WO2013120871A1

WO2013120871A1 - Process for the preparation of a viral protease inhibitor and its intermediates

Info

Publication number: WO2013120871A1
Application number: PCT/EP2013/052819
Authority: WO
Inventors: Murizio TADDEI; Emanuele Attolino; Evita BALDUCCI; Mario Michieletti
Original assignee: Dipharma Francis S.R.L.
Priority date: 2012-02-13
Filing date: 2013-02-13
Publication date: 2013-08-22
Also published as: ITMI20120192A1

Abstract

The application describes a process for the preparation of a compound of formula (II) useful for the preparation of a viral protease inhibitor. Intermediates of the process are also claimed.

Description

PROCESS FOR THE PREPARATION OF A VIRAL PROTEASE

INHIBITOR AND ITS INTERMEDIATES

FIELD OF INVENTION

The present invention relates to a process for the preparation of a viral protease inhibitor and intermediates thereof.

PRIOR ART

(I S, 3a , 6aS)-2-[(2S)-2-[[(2S)-2-Cyclohexyl-2-[(2-pyrazinylcarbonyl) amino]acetyl]amino-3,3-dimethylbutanoyl]-N-[(l S)- l-[(cyclopropylamino) (oxo)acetylbutyl]-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.

(I)

The preparation of telaprevir, reported in US 7,820,671 , involves the assembly of 6 different structural units to create 5 amide bonds (Scheme 1)

B c D E F

Scheme 1 and the subsequent oxidation of the alcohol hydroxyl group of compound of formula E. Compounds of formula A and F are pyrazine-carboxylic acid and cyclopropylamine respectively.

Compounds of formula B, C, D and E are four amino acids, all with configuration (S).

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

The preparation of the compounds of formula D and E is reported in

US 7,820,671.

While 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, the preparation of the key amino acid of formula D is rather complex, and uses expensive reagents which are often not commercially available, especially if the expensive chiral phase-transfer catalysts (i.e. chiral PTC) are used.

The synthesis of the dicyclic amino acid of formula D, reported more recently in US 7,776,887, involves the formation of the organolithium intermediate of the protected amine compound of formula G, and its subsequent carboxylation, according to Scheme 2 reported below.

G trans-cis CIS-CIS trans-cis

Scheme 2 The mixture of diastereoisomers of the carboxyl intermediate thus formed, due to the formation of three trans-cis, cis-cis stereocentres at the 1-, 2- and 3- positions, is then subjected to partial epimerisation of stereocentre 1 to give only the racemic trans-cis isomer, which is then subjected to resolution via diastereomeric salts to give the desired resolved amino acid compound of formula D.

In this case too, the process is rather long, laborious and inefficient in terms of chemical and stereochemical yields. Moreover, the specific reaction conditions prevent its use on an industrial scale. In addition, the starting amine compound of formula G, used in the synthesis, is not commercially available.

There is consequently a need for a more advantageous alternative method to prepare Telaprevir, and its synthetic intermediates, on an industrial scale. Said novel method should in particular be 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.

SUMMARY OF THE INVENTION

It has now surprisingly been found that a compound of formula (II) or a salt thereof

(Π)

where is H or an optionally substituted Ci -C₁₂ alkyl group, an optionally substituted C3-C10 cycloalkyl group or an optionally substituted aryl group, and P is an amino protecting group, can be advantageously prepared with a process comprising intramolecular cyclisation of a compound of formula (III)

(III)

where P is as defined above; i is an optionally substituted C C₁₂ alkyl group, an optionally substituted C3-C 10 cycloalkyl group or an optionally substituted aryl group, X is an OH group or a leaving group, and the asterisk * indicates the presence of a stereocentre with configuration (R) or (S) at the 1- position.

Said compound of formula (III), and the novel intermediates for its preparation, are further object of the invention.

The intramolecular cyclisation reaction of a compound of formula (III) in accordance with the process according to the invention is particularly advantageous because it allows operation at low temperatures under particularly mild reaction conditions, thus obtaining high yields of the desired compound of formula (II) with high chemical and stereochemical purity.

BRIEF DESCRIPTION OF ANAL YSIS METHODS

The 1H-NMR and 13C-NMR spectra were recorded with a Bruker AC 200 spectrometer with operational frequency of 200 MHz, and with a Bruker Avance spectrometer with operational frequency of 400 MHz. The chemical shift values (δ) are expressed in parts per million (ppm) from the tetramethylsilane (TMS) used as internal standard. The coupling constants (J) are expressed in Hz. When reporting the spectra, the following terminology has been used:

s = singlet; d = doublet; t = triplet; q = quartet; m = multiplet. The mass spectra were recorded by the LC/MSD chromatography system with an Agilent series 1 100 inline UV detector. Analysis conditions: 95% MeOH + 5% H2O, flow rate 0.4 mL/min, direct injection, ESI ionisation, desiccant gas (N2) flow rate 9 L/min, temperature 350°C, nebulisation pressure 40 psi, frag 70.

The flash chromatography separations were performed with the methodologies and instrumentation described by W.C. Still et al., using silica gel 60 with a particle size of 230-400 mesh (Fluka) and appropriate elements as stationary phase.

Thin-layer chromatography (TLC) was performed on sheets of glass covered with silica gel 60 F254 (Merck) and viewed by exposure under a UV lamp (254 nm) and development with a 1% alkaline solution of KMnO4 (yellow spots on a purple background).

An HPLC Varian Pro Star Model 210, with UV lamp detector at the wavelength of 220 nm, was used to monitor the enzymatic hydrolysis reactions.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for the preparation of a compound of formula (II) or a salt thereof,

(Π)

where is H or an optionally substituted Ci -C₁₂ alkyl group, an optionally substituted C3-C10 cycloalkyl group, or an optionally substituted aryl group; P is an amino protecting group; comprising the intramolecular cyclisation reaction of a compound of formula (III)

(III)

where P is as defined above, i is an optionally substituted Ci-C₁₂ alkyl group, an optionally substituted C₃-Ci₀ cycloalkyl group, or an optionally substituted aryl group; X is an OH group or a leaving group, and the asterisk * indicates the presence of a stereocentre with configuration (R) or (S) at the 1- position; and, if the case, the conversion of a compound of formula (II) into another compound of formula (II), and/or, the conversion of a compound of formula (II) to a salt thereof, and/or the conversion of a salt of a compound of formula (II) to the free acid.

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

A C₁-C₁₂ alkyl group, which may be straight or branched, can be for example a Ci-C₄ alkyl group optionally substituted by one to three substituents such as a halogen atom, typically fluorine, and tert-butyl, preferably tert-butyl.

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

An aryl group is, for example, phenyl, naphthyl or a heteromonocyclic or heterobicyclic group containing one to three heteroatoms independently selected from oxygen, nitrogen and sulphur, optionally substituted, for example, by one to three halogen atoms; preferably it is phenyl.

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

A leaving group according to the present invention can be, for example, a halogen atom, preferably bromine and iodine, or a Ci-C₆ alkyl- or aryl-sulphonyloxy group, possibly substituted by one or more fluorine atoms, preferably methanesulphonate (mesylate), p-toluenesulphonate (tosylate) or trifluoromethanesulphonate (triflate).

In a compound of formula (III) X is preferably an OH group.

The stereocentres at the 2- and 3- positions in a compound of formula (III) present on the cycle must have the (S) and ( ) configuration respectively, so that the two bonds present on the cyclopentane residue are cis to one another, namely on the same side as the cycle, whereas the absolute configuration of the stereocentre at the 1- position marked with an asterisk * can be (R) or (S), preferably (R).

If in a compound of formula (III) X is a leaving group as defined above, the cyclisation reaction can be effected in a solvent in the presence of a base.

A base can be organic or inorganic. An organic base can be, for example, a cyclic or acyclic tertiary amine, DBU (diazabicycloundecene), imidazole or a Ci-C₆ alkaline metal alkoxide, for example of sodium or potassium, preferably potassium tert-butoxide.

An inorganic base can be, for example, a carbonate, hydroxide or hydride of an alkali-metal or alkaline-earth metal, for example of sodium, potassium or calcium, preferably sodium or potassium.

The reaction can be effected in a polar aprotic solvent; an apolar aprotic solvent, such as toluene; or a polar protic solvent; or in a mixture of two or more, preferably two or three, of said solvents.

Examples of a polar aprotic solvent include amides, for instance dimethylformamide and dimethylacetamide; N-methylpyrrolidone; acetonitrile or dimethyl sulphoxide; cyclic and acyclic ethers, for example methyl tert-butyl ether, tetrahydrofuran, dioxane; chlorinated solvents, for examole dichloromethane, dichloroethane, chloroform, chlorobenzene; and esters, for example ethyl or methyl acetate. Examples of polar protic solvents include straight-chain or branched Ci-C₈ alkanols, such as Ci-C₅ alkanol or water.

The reaction can be effected in a mixture of two or more, preferably two or three, of said solvents.

The cyclisation reaction can be conducted at a temperature of between about 0°C and the reflux temperature of the solvent, preferably between about 10°C and about 40°C, and more preferably at room temperature.

Alternatively, if in a compound of formula (III) X is OH, the cyclisation reaction can be effected, for example, by treating a compound of formula (III) with iodine and triphenylphosphine in the presence of a base and a solvent as defined above; or with the use of a dialkylazadicarboxylate, such as diethylazadicarboxylate, and triphenylphosphine, under Mitsunobu cyclisation conditions. The cyclisation reaction can be typically carried out in a single step.

When in a compound of formula (III) X is OH, the cyclisation reaction is preferably carried out by treatment with iodine and triphenylphosphine, under the conditions reported above.

A compound of formula (II) can be converted to a salt thereof, and similarly, the conversion of a salt thereof to the free compound can be effected according to known methods.

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

For example the conversion of a compound of formula (II), where , being as defined above, is other than hydrogen, can be converted to a compound of formula (II), where R is H, by treatment in a strongly basic, aqueous or anhydrous, environment, such as treatment with an aqueous solution of sodium hydroxide or with a solution of potassium tert-butoxide in an organic solvent.

Regardless of the stereochemistry of the stereocentre at the 1- position in a compound of formula (III), the basic conditions of the cyclisation reaction are such that, even if the reaction begins with a compound of formula (III) wherein the absolute stereochemistry of the stereocentre at the 1- position is (R), a compound of formula (II) is obtained wherein the stereochemistry of the stereocentre at the 1- position is only (S). This is because total epimerisation of the stereocentre at the 1- position takes place according to the process to which the present invention relates.

A compound of formula (II) can be used in a process for the preparation of telaprevir of formula (I), as defined above, according to known methods, such as those reported in US 7,820,671.

The invention therefore also discloses a process for the preparation of a compound of formula (I), as defined above, comprising utilizing, as starting material, a compound of formula (II), obtained by the process according to the present invention.

According to a further object of the invention, a compound of formula (I), as defined above, can be obtained by a process comprising coupling of tripeptide ABC and tripeptide DEF in the presence of DMTMM (i.e. 4-(4,6- dimethoxy (1 ,3,5) triazin-2-yl)-4-methylmorpholinium chloride).

A compound of formula (III) is novel, and represents a further subject of the invention.

A compound of formula (III), where X is OH and the stereocentre at the 1- position is, for example, ( ), can be prepared by a process comprising the anti-Markovnikov hydration reaction of the olefin compound of formula (IV),

(IV)

where i and P are as defined above.

Said conversion can be performed, for example, by a hydroboration/oxidation process.

The hydroboration step can be performed by treating the olefin compound of formula (IV) with a commercially available hydroborating agent, for example using a complex of borane BH₃ with THF, Me₂S, or an amine, or using an alkylborane of formula R₂BH, where R is a Ci-C₆ alkyl, or 9-BBN.

The oxidation step can be conducted, for example, by treatment with hydrogen peroxide in a basic aqueous environment.

A compound of formula (III) where X is OH can be converted to a compound of formula (III) where X is a leaving group by known methods, such as by activating the alcoholic function to the leaving group, as defined above.

A compound of formula (IV) as defined above is novel, and represents a further subject of the invention.

A compound of formula (IV) can be prepared by resolution of its corresponding racemic mixture, comprising enantioselective enzymatic hydrolysis of the ester function of an enantiomer present in said racemic mixture in the presence of a enzyme, in a solvent mixture.

The enzyme can be a hydrolase, such as a lipase, a protease or an esterase.

Enantioselective enzymatic hydrolysis of the ester function in an enantiomer, present in the above-mentioned racemic mixture, can preferably be effected with a protease obtainable from various sources, such as bacteria, fungi, animals or plants, preferably a protease active at a pH of between about 5 and about 9.

In this way, one of the two enantiomers which is not a substrate for the enzyme remains unchanged, while the other, which is the substrate for the enzyme, is hydrolysed to obtain a compound of formula (V),

(V)

where P is as defined above.

The enantioselective enzymatic hydrolysis can preferably be effected using a protease, in particular a protease obtained from bacteria of the genus Bacillus, preferably Bacillus licheniformis, such as the protease called PROTIN® supplied by Amano®, or PROTEX® supplied by Genencor® and supplied by Clea®, or the alkalases® supplied by Clea® or Novozyme®, such as FE201®. A solvent mixture is formed, for example, by a solution comprising an aqueous buffer at a pH of between approximately 5.0 and approximately 9.0, more preferably around a pH of approximately 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 phosphate buffer, ammonium bicarbonate, ethanolamine/HCl or borate; the reaction is preferably conducted in phosphate buffer.

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

The reaction can be conducted at a temperature of between about 15 and about 60°C, preferably between about 20 and about 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 7.5, 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 1 -2 days.

A compound of formula IV) in racemic mixture can be prepared from a racemic mixture of formula .syft-configuration

(VI) where P is as defined above and each of the asterisks * indicates a stereocentre, by esterification of the carboxyl function with an alcohol of formula RiOH, where i is as defined above, according to known techniques.

The racemic mixture of formula (VI) with relative .syft-configuration can be prepared by chelate Claisen rearrangement of a compound of formula (VII)

(VII)

where P is as defined above.

Although up to 4 stereoisomers of formula (VI) can be obtained by rearranging a compound of formula (VII), in practice the chelate Claisen rearrangement reaction is highly diastereoselective (see Kazmaier, Angew. Chem. Int. And. 1994, 33, 998) and only leads to the formation of the racemic mixture of formula (VI) with relative syn stereochemistry, the two enantiomers having an absolute configuration (R,S) and (S,R).

The chelate Claisen rearrangement reaction can be conducted in a solvent in the presence of a strong organic base and a zinc salt, such as zinc chloride.

In this rearrangement reaction a solvent can be, for example, a polar or apolar aprotic solvent as specified above, either alone or as a mixture of two or more, preferably two or three, of said solvents. The reaction is preferably conducted in tetrahydrofuran.

A strong base can be, for example, lithium diisopropylamide (LDA) or lithium bis(trimethylsilyl)amide (LiHMDS).

The reaction can be conducted at a temperature of between about -78°C and the reflux temperature of the solvent, preferably between -78°C and about 0°C. A compound of formula (VII) can be prepared by condensing a compound of formula (VIII) where P is as defined above

(VIII)

with a compound of formula (IX)

(IX)

by known procedures.

The compound of formula (IX) is commercially available or can be prepared by reducing the compound of formula (X)

(X)

which is commercially available, or can be prepared in turn from methyl adipate by known methods.

The following examples illustrate the invention.

Example 1: Synthesis of compound of formula (IX)

A solution of methyl- 1-cyclopentene-l -carboxylate (X) (10 mL, 81.88 mmol) in CH₂C1₂ (200 mL) under N₂ atmosphere is brought to 0°C, and a solution of 1M DIBALH (diisobutylaluminium hydride) in hexane (204.7 mL, 204.7 mmol) is added drop by drop. The mixture is kept under stirring a 0°C for 1 hour. H₂O is added to the reaction, insolubles are filtered through celite, and the phases are separated. The organic phase is washed with brine, dried over Na₂SO₄, filtered and evaporated under low pressure. The pure compound of formula (IX) is obtained. Yield: 95% (7.62 g of a colourless liquid).

R_f: 0.28 (EtPet/EtOAc 9: 1).

^- MR 400 MHz (CDC1₃) δ (ppm): 5.56 (s, 1H); 4.13 (s, 2H); 2.76 (bs, 1H); 2.30-2.26 (m, 4H); 1.90-1.82 (m, 2H).

1³C-NMR 100 MHz (CDC1₃) δ (ppm): 143.67; 125.06; 61.60; 32.08;

31.86; 22.97.

MS(ES⁺): m/z 121 [M+Na]⁺.

Example 2: Preparation of cyclopentenylmethyl 2-(tert-butoxy carbonylamino) acetate of formula (VII)

Dimethylaminopyridine (828 mg, 6.78 mmol) is added to a solution of alcohol (IX) (6.65 g, 67.85 mmol) in CH₂C1₂ (100 mL) under N₂ atmosphere, followed by a solution of dicyclohexylcarbodiimide (15.4 g, 74.64 mmol) in CH₂C1₂ (80 mL) and a solution of Boc-glycine (VIII) (13 g, 74.64 mmol) in CH₂C1₂ (100 mL). The mixture is kept under magnetic stirring at room temperature for 3 h, then filtered through celite to remove insolubles. The solution is evaporated under low pressure. The crude product is purified by flash chromatography on silica gel (eluent: EtPet/EtOAc 85 : 15) to obtain a chemically pure product of formula (VII).

Yield: 95% (16.43 g of a colourless oil).

: 0.43 (EtPet/EtO Ac 85 : 15).

H-NMR 400 MHz (CDC1₃) δ (ppm): 5.53 (s, 1H); 5.26 (bs, 1H); 4.54 (s, 2H); 3.75 (d, J = 4, 2H); 2.20-2.14 (m, 4H); 1.79-1.74 (m, 2H); 1.29 (s, 9H).

¹³C-NMR 100 MHz (CDC1₃) δ (ppm): 169.76; 155.32; 138.00; 128.56; 79.14; 63.26; 41.83; 32.25; 31.82; 27.72; 22.69.

MS(ES⁺): m/z 278 [M+Na]⁺.

Example 3: Synthesis of (R,S)-(S,R) (2-(teri-butoxycarbonylamino)- 2-(2-methylenecyclopentyl) acetic acid racemic mixture of formula (VI)

Preparation of lithium diisopropylamide (LDA): a solution of diisopropylamine (24.6 mL, 174.12 mmol) in THF (150 mL) under N₂ atmosphere is brought to 0°C, and a 2.5 M solution of BuLi in hexane (69.65 mL, 174.12 mmol) is added drop by drop. The mixture is kept under stirring at 0°C for 20 minutes.

ZnCl₂ (8.7 g, 63.84 mmol), previously oven-dried at 140°C overnight, is added to a solution of compound (VII) (14.8 g, 58.04 mmol) in THF (200 mL) under N₂ atmosphere, and the mixture is brought to -78 °C. LDA is added drop by drop, and the temperature is allowed to rise gradually to room temperature. The reaction is left under magnetic stirring for 12 h. IN KHSO₄ is then added, and the mixture is diluted with Et₂O. The organic phase is washed with IN KHSO₄ (3x), and then extracted with IN NaOH (4x). The combined basic aqueous phases are adjusted to pH = 2 with IN KHSO₄ and extracted with Et₂O (4x). The combined organic phases are washed with brine (lx), dried over Na₂SO₄, filtered and evaporated under low pressure. The chemically pure product (VI) is obtained.

Yield: 90% (13.3 g of a white solid).

Pf: 1 19-121 °C.

R_f: 0.69 (EtOAc).

^-NMR 400 MHz (MeOD) δ (ppm): 4.94 (s, 1H); 4.86-4.85 (m, 2H); 2.89-2.87 (m, 2H); 2.32-2.29 (m, 2H); 1.77-1.70 (m, 2H); 1.61 -1.48 (m, 2H); 1.40 (s, 9H).

¹³C-NMR 100 MHz (MeOD) δ (ppm): 173.83; 155.75; 150.70; 105.39; 78.65; 54.78; 45.24; 32.46; 27.32; 26.88; 23.28.

MS(ES⁺): m/z 278 [M+Na]⁺.

Example 4: Synthesis of (R,S)-(S,R) (2-(teri-butoxycarbonylamino)-

2-(2-methylene cyclopentyl)acetic acid methyl ester racemic mixture of formula (IV)

2M trimethylsilyl diazomethane (TMSCH₂N₂) in Et₂O is added drop by drop to a solution of a compound of formula (VI) (380 mg, 1.49 mmol) obtained according to example 3 in toluene (9 mL) and MeOH (4.5 mL), until the colour remains yellow. The reaction is left under stirring at room temperature for 30 minutes. The mixture is evaporated under low pressure, and the chemically pure racemic mixture of formula (IV) is obtained.

Yield: 98% (393 mg of a colourless oil).

R_f: 0.62 (EtPet/EtOAc 4: 1 ).

^- MR 400 MHz (CDC1₃) δ (ppm): 4.93 (s, 2H); 4.82 (s, 1 H); 4.40 (m, 1H); 3.65 (s, 3H); 2.79-2.77 (m, 1H); 2.27-2.25 (m, 2H); 1 .70- 1.66 (m, 2H); 1.53- 1.50 (m, 2H), 1 .37 (s, 9H).

¹³C-NMR 100 MHz (CDC1₃) δ (ppm): 172.44; 155.13 ; 150.15 ; 106.87; 79.35; 54.78; 5 1.53; 46.07; 46.00; 32.67; 27.92; 23.54.

MS(ES⁺): m/z 292 [M+Na]⁺.

Example 5: Enzymatic resolution of the (R,S)-(S,R) of (2-(tert- butoxycarbonylamino)-2-(2-methylene cyclopentyl)acetic acid methyl ester racemic mixture of formula (IV)

The phosphate buffer (60 mM, pH 7.66, 285 mL) and the enzymatic solution P OTEX 6L ( 15 mL) are added to a solution of the racemic mixture of formula (IV) (2.3 1 g, 8.58 mmol) obtained according to example 4 in methyl tert-butyl ether (28.5 mL). The mixture is left under magnetic stirring at room temperature for 24 hours until 50% of the starting product has been converted (determined by HPLC). Et₂O is then added to the reaction, and the phases are separated. The organic phase is dried over Na₂SO₄, filtered and evaporated, to obtain enantiomerically pure methyl ester of formula (IV) (1.1 g, yield 50%). The buffered aqueous phase is adjusted to pH = 2 by adding IN KHSO₄, and ethyl ether is added. A large emulsion forms which is removed by adding celite to the mixture, leaving it under stirring for 30 minutes and filtering out the celite through a porous septum. The phases of the clear triphasic solution thus obtained are separated and the organic phase is dried over Na₂SO₄, filtered and evaporated, to obtain the enantiomerically pure compound of formula (V) (1 .1 g, yield 50%).

HPLC: T_r (methyl ester 1): 23.28 min, T_r(methyl ester 2): 26.70 min (hexane/isopropanol 99: 1 , flow rate 1 mL/min, Chiralpak IC chiral column).

Example 6: Synthesis of (R)-tert-butyl 2-(teri-butoxycarbonyl amino)-2-((₁S')-2-methylenecyclopentyl) acetate of formula (IV)

Boc₂O (5.7 g, 26.12 mmol) is added to a solution of a compound of formula (V) (3.33 mg, 13.06 mmol) in tBuOH (30 mL) under N₂ atmosphere, followed by dimethylaminopyridine (478 mg, 3.92 mmol). The reaction is left under stirring for 3 h. The mixture is evaporated under low pressure and the crude reaction product is purified by flash chromatography on silica gel (eluent: EtPet) to obtain the compound of formula (IV).

Yield: 40% (1.62 g of a yellow oil).

Rf. 0.40 (EtPet/EtOAc 95 :5).

^-NMR 400 MHz (CDC1₃) δ (ppm): 4.95 (s, 1H); 4.89 (s, 2H); 4.36- 4.34 (m, 1H); 4.82-4.80 (m, 1 H); 2.28-2.23 (m, 2H); 1.73- 1.70 (m, 2H); 1.49- 1.47 (m, 2H); 1 .42 (s, 9H); 1.39 (s, 9H).

¹³C-NMR 100 MHz (CDC1₃) δ (ppm): 170.97; 155.18; 150.28; 106.71 ; 81.20; 79.02; 55.01 ; 45.97; 32.99; 27.84; 27.57; 27.22; 23.70.

MS(ES⁺): m/z 334 [M+Na]⁺.

Example 7: Synthesis of (R)-tert-butyl 2-(tert-butoxy carbonylamino)-2-((15',2R)-2-hydroxymethyl) cyclopentyl)acetate of formula (III)

BH₃-THF (1M solution in THF, 17.1 mL, 17.10 mmol) is added drop by drop to a solution of a compound (IV) (2.66 g, 8.55 mmol), such as the one prepared in example 6, in THF (50 mL) at -25 °C under N₂ atmosphere. The mixture is brought to 0°C and left under magnetic stirring for 2 h. H₂O (7.5 mL), 20% NaOH (7.5 mL) and 35% H₂O₂ (17.5 mL) are then added, and the mixture is left under stirring at 0°C for 1 h. The mixture is diluted with EtOAc and washed with brine. The organic phase is dried over Na₂SO₄, filtered and evaporated under low pressure. The crude product is purified by flash chromatography on silica gel (eluent: EtPet/EtOAc 7:3) to obtain an optically pure compound of formula (III).

Yield: 65% (1.8 g of a white solid).

Pf: 101-104 °C.

R_f: 0.29 (EtPet/EtOAc 7:3).

^-NMR 400 MHz (CDC1₃) δ (ppm): 5.07 (bd, J = 8, 1H); 4.22-4.20

(m, 1H); 3.71 (dd, J, = 6, J₂ = 10, 1H); 3.41 (dd, J, = 7, J₂ = 10, 1H); 2.78-2.76 (m, 1H); 2.16-2.08 (m, 2H); 2.63-2.61 (m, 4H); 1.53-1.48 (m, 1H); 1.37 (s, 9H); 1.33 (s, 9H).

¹³C-NMR 100 MHz (CDC1₃) δ (ppm): 171.98; 155.21 ; 81.37; 79.32; 61.66; 54.23; 44.19; 43.06; 28.28; 27.81 ; 27.44; 26.62; 21.99.

MS(ES⁺): m/z 352 [M+Na]⁺.

Example 8: Synthesis of (lR,3aR,6a.S)-di-teri-biitylhexahydrocyclo- penta [c]pyrrol-l,2(l//)-dicarboxylate of formula (II)

A solution of triphenylphosphine (598 mg, 2.28 mmol), iodine (579 mg, 2.28 mmol) and imidazole (310 mg, 4.56 mmol) in CH₂C1₂ (8 mL) under N₂ atmosphere is left under magnetic stirring at 0°C for 15 minutes, and a solution of a compound of formula (III), obtained according to example 7 (500 mg, 1.52 mmol) in CH₂C1₂ (5 mL), is added. The reaction is brought to room temperature and left under magnetic stirring for 12 h. The mixture is washed with Na₂S₂O (2x). The organic phase is dried over Na₂SO₄, filtered and evaporated under low pressure. The crude product is purified by flash chromatography on silica gel (eluent: EtPet/EtOAc 85: 15) to obtain an enantiomerically pure compound of formula (II). Yield: 75% (354 mg of a yellow oil).

Rf. 0.42 (EtPet/EtOAc 85 : 15).

^- MR 400 MHz (CDC1₃) δ (ppm): 4.25 (d, J = 8, 1 H); 3.74-3.72 (m, 1H); 2.97 (t, J = 9, 1H); 2.91 -2.83 (m, 1H); 2.64-2.62 (m, 1 H); 1 .74- 1.66 (m, 2H); 1.60- 1.54 (m, 4H); 1.42 (s, 9H); 1.38 (s, 9H).

¹³C-NMR 100 MHz (CDC1₃) δ (ppm): 170.39; 153.30; 80.44; 78.94; 62.55; 5 1.56; 46.77; 42.10; 29.15; 27.90; 27.67; 27.26; 25.81.

MS(ES⁺): m/z 334 [M+Na]⁺.

Example 9: (lS,3aR,6aS)-2-(tert-butoxycarbonyl) octahydrocyclo- penta[c]pyrrole-l-carboxylic acid of formula (II)

Potassium tert-butoxide (159 mg, 1 .42 mmol) is added to a solution of an ester compound of formula (II) prepared according to example 8 (221 mg, 0.71 mmol) in THF (4 mL) in N₂ atmosphere, and left under magnetic stirring at room temperature for 12 hours. Ammonium chloride is added to the reaction diluted with EtOAc, and the phases are separated. The aqueous phase is extracted with EtOAc (2x) and the combined organic phases are washed with brine (lx), dried over Na₂SO₄, filtered and evaporated. The crude product is purified by flash chromatography on silica gel (eluent: EtPet/EtOAc 1 :4) to obtain an optically pure acid compound of formula (II).

Yield: 60% (109 mg of a pale yellow oil).

Rf. 0.23 (EtPet/EtOAc 3 :7).

^XH-NMR 400 MHz (CDC1₃) δ (ppm): 8.34 (bs, 1H); 4.09-3.97 (m, 1 H); 3.64-3.61 (m, 1H); 3.29-3.19 (m, 1H); 2.70-2.65 (m, 2H); 1.94- 1.92 (m, 1 H); 1.80- 1.74 (m, 2H); 1 .58- 1.56 (m, 2H); 1.39 (d, J = 14, 10H).

MS(ES⁺): m/z 278 [M+Na]⁺.

Claims

1. A process for the preparation of a compound of formula (II) or a salt thereof,

(Π)

wherein is H or an optionally substituted C C₁₂ alkyl group, an optionally substituted C₃-C₁₀ cycloalkyl group, or an optionally substituted aryl group; P is a amino protecting group; comprising the intramolecular cyclisation reaction of a compound of formula (III)

(HI)

wherein P is as defined above, i is an optionally substituted Ci-C₁₂ 0 alkyl group, an optionally substituted C -Ci₀ cycloalkyl group, or an optionally substituted aryl group; X is an OH group or a leaving group; and the asterisk * indicates the presence of the stereocentre in (R) or (S) configuration at the 1- position; and, if appropriate, the conversion of a compound of formula (II) into another compound of formula (II), and/or, the 5 conversion of a compound of formula (II) to a salt thereof, and/or the conversion of a salt of a compound of formula (II) to the free acid.

2. A process according to claim 1 , wherein when in a compound of formula (III) X is a leaving group, the cyclisation reaction is carried out in a solvent in the presence of a base.

3. A process according to claim 1, wherein when in a compound of formula (III) X is OH the cyclisation reaction is carried out by using iodine and triphenylphosphine in the presence of a base and a solvent or by using a dialkylazadicarboxylate and triphenylphosphine.

4. A process according to claim 3, wherein the cyclisation reaction is carried out in one step.

5. A process according to claims 2 or 3, wherein the base is an organic base selected from a cyclic or acyclic tertiary amine, DBU (diazabicycloundecene), imidazole and a Ci-C₆ alkoxide of an alkali metal or an inorganic base selected from an alkali or alkaline-earth metal carbonate, hydroxide and hydride.

6. A process according to claim 5, wherein the base is imidazole or potassium terbutoxide.

7. A process according to claims 2 or 3, wherein the solvent is selected from a polar aprotic solvent, an apolar aprotic solvent, a polar protic solvent, and a mixture of two or more of said solvents.

8. A process according to claim 7, wherein the apolar aprotic solvent is toluene; the polar aprotic solvent is selected from an amide, acetonitrile, dimethylsulphoxide, a cyclic or acyclic ether, a chlorinated solvent and an ester; and the polar protic solvent is selected from a straight or branched Ci-C₈ alkanol and water.

9. A process according to claim 8, wherein the solvent is selected from dichloromethane and tetrahydrofuran.

10. A process according to claims 1-9, wherein the cyclisation reaction is carried out at a temperature between about 0 °C and the reflux temperature of the solvent, preferably between about 10 °C and about 40°C.

1 1. A process according to claims 1 -10, further comprising the preparation of a compound of formula (III), as defined in claim 1 , wherein X is OH and the stereocentre at the 1 - position is (R), by a process comprising the anti-Markovnikov hydration reaction of an olefin compound of formula (IV),

(IV)

wherein R_! and P are as defined in claim 1.

12. A process according to claim 1 1 , wherein the anti-Markovnikov hydration reaction is performed by hydroboration/oxidation process, preferably by hydroboration of a compound of formula (IV) with a hydroborating agent selected from a complex of borane BH₃ with THF, Me₂S, or an amine, and an alkylborane of formula R₂BH, where R is a Ci-C₆ alkyl, or 9-BBN.

13. A process according to claims 1- 12, further comprising the conversion of a so obtai la (I)

(I)

14. A compound of formula (III) or (IV)

(IV)

wherein R_{l 5} P and X are as defined in claim 1.

15. A process for the preparation of a compound of formula (I), as defined in claim 13, comprising coupling between a tripeptide ABC and the tripeptide DEF

in the presence of 4-(4,6-dimethoxy (1 ,3,5) triazine-2-yl)-4- methylmorpholinium chloride (DMTMM).