DESC:RELATED APPLICATIONS
This application claims the benefit of priority of our Indian patent application numbers 201621026310 filed on 2nd August 2016 and 201621034372 filed on 7th October 2016 which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION
The present invention relates to a process for preparation of an optically pure (Sp)-Sofosbuvir of Formula-I and its intermediate namely (Sp)-isomer of isopropyl alanyl phosphoramidate of Formula (III) thereof. More particularly, the present invention is concerned with a process for epimerization of Rp isomer associated to the phosphorus atom.

Formula-(I) Formula (III)

BACKGROUND OF THE INVENTION
Globally, 130-150 million people have Chronic hepatitis C (CHC) infection. The prevalence of hepatitis C virus (HCV) is highest in Egypt at >10% of the general population; China has the most people overall with HCV (29.8 million); approximately 3.2 million are infected in the US. Of those with CHC, =20% develop serious morbidities ± mortality, that is, cirrhosis, end stage liver disease (ESLD), hepatocellular carcinoma (HCC). CHC infection leads to approximately 10,000 deaths per year in the US and has surpassed human immunodeficiency virus (HIV) as a cause of death.

Sofosbuvir is a nucleotide prodrug inhibitor of the HCV non structural protein 5B (NS5B) RNA dependent RNA polymerase, essential for HCV replication; as such, it is a direct-acting antiviral agent (DAA). Sofosbuvir having the chemical name (S)-Isopropyl 2-((S)-(((2R, 3R, 4R, 5R)-5-(2, 4-dioxo3,4-dihydropyrimidin-1(2H)-yl)-4-fluoro-3-hydroxy-4-methyltetrahydrofuran-2yl) methoxy)-(phenoxy) phosphorylamino) propanoate and its diastereomeric mixture is disclosed in WO2008121634 (family equivalent IN3658/KOLNP/2009). Diastereomeric mixtures of sofosbuvir, is subjected to crystallization, chromatography, or extraction in order to obtain purified compound of formula-I. General synthetic scheme of WO2008121634 can be described as below:

WO2010135569 (family equivalent IN4972/KOLNP/2011) discloses diastereomeric mixture and enantiomer in its solvates and hydrates thereof in crystalline, crystal-like, or amorphous form and process for preparation of the same.

WO2008121634 process provides racemic Sofosbuvir, while WO2010135569 process produces Sofosbuvir having diastereoisomeric mixture in a ratio of (Sp) relative to (Rp) of about 3:1. Therefore, there remains a need towards development of a process of (Sp)-Sofosbuvir having improved chiral selectivity and purity.

The inventors of the present invention have surprisingly identified an epimerization process in which (Sp)-Sofosbuvir and intermediates is obtained with high yield and purity.

SUMMARY OF THE INVENTION
The present invention provides an improved process for the preparation of optically pure (Sp)-Sofosbuvir and intermediates thereof.

In one aspect present invention relates to process for the preparation of optically pure (Sp)-Sofosbuvir (I) in high yield which comprises heating racemic sofosbuvir or (Rp)-isomer enriched Sofosbuvir or (Sp)-isomer enriched Sofosbuvir or (Rp)-isomer of Sofosbuvir (II) in a “solvent system in which both starting material and final product is not soluble” in presence or absence of base.

In another aspect, the present invention relates to a process for the preparation of optically pure (Sp)-isomer of isopropyl alanyl phosphoramidate (III) which comprises heating racemic isopropyl alanyl phosphoramidate or (Rp)-isomer enriched isopropyl alanyl phosphoramidate or (Sp)-isomer enriched isopropyl alanyl phosphoramidate or (Rp)-isomer of isopropyl alanyl phosphoramidate (IV) in a “solvent system in which both starting material and final product is not soluble” in the presence or absence of base.

wherein “R” is a leaving group.

DETAILED DESCRIPTION OF THE INVENTION
It is to be understood that the invention is not limited in its application to the details set forth in the following description or exemplified by the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

In an embodiment the term "optically pure (Sp)-Sofosbuvir (I)" represents that (Sp)-Sofosbuvir obtained according to the present invention contains less than 2 % (Rp)-isomer at phosphorous atom; preferably less than 1 % by HPLC at phosphorous atom; more preferably less than 0.5 % by HPLC at phosphorous atom. The term “optically pure (Sp)-isomer of isopropyl alanyl phosphoramidate (III)” obtained according to the present invention contains less than 3 % Rp-isomer at phosphorous atom; preferably less than 2 % by HPLC at phosphorous atom of compound of Formula (III); more preferably less than 1 % by HPLC at phosphorous atom of compound of Formula (III).

The term (Rp)-isomer to (Sp)-isomer represents the configuration at Phosphorus atom.

The term "(Rp)-isomer enriched Sofosbuvir” refers to a mixture of Sofosbuvir stereoisomers in which (Rp)-isomer is present in an amount greater than the (Sp)-isomer at phosphorous atom.

The term "(Rp)-isomer enriched isopropyl alanyl phosphoramidate” refers to a mixture of enriched isopropyl alanyl phosphoramidate stereoisomers in which (Rp)-isomer is present in an amount greater than the (Sp)-isomer at phosphorous atom.

The term "(Sp)-isomer enriched Sofosbuvir” refers to a mixture of Sofosbuvir stereoisomers in which (Sp)-isomer is present in an amount greater than the (Rp)-isomer at phosphorous atom.

The term "(Sp)-isomer enriched isopropyl alanyl phosphoramidate” refers to a mixture of enriched isopropyl alanyl phosphoramidate stereoisomers in which (Sp)-isomer is present in an amount greater than the (Rp)-isomer at phosphorous atom.

The term "racemic" or "racemate" refers to mixtures of isomers that are 1:1, to mixtures of isomers that are closer to 1:1.

In yet another embodiment of the invention the term "leaving group", as used herein in compound of formula (III) and (IV), is a group which is part of and attached to a substrate molecule; in a reaction where the substrate molecule undergoes a displacement reaction, the group is then displaced. Examples of leaving groups include, but are not limited to: halogen (F, Cl, Br, and I), tosylate, mesylate, trifiate, acetate, camphorsulfonate, aryloxide, and aryloxide substituted with at least one electron withdrawing group like, p-nitrophenoxide, 2-chlorophenoxide, 4-chlorophenoxide, 2,4-dinitrophenoxide, pentafluorophenoxide and like.

The present invention relates to epimerization of R-configuration at phosphorous atom of starting material so as to yield (Sp)-Sofosbuvir and intermediates thereof.

The present inventors surprisingly found that the epimerization of (Rp) isomer can be conveniently carried out in a “solvent system in which both starting material and final product is insoluble”. The “solvent system in which both starting material and final product is insoluble” as per the present invention includes but not limited to paraffin oil, mineral oil, vegetable oil, wax; preferably paraffin oil. The present inventors found that the use of a “solvent system in which both starting material and final product is soluble”, such as suitable organic solvents including but not limited to toluene, MDC, etc, for epimerization of (Rp) isomers of compound of formula (I) and/or formula (III) either did not proceed or did not completely epimerize the compound.

Accordingly in one embodiment the term “starting material” denotes racemic Sofosbuvir or (Rp)-isomer enriched Sofosbuvir or (Sp)-isomer enriched Sofosbuvir or (Rp)-isomer of Sofosbuvir (II) or a mixture thereof and the final product denotes (Sp)-Sofosbuvir. In another embodiment the term “starting material” denotes racemic isopropyl alanyl phosphoramidate or (Rp)-isomer enriched isopropyl alanyl phosphoramidate or (Sp)-isomer enriched isopropyl alanyl phosphoramidate or (Rp)-isomer of isopropyl alanyl phosphoramidate.

Starting material as used herein is prepared by either prior art process or by the process given in the example.

In one embodiment present invention relates to an improved process for the preparation of optically pure (Sp)-Sofosbuvir (I) in high yield comprising heating racemic Sofosbuvir or (Rp)-isomer enriched Sofosbuvir or (Rp)-isomer of Sofosbuvir (II) in liquid paraffin in presence or absence of base which leads to conversion of (Rp)-isomer to (Sp)-isomer without requiring any additional purification or separation step.

In another embodiment, the present invention relates to a process for the preparation of optically pure (Sp)-isomer of isopropyl alanyl phosphoramidate (III) in high yield comprising heating racemic isopropyl alanyl phosphoramidate or (Rp)-isomer enriched isopropyl alanyl phosphoramidate or (Rp)-isomer of isopropyl alanyl phosphoramidate (IV) in liquid paraffin in presence or absence of base which leads to conversion of (Rp)-isomer to (Sp)-isomer without requiring additional purification or separation step.

In another embodiment of the invention, the epimerization i.e the conversion of the (Rp)-isomer to (Sp)-isomer, according to the present invention is carried out at temperature in the range of 40-100°C; preferably about 50 to 80° C. The applicant found that the reaction above 100° C resulted with color change and formation of other related impurities even though the epimerization has happened. The epimerization reaction is monitored using HPLC, preferably chiral HPLC.

The term “base” as used herein includes but not limited to triethylamine, pyridine, ammonia, N,N-diisopropylethylamine, tri-n-butylamine, tri-n-propylamine, tetramethylethylene diamine and like. Inorganic base may also be employed.

The prior art process for preparation of (Sp)-Sofosbuvir and (Sp)-isomer isopropyl alanyl phosphoramidate involves fractional crystallization, that leaves the unwanted (Rp) isomer in mother liquor thereby affects the overall yield. Further, additional stages were required to convert the (Rp) into racemic compound. The present applicant found that those additional stage leads to yield loss and find a robust process for the preparation of (Sp)-Sofosbuvir in high yield.

The (Sp)-isomer isopropyl alanyl phosphoramidate obtained according to the present invention is converted to Sofosbuvir by following the process reported in the prior art or by following the examples given in this specification. In another aspect of the invention (Sp)-isomer isopropyl alanyl phosphoramidate prepared according to the present invention is converted to optically pure (Sp)-Sofosbuvir which comprises reacting (Sp)-isomer isopropyl alanyl phosphoramidate with (2’R)-2’-deoxy-2-fluoro- 2’-methyl uridine in the presence of lewis acid like tert-butylmagnesium halide.

In another aspect of the invention provides a process for preparing a stable crystalline Form-6 of Sofosbuvir which is substantially free from other polymorphic forms, comprising of following steps:
(i) dissolving Sofosbuvir in ethyl acetate to obtain solution;
(ii) isolating crystalline Form-6 of Sofosbuvir.

The term “Crystalline form 6 of Sofosbuvir” as used herein relates to crystalline
form of Sofosbuvir as disclosed in WO2011123645 and designated as form 6 and depicted in figure 21.

In one or more embodiments of the present invention, the obtained compound was isolated and purified, if needed, by conventional techniques. The term "conventional techniques" as used herein for isolation includes but not limited to distillation, distillation under reduced pressure or vacuum, filtration, evaporation, solvent-anti solvent, spray drying, lyophilization or freeze drying; for purification includes but not limited to acid-base purification, extraction, distillation, crystallization, recrystallization etc..

Examples
The following examples are provided to enable one skilled in the art to practice the invention and merely illustrate the process of this invention. However, it is not intended in any way to limit the scope of the present invention.
Chiral HPLC system (Waters-Alliance (manufactured by Waters India Ltd) LC system) equipped with UV-Visible detector, tertiary gradient pump, auto injector and Empower software system version 3.0, built 3471 (designed by Waters India) was used to determine chira purity of compunds.
Chromatographic Parameters:
Column : Chiralpak IG-3 (250 x 4.6 mm,3 µm)
Detector wavelength : 215 nm
Flow rate : 0.5 mL /min
Column oven temperature : 30ºC
Sample cooler temperature : 10ºC
Injection volume : 10 µL
Run time : 20 min
Diluent : Methanol
Elution : Isocratic



Needle wash : Methanol

Example-1: Preparation of pure (Sp)-isomer of isopropyl alanyl phosphoramidate intermediate (Formula (III), R is pentafluoro phenoxide)
L-alanine isopropyl ester hydrochloride (218.5 g) and toluene (1250 mL) were heated to reflux azeotropically till complete removal of water. The reaction mass was then cooled to 50°C and solvent was removed under reduced pressure. MDC was charged into the resultant mass was cooled to -60° to -50°C. Phenyldichloro phosphate (250.0 g) was charged into the reaction mass at -60°C to -50°C followed by trimethylamine (263.75 g) was added and the reaction mass was stirred for 2-3 hours at -60° to -50°C. Pentafluorophenol (240.0 g) was charged into the reaction mass followed by addition of triethylamine (143.39 g) at -60° to -50°C. The reaction mixture was allowed to warm to room temperature (RT) and stirred for 2-3 hours at RT. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 35 : 65). The reaction mixture was filtered and the filtrate was concentrated under reduced pressure. Paraffin oil (1250 mL) was charged into the residual mass and the reaction mixture was heated to 60°C. The reaction mixture was stirred for 3 hours at 60°C. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 99.67 : 0.37). The reaction mixture was filtered and washed with n-heptane (750 mL). The filtered solid was stirred with saturated bicarbonate solution (2500 mL) for 1 hour and filtered. The filtered solid was stirred with water (2500 mL) for 1 hour and filtered. The material was dried at 50-55°C in ATD to get pure (Sp)-isomer: (S)-2-[(S)-(2,3,4,5,6-pentafluoro-phenoxy)-phenoxy-phosphorylamino]propionic acid isopropyl ester (375 g) wherein (Rp)-isomer is 0.37 % on chiral HPLC.
Example-2 Preparation of pure (Sp)-isomer of isopropyl alanyl phosphoramidate intermediate (Formula (III), R is 2,4-dinitro phenoxide)
L-alanine isopropyl ester hydrochloride (87.4 g) and toluene (500 ML) were heated to reflux azeotropically upto complete water removal. The reaction mass was then cooled to 50°C and solvent was removed under reduced pressure. MDC was charged into the resultant solid and mass was cooled to -60° to -50°C. Phenyldichloro phosphate (100.0 g) was charged into the reaction mass at -60°C to -50°C. Triethylamine (105.5 g) was added and the reaction mass was stirred for 2-3 hours at -60° to -50°C. 2,4-dinitro phenol (95.98 g) was charged into the reaction mass followed by addition of triethylamine (71.87 g) at -60° to -50°C. The reaction mixture was allowed to warm to RT and stirred for 2-3 hours at RT. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 40 : 60). The reaction mixture was filtered and filtrate was concentrated under reduced pressure. Paraffin oil (500 mL) was charged into the residual mass and the reaction mixture was heated to 60°C. The reaction mixture was stirred for 3 hours at 60°C. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 99.6: 0.4). The reaction mixture was filtered and washed with n-heptane (300 mL). The filtered solid was stirred with saturated bicarbonate solution (1000 mL) for 1 hour and filtered. The filtered solid was stirred with water (1000 mL) for 1 hour and filtered. The material was dried at 50-55°C in ATD to get pure (Sp)-isomer: (S)-2-[(S)-(2,4-dinitro-phenoxy)-phenoxy-phosphorylamino]propionic acid isopropyl ester (150 g).

Example-3 Preparation of pure (Sp)-isomer of isopropyl alanyl phosphoramidate intermediate (Formula (III), R is p-nitro phenoxide)
L-alanine isopropyl ester hydrochloride (87.4 g) and toluene (500 ML) were heated to reflux azeotropically upto complete water removal. The reaction mass was then cooled to 50°C and solvent was removed under reduced pressure. MDC was charged into the resultant solid and mass was cooled to -60° to -50°C. Phenyldichloro phosphate (100.0 g) was charged into the reaction mass at -60°C to -50°C. Triethylamine (105.5 g) was added and the reaction mass was stirred for 2-3 hours at -60° to -50°C nitro phenol (72.53 g) was charged into the reaction mass followed by addition of Triethyl amine (57.5 g) at -60° to -50°C. The reaction mixture was allowed to warm to RT and stirred for 2-3 hours at RT. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 33 : 67). Filtered the reaction mixture and filtrate was concentrated under reduced pressure. Paraffin oil (500 mL) was charged into the residual mass and the reaction mixture was heated to 60°C. The reaction mixture was stirred for 3 hours at 60°C. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 99.5: 0.5). The reaction mixture was cooled to RT and stirred for 6 hours at RT. Reaction mixture was filtered and washed with n-heptane (300 mL). The filtered solid was stirred with saturated bicarbonate solution (1000 mL) for 1 hour and filtered. The filtered solid was stirred with water (1000 mL) for 1 hour and filtered. The material was dried at 50-55°C in ATD to get pure (Sp)-isomer: (S)-2-[(S)-(4-nitro-phenoxy)-phenoxy-phosphorylamino] propionic acid isopropyl ester (155 g).

Example-4 Preparation of pure (Sp)-isomer of isopropyl alanyl phosphoramidate intermediate (Formula (III), R is p-nitro phenoxide)
Isopropyl alanyl phosphoramidate (Distereomeric ratio of (Sp) : (Rp) = 35:65) and paraffin oil (300 mL), triethylamine (7.0 g) were charged and the reaction mixture was heated to 60°C. The reaction mixture was stirred for 3 hours at 60°C. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 99.6 : 0.4). n-heptane (300 mL) was charged into the reaction mixture and solid was filtered. The filtered solid was stirred with saturated bicarbonate solution (1000 mL) for 1 hour and filtered. Further the filtered solid was stirred with water (1000 mL) for 1 hour and filtered. The material was dried at 50-55°C in ATD to get pure (Sp)-isomer: (S)-2-[(S)-(4-nitro-phenoxy)-phenoxy-phosphorylamino] propionic acid isopropyl ester (145 g, HPLC purity: 99.99%).

Example-5 Preparation of pure (Sp)-isomer of isopropyl alanyl phosphoramidate intermediate (Formula (III), R is 2,4-dinitro phenoxide)
Isopropyl alanyl phosphoramidate (ratio of (Sp) : (Rp) = 39 : 61) and paraffin oil (400 mL), triethylamine (7.0 g) were charged and the reaction mixture was heated at 40°C. The reaction mixture was stirred for 6 hours at 40°C. The reaction was monitored by HPLC (Distereomeric ratio of (Sp) : (Rp) = 99.21 : 0.79). The reaction mixture was filtered and washed with n-heptane (300 mL). The filtered solid was stirred with saturated bicarbonate solution (1000 mL) for 1 hour and filtered. Further the filtered solid was stirred with water (1000 mL) for 1 hour and filtered. The material was dried at 50-55°C in ATD to get pure (Sp)-isomer: (S)-2-[(S)-(2,4-dinitro-phenoxy)-phenoxy-phosphorylamino]propionic acid isopropyl ester (145 g).

Example-6 Preparation of pure (Sp)-isomer of isopropyl alanyl phosphoramidate intermediate (Formula (III), R is pentafluoro phenoxide)
Isopropyl alanyl phosphoramidate (ratio of (Sp) : (Rp) = 51 : 49) and paraffin oil (500 mL), triethylamine (7.0 g) were charged and the reaction mixture was heated at 60°C. The reaction mixture was stirred for 3 hours at 60°C. The reaction mixture was cooled to RT and stirred overnight. The reaction was monitored by HPLC (ratio of (Sp) : (Rp) = 99.7 : 0.3). The reaction mixture was filtered and washed with n-heptane (300 mL). The filtered solid was stirred with saturated bicarbonate solution (1000 mL) for 1 hour and filtered. Further the filtered solid was stirred with water (1000 mL) for 1 hour and filtered. The material was dried at 50-55°C in ATD to get pure (Sp)-isomer: (S)-2-[(S)-(2,3,4,5,6-pentafluoro-phenoxy)-phenoxy-phosphorylamino]propionic acid isopropyl ester (145 g).

Example-7 Preparation of (Sp)-isomer of Sofosbuvir
L-alanine isopropyl ester hydrochloride (87.4 g) and toluene (500 mL) were heated to reflux azeotropically till complete removal of water. The reaction mass was then cooled to 50°C and solvent was removed under reduced pressure. MDC was charged into the resultant solid and mass was cooled to -60° to -50°C. Phenyldichloro phosphate (100.0 g) was charged into the reaction mass at -60°C to -50°C. N-methylimidazole (351.70 g) was charged into the reaction mass and the reaction mixture was stirred for 1 hour at -60° to -50°C. (2’R)-2’-Deoxy--2’-fluoro-2’-methyl uridine (94.89 g) was charged into the reaction mixture at -60° to -50°C. The reaction mixture was allowed to warm to RT and stirred overnight. Reaction mass was monitored by HPLC (ratio of (Sp) : (Rp) = 55 : 45).The reaction mixture was concentrated under reduced pressure. Paraffin oil (500 mL) was charged into the residual mass and the reaction mixture was heated to 60°C. The reaction mixture was stirred for 3 hours at 60°C. The reaction mixture was cooled to RT and stirred overnight. The reaction was monitored by HPLC (ratio of (Sp) : (Rp) = 100 : ND). The reaction mixture was filtered and washed with MTBE (300 mL). The solid was dissolved in ethyl acetate and washed with 1N HCl solution (500mL). Ethyl acetate layer was separated from aqueous layer. Ethyl acetate layer was washed with saturated bicarbonate solution (500 mL followed by brine solution (500 mL). The reaction mixture was concentrated under reduced pressure. To residue, MDC (330 mL) and diisopropyl ether (660 mL) were added and stirred overnight at RT. The reaction mixture was filtered and washed with MTBE. The solid was further purified in MDC (500 mL) and filtered. The material was dried at 40-45°C in under reduced pressure to get pure (Sp)-isomer (60 g) wherein (Rp)-isomer is not detectable on HPLC.

Example-8 Preparation of (Sp)-isomer of Sofosbuvir
(S)-2-[(S)-(2,3,4,5,6-pentafluoro-phenoxy)-phenoxy-phosphorylamino]propionic acid isopropyl ester (100 g) obtained by following example-1 or example-6 was charged to a solution of (2’R)-2’-Deoxy-2-fluoro-2’-methyluridine (40 g) in acetonitrile (250 mL) at 10?C to 20?C under nitrogen atmosphere. Solution of tert-butyl magnesium chloride (40 gm) in tetrahydrofuran (200 mL) was added at 10?C to 20?C and the reaction mass was stirred for 3-5 hours. Solution of ammomium chloride (20 g) in water was added at 10?C to 20?C. After completion of reaction, organic layer was separated and distilled under reduced pressure. MDC (130 ml) and diisopropyl ether (260 ml) were added to the residue and reaction mass was stirred for five hours. The reaction mixture was filtered and dried to get (Sp)-isomer of Sofosbuvir (48 g).

Example-9 Preparation of Form-6 of Sofosbuvir
Sofosbuvir (5.0 g) was stirred with ethyl acetate (50 mL) at 25-30°C for 3 hours. The reaction mixture was filtered and washed with ethyl acetate (5 mL). The material was dried for 12 hours at 50-55°C to get 3.0 g of Sofosbuvir Form-6.

Example-10 Preparation of Form-6 of Sofosbuvir
To a stirred solution of Sofosbuvir (10.0 g) in ethyl acetate (15 mL) was added to n-heptane (50 mL) at 25-30°C. The reaction mixture was stirred for 12 hours at 25-30°C. The reaction mixture was filtered and washed the wet cake with n-heptane (10 mL). The material was dried for 12 hours at 50-55°C to get 9.22 g of Sofosbuvir Form-6.

Example-11 Preparation of Form-6 of Sofosbuvir
Sofosbuvir (5.0 g) was stirred with isopropyl acetate (100 mL) at 25-30°C for 12 hours. The reaction mixture was filtered and washed with isopropyl acetate (5 mL). The material was dried for 12 hours at 50-55°C to get 2.9 g of Sofosbuvir Form-6.

Example-12 Preparation of Form-6 of Sofosbuvir
To a stirred solution of Sofosbuvir (10.0 g) in acetone (15 mL) was added to toluene (50 mL) at 25-30°C. The reaction mixture was stirred for 12 hours at 25-30°C. The reaction mixture was filtered and washed the wet cake with toluene (10 mL). The material was dried for 12 hours at 50-55°C to get 9.22 g of Sofosbuvir Form-6.

Example-13 Preparation of Form-6 of Sofosbuvir
Sofosbuvir (5.0 g) was dissolved in acetone (7.5 mL) and solution was filtered through 0.45 micron filter. The filtered reaction mass was added to toluene (50 mL) at 25-30°C. The reaction mixture was stirred for 12 hours at 25-30°C. The reaction mixture was filtered and washed the wet cake with toluene (5 mL). The material was dried for 12 hours at 50-55°C to get 3.1 g of Sofosbuvir Form-6.

Dated this 02nd August 2017
,CLAIMS:1. A process for the preparation of optically pure (Sp)-isomer of isopropyl alanyl phosphoramidate (III)

comprising heating racemic isopropyl alanyl phosphoramidate or (Sp)-isomer enriched isopropyl alanyl phosphoramidate or (Rp)-isomer enriched isopropyl alanyl phosphoramidate or (Rp)-isomer of isopropyl alanyl phosphoramidate (IV) in a solvent system in which both starting material and final product is not soluble in the presence or absence of base.
2. A process for the preparation of optically pure (Sp)-Sofosbuvir (I)

comprising heating racemic sofosbuvir or (Sp)-isomer enriched Sofosbuvir or (Rp)-isomer enriched Sofosbuvir or (Rp)-isomer of Sofosbuvir (II) in a solvent system in which both starting material and final product is not soluble in presence or absence of base.
3. The process as claimed in any of the proceeding claims wherein base is selected from group comprising of triethylamine, pyridine, ammonia, N,N-diisopropylethylamine, tri-n-butylamine, tri-n-propylamine, tetramethylethylene diamine.

4. The process as claimed in any of the proceeding claims solvent system in which both starting material and final product is not soluble is selected from paraffin oil, mineral oil, vegetable oil or wax; preferably paraffin oil.

5. The process as claimed in any of the proceeding claims wherein the heating is performed at a temperature in the range of 40?C to 100?C; preferably 40?C to 80?C.

6. The process as claimed in claim-2 wherein (Sp)-isomer of Sofosbuvir (I) is having optical purity greater than 99%.

7. The process as claimed in claim-1, wherein leaving group (R) is selected from group comprising of halogen, tosylate, mesylate, trifiate, acetate, camphorsulfonate, p-nitrophenoxide, 2-chlorophenoxide, 4-chlorophenoxide, 2,4-dinitrophenoxide, pentafluorophenoxide.

8. The process as claimed in claim-1, wherein (Sp)-isomer of isopropyl alanyl phosphoramidate (III) is having optical purity greater than 97%.

9. The process as claimed in claim-1 further comprising converting the optiacally pure (Sp)-isomer of isopropyl alanylphosphoramidate (III) into optically pure (Sp)-Sofosbuvir (I) by reacting with (2’R)-2’-Deoxy-2-fluoro- 2’-methyl uridine.

10. A process for preparing a stable crystalline Form-6 of Sofosbuvir which is substantially free from other polymorphic forms, comprising:
i) dissolving Sofosbuvir in ethyl acetate to obtain solution;
ii) isolating crystalline Form-6 of Sofosbuvir.

Dated this 02nd August 2017