FORM 2
THE PATENTS ACT 1970
(Act 39 of 1970)
&
THE PATENTS RULE, 2003
COMPLETE SPECIFICATION
(SECTION 10 and Rule 13)
"DEPROTECTION OF PHOSPHONATE ESTERS"
Emcure Pharmaceuticals Limited,
an Indian company, registered under the
Indian Company's Act 1957 and having its registered office at
Emcure House, T-184, M.I.D.C,
Bhosari, Pune-411026, India.
THE FOLLOWING SPECIFICATION DESCRIBES THE NATURE OF THE INVENTION

FIELD OF THE INVENTION
The invention relates to a process for the deprotection of esters by a simple, cost-effective process for the preparation of an antiviral.
BACKGROUND OF THE INVENTION
Tenofovir of formula (I) chemically known as [[(lR)-2-(6-amino-9H-purin-9-yl)-l-methylethoxy] methyl] phosphonic acid is an acyclic nucleoside phosphonate analog of adenosine-5'-phosphate utilized for the treatment of viral infections due to HIV reverse transcriptase. It is administered as the fumarate salt of bis-isopropoxycarbonylmethyl ester derivative of Tenofovir, which is marketed under the trade name Viread.

^T N OH
! OH
CH3
Tenofovir (I)
The preparation of tenofovir of formula (I) was first disclosed in US 5,922,695 (Scheme-I) comprising reaction of (R)-9-(2-hydroxypropyl)-adenine of formula (II) with dialkyl-p-toluenesulfonyloxymethylphosphonate of formula (III) in the presence of a base such as n-butyl lithium in DMF to give (R)-9-[2-(dialkylphosphonomethoxy)propyl]adenine of formula (IV). The compound of formula (IV) was then hydrolyzed with trimethylsilyl bromide in methyl cyanide as solvent to give tenofovir of formula (I).



NH2
W
OR TsO. J=0 n'BuLi
L^^QH OR DMF
CH,

(R)-9-(2-hydroxypropyl) Dialkyl-p-toluenesulfonyl- 9-[2(R)-(dialkyloxyphosphoryl
adenine (II) -oxymethylphosphonate (ID) methoxy)propyl]adenine (IV)
R = alkyl


(CH3)3SiBr/CH3CN
1
Hydrolysis
(Deprotection)
(I) Scheme-I: Method as disclosed in US 5,922,695 for the preparation of Tenofovir (I)
US 6,043,230 also discloses a method for the preparation of tenofovir of formula (I) by dealkylation of the phosphonate esters utilizing trimethylsilyl bromide in methyl cyanide as solvent. Trimethylsilyl bromide belonging to the class of trimethylsilyl halides is a very expensive reagent and increases the manufacturing cost of the life saving drug, which is abundantly required worldwide.
The utilization of trimethylsilyl halides requires anhydrous conditions, since these reagents are quite moisture sensitive and tend to decompose on contact with moisture. Isolation of the desired product requires careful work-up procedure to take care of the highly reactive nature of these trimethylsilyl halides with water.
Secondly, the use of methyl cyanide is extremely dangerous, certainly on an industrial scale.



(CH3)3SiBr/CH3CN
»■
Hydrolysis (Deprotection)
9-[2(R)-(alkoxyphosphoryl methoxy)propyl] adenine (IV) R = alkyl
Scheme-II: Method as disclosed in US 6,043,230 for the preparation of Tenofovir (I)

Further, the utilization of trimethylsilyl halide creates a heavy load on the effluent as the trialkyl silanol produced is difficult to decompose.
The use of trimethylsilyl halides for deprotection of phosphonate esters is also reported in US 4,808,716 (trimethylsilyl iodide), and Journal of American Chemical Society Volume No. 118(31), 1996, page 7421.
US 6,465,649 mentions that deprotection of phosphonate esters cannot be carried out with aqueous solutions of hydrohalic acids like hydrochloric acid and hydrobromic acid, due to the presence of acid labile functional groups like amino group as in the case of Tenofovir, which are not tolerant to harsh acidic conditions.
Nucleoside, Nucleotides & Nucleic acids 20(4-7), 1299-1302 (2001) discloses a method for the dealkylation of phosphonate esters with trimethylsilyl chloride.
It also mentions that trimethylsilyl chloride is less reactive and can be used only for the deprotection of the more labile dimethyl phosphonates. The deprotection of diethyl phosphonates requires long reaction times resulting in unsatisfactory yields. Utilization of trimethylsilyl chloride also requires the use of an activating agent like sodium or lithium iodide. However, the use of such salts leads to contamination of the resulting phosphonate product with lithium halide salts requiring further crystallization for removal of such salts.


Further, this reference also mentions that "Phosphonic acids are generated from their dialkyl esters by reaction with concentrated hydrochloric acid or hydrobromic acid, however the conditions are too harsh for many functional groups". They had carried out such reactions with concentrated hydrochloric acid and "observed 10-16% acid catalyzed deamination of adenine to hypoxanthine" without disclosing data relating to the same.

O
-N

= OH
CH3
(IV); R = alkyl Hypoxanthine impurity (V)
(10-16%)
Scheme-Ill: Disclosure in Nucleoside, Nucleotides & Nucleic acids 20(4-7), 1299-1302 (2001) regarding formation of tenofovir impurity of formula (V).
Removal of impurities in the range of 10-16% would require several steps of purification and is quite difficult and tedious on an industrial scale. Moreover, such removal of impurities in the final step results in significant loss in yield thereby making the process quite expensive.
It is now evident that prior art methods have largely relied on reagents belonging to the
class of trimethylsilyl halides, which are
i) quite expensive, therefore, increase the cost of the product,
ii) are moisture sensitive, therefore require stringent parameters like anhydrous
conditions,
iii) have low flash point, therefore are highly flammable,
iv) have effluent problems for disposal of waste like trialkyl silanol.
Additionally, use of methyl cyanide as solvent adds on to the risk.


In view of the high costs, stringent parameters and hazardous nature of trimethylsilyl halides, the present inventors have tried out various other reagents and methods. They have serendipitously found that dealkylation or deprotection of dialkyl phosphonates for preparation of Tenofovir of formula (I) can also be carried out with inorganic acids like hydrobromic acid without the formation of impurities like compound of formula (V).
The deprotection of dialkyl phosphonates using an inorganic acid like hydrobromic acid is potentially very useful in the large scale synthesis of phosphonic acid derivatives as a replacement for reagents like trimethylsilyl halides, which are expensive and more difficult to handle.
OBJECT OF THE INVENTION
It is an object of the invention to provide a simple, cost-effective process for deprotection of phosphonate esters of high purity without the formation of undesired impurities.
Another object of the invention is to provide a process for preparation of tenofovir of formula (I) in high yield and purity using an inorganic acid.
Yet another object of the invention is to utilize reagents for deprotection of phosphonate esters, which are less hazardous and safe to use on an industrial scale.
SUMMARY OF THE INVENTION
One aspect of the invention relates to a simple, cost-effective and non-hazardous process for the preparation of tenofovir of formula (I) in high yield and purity.
Another aspect of the invention relates to a cost-effective process for the preparation of Tenofovir of formula (I) comprising heating of (R)-9-[2-(dialkylphosphonomethoxy)propyl]adenine of formula (IV) in the presence of an


inorganic acid at a temperature between 60°-90°C and isolating compound of formula (I) in high yield and purity.
Advantages of the invention:
The instant invention overcomes the following shortcomings with respect to prior art,
i) avoids use of costly reagents like trimethylsilyl halides such as trimethylsilyl
chloride, trimethylsilyl bromide and trimethylsilyl iodide for deprotection of
the phosphonate esters for preparation of compound of formula (I) thereby
making the process economical and plant friendly,
ii) aqueous medium is used during the process of the invention and use of
stringent anhydrous conditions are avoided making the process simple and
cost effective;
iii) substitution of reagents like trimethylsilyl chloride having lower flash point
makes the process less hazardous and environment friendly,
iv) avoids load on the effluent treatment plant, and
v) yields tenofovir of formula (I) with acceptable pharmaceutical purity.
DETAILED DESCRIPTION OF THE INVENTION
The embodiment of the present invention is based on the utilization of an inorganic acid for the deprotection of phosphonate esters without the formation of the hypoxanthine impurity of formula (V) to give Tenofovir of formula (I) in high yield and purity.
The method of preparation of Tenofovir of formula (I) as per the present invention is summarized in Scheme-IV.


N

NH, 1 2
c> ^
N N ?^1
■ill 0V
CH3 NR

NH2

H
?:
-(X /P=0 OH
CH3

(R)-9-[2-(Dialkylphosphonomethoxy) Tenofovir (I)
propyl] adenine (IV); R = CMalkyl
Scheme - IV: Present invention for the preparation of Tenofovir (I).
An inorganic acid was added to (R)-9-[2-9(dialkylphosphonomethoxypropyl]adenine of formula (IV).
The inorganic acid was selected from the group comprising of hydrochloric acid, hydrobromic acid, hydroiodic acid, orthophosphoric acid, sulfuric acid etc.
Although all the inorganic acids helped in the desired deprotection of the phosphonate esters, hydrobromic acid (47 - 49 %) was preferred. The ease in carrying out the reaction, low amount of the acid required and the simplicity in isolation of the desired tenofovir of formula (I), are the main advantages.
The amount of the inorganic acid added was between 3.0 times and 20 times volume of acid per gram of compound of formula (IV).
The preferred volume of the acid added was between 6.0 times and 12.0 times volume of acid per gram of compound of formula (IV).
The reaction mass was heated to a temperature 60°C and 90°C.
The preferred temperature was between 82 ± 5°C.

The mixture was stirred till completion of reaction as monitored by HPLC.
The reaction mass was concentrated and the residue diluted with water.
The reaction mass was cooled to ambient temperature and optionally stirred with an organic solvent after which the aqueous layer was separated.
The pH of the aqueous layer was then adjusted at ambient temperature between 3.0 and 3.2.
The compound of formula (I) separating out was filtered, washed with water and dried.
The compound of formula (I) thus isolated was found to be devoid of impurities normally associated with utilization of an inorganic acid as a reagent for deprotection of phosphonate esters.
Tenofovir of formula (I) was optionally purified by adding the isolated tenofovir of formula (I) to water. The pH of the mixture was raised between 6.9 and 7.1 by addition of an alkali hydroxide solution like sodium hydroxide solution. The clear mixture after optional charcoal treatment was filtered and the pH of the filtrate was adjusted between 3.0 and 3.2 with an inorganic acid like hydrochloric acid. The resulting mixture was cooled, filtered and dried to give compound of formula (I) of high purity.
The invention can be further illustrated by the following examples, which however, should not be construed as limiting the scope of the invention.


EXAMPLE 1:
Preparation of Tenofovir (I) using hydrobromic acid.
Hydrogen bromide (48 %) (1750ml) was added to the flask containing (R)-9-[2-(diethylphosphonomethoxy) propyl] adenine (250 gms; 0.728moles). The reaction mass was heated to 75 to 85°C and stirred for 22 to 24 hrs, till completion of the reaction based on HPLC monitoring. The reaction mass was concentrated under reduced pressure. Water (1200ml) was added to the residue and cooled to 25 to 30°C. Optionally, dichloromethane (800 ml) was added to the reaction mixture, stirred, and the aqueous layer separated. The pH of the aqueous layer adjusted between 3.0 and 3.2 with 50% sodium hydroxide solution at ambient temperature. The reaction mass was stirred at 0 to 10°C and filtered. The wet cake was added to water (1750ml) and the pH adjusted between 6.9 and 7.1 by addition of sodium hydroxide solution (400ml). The clear solution was optionally stirred with activated carbon (12.5gms) and filtered. The pH of the filtrate was adjusted between 3.0 and 3.2 with hydrochloric acid (100ml) cooled between 5 and 15°C and the product separating out was filtered. The product of formula (I) was washed with acetone (175ml) and dried. Yield: 152.5 gms. Purity: 99%
EXAMPLE 2:
Preparation of Tenofovir (I) using hydrobromic acid.
Hydrobromic acid (48 %) (856ml) was added to the flask containing (R)-9-[2-(diethylphosphonomethoxy) propyl] adenine (50gms; 0.146moles). The reaction mass was heated to 60 to 65°C and stirred for 40 hours, till completion of the reaction. The reaction mass was concentrated under reduced pressure. Water (240ml) was added to the residue and cooled to 25 to 30°C. Optionally, dichloromethane (250 ml) was added to the reaction mixture, stirred, and the aqueous layer separated. The pH of the aqueous layer adjusted between 3.0 and 3.2 with 50% sodium hydroxide solution at ambient temperature. The reaction mass was stirred at 0 to 10°C and filtered. The wet cake was added to water (350ml) and the pH adjusted between 6.9 and 7.1 by addition of sodium hydroxide solution (40ml). The clear solution was optionally stirred with activated carbon


(12.5gms) and filtered. The pH of the filtrate was adjusted between 3.0 and 3.2 with hydrochloric acid (18.5ml) cooled between 5°C and 15°C and filtered. The product of formula (I) was washed with acetone (50ml) and dried. Yield: 30.6gms. Purity: 99.44%.
EXAMPLE 3
Preparation of Tenofovir (I) using hydrochloric acid.
Hydrochloric acid (48 %) (750ml) was added to the flask containing (R)-9-[2-(diethylphosphonomethoxy) propyl] adenine (50gms; 0.1456moles). The reaction mass was heated to 60 to 65 °C and stirred for 35 hours, till completion of the reaction. The reaction mass was concentrated under reduced pressure. Water (180 ml) was added to the residue and cooled to 25 to 30°C. Optionally, dichloromethane (170ml) was added to the reaction mixture, stirred, and the aqueous layer separated. The pH of the aqueous layer adjusted between 3.0 and 3.2 with 50% sodium hydroxide solution at ambient temperature. The reaction mass was stirred at 0 to 10°C, filtered, washed with acetone (50ml) and dried. Yield: 26gms. Purity: 97.33%.

.p=o
Claims:
1. A cost-effective process for the preparation of Tenofovir of formula (I) comprising heating of (R)-9-[2-(diethylphosphonomethoxy)propyl]adenine of formula (IV) in the presence of an inorganic acid at a temperature between 60°-90°C and isolating compound of formula (I) in high yield and purity. NH,
NH„

(R)-9-[2-(Dialkylphosphonomethoxy) propyl] adenine (IV);
C1-4 alkyl

Inorganic acid / 60-90°C

2. A process according to claim 1, wherein the inorganic acid is selected from the group selected from hydrobromic acid, hydroiodic acid and orthophosphoric acid.
3. A process according to claim 2, wherein the inorganic acid is preferably hydrobromic acid.
4. A process according to claim 1, wherein the temperature is preferably 82 ± 5°C.
5. A process according to claim 2 and 3, wherein the strength of hydrobromic acid is between 47% and 49%.
6. A process for preparing tenofovir of formula (I), substantially as herein described in the examples.