Claims:
1) A process for the preparation of a compound of formula (A) and hydrates thereof.

the process comprising ester hydrolysis of (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (B) in the presence of a mixture of acetic acid and concentrated sulfuric acid to provide (A) and hydrates thereof.

2) The process as claimed in claim 1, wherein the volume to volume ratio of acetic acid to (B) used in the ester hydrolysis is 1-3 : 3-1.

3) The process as claimed in claim 1, wherein the volume to volume ratio of acetic acid to (B) used in the ester hydrolysis is 1.5:1.

4) The process as claimed in claim 1, wherein the moles to moles ratio of sulfuric acid to (B) is 3-5 : 1.

5) The process as claimed in claim 1, wherein the moles to moles ratio of sulfuric acid to (B) is 4:1.

6) The process as claimed in claim 1, wherein the reaction is carried out at at a temperature ranging from about 80 oC to about120 °C.

7) The process as claimed in claim 1, wherein the reaction is carried out ata temperature ranging from about 100 oC to about105 °C .

8) The process as claimed in claim 1, wherein the hydrate is a monohydrate.

9) The process as claimed in claim 1, wherein (B) is prepared by reacting (R)-9-(2-hydroxy-propyl)adenine (C) with diethyltosyloxy methyl phosphonate (D) in the presence of a suitable complexing agent.

10) The process as claimed in claim 9, wherein the complexing agent is a mixture of a metal alkoxide and magnesium halide.

11) The process as claimed in claim 10, wherein the metal alkoxide is selected from the group consisting of sodium tert-butoxide, lithium tert-butoxide, potassium tert-butoxide and magnesium tert-butoxide.

12) The process as claimed in claim 10, wherein the magnesium halide is selected from the group consisting of magnesium chloride and magnesium bromide.

13) The process as claimed in claim 10, wherein the reaction is carried out in the presence of sodium tert-butoxide in combination with magnesium chloride.
14) The process as claimed in claim 13, wherein sodium tert-butoxide and magnesium chloride are used in aratio of 1 – 1.3 :1.

15) The process as claimed in claim 13, wherein sodium tert-butoxide and magnesium chloride are used in aratio of 1:1.

16) The process as claimed in claim 9, wherein the reaction is carried out in a solvent selected from the group consisting of dimethyl formamide, N-methyl pyrrolidone, tetrahydrofuran, tert-butanol, and n-butanol.

17) The process as claimed in claim 9, wherein the reaction is carried out at a temperature ranging from about 60 oC to about100°C.

18) The process as claimed in claim 9, wherein the reaction is carried out at a temperature ranging from about 70 oC to about 80°C.

19) The process as claimed in claim 9, wherein the isolation of (B) is carried out at a temperature ranging from about -8 oC to about -3°C.

20) The process as claimed in claim 9, wherein (C) is prepared by reacting adenine with (R)-propylene carbonate.
, Description:FORM 2
The Patent Act 1970
(39 of 1970)

&

The Patent Rules, 2003

COMPLETE SPECIFICATION
[See Section 10 and Rule 13]
PROCESS FOR PREPARATION OF (R)-([(ADENINE-9-YL)-PROPAN-2-OXY]-METHYL)-PHOSPHONIC ACID AND HYDRATES THEREOF;

AARTI INDUSTRIES LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, HAVING ADDRESS, 71, UDYOG KSHETRA, 2ND FLOOR, MULUND GOREGAON LINK ROAD, MULUND (W), MUMBAI - 400080, MAHARASHTRA, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

Field of the Invention
The present disclosure relates to (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid and more particularly an improved process for preparation of (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid and hydrates thereof.

Background and prior art
(R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid of formula (A), also known as PMPA or Tenofovir is the pharmaceutically active form of the prodrug Tenofovir diisoproxil fumarate.
Tenofovir diisoproxil fumarate is used as an anti-retroviral medication. It is used to prevent and treat HIV/AIDS and to treat chronic Hepatitis B. The structure of Tenofovir diisoproxil fumarate of formula (A’) is as follows:

Certain processes for preparation of compound (A) and (A’) are disclosed in US 5,922,695. The process disclosed in in US 5,922,695 involves reaction of adenine with (R)-1,2-propylene carbonate to form (R)-9-[2-hydroxy-propyl]adenine. (R)-9-[2-hydroxy-propyl]adenine is treated with Lithium-t-butoxide in tetrahydrofuran to form Lithium salt of (R)-9-[2-hydroxy-propyl]adenine. The slurry is then treated with diethyl-p-toluenesulfonyloxymethylphosphonate to form (R)-9-[2-diethylphosphonomethoxy)propyl] adenine. De-esterification of ethyl groups is carried out using bromotrimethyl silane in acetonitrile.
However the yields obtained are low and additional purification and crystallization steps are needed to remove unwanted impurities and solvents. Further, the process in US 5,922,695 involves the use of reagents like bromotrimethylsilane, which is highly expensive and corrosive and hence requires special handling procedures.
Various other processes are known in art for the preparation of an intermediate of formula (A). EP 2046792 B1 and Indian patent application IN 1218/CHE/2006 cover a process for the preparation of (R)-9-[2-phosphonomethoxy)propyl]adenine comprising reacting (R)-9-[2-(hydroxyl)propyl]adenine with diethyl p-toluene sulfonyloxy methylphosphonate in presence of magnesium tert-butoxide to get (R)-9-[2-(Diethyl phosphonomethoxy)propyl] adenine. Ester hydrolysis of (R)-9-[2-(Diethyl phosphonomethoxy)propyl]adenine with a suitable ester hydrolysis reagent yields (R)-9-[2-(phosphonomethoxy) propyl] adenine wherein the ester hyrolysing agent is aqueous hydrobromic acid, aqueous hydrochloric acid, HBr in acetic acid or HCl gas in isopropyl alcohol. However it was observed that during ester hydrolysis, the amino group in the 6-position of the purine ring gets cleaved and is replaced by a hydroxy group. This reaction leads to the generation of an impurity I shown below. The impurity I exists as keto or enol isomers and is chemically known as [(1R)-2-(6-hydroxypurin-9-yl)-1-methyl-ethoxy]methylphosphonic acid (enol form) or [(1R)-1-methyl-2-(6-oxo-1H-purin-9-yl)ethoxy]methylphosphonic acid (keto form).

The amount of impurity I formed is 4-5% and removal of this impurity from the final compound is difficult which results in lower yield of the final compound.
PCT application WO 2013072745 A1 and its Indian equivalent IN 3930/CHE/2011 disclose ester hydrolysis of (R)-9-[2-(di-Alk-phosphonomethoxy) propyl] adenine with a complex of amide and an acid, a complex of aluminum salt and an amide-acid reagent, and a complex of aluminum salt and an amine. HPLC purity of the compound obtained by the method is 98.5-99.5%, however the yield obtained is low i.e. around 50-60%, hence the process disclosed in PCT application WO 2013072745 is not industrially cost effective.
CN104262397 claims preparation of high purity Tenofovir comprising mixing adenine and (R)-propylene carbonate in a solvent under nitrogen, heating the reaction mixture in the presence of an alkali reagent to obtain (R)-9-(2-hydroxypropyl)adenine. The (R)-9-(2-hydroxypropyl)adenine is then treated with alkali (magnesium tert-butoxide, sodium isopropoxide, etc.) and di-ethyl tosylmethyl phosphonate in solvent, and the reaction is heated to obtain (R)-9-(2-(diethylphosphoryl) methoxypropyl)adenine. The diethyl ester is hydrolyzed with hydrobromic acid to obtain tenofovir.
However Tenofovir obtained in the reaction sequence described in CN104262397 requires acid purification preferably with an acid solution of hydrochloric acid and sulfuric acid and the yield obtained is low, about 55-60%.
There is a need for simple, efficient, industrially viable and cost effective methods for the preparation of (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid and hydrates thereof which overcome some drawbacks of previously known processes.
Summary of the invention
Provided herein is an improved process for synthesis of (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid and hydrates thereof, which avoids extensive purifications and work ups of previously described processes. Advantageously the process described herein avoids formation of impurity I, shown above, and other by-products, and provides (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid and hydrates thereof in higher yield and better purity.
An aspect of the process described herein is to provide an improved process for preparing (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid of formula (A) and hydrates thereof. The process comprises reaction of (R)-9-(2-hydroxy-propyl)adenine (B) with diethyltosyloxymethyl phosphonate in the presence of sodium tert-butoxide and magnesium chloride to yield diethyl phosphonate (B). The ester is then hydrolysed using a mixture of concentrated sulfuric acid and acetic acid to form the compound of formula (A).
The compound of formula (A) prepared by the process described herein is converted to Tenofovir di-isoproxyl fumarate.
Detailed description of the invention
In an embodiment of provided herein is a process to prepare (R)-([(adenine-9-yl)-propan-2-oxy]-methyl)-phosphonic acid of formula (A) and hydrates thereof as described hereinafter.
As used herein, “hydrates” includes monohydrates, dihydrates, semi-hydrates, and other similar forms.
The said process comprises ester hydrolysis of phosphonate ester (B) to provide the compound of formula (A), and hydrates thereof. In an embodiment, the compound of formula (A) is obtained as a monohydrate.
The schematic representation of the process is represented as follows-

Phosphonate ester (B) is treated with mixture of concentrated sulfuric acid and acetic acid. 1-3 volume of acetic acid is used and 3-5 moles of sulfuric acid is used with respect to one mole of diethyl phosphonate ester (B). Preferably 4 moles of sulfuric acid and 1.5 mol of acetic acid are used in the reaction. In a specific embodiment, the acetic acid is glacial acetic acid. As used herein, “glacial acetic acid” refers to > 99% acetic acid, or substantially anhydrous acetic acid. As used herein, “concentrated sulfuric acid” refers to = 98% sulfuric acid.
The reaction is carried out at 80-120°C, preferably at 100-105°C.
The phosphonate ester (B) used in the present process is prepared by reacting (R)-9-(2-hydroxy-propyl)adenine with diethyltosyloxymethyl phosphonate in the presence of a suitable complexing agent. The complexing agent is mixture of metal alkoxides and magnesium halide. The metal alkoxide is selected from the group consisting of sodium tert-butoxide, lithium tert-butoxide, potassium tert-butoxide and magnesium tert-butoxide. The magnesium halide is selected from the group consisting of magnesium chloride and magnesium bromide. The preferred complexing agent is a mixture of sodium-tert-butoxide and magnesium chloride.
Without being bound by any theory, it is believed that the reaction proceeds through the formation of a complex due to coordination of divalent magnesium cation, phosphonate oxygen and oxygen in the tosyl group of diethyltosyloxymethyl phosphonate. The complex formation is shown below.

The complex formed promotes SN2 type displacement with (R)-9-(2-hydroxy-propyl)adenine.
(R)-9-(2-hydroxy-propyl)adenine is added to mixture of sodium tert-butoxide and magnesium chloride. The ratio of sodium tert-butoxide and magnesium chloride used is 1 – 1.3 : 1, preferably 1:1.
The reaction is carried out at 60-100°C, preferably at 70-80°C. The solvent is selected from the group consisting of dimethyl formamide, N-methyl pyrrolidone, tetrahydrofuran, tert-butanol, n-butanol, preferably dimethyl formamide.
After completion of the reaction, the reaction mass was cooled gradually to room temperature and further chilled to -8 to -3°C. Advantageously, it was found that when isolation of phosphonate ester (B) is carried out at about -8 to about-3°C, ester hydrolysis is minimal, or avoided, and results in improvement of the yield and purity.
The phosphonate ester (B) formed is isolated as a white solid or is optionally hydrolysed in the same pot without isolation. The phosphonate ester (B) is subjected to ester hydrolysis using a mixture of acetic acid and sulfuric acid, instead of reagents like trimethylbromosilane, which simplifies the process while avoiding extensive purification steps and work up. Surprisingly, the use of a mixture of concentrated sulfuric acid and glacial acetic acid for ester hydrolysis led to a cleaner reaction compared to previously reported ester hydrolyses using aqueous mineral acids.
(R)-9-(2-hydroxy-propyl)adenine used in the process described herein is prepared by reacting adenine with (R)-propylene carbonate.

The preparation of (R)-9-(2-hydroxy-propyl)adenine is carried out in presence of an alkali selected from the group consisting of sodium hydroxide, potassium hydroxide, potassium carbonate, cesium carbonate, sodium methoxide and sodium ethoxide. The reaction is carried out in a solvent selected from the group consisting of dimethyl formamide, N-methyl pyrollidone, Dimethyl sulfoxide, t-butanol and xylene, preferably dimethyl formamide.
EXAMPLES
The invention can be specifically described further with the following non-limiting examples.

Example 1
Preparation of (R)-9-(2-hydroxy-propyl)adenine (C)
Adenine (100 g) was added to dimethyl formamide (500 ml) at 25-30°C under inert atmosphere. The reaction mixture was stirred and sodium hydroxide (2.95 g) was added at 25-30°C. The mixture was stirred for 10 minutes to form a thick slurry. (R)-propylene carbonate (90 g) was added in 10-20 minutes at 25-30°C. The reaction mass was heated to 122-128°C and maintained under inert atmosphere for 10-12 hours. The reaction mass was cooled to 80-85°C and dimethyl formamide was distilled under vacuum.
Strippings of toluene (100 ml x 2 times) were conducted below 90°C to azeotropically distill out traces of dimethyl formamide. Toluene (300 ml) was added to the residue at 80-85°C. The mixture was cooled gradually to 25-30°C and stirred for 1 hour. The solid obtained was filtered, dried under suction and washed again with toluene (100 ml). The solid was further dried under vacuum at 65-70°C to yield (R)-9-(2-hydroxy-propyl)adenine (130 g).
Yield: 90.93%
HPLC purity: 90%
Chiral Purity: 99.5%

Example 2
Preparation of (R)-9-(2-hydroxy-propyl)adenine (C)
Under inert atmosphere dimethyl formamide (5 L) was added to adenine (1 kg) at 25-30°C. The reaction mixture was stirred, sodium hydroxide (29.5 g) was added at 25-30°C and stirred for 10 minutes to form thick slurry. To the reaction mass was added (R)-propylene carbonate (900 g) in 10-20 minutes at 25-30°C. The reaction mass was heated to 122-128°C and maintained under inert atmosphere for 10-12 hours. The reaction mass was cooled to 80-85°C and dimethyl formamide was distilled under vacuum.
Strippings of Toluene (1 L) were conducted twice below 90°C to azeotropically distill out traces of dimethyl formamide. Toluene (3 L) was added to the residue at 80-85°C. The mixture was cooled gradually to 25-30°C and stirred for 1 hour. The solid obtained was filtered, dried under suction and washed again with toluene (1 L). The solid was further dried under vacuum at 65-70°C to yield (R)-9-(2-hydroxy-propyl)adenine (1.32 kg).
Yield: 92.32%
HPLC purity: 90.5%
Chiral Purity: 99.6%

Example 3
Preparation of (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (B)
Sodium tert-butoxide (100 g) was added to dimethyl formamide (400 ml) at 25-30°C. To the reaction mass, magnesium chloride (50 g) was added in portions and the mixture was heated gradually to 75-80°C over 3 hours. (R)-9-(2-hydroxy-propyl)adenine (C) (100 g) obtained in example 1 was added at 75-80°C for one hour under inert atmosphere. To the slurry formed, diethyl tosyloxy methyl phosphonate (D) (200 g) was added drop wise over a period of 2 hours. The reaction mass was maintained at 75-80°C under inert atmosphere for 2 hours. The mass was cooled to 30°C and then to -8 to -3°C. Acetic acid (63 g) was added over 10-20 minutes and the reaction mass was stirred for 10 minutes at -8 to -3°C.
Dimethyl formamide was distilled out under vacuum below 60°C and then stripped out with toluene (100 x 2 times). Chloroform (500 ml) was added at 35-40°C, the mixture was stirred well and cooled to -8 to -3°C. Water (100 ml) chilled to 0-5°C was added and the mixture stirred at -5°C for 30 minutes. The salt was filtered, dried under suction and washed with water (100 ml).
The crude diethyl phosphonate salt obtained was slurried in chloroform (400 ml x 2). The chloroform layers were separated, combined and the aqueous layer was extracted again with chloroform (200 ml x 2 times) below 5°C. The combined chloroform layers were washed with chilled water (100 ml) at -8 to -3°C and stirred for 10 minutes. The chloroform layer was dried over sodium sulfate (20 g).
Chloroform was removed by distillation under reduced pressure at 70°C and the residue was stripped out with toluene (100 ml x 2 times). The oil obtained was degassed well to yield (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (200 g) (HPLC assay: 61.5%).
Yield: 81.45%
HPLC purity: 90.5%

Example 4
Preparation of (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (B)
At room temperature, to dimethyl formamide (4 L) was added sodium tert-butoxide (1 Kg). Magnesium chloride (500 gm) was then added in portions to the reaction mixture. The mass was heated gradually to 75-80°C for 3 hours and (R)-9-(2-hydroxy-propyl)adenine (C) (1 kg) obtained in example 3 was added at 75-80°C under inert atmosphere. Diethyl tosyloxy methyl phosphonate (D) (2 kg) was charged drop wise over a period of 2 hours to the slurry formed. Under inert atmosphere the reaction mass was maintained at 75-80°C for 2 hours. The mass was cooled to 30°C and then to -8 to -3°C. Acetic acid (630 g) was added in 10-20 minutes and the reaction mass was stirred for a further10 minutes at -8 to -3°C.
Dimethyl formamide was distilled out under vacuum below 60°C and then stripped out with toluene (1000 x 2 times). Chloroform (5 L) was added at 35-40°C, stirred well and cooled to -8 to -3°C. Chilled water (10 L) was added and stirred at -5°C for 30 minutes. The salt was filtered, dried under suction well and washed with water (100 ml).
The diethyl phosphonate salt obtained was slurried in chloroform (4 L x 2). The chloroform layers were separated and combined. The aqueous layer was extracted with chloroform (2 L x 2 times) below 5°C. The combined chloroform layers were washed with chilled water (1 L) at -8 to -3°C and stirred for 10 minutes. The chloroform layer was dried over sodium sulfate (200 g).
The chloroform was removed by distillation under reduced pressure at 70°C and the residue was stripped out with toluene (1 L x 2 times). The oil obtained was degassed well to yield (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (1.9 Kg) (HPLC assay: 65%).
Yield: 81.78%
HPLC purity: 91.5%
Example 5
Preparation of (R)-(((Adenine-9-yl)-propan-2-oxy)-methyl-phosphonic acid hydrate (A)
To (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (B) (100 g) (HPLC assay: 61.5%) as prepared in example 2 was added glacial acetic acid (90 ml) at 25-30°C. Concentrated sulfuric acid (68.4 g) was added drop wise to the reaction mixture. After addition of sulfuric acid the temperature was raised to 100-110°C and reaction mass was maintained for 12 hours. The reaction mass was cooled to 30-35°C and water (100 ml) was added. The mass was cooled to 0-5°C and sodium hydroxide (28 g) was added. Acetic acid was distilled under vacuum below 65-70°C and water (200 ml) was added and stirred well. Charcoal (10 g) was added and the mixture was stirred for 30 minutes at 30°C. The mixture was filtered and washed with water (100 ml) and cooled to 0-5°C. The aqueous layer was washed with chloroform (200 ml x 2 times). Both the layers were separated and the aqueous layer was cooled to 0-5°C and the pH was adjusted to 2 to 2.5 using 50% NaOH solution. The mixture was stirred at 0-5°C for 4 hours and the solid was filtered and dried under suction. Water (420 ml) was added to the solid obtained and heated to 90-95°C to obtain a clear solution, stirred for 30 minutes and cooled gradually to 30°C. The slurry was maintained at 25-30°C for 4-5 hours and the solid was filtered and dried under suction. Further it is dried under vacuum at 60-65°C to yield title compound (41 g).
Yield: 75%
HPLC purity: 99.85%
Chiral Purity: 99.8%
Specific Optical Rotation: 21.9° (1% solution in 0.1M HCl at 589 nm)

Example 6
Preparation of (R)-(((Adenine-9-yl)-propan-2-oxy)-methyl-phosphonic acid hydrate (A)
At 25-30°C, glacial acetic acid (1.852 L) was added to the (R)-9-(2-(diethyl phosphonomethoxy)propyl)adenine (B) (1.9 kg) (HPLC assay: 65%) as prepared in example 2. Concentrated sulfuric acid (1.407 L) was added drop wise to the reaction mixture. The temperature was raised to 100-110°C and reaction mass was maintained for 12 hours. The reaction mass was cooled to 30-35°C and water (1.9 L) was added. Sodium hydroxide (575.5 g) was added after cooling the mass to 0-5°C. Acetic acid was distilled under vacuum below 65-70°C and water (3.8 L) was added and stirred well. Charcoal (205.8 g) was added and stirred for 30 minutes at 30°C. The mixture was filtered, washed with water (2 L) and cooled to 0-5°C. The aqueous layer was washed with chloroform (4 L x 2 times). Both the layers were separated and aqueous layer was cooled to 0-5°C and the pH was adjusted to 2 to 2.5 using 50% NaOH solution. The mixture was stirred at 0-5°C for 4 hours and solid was filtered and dried under suction. To the solid obtained, water (8.4 L) was added. The mixture was heated to 90-95°C to obtain a clear solution, stirred for 30 minutes and cooled gradually to 30°C. The slurry was maintained at 25-30°C for 4-5 hours and solid was filtered and dried under suction. Further it is dried under vacuum at 60-65°C to yield (R)-((Adenine-9-yl)-propan-2-oxy)-methyl-phosphonic acid hydrate (700 g).
Yield: 75%
HPLC purity: 99.85%
Chiral Purity: 99.8%
Specific Optical Rotation: 22° (1% solution in 0.1M HCl at 589 nm)