This invention relates to an improved process for the preparation of Remdesivir and intermediates thereof. Specifically, the present invention discloses a process for the preparation of Remdesivir substantially free of impurities. Further, the improved aspect of the present invention is to provide a process for preparation of Remdesivir to minimize genotoxic impurities.
BACKGROUND OF THE INVENTION
Remdesivir is a broad-spectrum antiviral medication developed by the biopharmaceutical company Gilead Sciences. Remdesivir is useful for the treatment of Filoviridae virus infections such as Marburg virus, Ebola virus and Cueva virus infections. Remdesivir is an investigational antiviral compound undergoing clinical trials in a number of countries as a potential treatment for Corona Virus Disease (COVID-19). The chemical name for Remdesivir is (2S)-2-{(2R,3S,4R,5R)-[5-(4-aminopyrrolo[2,1-f] [1,2,4] triazin-7-yl)-5-cyano-3,4-dihydroxy-tetrahydro-furan-2-ylmethoxy]phenoxy-(S)-phosphorylamino} propionic acid 2-ethyl-butyl ester. The CAS Registry Number of Remdesivir is [1809249-37-3] which has the following structure

U.S. Patent No. 9724360 discloses riboside phosphates and prodrugs thereof, which includes Remdesivir and pharmaceutically acceptable salts thereof. Synthetic schemes for the preparation of these riboside derivatives are disclosed in this patent.

PCT Publication WO2009132135A1 discloses the process for the preparation of compound of Formula IV by coupling compound of Formula II with halopyrazole compound of Formula III and finally purifying compound of Formula IV by Column chromatography using (0-50% Ethyl acetate: hexanes).

PCT Publication WO2011035250A1 discloses the process for the preparation of compound of Formula IV by coupling compound of Formula II with halopyrazole compound of Formula III in the presence of tetrahydrofuran as solvent. The purity of thus obtained compound of Formula IV is very low, which is further purified by Column chromatography to provide 1:1 mixture of anomers. Therefore, this route is not commercially viable.

The known processes suffer from problems, such as a cumbersome purification of the product by column chromatography and low yields. In view of the preparation methods available for Remdesivir, there is a need for simple, industrially scalable, cost effective and environmentally-friendly processes for the preparation of Remdesivir that is free from above mentioned drawbacks and achieves high yield and purity.

Despite the progress in the manufacturing operations for Active Pharmaceutical Ingredients various controls applied to make the drugs acceptable for human consumption. In the past several drug companies voluntarily recalled their drugs after finding trace amount of unexpected impurities i.e. nitrosamine, thus in light of probable contamination of nitrosamine impurities particularly N-nitrosodimethylamine (NMDA/NDMA) of and N-nitrosodiethylamine (NEDA/NDEA) of, particularly when the manufacturing process lead to the formation of a nitrosamine or when recycled raw materials / solvents can create unacceptable contamination or due to probable saturation of nitrosamine in environment, it becomes inevitable to identify potential cross contamination risks for drugs manufactured, to include enhanced evaluation of impurity controls and to demonstrate a capability of predicting, controlling, and preventing impurities in the drug substance and subsequently in the drug product.

In the process of the prior arts diisopropylethyl amine is used during coupling of compound of Formula VII and compound of Formula VIII. Diisopropylethyl amine is direct source of secondary amines resulting in the formation of highly toxic Nitrosamines (N-Nitrosodipropylamine, N-Nitrosodiisopropylamine, N-Nitrosoethylisoproplymiane, N-Nitrosodimethylamine, N-Nitrosodiethylamine) which are highly carcinogenic and are point of concern as per ICH M7. The processes disclosed in the prior art fail to provide the control of impurities/genotoxic nitrosamine impurities. Consequently, there is a need for an improved process for the preparation of Remdesivir, which not only overcomes the problems in the prior art processes as mentioned above, but also is simple, environment friendly, economically viable and industrially feasible for the preparation of Remdesivir having a good control over control over impurities/genotoxic nitrosamine impurities.

OBJECT OF THE INVENTION

It is a principal object of the present invention to improve upon limitations in the prior arts by providing a process for the preparation of Remdesivir.

It is another object of the present invention to provide a simple, commercially viable and environment friendly process for preparing Remdesivir, in high yield and purity.
It is yet another object of the present invention is to overcome or alleviate at least one of the deficiencies of prior art and provide a useful alternative for the preparation of Remdesivir suitable for human consumption.
It is another object of the present invention to provide Remdesivir substantially free of nitrosamines, wherein nitrosamine is selected from the group comprising of N-Nitrosodimethylamine, N-Nitrosodiethylamine, N-Nitrosodiisopropylamine (NDIPA/DIPNA), N-Nitrosoethylisopropylamine (EIPNA), N-Nitrosomethylethylamine (NMEA), N-Nitrosodipropylamine (NPDA), N-Nitrosodibutylamine (NBDA), N-Nitrosomethyldodecylamine, N-Nitroso-N-methyl-N-tetradecylamine, N-Nitroso-N-methyl-4-fluoroaniline, N-Nitroso-N-methyl-N-(2-phenyl) ethylamine, N-[methyl(nitroso)amino]butanoic acid (NMBA).

It is another object of the present invention to provide a simple and efficient process for the preparation of Remdesivir substantially free of nitrosamines, wherein nitrosamine is selected from the group comprising of N-Nitrosodimethylamine, N-Nitrosodiethylamine, N-Nitrosodiisopropylamine (NDIPA/DIPNA), N-Nitrosoethylisopropylamine (EIPNA), N-Nitrosomethylethylamine (NMEA), N-Nitrosodipropylamine (NPDA), N-Nitrosodibutylamine (NBDA), N-Nitrosomethyldodecylamine, N-Nitroso-N-methyl-N-tetradecylamine, N-Nitroso-N-methyl-4-fluoroaniline, N-Nitroso-N-methyl-N-(2-phenyl) ethylamine, N-[methyl(nitroso)amino]butanoic acid (NMBA).

It is still another object of the present invention to provide an improved process for the preparation of Remdesivir, via intermediate compound of Formula III and IV.

It is yet another object of the present invention to provide intermediate compound of Formula III and IV.

It is still another object of the present invention to provide a process for the preparation of an intermediate compound of Formula IV by reacting compound of Formula II and III.

It is yet another object of the present invention to provide Remdesivir substantially free of nitrosamines, which is efficiently prepared by coupling compound of Formula VII and compound of Formula VIII in presence of an inorganic base.

It is yet another object of the present invention to provide Remdesivir of purity higher than 99.5%.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a simple and efficient process for the preparation of Remdesivir substantially free of nitrosamines, wherein nitrosamine is selected from the group comprising of N-Nitrosodimethylamine, N-Nitrosodiethylamine, N-Nitrosodiisopropylamine (NDIPA/DIPNA), N-Nitrosoethylisopropylamine (EIPNA), N-Nitrosomethylethylamine (NMEA), N-Nitrosodipropylamine (NPDA), N-Nitrosodibutylamine (NBDA), N-Nitrosomethyldodecylamine, N-Nitroso-N-methyl-N-tetradecylamine, N-Nitroso-N-methyl-4-fluoroaniline, N-Nitroso-N-methyl-N-(2-phenyl) ethylamine, N-[methyl(nitroso)amino]butanoic acid (NMBA).

According to one aspect of the present invention there is provided an improved and commercially viable process for the preparation of Remdesivir, via intermediate compound of Formula III and IV, as shown in Scheme 1.

Scheme 1

wherein each R1, R2 and R3 is independently a hydroxy protecting group, alternatively, R2 and R3 groups on adjacent carbons can be combined to form a -C(R’)- group, and R' is H, C1 -C6 alkyl, phenyl or substituted phenyl.
R4 and R5 are both independently H or N-protecting group, wherein N-protecting group is preferably tert-butyloxycarbonyl group.

It has been found that Remdesivir substantially free of nitrosamines is efficiently prepared by coupling compound of Formula VII and compound of Formula VIII in presence of an inorganic base.
DESCRIPTION OF THE INVENTION
While this specification concludes with claims particularly pointing out and distinctly claiming that, which is regarded as the invention, it is anticipated that the invention can be more readily understood through reading the following detailed description of the invention and study of the included examples.
Nitrosamines are potent carcinogens in animals and probable carcinogens in humans. The probable reason of formation of nitrosamine as impurities is that they can form when certain reaction conditions are met such as use of organic amines during the reaction. According to one of the possible concepts, use of secondary amines resulting in the formation of highly toxic Nitrosamines (N-Nitrosodipropylamine, N-Nitrosodiisopropylamine, N-Nitrosoethylisoproplymiane, N-Nitrosodimethylamine, N-Nitrosodiethylamine) which are highly carcinogenic and are cohort of concern as per ICH M7. ICH M7 recommends that these mutagenic carcinogens be controlled at or below the acceptable cancer risk level. Due to their known potent carcinogenic effects, and because it is feasible to limit these impurities by taking reasonable steps to control or eliminate their presence, the goal is to have no quantifiable nitrosamine impurities or well within the declared limits which is safe for human consumption.
Understanding the limitations of prior art, there is a need for the development of an advantageous process for the preparation of Remdesivir and the best optimized process would always have better control on the formation of impurities/nitrosamine impurity rather than the removal of such impurities by purification, since such impurities once formed are difficult to remove and their removal adds up to the cost of process, making the process inefficient in several ways.
The inventors of the present invention devised the conditions suitable for preparing Remdesivir substantially free of nitrosamine impurities by involving the use of inorganic base in place of organic base (amine) as used in the process of the prior arts. Specifically, use of an inorganic base during the coupling of compound of Formula VII and compound of Formula VIII completely eliminates the source of Nitrosamines resulting in Nitrosamines free process.
This approach has shown very efficient control on formation of nitrosamine impurities. Furthermore, the process of present invention also reduces the requirement of purifications and thereby reduces solvent consumption, operational step and the time cycle and effluent generation, making the process cost effective and environmentally efficient.
The present invention relates to a process for preparing Remdesivir substantially free of nitrosamines comprising the steps of:
(a) coupling the compound of Formula VII with compound of Formula VIII in presence of an inorganic base to obtain compound of the Formula IX; and

(b) converting the compound of Formula IX to Remdesivir.

Coupling of compound of Formula VII with compound of Formula VIII to obtain compound of Formula IX is carried out in presence of an inorganic base selected from group comprising of alkali or alkaline earth metal hydroxide, carbonate or bicarbonate and the like, preferably sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate, lithium carbonate, sodium bicarbonate and potassium bicarbonate, more preferably sodium hydroxide, potassium hydroxide and lithium carbonate. This coupling is preferably carried out in presence of a reagents such as magnesium chloride, aluminum chloride, ferric chloride, cuprous chloride and the like or mixture thereof.

Compound of Formula IX is converted to Remdesivir in presence of an acid such as hydrochloric acid, sulfuric acid and the like in an organic solvent.

Further, the present invention (Scheme 1) relates to a process for the preparation of Remdesivir comprising the steps of:
(a) coupling compound of Formula II with halopyrazole compound of Formula III to obtain compound of Formula IV with or without its isolation;

wherein Y is halogen selected from the group comprising of Cl, Br or I.
R1, R2 and R3 is independently a hydroxy protecting group, alternatively, R2 and R3 groups on adjacent carbons can be combined to form a -C(R’)- group, and R' is H, C1 -C6 alkyl, phenyl or substituted phenyl.
R4 and R5 are both independently H or N-protecting group, wherein N-protecting group is preferably tert-butyloxycarbonyl group.

(b) optionally, subjecting the said compound of Formula IV (when both R4 and R5 are H) or reaction mixture of step (a) with an acid to obtain acid addition salt of compound of Formula IV;

wherein A is an organic acid selected from the group comprising of oxalic acid, tartaric acid, maleic acid, succinic acid, acetic acid, triflic acid, benzoic acid, trifluoroacetic acid, fumaric acid, gluconic acid, citric acid, malic acid, ascorbic acid, benzoic acid, isethionic acid, lactobionic acid, tannic acid, palmitic acid, alginic acid, polyglutamic acid, methanesulfonic acid, p-toluenesulfonic acid, naphthalenesulfonic acid, naphthalenedisulfonic acid, benzenesulfonic acid, ethanesulfonic acid, polygalacturonic acid, malonic acid, sulfosalicylic acid, glycolic acid, 2-hydroxy-3-naphthoate, pamoate, salicylic acid, stearic acid, phthalic acid, mandelic acid, lactic acid, lysine, arginine, glutamic acid, glycine, serine, threonine, alanine, isoleucine, leucine and the like.

(c) converting compound of Formula IV or its salt to compound of the Formula V;

(d) converting the compound of Formula V to compound of the Formula VI;

(e) converting the compound of Formula VI to compound of the Formula VII;

(f) coupling the compound of Formula VII with compound of Formula VIII in presence of a base, preferably an inorganic base to obtain compound of the Formula IX; and

(g) converting the compound of Formula IX to Remdesivir.

In step (a) of Scheme 1, compound of Formula II is coupled with halopyrazole compound of Formula III with or without isolation of compound of Formula IV. This coupling is carried out with Grignard reagent selected from the group comprising of CH3MgBr, iPrMgCl, tBuMgCl, PhMgCl or iPrMgCl.LiCl or combinations thereof; or trimethylsilyl chloride (TMSCl). This coupling is carried out in presence of an organic solvent selected from the group comprising of ethers such as dimethyl ether, diethyl ether, methyl tert-butyl ether (MTBE), diisopropyl ether, tetrahydrofuran (THF), dioxane and the like or mixtures thereof. Preferably, an organic solvent is tetrahydrofuran (THF).

In the optional step (b) of Scheme 1, compound of Formula IV is converted to salt of compound of Formula IV with an acid in an organic solvent. Acid is an organic acid selected from the group comprising of oxalic acid, tartaric acid, maleic acid, succinic acid, acetic acid, triflic acid, benzoic acid, trifluoroacetic acid, fumaric acid, gluconic acid, citric acid, malic acid, ascorbic acid, benzoic acid, isethionic acid, lactobionic acid, tannic acid, palmitic acid, alginic acid, polyglutamic acid, methanesulfonic acid, p-toluenesulfonic acid, naphthalenesulfonic acid, naphthalenedisulfonic acid, benzenesulfonic acid, ethanesulfonic acid, polygalacturonic acid, malonic acid, sulfosalicylic acid, glycolic acid, 2-hydroxy-3-naphthoate, pamoate, salicylic acid, stearic acid, phthalic acid, mandelic acid, lactic acid, lysine, arginine, glutamic acid, glycine, serine, threonine, alanine, isoleucine, leucine and the like. Organic acid is preferably oxalic acid. The organic solvent is selected from the group comprising of alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol and the like, ketones such as acetone, butanone, methyl ethyl ketone, methyl isobutyl ketone and the like, esters such as ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate and the like, halogenated hydrocarbons such as dichloromethane (DCM), chloroform, dichloroethane, chlorobenzene and the like or mixtures thereof.

In step (c) of Scheme 1, conversion of compound of Formula IV or its salt to compound of Formula V is carried out in an organic solvent with reagent selected from the group comprising of trimethylsilyl cyanide (TMSCN), p-Toluenesulfonylmethyl Isocyanide (TosMIC), acetone cyanohydrin, Diethyl Cyanophosphonate and the like; trimethylsilyl)trifluoromethylsulfonate (TMSOTf), trimethylsilyl chloride and the like or combinations thereof. The organic solvent is selected from the group comprising of halogenated hydrocarbons such as dichloromethane (DCM), chloroform, dichloroethane, chlorobenzene and the like, aliphatic hydrocarbons such as alkanes or cycloalkanes such as pentane, hexane, heptane, octane, cyclohexane, cyclopentane and the like; aromatic hydrocarbons such as toluene, xylene and the like or mixture thereof.

In step (d) of Scheme 1, conversion of compound of Formula V to compound of Formula VI is carried out in an organic solvent selected from the group comprising of halogenated hydrocarbons such as dichloromethane (DCM), chloroform, dichloroethane, chlorobenzene and the like or mixtures thereof; alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol and the like. Preferably, an organic solvent is dichloromethane.

In step (e) of Scheme 1, conversion of compound of Formula VI to compound of Formula VII is carried out in an organic solvent selected from the group comprising of acetone, butanone, methyl ethyl ketone, methyl isobutyl ketone, methyl amyl ketone, di-isobutyl ketone, cyclohexanone methanol, ethanol, n-propanol, isopropanol, n-butanol.

In step (f) of Scheme 1, coupling of compound of Formula VII with compound of Formula VIII to obtain compound of Formula IX is carried out in presence of an inorganic base selected from group comprising of alkali or alkaline earth metal hydroxide, carbonate or bicarbonate and the like, preferably sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate, lithium carbonate, sodium bicarbonate, potassium bicarbonate and more preferably sodium hydroxide, potassium hydroxide and lithium carbonate. This coupling is preferably carried out in presence of a reagents such as magnesium chloride, aluminum chloride, ferric chloride, cuprous chloride and the like or mixture thereof.

In step (g) of Scheme 1, compound of Formula IX is converted to Remdesivir with reagents such as hydrochloric acid, sulfuric acid and like thereof in an organic solvent. The organic solvent is selected from the group comprising of ethers such as dimethyl ether, diethyl ether, methyl tert-butyl ether (MTBE), diisopropyl ether, tetrahydrofuran (THF), dioxane and the like or mixtures thereof.
The solvents as used above in any of the steps or for the crystallization of Remdesivir is independently selected from the group comprising of nitriles, alcohols, ketones, esters, halogenated hydrocarbons, ethers, amides, dialkylsulfoxides, hydrocarbons, organic acids, water or the mixtures thereof. Nitriles are selected from the group comprising of acetonitrile, propionitrile, butyronitrile, valeronitrile and the like. Alcohols are selected from the group comprising of methanol, ethanol, n-propanol, isopropanol, n-butanol and the like. Ketones are selected from the group comprising of acetone, methyl ethyl ketone, methyl isobutyl ketone and the like. Esters are selected from the group comprising of ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate and the like. Halogenated hydrocarbons are selected from the group comprising of dichloromethane (DCM), chloroform, dichloroethane, chlorobenzene and the like. Ethers are selected from the group comprising of diethyl ether, methyl tert-butyl ether (MTBE), diisopropyl ether, tetrahydrofuran (THF), dioxane and the like. Amides are selected from the group comprising of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), Nmethylformamide, N-methylpyrrolidone and the like. Dialkyl sulfoxides can be selected from the group comprising of dimethylsulfoxide, diethylsulfoxide, dibutylsulfoxide and the like. Aliphatic hydrocarbons are selected from the group comprising of alkanes or cycloalkanes such as pentane, hexane, heptane, octane, cyclohexane, cyclopentane and the like. Aromatic hydrocarbons are selected from the group comprising of toluene, xylene and the like. Organic acids are selected from the group comprising of acetic acid, formic acid, propionic acid and the like.
Compound of Formula VI is converted to Remdesivir by process of the instant invention or by any methods known in prior art.

Compound of Formula II, used in the preparation of compound of Formula III is prepared by the process comprising the steps of converting lactol of Formula I to compound of Formula II;

Conversion of lactol of Formula I is carried out in an organic solvent with reagent selected from the group comprising of anhydrides such as acetic anhydride, acetic propionic anhydride, acetic butyric anhydride, maleic anhydride, glutaric anhydride and the like or mixtures thereof, preferably acetic anhydride. In step a) an organic solvent is selected from the group comprising of sulfoxides such as dimethyl sulfoxide, dimethyl sulfone; amides such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N-methylformamide, N-methylpyrrolidone and the like or mixtures thereof.

Remdesivir obtained by process of the present invention being substantially free of nitrosamines, preferably nitrosamine impurity is less than 0.01 ppm. Nitrosamine impurities are well within the specifications prescribed safe for human consumption, preferably nitrosamine impurity is less than 0.01 ppm, wherein the limit of detection is 0.01 ppm.
In another aspect Remdesivir of the present invention is pharmaceutically acceptable salts or hydrates thereof of Remdesivir.
In another aspect there is provided a pharmaceutical composition that includes a therapeutically effective amount of Remdesivir its pharmaceutically acceptable salts or hydrates thereof according to the process of the present invention and one or more pharmaceutically acceptable carriers, excipients or diluents.
In yet another aspect there is provided use of a pharmaceutical composition that includes a therapeutically effective amount of Remdesivir its pharmaceutically acceptable salts or hydrates thereof according to the process of the present invention and one or more pharmaceutically acceptable carriers, excipients or diluents to treat conditions in a subject in need thereof.
The process for the preparation of Remdesivir described in the present invention is demonstrated in the examples illustrated below. These examples are provided as illustration only and therefore should not be construed as limitation of the scope of the invention.

EXAMPLES
Example 1: Preparation of compound of Formula II (wherein R1, R2 and R3 are benzyl)
The commercially available lactol of Formula I (wherein R1, R2 and R3 are benzyl) (10 g) was dissolved in anhydrous DMSO (30 mL) under nitrogen. Acetic anhydride (20 mL) was added and the resultant reaction mixture was stirred at RT for about 48 h. The reaction mixture was poured onto ice water (500 mL) and the mixture stirred for 20 min. The mixture was extracted with ethyl acetate and the combined organic extracts were then washed with water (3x200 mL). The organic extract was dried over anhydrous magnesium sulphate, filtered and concentrated under reduced pressure to provide the lactone of Formula II.

Example 2: Preparation of compound of Formula IV (wherein R1, R2 and R3 are benzyl; R4 and R5 are H)
The iodopyrazole of Formula III (wherein R4 and R5 are H; and Y is Iodo group) (75.0 g) was dissolved in THF (1500 ml) and the solution was cooled to about 0°C with stirring under nitrogen. Trimethylsilyl chloride (TMSC1) (62 g) was added and after about 30 min phenyl magnesium chloride (2.0 ? in THF, 280 ml) was slowly added. The reaction mixture was cooled to about -20°C to -15°C and iso-propyl magnesium chloride-lithium chloride complex (1.3 ? in THF, 233 ml) was slowly added. After about 1 h, solution of lactone of Formula II (120 g) in dichloromethane (375 g) was added slowly and the resulting reaction mixture was stirred for about 1.5 h. Then ammonium chloride solution (1100 ml) was added and the mixture was warmed to about 15°C and ethyl acetate (1500 ml) was added and washed the organic layer with 1N hydrochloric acid and then with 5% NaHCO3 solution. Dried the organic layer over sodium sulfate. Finally, the organic layer was evaporated to give the product as an oily mass.
Yield: 120 g

Example 3: Preparation of Oxalate salt of compound of Formula IV (wherein R1, R2 and R3 are benzyl; R4 and R5 are H)
The compound of Formula IV (10 g) was heated with MTBE (100 mL) at 60-65°C for 1 h and gradually cooled to 0-5°C and stirred for 10-12 h. Precipitated solid was filtered, washed with MTBE and suck dried. The suck dried material was dissolved in MTBE (80 mL) and treated with oxalic acid (0.81 g) at ambient temperature for 2.5 h. Then the reaction mixture was cooled to 0-5oC and the precipitated solid was filtered. Finally, dried the product under vacuum to yield the desired oxalate salt.
Yield: 3.8 g; HPLC Purity: 97.8%

Example 4: Preparation of compound of Formula V (wherein R1, R2 and R3 are benzyl)
Charged oxalate salt of compound of Formula IV (wherein R1, R2 and R3 are benzyl; R4 and R5 are H) (10 g) and dichloromethane (150 mL) at 25-30°C and stirred for 25-30 min. Cooled the solution to -80° to -78°C. Slowly added trifluoromethane sulfonic acid (5.32 g) at -80° to -78°C drop wise over a period of 25-30 min. Slowly added trimethylsilyl trifluoromethane sulfonate (20.75 g) at -80° to -78°C drop wise over a period of 25-30 min. Prepared a solution of trimethylsilyl cyanide (9.26g) in dichloromethane (500 mL) and added to the above reaction mass at -80 to -78°C. Stirred the reaction mass at -80 to -78°C for 20-30 min. Prepared 20% w/w aqueous potassium hydroxide solution (1200 mL), cooled the solution to -10° to -5°C, and added this aqueous potassium hydroxide solution to the above reaction mass. Gradually raised the temperature of the reaction mass to 25-30°C in 1-2 h. organic layer was separated and washed with 10% sodium chloride solution (500 mL x 3). Filtered the organic layer through Hyflo bed and washed with dichloromethane (100 mL). Concentrated the reaction mass at NMT 40°C under vacuum. Charged IPA (50 mL) and heated the reaction mass to 45-50°C. Gradually cooled the reaction mass to 25-30°C and stirred for 3-4h. Filtered the product and washed with IPA (10 mL) Dried the product under vacuum at 35-40°C for 5-6 h.
Yield: 7.5 g

Example 5: Preparation of compound of Formula VI
Charged the compound of Formula V (wherein R1, R2 and R3 are benzyl) (60 g) and Dichloromethane (300 mL) at 25-30°C and stirred for 25-30 min. Cooled the reaction mixture to -30° to -20°C. Slowly added solution of 1M boron trichloride solution (510g) at -30°C to -20°C in 3h. Slowly methanol (420 mL) was added to it and raised the temperature to 15-20°C and stirred for NLT 12h. Filtered the reaction mass and washed with methanol (120 mL) and sucked to dryness the wet cake for 30-60 min.(wet cake:35.0g). Charged wet cake & charged premixed 1N HCl (80.4g) & MeOH (120mL) at RT and stirred for 8-10h at 15-20°C (wet cake:30.0g). Charged wet cake at 19-25°C and water (300mL) and adjust the pH (NLT 8.0) with 20% aqueous K2CO3 solution (12g K2CO3 dissolved in 60 mL water) at RT. Stirred for 1h at 20-25°C and filtered the solid and washed with DM water (180mL) & methanol (60mL). Sucked to dryness under vacuum for 45-60 min (wet cake: 25g). Finally, dried the product at NMT 60°C under vacuum for 4-6 hr.
Yield: 24 g; HPLC Purity: 99.11%

Example 6: Preparation of compound of Formula VII
Charged the compound of Formula VI (100 g) and acetone (700 mL) at 25-30°C. Charged 2,2-dimethoxy propane (178 g) and p-TSA (72 g) to reaction mixture at RT and stirred for 6h. Charged isopropyl acetate (700 mL) and stirred for 2-3h. Filtered the reaction mixture, washed the wet cake with of isopropyl acetate (100 mL). Charged wet cake, ethyl acetate (1000mL), water (100mL) and adjusted the pH (NLT 9.0) with 10% w/v aqueous potassium carbonate solution (50g K2CO3 dissolved in 500mL DM water at RT. Separated the layer and washed the organic layer with water (2x500mL). Charged organic layer and charcoal (10g) and heated to 35-40°C under stirring for 25-30min. Filtered the reaction mixture through hyflo bed & washed with ethyl acetate (100mL). Charged the filtrate and distilled out under vacuum at temperature NMT 50ºC up to 4-5V residual mass. Gradually cooled the mass to 25-30°C over a period of 2h. Charged n-Heptane (1000mL) over a period of 4h at 25-30°C and cooled it to 10-15°C and stirred for 2h. Filtered the solid at 10-15°C and washed with n-heptane (200 mL). Sucked to dryness under vacuum for 45-60 min and finally dried the product under vacuum at temperature NMT 50°C for 4-6h.
Yield: 93 g; HPLC Purity: 99.8%

Example 7: Preparation of Crude Remdesivir
Compound of Formula VII (100 g) followed by compound of Formula VIII (149.54 g) and tetrahydrofuran (1.0 L) are charged in reactor at ambient temperature and anhydrous magnesium chloride (43.10 g) is added to it. Diisopropyl ethyl amine (97.64 g) is added dropwise over a period of 1-2 h and the reaction mass is stirred at ambient temperature for 2-4 h. Then ethyl acetate (1.0 L) is added and washed the organic layer with 10% w/v aqueous citric acid solution, followed by 10% aqueous potassium carbonate solution and then with aqueous ammonium chloride solution. The organic layer is concentrated at 50°C under vacuum and charged acetonitrile (1.0 L) to the residue and cooled the solution of Formula IX in acetonitrile. Then conc. HCl was added dropwise during 1 h at 0°C. After stirring for 3 h at 0°C, the reaction mass was diluted with ethyl acetate (500 mL) and neutralized with aqueous potassium bicarbonate solution by adjusting pH ~6. The organic layer was washed with brine, charcoalized and concentrated. Finally, crystallization from ethyl acetate and di-isopropylether give crude Remdesivir.
Yield: 160 g; HPLC purity: 99.1%; Nitrosamine content: ND (not detected)

Example 8: Preparation of Crude Remdesivir
Compound of Formula VII (100 g) followed by compound of Formula VIII (149.54 g) and acetonitrile (1.0 L) are charged in reactor at ambient temperature and anhydrous magnesium chloride (43.10 g) is added to it. Diisopropyl ethyl amine (97.64 g) is added dropwise over a period of 1-2 h and the reaction mass is stirred at ambient temperature for 2-4 h. Then ethyl acetate (1.0 L) is added and washed the organic layer with 10% w/v aqueous citric acid solution, followed by 10% aqueous potassium carbonate solution and then with aqueous ammonium chloride solution. The organic layer is concentrated at 50°C under vacuum and charged tetrahydrofuran (1.0 L) to the residue and concentrated, then acetonitrile (900 mL) was added and cooled the solution of Formula IX in acetonitrile. Then conc. HCl was added dropwise during 1 h at 0°C. After stirring for 3 h at 0°C, the reaction mass was diluted with ethyl acetate (500 mL) and neutralized with aqueous potassium bicarbonate solution by adjusting pH ~6. The organic layer was washed with brine, charcoalized and concentrated. Finally, crystallization from ethyl acetate and di-isopropylether give crude Remdesivir.
Yield: 160 g; HPLC purity: 99.0%; Nitrosamine content: ND (not detected)

Example 9: Preparation of Crude Remdesivir
Compound of Formula VII (100 g) followed by compound of Formula VIII (149.54 g) and tetrahydrofuran (1.0 L) are charged in reactor at ambient temperature and anhydrous magnesium chloride (43.10 g) is added to it. Diisopropyl ethyl amine (59.06 g) is added dropwise over a period of 1-2 h and the reaction mass is stirred at ambient temperature for 2-4 h. Then ethyl acetate (1.0 L) is added and washed the organic layer with 10% w/v aqueous citric acid solution, followed by 10% aqueous potassium carbonate solution and then with aqueous ammonium chloride solution. The organic layer is concentrated at 50°C under vacuum and charged acetonitrile (1.0 L) to the residue and cooled the solution of Formula IX in acetonitrile. Then conc. HCl was added dropwise during 1 h at 0°C. After stirring for 3 h at 0°C, the reaction mass was diluted with ethyl acetate (500 mL) and neutralized with aqueous potassium bicarbonate solution by adjusting pH ~6. The organic layer was washed with brine, charcoalized and concentrated. Finally, crystallization from ethyl acetate and di-isopropylether give crude Remdesivir.
Yield: 115 g; HPLC purity: 99.2%; Nitrosamine content: ND (not detected)

Example 10: Preparation of Pure Remdesivir
A solution of crude Remdesivir (100 g) in a mixture of IPA-water (3:1, 400 mL) was treated with activated charcoal for 30 min at 50°C and the resulting solution after filtration was diluted with water. The precipitated solid was filtered, washed with water and finally dried to yield pure Remdesivir.
Yield: 96 g; HPLC purity: 99.6%

Example 11: Preparation of Pure Remdesivir
A solution of crude Remdesivir (100 g) in a mixture of acetone-water (3:1, 400 mL) was treated with activated charcoal for 30 min at 50°C and the resulting solution after filtration was diluted with water. The precipitated solid was filtered, washed with water and finally dried to yield the desired pure Remdesivir.
Yield: 80 g; HPLC purity: 99.5%

Example 12: Preparation of Pure Remdesivir
A solution of crude Remdesivir (100 g) in a mixture of acetonitrile-water (3:2, 400 mL) was treated with activated charcoal for 30 min at 50°C and the resulting solution after filtration was diluted with water. The precipitated solid was filtered, washed with water and finally dried to yield the desired pure Remdesivir.

WE CLAIM:

1. Remdesivir substantially free of nitrosamines, wherein nitrosamine is selected from the group comprising of N-Nitrosodimethylamine, N-Nitrosodiethylamine, N-Nitrosodiisopropylamine (NDIPA/DIPNA), N-Nitrosoethylisopropylamine (EIPNA), N-Nitrosomethylethylamine (NMEA), N-Nitrosodipropylamine (NPDA), N-Nitrosodibutylamine (NBDA), N-Nitrosomethyldodecylamine, N-Nitroso-N-methyl-N-tetradecylamine, N-Nitroso-N-methyl-4-fluoroaniline, N-Nitroso-N-methyl-N-(2-phenyl) ethylamine, N-[methyl(nitroso)amino]butanoic acid (NMBA).

2. A process for preparing Remdesivir substantially free of nitrosamines comprising the steps of:
(a) coupling the compound of Formula VII with compound of Formula VIII in presence of an inorganic base to obtain compound of the Formula IX; and

(b) converting the compound of Formula IX to Remdesivir.

3. The process as claimed in claim 2 wherein inorganic base selected from group comprising of alkali or alkaline earth metal hydroxide, carbonate or bicarbonate.

4. A process for the preparation of Remdesivir comprising the steps of:
(a) coupling compound of Formula II with halopyrazole compound of Formula III to obtain compound of Formula IV with or without its isolation;

wherein Y is halogen selected from the group comprising of Cl, Br or I.
R1, R2 and R3 is independently a hydroxy protecting group, alternatively, R2 and R3 groups on adjacent carbons can be combined to form a -C(R’)- group, and R' is H, C1 -C6 alkyl, phenyl or substituted phenyl.
R4 and R5 are both independently H or N-protecting group;

(b) optionally, subjecting the compound of Formula IV (when both R4 and R5 are H) or reaction mixture of step (a) with an acid to obtain acid addition salt of compound of Formula IV;

wherein A is an organic acid selected from the group comprising of oxalic acid, tartaric acid, maleic acid, succinic acid, acetic acid, triflic acid, benzoic acid, trifluoroacetic acid, fumaric acid, gluconic acid, citric acid, malic acid, ascorbic acid, benzoic acid, isethionic acid, lactobionic acid, tannic acid, palmitic acid, alginic acid, polyglutamic acid, methanesulfonic acid, p-toluenesulfonic acid, naphthalenesulfonic acid, naphthalenedisulfonic acid, benzenesulfonic acid, ethanesulfonic acid, polygalacturonic acid, malonic acid, sulfosalicylic acid, glycolic acid, 2-hydroxy-3-naphthoate, pamoate, salicylic acid, stearic acid, phthalic acid, mandelic acid, lactic acid, lysine, arginine, glutamic acid, glycine, serine, threonine, alanine, isoleucine, leucine and the like.

(c) converting compound of Formula IV or its salt to compound of the Formula V;

(d) converting the compound of Formula V to compound of the Formula VI;

(e) converting the compound of Formula VI to compound of the Formula VII;

(f) coupling the compound of Formula VII with compound of Formula VIII in presence of inorganic base to obtain compound of the Formula IX; and

(g) converting the compound of Formula IX to Remdesivir.

5. The process as claimed in claim 4, wherein N-protecting group is tert-butyloxycarbonyl group.

6. The process as claimed in claim 4, wherein Grignard reagent in step (a) is selected from the group comprising of CH3MgBr, iPrMgCl, iPrMgCl.LiCl, tBuMgCl, PhMgCl and mixtures thereof.

7. The process as claimed in claim 4, wherein reagent in step (c) is selected from the group comprising of trimethylsilyl cyanide (TMSCN), p-toluenesulfonylmethyl isocyanide (TosMIC), acetone cyanohydrin, diethyl cyanophosphonate, trimethylsilyl)trifluoromethylsulfonate (TMSOTf) and trimethylsilyl chloride.

8. The process as claimed in claim 4, wherein step (d) is carried out in presence of an organic solvent selected from the group comprising of dichloromethane (DCM), chloroform, dichloroethane, chlorobenzene, methanol, ethanol, n-propanol, isopropanol, n-butanol and mixtures thereof.

9. The process as claimed in claim 4, wherein step (e) is carried out in presence of an organic solvent selected from the group comprising acetone, butanone, methyl ethyl ketone, methyl isobutyl ketone, methyl amyl ketone, di-isobutyl ketone, cyclohexanone methanol, ethanol, n-propanol, isopropanol or n-butanol.

10. The process as claimed in claim 4, wherein inorganic base in step (f) is selected from group comprising of alkali or alkaline earth metal hydroxide, carbonate or bicarbonate.