The present invention relates to an improved process for the preparation of N2-acetyl-9-(1,3-diacetoxy-2-propoxymethyl) guanine referred to here as N-9 alkylated isomer, useful as intermediate for the preparation of antiviral compound, ganciclovir.
Ganciclovir, chemically known as 9-(1,3-dihydroxy-2-propoxymethyl) guanine has the structural formula i as shown in the accompanied drawings, is one of the most important
acyclic nucleosides having significant antiviral properties, especially effective against members of the herpes family and a few other DNA viruses.
• A number of methods are reported in the literature for the production of acylic purine nucleosides such as acyclovir and ganciclovir for example, methods which use guanine, fdiacetyl guanine, 2,6-dichloropurine, 2-amino-6-chloropurine as the starting material (US 4,146,715); tetraacetylguanosine as the starting material (J. Boryski et. al., Nucleosides and Nucleotides, 1989, Vol. 8, 529); acetylguanine as the starting material (Japanese Patent Application No. 84-80685) or 2-chloro-6-iodopurine as the starting material (J. R. Barrio et. Al, J. Med. Chem. 1980, Vol. 23, 572).
However, the simplest synthetic approach to the N-9 substituted guanine compounds involves the direct alkylation of appropriately substituted 2-aminopurines e.g. guanine derivatives. There are significant drawbacks to this approach as it always results in a mixture of N-9 and N-7 alkylation products. The undesirable N-7 isomers are produced in large amounts, which in turn necessitate tedious and in-efficient methods of separation such as silica gel chromatography in order to obtain the desired N-9 isomer at a favourable degree of purity. Use of halogenated purines can. hardly be selected as methods for industrial production, as their starting materials are expensive and difficult to acquire, and they demand a reaction with ammonia at high temperature and pressure in order to obtain guanine nucleosides, such as acyclovir, ganciclovir etc. Hence, it is highly desirable to develop a regioscpecific process for the manufacture of ganciclovir.
One such process which is regiospecific for the N-9 position was recently reported in US Patent..No, 5,821, 367 which comprises reacting protected guanine derivative with an alkylating agent selected from 2-oxa-1,4-butanediol diacetate; 1,4-diacetoxy-3-acetoxymethyl-2-oxabutane; 1,4-dibenzyloxy-3-acetoxymethyl-2-oxabutane in the absence of solvent or any acid catalyst to obtain the penultimate intermediates which are converted to acyclic nucleosides (acyclovir and ganciclovir). Inventors have also reported
the conversion of N-7 isomer to N-9 isomer by heating a suspension of the N-7 isomer in an alkylating agent in the US Pat. No. 6,043,364.
Another US Patent No. 5,583,225 describes a process for the synthesis of purine nucleosides, particularly ganciclovir and aciclovir where the deprotected guanine (diacetyl guanine) is reacted with the desired side chain in the presence of phosphoric or polyphosphoric acid at 120°C. Though the reaction time is very short (3 hours) as compared to the prior art processes, the yields are not encouraging due to the low ratio of desired N-9 isomer in case of ganciclovir.
Each of the above methods has drawbacks in that the desired compound are not obtained in high yield and high purity, thus making the process complicated from the industrial point of view. Shortcomings in any of the parameters result in increased manufacturing cost, which impacts negatively on the desirability of the process.
It is therefore, desirable to solve the problems associated with the prior art and to provide an efficient process for the preparation and isolation of the desired N-9 isomer which improves the economics by resulting in higher yields of the desired isomer and less reaction time. The process should be easy to handle at commercial scale and avoid chromatographic separation of the N-9 and N-7 isomers and the increased cost associated with such a separation (cost of solvents, stationary phase).
Surprisingly, such an efficient and selective process has been discovered for preparing ganciclovir. During the course of the present investigation, it has been discovered that the presence of monoacetyl guanine (MAG) in the condensation reaction of diacetyl guanine with the side chain plays a crucial role in reaction completion MAG itself is not a suitable starting material for the alkylation, but addition of some quantity to the reaction mixture facilitates the reaction completion.
A possible explanation for the better yields is that the reaction of monoacetyl guanine with the side chain produces acetic acid as a side product, which probably catalyzes the reaction between the side chain and diacetyl guanine. This is evident by the reaction of these intermediates in the presence of acetic acid. Alkylation reaction done in this mode gives better yield and quality of the desired N-9 isomer which not only reduces the cost of the product but also reduces the waste formation which is otherwise unavoidable .
Alkylation when performed below 100°C was completed in 30-45 hours and resulted in better yields of the desired N-9 isomer than when performed at a temperature above 100°C. The time required for alkylation at high temperature was less (6-10 hours) but the yields were less due to decomposition of the product of higher temperature.
The addition of the N-7 isomer during alkylation enhances the formation of N-9 isomer and suppresses the further formation of N-7 isomer. This has been described in our earlier pending patent application no. 592/Del/01 filed on May 18, 2001.
The crystallization procedure of the present invention is very simple and produces the N-9 isomer having more than 95% purity which can directly be used for the preparation of ganciclovir. The present process is cost effective and obviates the need for chromatographic separation.
Thus the present invention provides an efficient process for the preparation of ganciclovir which is convenient to operate on a commercial scale which reduces overall production costs and also gives the desired product in good yield and quality.
Accordingly, the present invention provides a process for the preparation of N2-acetyl-9-(1,3-diacetoxy-2-propoxymethyl) guanine (N-9 alkylated isomer) of structural formula V as shown in the accompanied drawings, useful intermediate for the preparation of antiviral compound, ganciclovir, which comprises alkylation of diacetyl guanine of structural formula III in the presence of monoacetyl guanine of structural Formula II or acetic acid, with 2-acetoxymethoxy-1,3-diacetoxy propane of structural formula IV in an inert organic solvent optionally containing a catalyst and N2-acetyl-7-(1,3-diacetoxy-2-propoxymethyl) guanine of structural formula VI for 3-60 hours at a temperature from about 50 to 150°C and isolating the N-9 alkylated isomer.
Alkylation is done in the presence of catalyst. Alkylation catalyst used in the reaction is selected from sulfuric acid, methanesulfonic acid, p-toluene sulfonic acid and the like. Most preferred is p-toluene sulfonic acid.
The formation of N-9 isomer is enhanced by adding N-7 isomer of structural formula VI to the reaction mixture.
Alkylation reaction is performed in an inert organic solvent followed by a suitable work up and crystallization of N-9 and N-7 isomers from organic solvent or a mixture thereof.
The inert organic solvent for alkylation reaction is selected from benzene, toluene, xylene, acetonitrile, tetrahydrofuran, dimethylformamide, chloroform, dichloromethane, methyl iso-butyl ketone or pyridine. Most preferred is dimethylformamide.
N-9 and N-7 alkylated isomers of structural formula V and VI are separated from each other by methods known in the literature or by crystallization methods as per our pending patent application No.592/Del/01filed on May 18, 2001.
Methods known in the art may be used with the process of this invention to enhance any aspect of this invention. For example, the solution containing the mixture of N-7 and N-9 isomers may be heated for dissolution, or it may be cooled to separate out the product or the slurry may further be cooled prior to filtration.
The N-9 isomer so obtained after separation is hydrolyzed to yield ganciclovir by the methods known in the literature (J.E. Martin et.al. J. Med. Chem., 1983,26, 759-761).
Other features of the invention will become apparent in the course of the following description of exemplary embodiment which is given for illustration of the invention and are not intended to be limited thereof.
PREPARATION OF N2-ACETYL-9-(1,3-DIACETOXY-2-PROPOXYMETHYL)GUANINE
EXAMPLE 1
To a mixture of diacetylguanine (8g, 34 mmol), monoacetylguanine (2g, 10 mmole) and 2-acetoxymethoxy-1,3-diacetoxy propane (18g, 72 mmole) in dimethylformamide (30 ml) was added p-toluene sulphonic acid (0.5g, 2.6 mmole) at 90-100°C and the reaction mixture was stirred for 35-45 hours. After reaction completion, solvent was distilled off and N-7 isomer (8.1g) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (7.62g), was obtained by crystallization of the residue with toluene and methanol in 45% yield.
EXAMPLE 2
To a mixture of diacetylguanine (24g, 102 mmol), monoacetylguanine (6g, 31 mmole) and 2-acetoxymethoxy-1,3-diacetoxy propane (54g, 217 mmole) in dimethylformamide (90 ml) was added p-toluene sulphonic acid (1.5g, 7.9 mmole) and N-7 isomer (15g, 39 mmole) at 90-100°C and the reaction mixture was stirred for 35-45 hours. After reaction completion, solvent was distilled off and N-7 isomer (12.5g) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (37g) was obtained by crystallization of the residue from toluene and methanol in 73% yield.
EXAMPLE 3
To a mixture of monoacetylguanine (20g, 103 mmole) and 2-acetoxymethoxy-1,3-diacetoxypropane (36g, 145 mmole) in dimethylformamide (60ml) was added p-toluene sulphonic acid (1g, 5.2 mmole) and stirred at 90-100°C. Reaction was not complete and even after 38 hours. The reaction mixture was filtered to remove unreacted monoacetylguanine (MAG) (3.85g). Solvent was distilled off and N-7 isomer (4.65g) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (21.6g) was obtained by crystallization of the residue from toluene and methanol. This N-9 isomer had 5.81% of MAG in it.
EXAMPLE 4
To a mixture of diacetylguanine (20g, 85 mmole), acetic acid (3.5g, 58 mmole) and 1,3-diacetoxy-2-acetoxy-methoxy propane (36g, 145 mmole) in dimethylformamide (60ml) was added p-toluene sulphonic acid (1.0g, 5 mmole) alongwith N-7 isomer (10g) and the reaction mixture was stirred at 90-100°C 35-45 hours.
After reaction completion, the solvent was distilled off and N-7 isomer (8.3g) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (20.1g) was obtained by crystallization of the residue from toluene and methanol in 62 % yield.
EXAMPLE 5
To a mixture of diacetylguanine (9g, 38 mmol), and 2-acetoxymethoxy-1,3-diacetoxypropane (18g, 72.6 mmole) in dimethylformamide (27 ml) was added p-toluene sulphonic acid (1.35g, 7.1 mmole) and N-7 isomer (4.5g, 12 mmole) at 120-125°C and the reaction mixture was stirred for 6-10 hours. After reaction completion, solvent was distilled off and N-7 isomer (3.5g) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (7.8g,) was obtained by crystallization of the residue from toluene and methanol in 53.4% yield.
EXAMPLE 6
To a mixture of diacetylguanine (10g, 42.5 mmol), acetic acid (1.5g, 25 mmole) and 2-acetoxymethoxy-1,3-diacetoxypropane (20g, 80.6 mmole) in dimethylformamide (30 ml) was added p-toluene sulphonic acid (0.5g, 2.6 mmole) and N-7 isomer (5g, 13 mmole) at 120-125°C and the reaction mixture was stirred for 6-10 hours. After reaction completion, solvent was distilled off and N-7 isomer (3.95) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (8.5g) was obtained by crystallization of the residue from toluene and methanol in 52.4% yield.
EXAMPLE 7
To a mixture of diacetylguanine (8g, 34 mmol), monoacetylguanine (2g, 10 mmole) and 2-acetoxymethoxy-1,3-diacetoxypropane (20g, 80 mmole) in dimethylformamide (30 ml) was added p-toluene sulphonic acid (0.5g, 10 mmole) and N-7 isomer (5g, 13 mmole) at 120-125°C and the reaction mixture was stirred for 6-10 hours. After reaction completion, solvent was distilled off and N-7 isomer (4.1g) was isolated using methanol as crystallization solvent. Solvent was recovered to get the residue and the required N-9 isomer (10.2g) was obtained by crystallization of the residue from toluene and methanol in 60.8% yield.

WE CLAIM :
1. A process for the preparation of N2-acetyl-9-(1,3-diacetoxy-2-propoxymethyl) guanine (N-9 alkylated isomer) of structural formula V as shown in the accompanied drawings, useful intermediate for the preparation of antiviral compound, ganciclovir, which comprises alkylation of diacetyl guanine of structural formula III in the presence of monoacetyl guanine of structural Formula II or acetic acid, with 2-acetoxymethoxy-1,3-diacetoxy propane of structural formula IV in an inert organic solvent optionally containing a catalyst and N2-acetyl-7-(1,3-diacetoxy-2-propoxymethyl) guanine of structural formula VI for 3-60 hours at a temperature from about 50 to 150°C and isolating the N-9 alkylated isomer.
2. The process as claimed in claim 1 wherein the catalyst is selected from sulfuric acid, methanesulfonic acid or p-toluene sulfonic acid.
3. The process as claimed in claim 1 which comprises the addition of N-7 isomer of structural formula VI to the reaction mixture at a temperature of 60-120°C.
4. The process as claimed in claim 1 wherein the inert organic solvent is selected from benzene, toluene, xylene, acetonitrile, tetrahydrofuran, dimethylformamide, chloroform, dichloromethane, methyl iso-butyl ketone or pyridine.
5. The process as claimed in claim 1 wherein isolation comprises of deprotecting the N-9 isomer after the alkylation reaction.

6. A process for the preparation of N2-acetyl-9-(1,3-diacetoxy-2-propoxymethyl) guanine (N-9 alkylated isomer) of structural formula V as shown in the accompanied drawings substantially described herein and exemplified by the examples.