WO2007077505A2

WO2007077505A2 - Crystalline l-menthyl (2r, 5s)-5-(4-amino-5-fluoro-2-oxo-2h-pyrimidin-1-yl)[1, 3]oxathiolan-2-carboxylate and process for preparation thereof

Info

Publication number: WO2007077505A2
Application number: PCT/IB2006/054981
Authority: WO
Inventors: Santosh Richhariya; Kaptan Singh; Mohan Prasad
Original assignee: Ranbaxy Laboratories Ltd
Current assignee: Ranbaxy Laboratories Ltd
Priority date: 2005-12-30
Filing date: 2006-12-20
Publication date: 2007-07-12
Anticipated expiration: 2008-06-30
Also published as: WO2007077505A3

Abstract

Provided herein is crystalline L-menthyl (2R, 5S)-5-(4-amino-5-fluoro-2-oxo-2H- pyrimidin-l-yl)[l,3]oxathiolan-2-carboxylate and a process for preparation thereof.

Description

CRYSTALLINE L-MENTHYL (2R, 5S)-5-(4-AMINO-5-FLUORO-2-OXO-2H- PYRIMIDIN-1-YL)[1, 3]OXATHIOLAN-2-CARBOXYLATE AND PROCESS FOR

PREPARATION THEREOF

Field of the Invention

Background of the Invention L-menthyl (2R,5S)-5-(4-amino-5-fluoro-2-oxo-2H-pyrimidin-l- yl)[l,3]oxathiolane-2-carboxylate is a useful intermediate in the synthesis of the antiviral drug emtricitabine.

Emtricitabine of Formula I, also known as FTC, is chemically (-)-β-L-4-amino-5- fluoro-l-[(2R,5S)-2-(hydroxymethyl)-l,3-oxathiolan-5-yl]-2(lH)-pyrimidinone.

Nucleoside analogues such as 3'-azido-3'-deoxythymidine (AZT), 2',3'- dideoxycytidine (DDC), 2',3'-didehydro-3'-deoxythymidine (D4T), 2',3'-dideoxyinosine (DDI), and various 2'-fluoro-derivatives of these nucleosides are reportedly effective in halting human immunodeficiency virus (HIV) replication by inhibiting reverse transcription. These compounds represent an important development in the number of drugs against HIV, particularly HIV-I. Emtricitabine and its 5-defluorocytosine analogue also known as lamivudine or

3TC are promising antiviral drugs having activity in particular against retroviruses such as human immunodeficiency virus (HIV), which is recognized as an etiologic agent of autoimmune deficiency syndrome (AIDS) and hepatitis B virus (HBV). Both emtricitabine and lamivudine exert protective activity against HIV induced cytopathogenicity.

Emtricitabine has two chiral centers and therefore four stereoisomers exist namely (2R, 5S) and (2S, 5R) which are the cis isomers and (2R, 5R) and (2S, 5S) which are the trans isomers. The cis (2R, 5S) isomer is found to have profound antiviral activity while the other cis (2S, 5R) isomer and the trans isomers (2S, 5S) and (2R, 5R) have lower therapeutic activity and are found to be of reduced interest in therapeutics. The synthetic approach adopted for emtricitabine involves introduction of the pyrimidine base in the 1,3- oxathiolane ring. Several stereoselective processes employing chiral auxiliaries have been developed to selectively obtain the cis (2R, 5S) isomer.

European Patent No. 0 515 157 (herein after the '157 patent) discloses a process for preparing emtricitabine, which involves condensation of L-menthyl cis-l,3-oxathiolan- 5S-acetoxy-2R-carboxylate with 5-fluorocytosine in the presence of iodotrimethylsilane, reduction of the L-menthyl ester group of the product so formed with lithium aluminum hydride to afford emtricitabine.

PCT Application No. WO 04/085432 (herein after the '432 PCT) discloses a process for preparing emtricitabine by isolation of the intermediate L-menthyl 1,3- oxathiolan-5S-(5-fluorocytosin-l-yl)-2R-carboxylate as the oxalate salt. The L-menthyl group in the oxalate salt is reduced with sodium borohydride to obtain emtricitabine. The '432 PCT discloses that L-menthyl ester of emtricitabine is a gel that is inseparable from the mother liquors by simple filtration and has to be isolated by chromatographic purification. As per the disclosure of the '432 PCT, the '157 patent does not report preparing menthyl emtricitabine intermediate with desired stereochemistry. The '432 PCT further discloses that in order to obtain emtricitabine which is the cis (2R, 5S) isomer, the corresponding L-menthyl ester having configuration (l'R, 2'S, 5'R) on the menthyl ring and (2R, 5S) configuration on the oxathiolane ring must be employed. Example 18 of the '157 patent refers to the L-menthyl derivative having inverted configuration on the oxathiolane ring whereas Example 19 refers to the derivative having inverted configuration on the menthyl ring. In view of the above, there remains a need for novel crystalline forms of emtricitabine and derivatives thereof. Summary of the Invention

It has been found that the L-menthyl ester of emtricitabine of Formula II (herein after "crystalline L-methyl emtricitabine") can be isolated in crystalline form by simple crystallization from the reaction mass thereof. This isolation process does not utilize tedious or time-consuming chromatographic purification or salt formation.

FORMULA II

Figure 1: Powder X-ray diffraction pattern of crystalline L-menthyl emtricitabine Figure 2: Differential Scanning Calorimetric thermogram of crystalline L-menthyl emtricitabine Figure 3: HPLC chromatogram of crystalline L-menthyl emtricitabine

Detailed Description of the Invention

Provided herein is crystalline L-menthyl emtricitabine of Formula II.

FORMULA II

Also provided is crystalline L-menthyl emtricitabine having characteristic powder X-ray diffraction pattern as depicted in Figure 1. 2. Crystalline L-menthyl emtricitabine can exhibit a powder X-ray diffraction pattern with characteristic 2Θ values (°) at: 12.72, 17.60, 18.30, 21.78, 22.64, and 26.06. Crystalline L-menthyl emtricitabine can also exhibit a powder X-ray diffraction pattern with characteristic 2Θ values (°) at: 12.72, 14.44, 15.74, 16.84, 17.60, 18.30, 21.78, 22.08, 22.64, 24.38, 26.06, 37.20 and 37.50. Crystalline L-menthyl emtricitabine can also exhibit a powder X-ray diffraction pattern showing characteristic 2Θ values (°) at: 4.38, 7.36, 10.54, 12.00, 12.72, 13.84, 14.44, 14.72, 15.08, 15.74, 16.84, 17.34, 17.60, 18.30, 18.48, 19.44, 19.80, 20.98, 21.78, 22.08, 22.64, 23.12, 24.04, 24.38, 24.88, 25.42, 25.80, 26.06, 26.80, 27.30, 27.88, 28.26, 28.48, 28.82, 29.28, 29.54, 29.90, 30.16, 30.46, 31.12, 31.42, 31.76, 32.30, 32.68, 33.04, 33.32, 33.90, 34.96, 35.66, 36.64, 37.20, 37.50, 38.00, 39.06, 39.46, and 39.78.

Also provided is crystalline L-menthyl emtricitabine having characteristic differential scanning calorimetric thermogram as depicted in Figure 2. Crystalline L- menthyl emtricitabine can also exhibit a differential scanning calorimetric thermogram showing characteristic endothermic peak at about 188-198 ⁰C.

Also provided is pure crystalline L-menthyl emtricitabine. As used herein the term "pure" refers to a purity of 99 % or more, a purity of 99.5 % or more and even a purity of 99.9% or more.

Also provided is a process for preparing crystalline L-menthyl emtricitabine of Formula II,

FORMULA II comprising the steps of: a) forming a mixture by adding a first organic solvent to a reaction mass or concentrate or mother liquor comprising L-menthyl emtricitabine, b) adding a second organic solvent to the mixture to form a second mixture, c) isolating crystalline L-menthyl emtricitabine from the second mixture thereof.

Crystalline L-menthyl emtricitabine can be isolated directly from a reaction mass thereof. The term "reaction mass" as used herein is understood to mean a reaction mixture comprising L-menthyl emtricitabine obtained by any process or a residue obtained by evaporation of solvent(s) from a reaction mixture comprising L-menthyl emtricitabine.

A suitable first organic solvent can be added to a reaction mass or concentrate or mother liquor comprising L-menthyl emtricitabine. The mixture can be refluxed, a second organic solvent can be added and the resultant mixture can be cooled to about 20 ⁰C to about 30 ⁰C. The solid so obtained can be filtered and dried under vacuum at about 30 ⁰C to about 40 ⁰C to yield crystalline L-menthyl emtricitabine.

Suitable first organic solvents can be selected from C₃_₈ esters, such as, for example, ethyl acetate, propyl acetate, Isopropyl acetate, butyl acetate, isobutyl acetate and the like. A mixture of two or more such first organic solvents can also be effectively utilized. Suitable second organic solvents can be selected from Ci_₄ alkanols such as for example methanol, ethanol, n-propanol, isopropanol and the like. A mixture of two or more such second organic solvents can also be effectively utilized.

FORMULA II comprising the steps of: a) reacting 5-fluorocytosine derivative of Formula III,

Formula III wherein P is hydrogen or protecting group, with a 1,3-oxathiolane derivative of Formula IV, wherein L is a leaving group,

Formula IV to form a compound of Formula V, wherein P is hydrogen or protecting group,

b) optionally deprotecting the compound of Formula V when P is a deprotecting group, c) adding a first organic solvent to the reaction mixture obtained in step a) or b), d) adding a second organic solvent to form a reaction mass, and e) isolating crystalline L-menthyl emtricitabine from the reaction mass thereof.

The 5-fluorocytosine derivative of Formula III and 1,3-oxathiolane derivative of Formula IV can be prepared by processes known in the art, such as for example, in EP 515157, U.S. Patent No. 6,051,709 and PCT Publication No. WO 04/085432. A solution of 5-fluorocytosine derivative of Formula III (wherein P is hydrogen or protecting group, wherein suitable protecting groups include those known in the art, e.g., silyl, benzyl, benzoyl, benzyloxy carbonyl and the like) in an organic solvent can be reacted with 1,3- oxathiolane derivative of Formula IV (wherein L is a leaving group, which includes an atom or group that is displaceable upon reaction with 5-fluorocytosine derivative of Formula III, such as for example, a halogen, e.g., I, Cl and Br; amide; azide; acyloxy; alkoxy; alkoxy carbonyl; and the like). The reaction mixture can be heated to reflux for about 15-20 hours and cooled to about 25 ⁰C to about 30 ⁰C. Deionized water at about 25 ⁰C to about 30 ⁰C is added to the cooled solution and the mixture stirred for about 0.5-1.0 hours. The organic layer can be separated and washed with concentration hydrochloric acid and deionized water and then with aqueous sodium chloride solution. The organic layer can be recovered under vacuum. The residue so obtained can be treated with a first organic solvent followed by a second organic solvent by the method described above in obtaining crystalline L-menthyl emtricitabine. Suitable protecting groups, as well as the method of removing protecting groups, are known to a person of ordinary skill in the art.

Crystalline L-menthyl emtricitabine can be converted to emtricitabine by processes known in the art, for example, a process reported in European Patent No. 0 515 157 or by processes exemplified herein. For example, crystalline L-menthyl emtricitabine can be prepared by a process described above and then b) the product of step a) can be reduced in the presence of one or more reducing agents, and c) emtricitabine of Formula I can be isolated from the reaction mixture thereof.

Suitable reducing agents can be selected from sodium borohydride, potassium borohydride, lithium aluminum hydride or a mixture thereof.

While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are included within the scope of the present invention. The examples are provided to illustrate particular aspects of the disclosure and do not limit the scope of the present invention as defined by the claims.

Examples

Example 1: Preparation of (5-Fluoro-2-trimethylsilanyloxy-pyrimidin-4-yl)- trimethylsilanyl-amine

5-fluorocytosine (100 g), hexamethyldisilazane (400 mL) and ammonium sulphate (5 g) were charged at 25 ⁰C. The mixture was refluxed at 135 ⁰C for 4 hours where a clear solution was obtained. The solution was cooled to 80 ⁰C and excess hexamethyldisilazne was recovered under vacuum at a pressure of 700 mm Hg to form a residue. The residue was cooled to 30 ⁰C and methylene chloride (1000 mL) was added. To this mixture triethylamine (140 mL) was added over 0.5 hours at 25 ⁰C and the resultant mixture stirred for 0.5 hours at 25 ⁰C and used as such in the next step.

Example 2: Preparation of 5S-Chloro-[l,3]oxathiolane-2R-carboxylic acid 2-(l'R, 2'S, S'RHsopropyl-S-methyl-cyclohexyl ester

Methylene chloride (3000 mL), 5R-Hydroxy-[l,3]oxathiolane-2S-carboxylic acid 2-isopropyl-5-methyl-cyclohexyl ester (300 g), methanesulphonic acid (1 mL) and dimethylformamide (85 mL) were charged at 25 ⁰C and the mixture stirred for 0.5 hours at 25 ⁰C. The mixture was cooled to 5 ⁰C to 10 ⁰C and thionyl chloride (80 mL) was added at 5 ⁰C to 10 ⁰C over 0.5 hours. The reaction mixture was stirred 10 ⁰C to 15 ⁰C for 2 hours. Excess methylene chloride was recovered under vacuum at a pressure of 650-700 mmHg and 40 ⁰C to 45 ⁰C to get a residue. To the residue methylene chloride (600 mL) was added and the excess methylene chloride was recovered under vacuum at a pressure of 650-700 mm Hg and 40 ⁰C to 45 ⁰C to get a residue. To the residue methylene chloride (600 mL) was added and the mixture used as such in the next step.

Example 3: Preparation of crystalline L-menthyl emtricitabine

The mixture obtained in Example 1 was refluxed at 42 ⁰C to 45 ⁰C. The mixture obtained in Example 2 was slowly charged over 1 hour at refluxing temperature. The combined reaction mixture was refluxed for 18 hours at 42 ⁰C to 45 ⁰C and the reaction was monitored by high performance liquid chromatography. The mixture was cooled to 25 ⁰C and deionized water (500 mL) was added at 25 ⁰C. The resultant mixture was stirred for 0.5 hours at 25 ⁰C. The organic layer was separated and washed twice with a concentrated hydrochloric acid solution (1 mL) and deionized water (500 mL) and then with aqueous sodium chloride solution (10%, 500 mL) at 25 ⁰C. The organic layer was recovered under vacuum at a pressure of 650-700 mm at 40 ⁰C to 45 ⁰C to get a residue. To the residue ethyl acetate (1000 mL) was charged at 25 ⁰C and the mixture refluxed at 78 ⁰C. To the mixture methanol (200 mL) was charged slowly and the resultant mixture slowly cooled to 25 ⁰C over 2 hours. The product so obtained was filtered and dried under vacuum at 30 ⁰C to 35 ⁰C to obtain the title compound. Yield: 25O g (81%) XRD as per Figure 1 DSC as per Figure 2 Purity: 100% (by HPLC)

Example 4: Preparation of Emtricitabine

Denatured spirit (210 mL) and menthyl emtricitabine (24.5 g) were charged at 25- 30 ⁰C. The mixture was cooled to 15-20 ⁰C and charged with a solution of dipotassium hydrogen phosphate (30 g) and water (30 mL) at 25-30⁰C and stirred for 0.5 hour at 25-30 ⁰C. The stirred mixture was cooled to 15 ⁰C and charged with sodium borohydride (4.66 g) and deionized water (30 mL) over 1.5 hour maintaining the temperature of the reaction mixture below 25 ⁰C. The reaction mixture was stirred at the same temperature till the level of menthyl emtricitabine was less than 0.5 % as measured by High Performance Liquid Chromatography. The reaction mixture was allowed to stand for 0.5 hour at 25-30 ⁰C. The organic layer was separated and the pH adjusted to 7.2 with concentrated hydrochloric acid (0.5 mL) at 25-30 ⁰C. To the resultant mixture activated carbon (3 g) was added at 25-30 ⁰C. The mixture was stirred for 0.5 hour, filtered through hyflo bed and washed with denatured spirit (30 mL) at 25-30 ⁰C. The stirred mixture was concentrated under vacuum (720 mmHg) at 45 ⁰C to an oily residue. The residue was charged with absolute ethanol (75 mL) and the resultant mass was concentrated under vacuum (720 mm Hg) at 45 ⁰C to a foamy residue. The foam was charged with absolute ethanol (75 mL) and the resultant mass was concentrated under vacuum (720 mmHg) at 45 ⁰C to a foamy residue. The residue was charged with absolute ethanol (60 mL) at 25-30 ⁰C and the temperature of the resultant mixture raised to 78 ⁰C and the mixture was refluxed for 0.5 hour. The mixture was cooled slowly to 60 ⁰C in 1 hour and stirred for 1 hour at 60 ⁰C and further cooled to 25-30 ⁰C in 1 hour and stirred at the same temperature for 4 hours. The solid so obtained was filtered, washed with ethyl acetate (30 mL) at 25- 30 ⁰C and dried under vacuum at 40-45 ⁰C for 12 hours to afford the title compound. Yield: 8.6 g (57.4%) Purity: 99.65 % (by HPLC)

Claims

WE CLAIM: 1. Crystalline L-menthyl emtricitabine of Formula II.

FORMULA II 2. Crystalline L-menthyl emtricitabine of claim 1 exhibiting a powder X-ray diffraction pattern with characteristic 2Θ values (°) at: 12.72, 17.60, 18.30, 21.78, 22.64, and 26.06.

3. Crystalline L-menthyl emtricitabine of claim 1 exhibiting a powder X-ray diffraction pattern with characteristic 2Θ values (°) at: 12.72, 14.44, 15.74, 16.84, 17.60, 18.30, 21.78, 22.08, 22.64, 24.38, 26.06, 37.20 and 37.50.

4. Crystalline L-menthyl emtricitabine of claim 1 exhibiting a powder X-ray diffraction pattern with characteristic 2Θ values (°) at : 4.38, 7.36, 10.54, 12.00, 12.72, 13.84, 14.44, 14.72, 15.08, 15.74, 16.84, 17.34, 17.60, 18.30, 18.48, 19.44, 19.80, 20.98, 21.78, 22.08, 22.64, 23.12, 24.04, 24.38, 24.88, 25.42, 25.80, 26.06, 26.80, 27.30, 27.88, 28.26, 28.48, 28.82, 29.28, 29.54, 29.90, 30.16, 30.46, 31.12, 31.42, 31.76, 32.30, 32.68, 33.04, 33.32, 33.90, 34.96, 35.66, 36.64, 37.20, 37.50, 38.00, 39.06, 39.46, 39.78.

5. Crystalline L-menthyl emtricitabine of claim 1 exhibiting characteristic powder X- ray diffraction pattern as depicted in Figure 1.

6. Crystalline L-menthyl emtricitabine of claim 1 exhibiting differential scanning calorimetric thermogram showing characteristic endothermic peak at about 188- 198 ⁰C.

7. Crystalline L-menthyl emtricitabine of claim 1 exhibiting characteristic differential scanning calorimetric thermogram as depicted in Figure 2.

8. Pure crystalline L-menthyl emtricitabine.

9. Crystalline L-menthyl emtricitabine of claim 8 having a purity of 99 % or more.

10. Crystalline L-menthyl emtricitabine according to claim 8 having a purity of 99.5 Ψc or more.

11. Crystalline L-menthyl emtricitabine of claim 8 having a purity of 99.9 % or more.

12. A process for preparing crystalline L-menthyl emtricitabine of Formula II,

comprising the steps of: a) forming a mixture by adding a first organic solvent to a reaction mass or concentrate or mother liquor comprising L-menthyl emtricitabine, d) adding a second organic solvent to the mixture to form a second mixture, b) isolating crystalline L-menthyl emtricitabine from the second mixture thereof, and c) isolating crystalline L-menthyl emtricitabine from the second mixture thereof.

13. The process of claim 12, wherein the first organic solvent is selected from one or more C₃-₈ esters or mixtures thereof.

14. The process of claim 13, wherein the one or more C₃_₈ esters are selected from ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate or mixtures thereof.

15. The process of claim 12, wherein the second organic solvent is selected from Ci^ alkanols or mixtures thereof.

16. The process of claim 15, wherein the Ci_₄ alkanol is selected from methanol, ethanol, n-propanol, isopropanol or mixtures thereof.

17. The process according to claim 12, wherein the crystalline L-menthyl emtricitabine obtained has a purity of 99.5 % or more.

18. A process for preparing crystalline L-menthyl emtricitabine of Formula II,

FORMULA V b) optionally deprotecting the protecting group in the compound of Formula V when P is a deprotecting group, c) adding a first organic solvent to the reaction mixture obtained in step a) or b), d) adding a second organic solvent to form a reaction mass, and e) isolating crystalline L-menthyl emtricitabine from the reaction mass thereof.

19. The process of claim 18, wherein the first organic solvent is selected from one or more C₃_₈ esters or mixtures thereof.

20. The process of claim 19, wherein the one or more C₃-₈ esters are selected from ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate or mixtures thereof.

21. The process of claim 18, wherein the second organic solvent is selected from Ci_-4 alkanols or mixtures thereof.

22. The process of claim 21, wherein the C^₄ alkanol is selected from methanol, ethanol, n-propanol, isopropanol or mixtures thereof.

23. The process according to claim 18, wherein the crystalline L-menthyl emtricitabine obtained has a purity of 99.5 % or more.

24. A process for preparing emtricitabine of Formula I

FORMULA I comprising the steps of: a) preparing crystalline L-menthyl emtricitabine by the process of claim 10 or 11 b) reducing the product of step a) in the presence of one or more reducing agents, and c) isolating emtricitabine of Formula I from the reaction mixture thereof.

25. The process according to claim 24, wherein the one or more reducing agents are selected from sodium borohydride, potassium borohydride, lithium aluminum hydride or a mixture thereof.