97 (m, 1H);

Diagnostic signals of the diastereoisomer designated herein as B:

1H-NMR, CDCI₃ δ: 3.77-3.74 (m, 1Η)3.35-3.27 (m, 1H);

Overlapping signals of the diastereoisomers designated herein as A and B:

1H-NMR, CDCl₃/D₂O δ: 2.75-2.72 (m, 2H); 1.61-1.25 (m, 8H); 0.95- 0.89 (m, 6 H); 0.78-0.73 (m, 4H); 0.52-0.48 (m, 4H).

XRPD analysis of the 3-(S)-amino-2-hydroxy-hexanoyl cyclopropylamide, Form β, thus obtained, as shown in the Figure, detects main peaks (expressed in 20°) at 7.17, 1 1.82, 14.34, 18.03, 18.63, 18.99, 19.98, 20.88, 21.30, 21.69, 22.05 and 29.01.

Claims

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

wherein Y is H or an amino protecting group; and the asterisk * identifies the presence of a stereocentre in (R) or (S) configuration or a racemic mixture thereof; comprising reacting a compound of formula (III), or a salt thereof

wherein X is an -ORi group wherein Ri is an optionally substituted C C₆ alkyl group, an optionally substituted aryl group, or an optionally substituted heteroaryl group; or X is an -SR₂ group wherein R₂ is an optionally substituted C C₆ alkyl group, an optionally substituted aryl group, or an optionally substituted heteroaryl group; or X is the reactive residue of a carboxylic acid; and Y and the asterisk * are as defined above; with a cyclopropylamine of formula (IV), or a salt thereof,

and, if the case, converting a compound of formula (II) to another compound of formula (II), or a salt thereof.

2. Process as claimed in claim 1 , wherein in a compound of formula (III), Ki is a C_rC₄ alkyl group, in particular methyl or ethyl, optionally substituted by phenyl.

3. Process as claimed in claim 1 or 2, wherein the reaction is carried out in a solvent selected from the group comprising a polar aprotic solvent; an acyclic or cyclic ether; a chlorinated solvent; an apolar aprotic solvent; a polar protic solvent; water; a tertiary amine; and a mixture of two or more, preferably two or three, of said solvents.

4. Process as claimed in claim 1 , wherein the compound of formula (II) thus obtained, wherein Y is H and the stereocentre indicated by asterisk * at the 3 position is in (S) absolute configuration, namely 3-(S)-amino-2-hydroxy- hexanoyl cyclopropylamide, is in crystalline form β, and has an XRPD spectrum wherein the main peaks (expressed in 29°) are found at 7.17, 1 1.82, 14.34, 18.03, 18.63, 18.99, 19.98, 20.88, 21.30, 21.69, 22.05 and 29.01.

5. Process as claimed in claim 1 , further comprising the preparation of a compound of formula (IX)

wherein Y and asterisk * are as defined in claim 1 , by the oxidation reaction of an alcohol of formula (X)

wherein Y and asterisk * are as defined in claim 1 , by treatment with pyridine-sulphur trioxide complex (pyridine- SO₃) in the presence of dimethyl sulphoxide and a base.

6. A process according to claim 1, wherein the reaction is carried out at atmospheric pressure or preferably at a pressure of between about 1.5 and 5 bar.

7. A process according to claim 1, further comprising preparing a compound of formula (I), or a pharmaceutically acceptable salt thereof,

comprising utilising, as starting material, a compound of formula (II), as defined in claim 1, wherein the stereocentre identified by the asterisk * at the 3 position is in (S) absolute configuration, or a salt thereof, or a compound of formula (II) as defined in claim 4.

8. A process according to claim 1, wherein a compound of formula (III)

(III)

wherein Y is H, and X is as defined in claim 1 , is obtained by a process comprising preparing a compound of formula (V) or formula (VI), respectively, in optic

(V) (VI)

wherein R₃ is H or a Ci-C₆ alkyl group, and the stereocentre identified by the asterisk * at the 3 position is in (S) or (R) absolute configuration, by enantioselective enzymatic hydrolysis of the ester function in one of the two enantiomers of a compound of formula (V) or formula (VI), wherein R₃ is a C C6 alkyl group, in a solvent mixture; and optionally converting a compound of formula (III) to another compound of formula (III).

9. A process according to claim 8, wherein the enzymatic hydrolysis is carried out with a protease or a lipase.

10. A process according to claim 8 or 9, wherein the solvent mixture is a solution comprising an aqueous buffer at a pH ranging about between 6.0 and 9.0, more preferably at a pH of about 7.5; and optionally an organic cosolvent, miscible or immiscible with the buffer.

1 1. A process according to claim 10, wherein the organic cosolvent is preferably an aprotic polar solvent; a ketone, an ether; an apolar aprotic solvent, such as toluene; more preferably an apolar aprotic solvent.

12. A process according to each of claims 8 to 1 1, wherein a compound of formula (V) or formula (VI), in optically active form,

(V) (VI)

wherein R₃ is H or a C C₆ alkyl group, and the stereocentre identified by the asterisk * at the 3 position is in (S) or (R) absolute configuration, is obtained with a chemical purity evaluated by HPLC as equal to or higher than 95%, preferably equal to or higher than 98%; and an enantiomeric purity of the isolated enantiomers of formula (V) or (VI), evaluated by chiral HPLC, and expressed as an enantiomeric ratio, typically equal to or higher than 98:2, preferably equal to or higher than 99: 1.

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

comprising utilising, as starting material, a compound of formula (V), as defined in claim 8, wherein R₃ is H or a C_rC₆ alkyl group and wherein the stereocentre identified by the asterisk * at the 3 position is in (S) configuration, or a compound of formula (VI), as defined in claim 8, wherein R₃ is H or a C_rC₆ alkyl group and wherein the stereocentre identified by the asterisk * at the 3 position is in (R) configuration; or a salt thereof, as obtained according to the enzymatic hydrolysis process of claim 8.

14. A process for preparing a compound of formula (I), or a pharmaceutically acceptable salt thereof, as defined in claim 13, comprising utilising as starting material a compound of formula (III), wherein Y is hydrogen and X is an -ORi group as defined in claim 1, wherein the stereocentre identified by the asterisk * at the 3 position is in (S) absolute configuration, as obtained according to the enzymatic hydrolysis process of claim 8.

15. A compound of formula (II)

wherein Y is H and the stereocentre indicated by asterisk * at the 3 position is in (S) absolute configuration, namely 3-(S)-amino-2-hydroxy- hexanoyl cyclopropylamide, in crystalline form β, and presents an XRPD spectrum wherein the main peaks (expressed in 29°) are found at 7.17, 1 1.82, 14.34, 18.03, 18.63, 18.99, 19.98, 20.88, 21.30, 21.69, 22.05 and 29.01.