FIELD OF INVENTION
The present invention relates to an improved process for the preparation of pure 2-[(2-acetylamino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]-1 -benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of formula (III).

The compound of formula (III) is a key intermediate in the preparation of Valganciclovir hydrochloride of formula I and Ganciclovir of formula II.
BACKGROUND OF THE INVENTION
9-(2-Hydroxy-l-hydroxymethyl)ethoxy]methyl]guanine is generically known as Ganciclovir (II)].

Ganciclovir is an acyclic nucleoside analogue of 2'-deoxyguanosine, which inhibits replication of herpes viruses. Ganciclovir has been shown to be active against cytomegalovirus (CMV) and herpes simplex virus (HSV) in human. Ganciclovir is marketed under the name Cytovene®. It has been indicated for the treatment of CMV retinitis in immunocompromised patients, including patients with acquired

immunodeficiency syndrome (AIDS) and also indicated for the prevention of CMV disease in transplant recipients at risk for CMV disease.
Ganciclovir has a relatively low rate of absorption when administered orally and must be used at high dosages when administered by that route.
L-Valine, 2-[(2-amino-1,6-dihydro-6-oxo-9/f-purin-9-yl)methoxy]-3-hydroxypropyl ester monohydrochloride is generically known as Valganciclovir (I).

Valganciclovir (la) is a mono-L-valyl ester (prodrug) of the antiviral compound Ganciclovir (II), which exists as a mixture of two diastereomers. After the oral administration, both diastereomers are rapidly converted to Ganciclovir (II) by intestinal and hepatic esterases.
Valganciclovir hydrochloride (I) is marketed in the US under the name Valcyte®. It has been approved for the treatment of cytomegalovirus (CMV) retinitis in patients with acquired immunodeficiency syndrome (AIDS) and also indicated for the prevention of cytomegalovirus (CMV) disease in kidney, heart, and kidney-pancreas transplant patients at high risk (Donor CMV seropositive/Recipient CMV seronegative [(D+/R-)]).
Roche disclosed Ganciclovir (II) in US 4,355,032. US '032 also discloses a process for the preparation of Ganciclovir by reacting Guanine (IV) with acetic anhydride and recrystallized from DMSO to produce 9-acetyl-2-acetylamino-l,9-dihydro-6H-purin-6-one (IVa), which is further condensed with 2-0-acetoxymethyl-l,3-di-0-

benzylglycerol (V) in presence of bis(p-nitrophenyl)phosphate to produce a compound of formula (VI). Compound (VI) is hydrogenated by palladium hydroxide on carbon in presence of methanol and recrystallized from methanol/ethyl acetate to produce compound (VII), which is further treated with methanolic ammonia and recrystallized from methanol to produce Ganciclovir (II). The process is shown in Scheme I.

The above process involves the use of expensive bis(p-nitrophenyl)phosphate. Hence, the above process is not suitable for commercial scale synthesis of 2-[(2-acetylamino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]-1 -benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) (III).
Various mono and diacyl esters of Ganciclovir are disclosed in J. Pharm. Sci. 1987, 76 (2), 180-184. The preparation of these esters is also mentioned in this article.

However, L-valyl ester of Ganciclovir and its process of preparation is not discussed in this article. The process is shown in Scheme II.

wherein, R represents methyl, ethyl, propyl, butyl, t-butyl, n-pentyl, CH3OCH2-,
CH3-(CH2)l4.
SCHEME-II
EP 0 375 329 Bl generically discloses ester prodrugs of Ganciclovir including Valganciclovir. However, EP '329 does not disclose the utility as well as process for the preparation of Valganciclovir.
Roche specifically disclosed crystalline Valganciclovir hydrochloride in US 6,083,953. US '953 also discloses a process for the preparation of Valganciclovir hydrochloride (I), by reacting benzyloxymethyloxirane (VIII) with paraformaldehyde in dichloromethane to provide chloromethyl ether intermediate (IX) followed by reaction with potassium acetate in acetone to produce (l-chloro-2-acetoxymethoxy-3-benzyloxy)propane (X). Compound (X) is further reacted with persilylated guanine to produce chlorobenzyl ganciclovir (XI) along with its N-7 isomer, which is further reacted with potassium acetate and DMF to produce mono acetyl benzyl ganciclovir (XII), which is treated with ammonia in methanol to produce mono benzyl ganciclovir (XIII). Compound (XIII) is then reacted with N-(benzyloxycarbonyl)-L-valine to produce compound (XIV), which is further converted to Valganciclovir hydrochloride (I) by reacting with Pd/C and crystallized from isopropanol. The process is shown in scheme-Ill.


The major disadvantage with the above process is that persilylation of guanine and its subsequent reaction with (l-chloro-2-acetoxymethoxy-3-benzyloxy)propane (X) to produce chlorobenzyl ganciclovir (XI) involves longer reaction time and low yield due to the formation of undesired impurities, which requires chromatographic purification. Further, the above process involves the use of expensive trimethylsilyl trifluoromethane sulfonate, which is not suitable for the commercial production of chlorobenzyl ganciclovir (XI).

us 5,756,736 discloses a process for the preparation of Valganciclovir, by reacting Ganciclovir (II) with a trityl halide to produce a compound (XV) with trityl protection at one of the aliphatic hydroxyl groups and at the 2-amino moiety of the guanine group of Ganciclovir, which is further reacted with L-valine derivative, to produce an N,0-bistrityl-monovaline ester of Ganciclovir (XVI), followed by the removal of the protecting groups to produce Valganciclovir hydrochloride (I). The process is shown in scheme IV.

Further, US 6,040,446 discloses an alternative process for the preparation of Valganciclovir, which involves the condensation of a silylated guanine compound (IVb) with a substituted glycerol derivative (XVII) to produce mono-hydroxy protected Ganciclovir (XVIII), which is further reacted with L-valine derivative followed by the removal of protecting groups to produce Valganciclovir hydrochloride (I). The process is shown in scheme V.


Wherein, T), T}, and l} are independently silyl group; Y' and Y^ are aralkyloxy, or one of Y and Y is halo or acyloxy, Z is a leaving group selected from acyloxy, benzoyloxy, halo, mesyloxy or tosyloxy.
SCHEME-V
Hence, there is a need to develop a process, which provides 2-[(2-acetylamino-l,6-dihydro-6-oxo-9H-purin-9-yl)methoxy] -1 -benzyloxy-3 -chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III) using commercially available, less expensive reagents and easy to handle at industrial scale.
Further, there is a need to develop a purification process, which reduces the unwanted regio- isomer (N-7 isomer of N-acetyl chlorobenzyl ganciclovir (IIIa) and

an undesired homologue of N-acetyl chlorobenzyl ganciclovir (XXI) to a pharmaceutically acceptable limit, which in turn used as such in the preparation of Valganciclovir.

OBJECTIVE OF INVENTION
The main objective of the present invention is to provide a simple and effective process for the preparation of 2-[(2-acetylamino-l,6-dihydro-6-oxo-9//-purin-9-yl)methoxy]-l-benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III) and its use in the preparation of Valganciclovir hydrochloride (I).
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for the preparation of 2-[(2-acetylamino-l,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]-l-benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III),


which comprises:
(i) condensing 9-acetyl-2-acetylamino-1,9-dihydro-6H-purin-6-one of formula (IVa),

with (2RS)-(1 -chloro-2-acetyloxymethoxy-3-benzyloxy)propane of formula (XXII).

in the presence of an acid catalyst in a solvent to produce a mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa),

(ii) treating the mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa) with a solvent or/and a solvent mixture or/and treating with acid to produce pure N-acetyl chlorobenzyl ganciclovir of formula (III) or its acid addition salt.


In another embodiment, the present invention also relates to the use of above 2-[(2-acetylamino-1,6-dihydro-6-oxo-9//-purin-9-yl)methoxy]-1 -benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III) to produce Valganciclovir hydrochloride (I),
In another embodiment, the present invention also relates to the use of above 2-[(2-acetylamino-1,6-dihydro-6-oxo-9//-purin-9-yl)methoxy]-1 -benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III) to produce Ganciclovir (II).
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improved process for the preparation of 2-[(2-acetylamino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]-1 -benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III).
9-Acetyl-2-acetylamino-l,9-dihydro-6H-purin-6-one of formula (IVa) is condensed with (2RS)-(l-chloro-2-acetyloxymethoxy-3-benzyloxy)propane of formula (XXII) in presence of an acid catalyst in a suitable organic solvent to produce a mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa).
The suitable organic solvents for the above reaction include but are not limited to N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, sulfolane. The

acid catalyst is selected from p-toluenesulfonic acid monohydrate, methanesulfonic acid, benzenesulfonic acid.
The reaction may be performed at a temperature ranging from about 50°C to about 150°C. More preferably the reaction is carried out at a temperature of about 100°C to 120°C. The reaction time is about 1 to about 10 hours, more preferably about 2 to about 4 hours. After completion of the reaction, the reaction mass is cooled to 10°C to about 30°C and aqueous solution of sodium chloride solution followed by solvent is added to extract the mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and Ilia) from the reaction mass. Aqueous layer is extracted further with organic solvent and the combined organic layer containing mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and Ilia) is treated with carbon and solvent is removed either by distillation or under reduced pressure.
The solvent used for extraction is selected from ethyl acetate, methylene chloride, chloroform.
According to one embodiment, the residue containing a mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa) is treated with solvent at a temperature of about 30 to about 100°C, more preferably at 40-60°C for about 0.5 hrs to 3 hrs, which is further cooled to 10°C to about 30''C. The precipitated product is filtered, followed by washing with a solvent and dried the product to produce crude N-acetyl chlorobenzyl ganciclovir (III).
The solvent used in the above isolation is selected from toluene, isopropyl alcohol.
Crude N-acetyl chlorobenzyl ganciclovir (III) is slurried from a solvent mixture, which is selected from toluene/ethanol, toluene/isopropyl alcohol to produce pure N-acetyl chlorobenzyl ganciclovir (III). The slurry is performed at a temperature of 30°C to about 100°C, more preferably 40 to 60°C. Pure N-acetyl chlorobenzyl

ganciclovir (III) is isolated by techniques well known in the art, such as filtration, and optionally dried.
It has been observed that N-acetyl chlorobenzyl ganciclovir (III) isolated by the above process having N-7-isomer of N-acetyl chlorobenzyl ganciclovir (IIIa) less than 0.5% and homologue of N-acetyl chlorobenzyl ganciclovir less than 4%.
According to another embodiment, the residue containing a mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa) is treated with a solvent mixture selected from toluene/isopropanol, toluneA)utanol, at a temperature of about SOT to about 100°C, more preferably 40 to 60°C followed by cooling to a temperature of about 10°C to about 30° for about 5 to about 20 hrs, more preferably 14 to 16 hrs. The product obtained is filtered and dried to produce crude N-acetyl chlorobenzyl ganciclovir (III).
Crude N-acetyl chlorobenzyl ganciclovir (III) is treated with an acid in a solvent to produce acid addition salt of N-acetyl chlorobenzyl ganciclovir (III). The salt formation is performed at a temperature of about 30°C to about 100°C, more preferably 40 to 60°C followed by cooling to a temperature of about 10°C to about 30° for about 1 to about 8 hrs, more preferably 2 to 3 hrs.
The acid used to prepare acid addition salt is selected from hydrochloric acid, hydrobromic acid. The solvent used in the salt formation is selected from methanol, ethanol, isopropanol, toluene, 1-butanol, acetone, ethyl acetate.
It has been observed that N-acetyl chlorobenzyl ganciclovir (III) isolated by the above process having N-7-isomer of N-acetyl chlorobenzyl ganciclovir (Ilia) less than 0.1% and homologue of N-acetyl chlorobenzyl ganciclovir less than 0.8%.
According to another embodiment, the residue containing mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa) is treated with an acid in a

solvent to produce acid addition salt of N-acetyl chlorobenzyl ganciclovir (III). The salt formation is performed at a temperature of about 30°C to about 100°C, more preferably 40 to 60°C followed by cooling to a temperature of about 10°C to about 30° for about 1 to about 8 hrs, more preferably 2 to 3 hrs.
The acid used to prepare acid addition salt is selected from hydrochloric acid, hydrobromic acid. The solvent used in the salt formation is selected from methanol, ethanol, isopropanol, toluene, 1-butanol, acetone, ethyl acetate.
The product obtained is further purified in a solvent mixture to produce pure acid addition salt of N-acetyl chlorobenzyl ganciclovir (III).
It has been observed that N-acetyl chlorobenzyl ganciclovir (III) isolated by the above process having N-7-isomer of N-acetyl chlorobenzyl ganciclovir (IIIa) less than 0.1% and homologue of N-acetyl chlorobenzyl ganciclovir less than 0.5%.
In another embodiment, the present invention also relates to the use of pure N-acetyl chlorobenzyl ganciclovir (III) or its acid addition salt thereof to produce Valganciclovir hydrochloride (I) by the process reported in US 6,083,953.
The following examples illustrate the nature of the invention and are provided for illustrative purposes only and should not be construed to limit the scope of the invention.

EXAMPLE -1
PREPARATION OF 2-[2-ACETYLAMINO-l,6-DIHYDRO-6-OXO-9H-
PURIN-9-YL)-METHOXY]-l-BENZYLOXY-3-CHLOROPROPANE (N-
ACETYL CHLOROBENZYL GANCICLOVIR)
A mixture of N,N-dimethyIfonnamide (1500ml), 9-acetyl-2-acetylamino-l,9-dihydro-6H-purin-6-one (300 g) and (2RS)-(l-chloro-2-acetyloxymethoxy-3-benzyloxy)propane (695.70 g) were added to p-toluenesulfonic acid monohydrate (15 g) and heated for 3 hr at 110-115°C. The ratio of N-9 to N-7 isomers of N-acetyl chlorobenzyl ganciclovir at this stage was about 2.5:1. Reaction mass was cooled to 25° C, then 8% w/w aqueous sodium chloride solution (2100 ml) and ethyl acetate (2100 ml) were added. The mass was stirred for 20 min and the organic layer was separated. Aqueous layer was extracted with ethyl acetate (1000 ml) and the combined organic layer was washed with water (1500 ml), and then treated with carbon (30 g) at 25-27° C for 30 min. Carbon was removed by filtration and the residue was washed with ethyl acetate (300 ml). The filtrate was concentrated at 40-50°C imder reduced pressure to remove ethylacetate completely. Toluene (1200 ml) was added to the residue and stirred at 27-29°C for 16 hr. Crude N-acetyl chlorobenzyl ganciclovir was filtered, washed with toluene (300 ml) and dried at 50-55°C under reduced pressure.
(Content of 7-isomer of N-acetyl chlorobenzyl ganciclovir: 4.21% Homologue of N-acetyl chlorobenzyl ganciclovir: 5.07%)
Crude N-acetyl chlorobenzyl ganciclovir was stirred in a mixture of toluene (405 ml) and ethanol (45 ml) at 50-55°C for 1 hr, then cooled to 27°C and stirred at 27-28° C for 2 hr. The product was filtered and washed with a mixture of toluene (135 ml) and ethanol (15 ml). The wet product was dried at 50-55°C under reduced pressure to obtain 135.20 gm of N-acetyl chlorobenzyl ganciclovir. (Content of 7-isomer of chlorobenzyl ganciclovir: 0.26% Homologue of N-acetyl chlorobenzyl ganciclovir: 3.94%)

EXAMPLE - 2
PREPARATION OF 2-[2-ACETYLAMIN0.1,6-DIHYDRO-6-OXO-9H-PURIN-9-YL)-METHOXY]-l-BENZYLOXY-3-CHLOROPROPANE HYDROCHLORIDE (N-ACETYL CHLOROBENZYL GANCICLOVIR HYDROCHLORIDE)
A mixture of N,N-dimethylfonnamide (800 ml), 9-acetyl-2-acetylamino-l,9-dihydro-6H-purin-6-one (200 g) and (2RS)-(l-chloro-2-acetyloxymethoxy-3-benzyloxy)propane (463.80 g) were added to p-toluenesulfonic acid monohydrate (10 g) and heated for 3 hr at 110-115° C. The ratio of N-9 to N-7 isomers of N-acetyl chlorobenzyl ganciclovir at this stage was about 2.5:1. Reaction mass was cooled to 28° C, then 8% w/w aqueous sodium chloride solution (1400 ml) and ethyl acetate (1400 ml) were added. The mass was stirred for 20 min and the organic layer was separated. Aqueous layer was extracted with ethyl acetate (600 ml) and the combined organic layer was washed with water (1000 ml), and then treated with carbon (20 g) at 27-28° C for 30 min. Carbon was removed by filtration and the residue was washed with ethyl acetate (200 ml). The filtrate was concentrated at 40-50''C under reduced pressure to remove ethyl acetate completely. Toluene (720 ml) and isopropyl alcohol (80 ml) were added to the residue and stirred at 50-55° C for 1 hr. The resulting product slurry was cooled to 26° C and stirred at 26-28° C for 16 hr. Crude N-acetyl chlorobenzyl ganciclovir was filtered, washed with mixture of toluene (180 ml) and isopropyl alcohol (20 ml) and dried at 50-55°C under reduced pressure.
(Content of 7-isomer of N-acetyl chlorobenzyl ganciclovir: 0.17% Homologue of N-acetyl chlorobenzyl ganciclovir: 2.54%)
A mixture of crude N-acetyl chlorobenzyl ganciclovir (85 g), ethanol (600 ml) and concentrated hydrochloric acid (21.86 g, 35% w/w) were stirred at 50-55° C for 45 min, then cooled to 28° C and stirred at 28-30°C for 2 hr. The product was filtered

and washed with ethanol (85 ml) and dried at 50-55° C under reduced pressure to obtain 100g of N-acetyl chlorobenzyl ganciclovir hydrochloride. (Content of 7-isomer of N-acetyl chlorobenzyl ganciclovir: Not detected, Homologue of N-acetyl chlorobenzyl ganciclovir: 0.76%)
EXAMPLE-3
PREPARATION OF 2-[2-ACETYLAMINO-l,6-DIHYDRO-6-OXO-9H-PURIN-9.YL)-METHOXYI-l-BENZYLOXY-3-CHLOROPROPANE HYDROCHLORIDE (N-ACETYL CHLOROBENZYL GANCICLOVIR HYDROCHLORIDE)
A mixture of N,N-dimethylformamide (1000 ml), 9-acetyl-2-acetylamino-l,9-dihydro-6H-purin-6-one (250 g) and (2RS)-(l-chloro-2-acetyloxymethoxy-3-benzyloxy)propane (579.80 g) were added to p-toluenesulfonic acid monohydrate (12.50 g) and heated for 3 hr at 110-115° C. The ratio of N-9 to N-7 isomers of N-acetyl chlorobenzyl ganciclovir at this stage was about 2.5:1. Reaction mass was cooled to 28°C, then 8% w/w aqueous sodium chloride solution (1750 ml) and ethyl acetate (1750 ml) were added. The mass was stirred for 20 min and the organic layer was separated. Aqueous layer was extracted with ethyl acetate (830 ml) and the combined organic layer was washed with water (1250 ml), and then treated with carbon (25 g) at 25-30° C for 30 min. Carbon was removed by filtration and the residue was washed with ethyl acetate (250 ml). The filtrate was concentrated at 40-50°C under reduced pressure to remove ethyl acetate completely, and the residual mass was co-distilled with isopropyl alcohol (250 ml). Isopropyl alcohol (1250 ml) was added to the residue and heated to 48°C, and then concentrated hydrochloric acid was added (61.0 g, 35% w/w). Clear solution of N-acetyl chlorobenzyl ganciclovir hydrochloride was formed and slowly cooled to 30°C, and then the product was crystallized. The slurry was stirred at 28-30°C for 2 hr, filtered and washed with isopropyl alcohol (250 ml). (Content of 7-isomer of N-acetyl chlorobenzyl ganciclovir: 0.98%

Homologue of N-acetyl chlorobenzyl ganciclovir: 1.34%)
The N-acetyl chlorobenzyl ganciclovir hydrochloride was further purified from a
mixture of methanol (430 ml) and isopropyl alcohol (430 ml). 120 g of N-acetyl
chlorobenzyl ganciclovir hydrochloride was obtained.
(Content of 7-isomer of N-acetyl chlorobenzyl ganciclovir: 0.02%)
Homologue of N-acetyl chlorobenzyl ganciclovir: 0.11%,

1. A process for the preparation of 2-[(2-acetylamino-l,6-dihydro-6-oxo-9H-purin-9-yl)-methoxy]-1 -benzyloxy-3-chloropropane (N-acetyl chlorobenzyl ganciclovir) of Formula (III),

which comprises:
(i) condensing 9-acetyl-2-acetylamino-1,9-dihydro-6H-purin-6-one of formula (IVa),

with (2RS)-(l-chloro-2-acetyloxymethoxy-3-benzyloxy)propane of formula (XXII),

in the presence of an acid catalyst in a solvent to produce a mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and IIIa),


(ii) treating the mixture of N-9 and N-7 isomers of N-acetyl chlorobenzyl ganciclovir (III and Ilia) vdth a solvent or/and a solvent mixture or/and treating with acid to produce pure N-acetyl chlorobenzyl ganciclovir of formula (III) or its acid addition salt.

2. A process according to claim 1, wherein the acid catalyst is selected from p-toluenesulfonic acid monohydrate, methanesulfonic acid, benzenesulfonic acid.
3. A process according to claim 1, wherein the solvent used in step (i) is selected from N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, sulfolane.

4. A process according to claim 1, wherein the solvent used in step (ii) is selected from toluene, ethanol, isopropyl alcohol.
5. A process according to claim 1, wherein the solvent mixture used in step (ii) is selected from toluene/ethanol, toluene/isopropyl alcohol.

6. A process according to claim 1, wherein the acid used in step (ii) is selected
from hydrochloric acid, hydrobromic acid,
7. A process according to claim 1, wherein the N-acetyl chlorobenzyl ganciclovir
(III) isolated by the above process having N-7-isomer of N-acetyl chlorobenzyl
ganciclovir (Ilia) less than 0.1% and homologue of N-acetyl chlorobenzyl
ganciclovir (XXI) less than 0.8%.
8. A process according to claim 1, wherein use of pure N-acetyl chlorobenzyl
ganciclovir (III) or its acid addition salt thereof to produce Valganciclovir (la)
or its hydrochloride salt.
9. A process according to claim 1, wherein use of pure N-acetyl chlorobenzyl
ganciclovir (III) or its acid addition sah thereof to produce Ganciclovir (II).