This application claims priority to Indian patent application No. 1624/CHE/2013 filed on April 10, 2013 and 2462/CHE/2013 filed on June 05,2013 the contents of which are incorporated by reference in their entirety.
FIELD OF THE INVENTION
The present invention relates to an improved process for the preparation of pure mono protected L-valine ester of ganciclovir of formula (A) and its further conversion to Valganciclovir Hydrochloride.
Further, the present invention relates to an improved process for the preparation of pure Valganciclovir hydrochloride having less content of unreacted guanine and ganciclovir.
BACKGROUND OF THE INVENTION
Valcyte (Valganciclovir HCI tablets) contains Valganciclovir hydrochloride, a hydrochloride salt of the L-valyl ester of ganciclovir that exist as a mixture of two diastereomers. Ganciclovir is a synthetic guanine derivative active against cytomegalovirus (CMV).Valganciclovir hydrochloride is a white to off-white crystalline powder with a molecular formula of C14H22N6O5.HCI and a molecular weight of 390. 83. The chemical name for Valganciclovir hydrochloride is L-valine, 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)-methoxy-3-hydroxy-1-propanyl ester monohydrochloride. Valganciclovir hydrochloride is a polar hydrophilic compound with solubility of 70 mg/ml in water at 25°C at a pH 7.0 and an n-octanol/water partition coefficient of 0.0095 at pH 7.0. The pKa for Valganciclovir hydrochloride is 7.6. The chemical structure of Valganciclovir hydrochloride having the formula (1)
There is a need for an improved process for the preparation of pure mono protected L-valine ester of ganciclovir from Ganciclovir which is then deprotected to obtain Valganciclovir Hydrochloride. Further there is a need to develop an improved process for preparation of pure Valganciclovir hydrochloride having less contamination of unreacted guanine and ganciclovir compounds.

OBJECT AND SUMMARY OF THE INVENTION
The present invention provides an improved process for the preparation of pure mono protected L-valine ester of ganciclovir of formula (A).
Another aspect of the present invention provides an improved process for the preparation of Valganciclovir hydrochloride comprising the steps of:
a) reacting ganciclovir, silylating agent and Cbz-L-valine in an organic solvent,
b) adding a base and a coupling agent, and
c) isolating pure mono protected L-valine ester of ganciclovir.
The present invention further relates to the conversion of mono protected L-valine ester of ganciclovir to Valganciclovir hydrochloride by deprotecting the protecting group and converting to the hydrochloride salt.
Yet another aspect of the present invention provided an improved process for the preparation of pure Valganciclovir hydrochloride having reduced quantities of starting materials.
Yet another aspect of present invention provided an improved process for the preparation of pure Valganciclovir hydrochloride having reduced quantities of unreacted guanine and ganciclovir comprising the steps of;
a) converting ganciclovir to N-benzyloxycarbonyl-L-valinate ester of ganciclovir
b) deprotection of N-benzyloxycarbonyl-L-valinate ester of ganciclovir by hydrogenation,
c) isolating Valganciclovir hydrochloride,
d) dissolving Valganciclovir hydrochloride in an organic solvent,
e) adding carbon, and
f) isolating pure Valganciclovir hydrochloride having less content of guanine and ganciclovir.
The process of the present invention is industrially viable for the production of mono protected L-valyl ester with high yield and purity.

DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improved process for the preparation of pure mono protected L-valine ester of ganciclovir by treating Ganciclovir with a silylating agent and Cbz-L-Valine in presence of an organic solvent, base and a coupling agent.
In one embodiment, the present invention relates to an improved process for the preparation of pure mono protected L-valine ester of ganciclovir as shown in scheme I below:
Scheme-I
In another embodiment, the present invention relates to an improved process for the preparation of pure mono protected L-valine ester of ganciclovir comprising the steps of:
a) reacting ganciclovir, silylating agent and Cbz-L-valine in presence of organic solvent,
b) adding a base and a coupling agent, and
c) isolating pure mono protected L-valine ester of ganciclovir.
According to the present invention, ganciclovir is dissolved in a solvent selected from water,
ketones such as acetone, methyl isobutyl ketone, methyl ethyl ketone and cyclohexanone; alcohols such
as methanol, ethanol, isopropanol, n-propanol, n- butanol, tertiary-butyl alcohol, cyclohexanol; chlorinated
solvents such as dichloromethane, chloroform, carbon tetrachloride; hydrocarbon solvents such as
toluene, xylene, n-hexane, n-heptane, cyclohexane; esters such as ethyl acetate, n-propyl acetate, n-butyl
acetate, t-butyl acetate; ethers such as diethyl ether, dimethyl ether, diisopropyl ether nitriles such as
acetonitrile, propionitrile; or polar aprotic solvents like dimethyl formamide, dimethyl sulfoxide and dioxane
or a mixture of thereof. Adding silylating agent selected from trimethylchlorosilane, N,0-
Bis(trimethylsilyl)acetamide, N,0-Bis(trimethylsilyl)trifluoroacetamide, Bromotrimethylsilane,

Heptamethyldisilazane, Hexamethyldisilane Hexamethyldisilazane, odotrimethylsilane, preferably trimethylchlorosilane and a base selected from imidazole, triethylamine, n-propyl amine, N-methyl imidazole, etc. and inorganic bases like sodium carbonate, potassium carbonate, sodium bicarbonate, sodium hydroxide, potassium hydroxide, preferably imidazole followed by addition of Cbz -L-valine and a coupling agent selecting from dicyclohexylcarbodiimide, 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide, Carbonyldiimidazole to afford mono protected L-valine ester of ganciclovir.
According to the present invention, after completion of the reaction confirmed by TLC, the reaction mass is added to an organic solvent selected from chlorinated solvents such as dichloromethane, chloroform, carbon tetrachloride and dil HCI or a mixture thereof followed by separation of aqueous layer and organic layers. Upon adjusting the pH of aqueous layer with ammonium solution a solid is obtained. The organic layer and solid obtained may be dissolved in a second solvent selected from alcohols such as methanol, ethanol, isopropanol, n-propanol, n- butanol, tertiary-butyl alcohol or a mixture thereof. Upon adjusting the pH of the reaction mass with ammonia solution-the desired compound is obtained.
In yet another embodiment, the present invention relates to pure mono protected L-valine ester of ganciclovir having purity more than 98.5% by HPLC.
In yet another embodiment, the present invention relates to an improved process for the preparation of Valganciclovir and its pharmaceutically acceptable salts comprising the steps of:
a) reacting ganciclovir, silylating agent and Cbz-L-Valine in an organic solvent,
b) adding a base and a coupling agent,
c) isolating pure mono protected L-valine ester of ganciclovir,
d) deprotecting the pure mono protected L-valine ester of ganciclovir, and
e) isolating pure valganciclovir or its pharmaceutically acceptable salts.
According to the present invention, deprotection is carried out in a hydrogenation reactor using a catalyst selected from palladium, platinum, and Raney nickel. In one embodiment, the catalyst is palladium on charcoal in presence of an acid selected from hydrochloric acid, sulphuric acid, phosphoric, maleic, fumaric, citric, and tartaric acid. In one embodiment, the acid is hydrochloric acid. The hydrogenation is carried out under hydrogen pressure thus isolating pure Valganciclovir or its pharmaceutically acceptable salts.
In yet another embodiment, the present invention relates to an improved process for the preparation of Valganciclovir and its pharmaceutically acceptable salts using the pure mono protected L-valine ester of ganciclovir as shown in scheme-ll.

HA: salt selected from HC1, HBr, H2S04 etc
Yet another embodiment of the present invention relates to an improved process for the preparation of Valganciclovir hydrochloride having reduced quantities of unreacted guanine and ganciclovir comprising the steps of;
a) converting ganciclovir to N-benzyloxycarbonyl-L-valinate ester of ganciclovir
b) deprotection of N-benzyloxycarbonyl-L-valinate ester of ganciclovir by hydrogenation
c) isolating Valganciclovir hydrochloride,
d) dissolving Valganciclovir hydrochloride in an organic solvent,
e) adding carbon, and
f) isolating pure Valganciclovir hydrochloride having reduced quantities of unreacted guanine and ganciclovir.
According to the present invention, the amino group of Ganciclovir is protected with a silylated ganciclovir intermediate, which is further reacted with Cbz-L-valine in an organic solvent to give N-benzyloxycarbonyl-L-valinate ester of ganciclovir. It is subjected to hydrogenation reaction under hydrogen pressure in alcoholic solvent selected from methanol, ethanol, n-propanol, isopropanol, n-butanol. The resultant solution is given carbon treatment. Removing the solvent obtains Valganciclovir hydrochloride having less content of guanine and ganciclovir.

According to the present invention, the unreacted guanine and ganciclovir are removed to the highest possible extent by using the carbon treatment. The below [table 1] furnishes the quantitative information of the guanine and ganciclovir quantities present before and after the carbon treatment. This clearly indicates that, substantial amounts of unreacted guanine and ganciclovir are moved by giving carbon treatment, from one to five times, thus increasing the purity of the final products viz., amorphous valganciclovir hydrochloride and crystalline valganciclovir hydrochloride.
Table-1
Content of Ganciclovir in Valganciclovir Amorphous

According to the present invention, the isolated pure valganciclovir or its pharmaceutical acceptable salt obtained is pure valganciclovir hydrochloride having a purity more than 99.0% by HPLC.
Advantage(s):
The present process improves the purity of the final product by substantial reduction in the unreacted
ganciclovir [to an extent of 25-30%] and guanine [to an extent of 50 -60%] without making any impact on
yield.
The process of the present invention is industrially viable for the production of mono protected L-valyl ester
with high yield and purity.
The following non-limiting examples illustrate specific embodiments of the present invention. They should
not be construed-as limiting the scope of present invention in any way.
Examples: Example 1
N,N-dimethylformamide (500 mL) and Ganciclovir (100g) was charged into a clean reactor at 25-35°C under nitrogen atmosphere. The reaction mass was cooled to 0-5°C, and trimethylchlorosilane (38.4gm, 0.35 moles) was added at a rate so that the temperature swhould not exceed more than 0°-5°C. The reaction mass was stirred untill clear solution was obtained. -Imidazole (37.4gm, 0.55 moles) was added under nitrogen atmosphere. Cbz-L-valine (216.5g, 0.86 moles) was added to the reaction mass and followed by the addition of dicyclohexylcarbodiimide (73.2g, 0.35moles). The mass was stirred, and-the temperature was gradually raised to 55-65°C. The reaction mass was maintained at 6-7hrs at 55° to 65°C. After completion of the reaction, the reaction mass was cooled to room temperature and dichloromethane (1100ml), dil. hydrochloric acid (700 ml) were added slowly. The reaction mass was allowed to settle for 30 minutes; the layers were allowed to separate. The aqueous layer pHwas adjusted to1.5 -2.5 with ammonia solution. The resultant precipitate was collected as a solid, and the wet solid was washed with water (500 mL).
In another reactor the organic layer was collected, and diluted hydrochloride was charged and stirred for 20-30 minutes at room temperature. The above obtained solid was dissolved in diluted hydrochloride solution and methanol. This solution was added to the well stirred solution of the collected organic layer solution.The reaction mass was stirred for 30 minutes and the separated both organic and aqueous layers. The organic layer was re-extracted with dilute hydrochloric acid and methanolic solution.AII aqueous methanol solution layers were combined, The reaction was stirred, then cooled to 10-15°C and the pH adjusted to 5 to 6.5 with ammonia solution at a temperature of below 10°C. The reaction mass was stirred for 1-2 hr at 10°-30°C. The reaction mass was fed into a centrifuged and allowed for spin dry-at 15-30°C. The wet cake was washed with DM water and spin dried for 30-40 minutes. The purity by HPLC was greater than ->98.5 %, and the yield was 60.0 g.

Example 2
Methanol(1000 mL) and DM water(450 mL) were charged into a clean reactor at 25-35°C and cooled to 0°C to 15°C. Hydrochloric acid (30 mL) was added slowly, followed by mono protected L-valine ester of ganciclovir (100 g) and methanol (300 mL). The temperature of the reaction mass was raised to 15-30°C and stirred for 30-60 minutes to obtain a clear solution. -10% palladium carbon (5 g) was charged into the reaction mass at 25-35°C and hydrogen gas is purged over a period of 2-6 hours. After completion of reaction, the mass was filtered.-Methanol was charged into the reaction mass at 25-30°C, and methanol was distilled off under vacuum at a temperature not more than 40°C. The reaction mass was cooled below 30°C and vacuum was released under nitrogen. The reaction mass was degassed for 2-8 hours under vacuum at below 30°C to get a solid. DM water was added to the reaction mass at 20-30°C and stirred for 30-45 minutes to get a clear solution. The reaction mass was filtered. DM water and Isopropyl alcohol were charged to the reaction mass at 25-35°C over a period of 60-90 minutes and stirred for 30-45 minutes. The temperature of the reaction mass was cooled to 0-5°C and stirred for 30-60 minutes. Isopropyl alcohol was charged to the reaction mass at 0-10°C over a period of 30-60 minutes and stirred for 4-5 hours. The reaction mass was fed into centrifuge at 0-10°C and stirred for 30-60 minutes. The wet cake was washed with chilled DM water and Isopropyl alcohol at 0-10°C. The wet cake was washed with chilled isopropyl alcohol. The purity by MPLC was greater than 98.5 %, and the yield was 50.0 g
Example 3
N,N-dimethylformamide (500ml) and Ganciclovir (100g) were charged into a clean reactor at 25-35°C under nitrogen atmosphere, and the reaction was cooled mass to 0-5°C. Trimethylchlorosilane (38.4gm, 0.35 moles) was added at such a rate so that the temperature would not exceed more than 0°-5°C. The reaction mass was stirred untill a clear solution was obtained. Imidazole (37.4gm, 0.55 moles) was added under nitrogen atmosphere.Cbz-L-valine (216.5g, 0.86 moles) was added to the reaction mass, followed by dicyclohexylcarbodiimide (73.2g, 0.35moles). The mass was stirred and the temperature was gradually raised to 55-65°C, and the reaction mass was maintained for 6-7hrs at 55° to 65°C. After completion of the reaction, the reaction mass was cooled to room temperature and dichloromethane (1100 mL), and dilute, hydrochloric acid (700 mL) were added slowly. The reaction mass was allowed to settle for 30 minutes, and the layers were allowed to separate. The pH of the aqueous layer-was adjusted to1.5 -2.5 with ammonia solution. The resultant precipitate was collected as a solid, and the wet solid was washed with water (500ml).
In another reactor the organic layer was collected and diluted hydrochloride was charged and stirred for 20-30 minutes at room temperature. The above obtained solid was dissolved in diluted hydrochloride solution and methanol. This solution was added to the well stirred solution of the collected organic layer solution. The reaction mass was stirred for 30 minutes and the separated both organic and aqueous layers. The organic layer was re-extracted with dilute Hydrochloric acid and methanolic solution. All aqueous

methanol solution layers were combined, The reaction mass was stirred and cooled to 10-15°C and the pH was adjusted the-to 5 to 6.5 with ammonia solution at a temperature ef below 10°C. The reaction mass was stirred for 1-2 hr at 10°-30°C. The reaction mass was fed into a centrifuged and allowed for spin dry at 15-30°C. The wet cake was washed with DM water and spin dried for 30-40 minutes. The purity by HPLC was greater than-98.5 %, and the yield was 60.0 g. Example 4
Methanol (1000 mL) and DM water(450 mL) was charged into a clean reactor at 25-35°C, and cooled to 0°C to 15°C. Hydrochloric acid (30 mL) was added slowly, followed by mono protected L-valine ester of ganciclovir (100 g) and methanol (300 ml). The temperature of the reaction mass was raised to 15-30°C and stirred for 30-60 minutes to get a clear solution. 10% palladium carbon (5 g) was charged into the reaction mass at 25-35°C and hydrogen gas was purged over a period of 2-6 hours. After completion of reaction, the mass was filtered. Methanol was charged into the reaction mass at 25-30°C, and methanol was distilled off under vacuum at a temperature not more than 40°C. The reaction mass was cooled to below 30°C and vacuum was released under nitrogen. The reaction mass was degassed for 2-8 hours under vacuum at below 30°C to obtain a solid. DM water was charged into the reaction mass at 20-30°C and stirred for 30-45 minutes to get a clear solution. The reaction mass was filtered. DM water and Isopropyl alcohol were charged to the reaction mass at 25-35°C over a period of 60-90 minutes and stirred for 30-45 minutes. The temperature of the reaction mass was cooled to 0-5°C and stirred for 30-60 minutes. Isopropyl alcohol was charged to the reaction mass at 0-10°C over a period of 30-60 minutes and stirred for 4-5 hours. The reaction mass was fed into a centrifuge at 0-10°C and stirred for 30-60 minutes. The wet cake was washed with chilled DM water and isopropyl alcohol at 0-10°C. The wet cake was washed with chilled isopropyl alcohol. The purity by HPLC was greater than 98.5 % and the yield was 50.0 g
Example 5
Crystalline Valganciclovir hydrochloride (100g) was dissolved in methanol in a clean reactor at 25-35°C and stirred for 30-60 minutes to get a clear solution. Activated carbon (BW-280) was charged to the reaction mass at 25-35°C and stirred over a period of 1-2 hours. The reaction mass was filtered through a hyflow bed. Methanol was charged to the reaction mass at 25-30°C and solvent was distilled off under vacuum at temperature not more than 40°C. The reaction mass was degassed for 1-2 hours under vacuum at below 30°C to get solid and the vacuum was released under nitrogen.
The product is dried under vacuum to obtain Crystalline Valganciclovir hydrochloride is obtained having less content of Ganciclovir.
Example 6
Amorphous Valganciclovir hydrochloride (100g) was dissolved in methanol in a clean reactor at 25-35°C, stirred for 30-60 minutes to get clear solution. Activated carbon (BW-280) was charged to the reaction mass at 25-35°C and stirred over a period of 1-2 hours. The reaction mass was filtered through a hyflow bed.

Methanol was charged to the reaction mass at 25-30°C and solvent was distilled off under vacuum at temperature not more than 40°C. The reaction mass was degassed for 1-2 hours under vacuum at below 30°C to get solid and the vacuum was released under nitrogen.
The product is dried at under vacuum, thus obtaining amorphous Valganciclovir hydrochloride having less content of Ganciclovir.
We claim:
1. An improved process for the preparation of pure mono L-valine ester of ganciclovir comprising the
Steps of:
a) reacting ganciclovir, a silylating agent and Cbz-L-valine in presence of an organic solvent;
b) adding a base and a coupling agent;
c) isolating pure mono protected L-valine ester of ganciclovir;
d) deprotecting the pure mono protected L-valine ester of ganciclovir;
e) optionally adding carbon; and
f) isolating pure valganciclovir or its pharmaceutically acceptable salts.

2. The process of claim 1, wherein a) the silylating agent is trimethylchlorosilane, b) the base is imidazole and the coupling agent is dicyclohexylcarbodiimide, and d) deprotection is carried out in a hydrogenation reactor using a 5% palladium on charcoal under hydrogen pressure.
3. The process of claim 1, wherein the organic solvent is selected from methanol, N,N-dimethylformamide, dichloromethane, and a mixture thereof
4. The process of claim 1, wherein isolating pure mono protected L-valine ester of ganciclovir results in mono protected L-valine ester of ganciclovir having a purity more than 98.5% by HPLC.
5. The process of claim 1, wherein isolating pure valganciclovir or its pharmaceutical acceptable salt results in valganciclovir or its pharmaceutical acceptable salt having a purity of more than 99% by HPLC.
6. An improved process for the preparation of Valganciclovir hydrochloride having lower content of guanine and ganciclovir comprising the steps of:

a) dissolving Valganciclovir hydrochloride in an organic solvent;
b) adding carbon; and
c) isolating Valganciclovir hydrochloride having less content of guanine and ganciclovir.
7. The process of claim 6, wherein the organic solvent is selected from methanol, ethanol, N,N-
dimethylformamide, dichloromethane, and a mixture thereof.