PROCESS FOR PREPARING PURINE DERIVATIVES

Abstract: Process for preparing the L-monovaline ester of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)-methoxy-1,3-propanediol and its pharmaceutically acceptable salts. The present process provides an N,O-bis-trityl intermediate of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)-methoxy-1,3-propanediol which allows for monoesterification by an L-valine derivative. These products are of value as antiviral agents with improved absorption.

Description: PROCESS FOR PREPARING PU INE DERIVATIVES BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a process for preparing a prodrug formulation of ganciclovir and its pharmaceutically acceptable salts. More specifically, the invention relates to a process for preparing the L- monovaiine ester derived from 2- (2-amino-l, 6-dihydro-6- oxo-purin-9-yl)methoxy-l,3-propane-diol and its pharmaceutically acceptable salts. Background Information British Patent 1523865 describes antiviral purine derivatives with an acyclic chain in the 9-position. Among those derivatives 2- (2-amino-l, 6-dihydro-6-oxo-purin-9- yl) -methoxy-ethanol with the INN name acyclovir has been found to have good activity against herpes viruses such as herpes simplex. U.S. Patent 4,355,032 discloses the compound 9-[(2- hydroxy-1-hydroxymethyl-ethoxy) -methyl] -guanine or 2- (2- amino-1, 6-dihydro-6-oxo-purin-9-yl) -me hoxy-1, 3-propane¬ diol or 9-[(1, 3-dihydroxy-2-propoxy) -methyl] -guanine (DHPG) with the INN name ganciclovir. Ganciclovir is highly efficacious against viruses of the herpes family, for example, against herpes simplex and cytomegalovirus. European Patent Application EP 0 375 329 discloses prodrug compounds with the following formula B. wherein R and R are independently selected from a hydrogen atom and an amino acyl residue providing at least one of R and R represents an amino acid acyl residue and B represents a group of the formulae in which R 2 represents a Ci-6 straight chain, C3-6 branched chain or C3-6 cyclic alkoxy group, or a hydroxy or amino group or a hydrogen atom and the physiologically acceptable salts thereof. These prodrug compounds are described as having advantageous bioavailability when administered the oral route, resulting in high levels of the parent compound in the body. Example 3 (b) European Patent Application EP 0 375 329 discloses the preparation of the bis(L-isoleucinate) ester of ganciclovir as a white foam. Example 4 (b) discloses the preparation of the bis (glycinate) ester of ganciclovir as a white solid. Example 5 (b) discloses the preparation of the bis (L-valinate) ester of ganciclovir as a solid. Example 6 (b) discloses the preparation of the bis (L-alaninate) ester of ganciclovir as a syrup containing 90% of the bis ester and 10% of the monoester. The bis-esters are prepared by reacting ganciclovir with an optionally protected amino acid or functional equivalent thereof; the reaction may be carried out in a conventional manner, for example in a solvent such as pyridine, dimethylformamide, etc., in the presence of a coupling agent such as 1,3-dicyclohexylcarbodiimide, optionally in the presence of a catalytic base such as 4- dimethylaminopyridine. The described bis esters are non- crystalline materials which are difficult to process for the manufacture of oral pharmaceutical dosage forms. British Patent Application No. 8829571 is the priority patent application for European Patent Application EP 0 375 329 and US Patent No. 5,043,339, and discloses amino acid esters of the compounds of the formula (wherein R represents a hydroxy or amino group or a hydrogen atom) and the physiologically acceptable salts thereof. Examples of preferred amino acids include aliphatic acids e.g. containing up to 6 carbon atoms such as glycine, alanine, valine and isoleucine. The amino acid esters include both mono and diesters. The preparation of the diesters is identical to the preparation in European Patent Application EP 0 375 329; however, this patent application as well as European Patent Application EP 0 375 329 and US Patent No. 5,043,339 do not disclose the preparation of monoesters, or any data suggesting their usefulness. Leon Colla et. al. , J. Med. Chem. (1983) 26, 602-604 disclose several water-soluble ester derivatives of acyclovir and their salts as prodrugs of acyclovir. The authors indicate that acyclovir cannot be given as eye drops or intramuscular injections because of its limited solubility in water and have therefore synthesized derivatives of acyclovir which are more water soluble than the parent compound. The authors disclose the hydrochloride salt of the glycyl ester, the hydrochloride salt of the alanyl ester, the hydrochloride salt of the β- alanyl ester, the sodium salt of the succinyl ester, and the azidoacetate ester. The alanyl esters were prepared by conventional esterification methods, including reacting acyclovir with the corresponding N-carboxy-protected amino acid in pyridine, in the presence of 1, 3-dicyclohexyl- carbodiimide and a catalytic amount of p-toluenesulfonic acid and subsequently catalytic hydrogenation to give the alpha- and beta-alanyl esters as their hydrochloride salts. L. M. Beauchamp et. al. , Antiviral Chemistry & Chemotherapy (1992), 3 (3), 157-164 disclose eighteen amino acid esters of the antiherpetic drug acyclovir and their effectiveness as prodrugs of acyclovir, evaluated in rats by measuring the urinary recovery of acyclovir. Ten prodrugs produced greater amounts of the parent drug in the urine than acyclovir itself: the glycyl, D,L-alanyl, L-alanyl, L-2-aminobutyrate, D,L-valyl, L-valyl, DL- isoleucyl, L-isoleucyl, L-methionyl, and L-prolyl ester. According to the authors the L-valyl ester of acyclovir was the best prodrug of the esters investigated. These esters were prepared by methods similar to those employed by Colla et. al. European Patent Publication 308 065 discloses the valine and isoleucine esters of acyclovir, preferably in the L-form, as showing a large increase in absorption from the gut after oral administration, when compared with other esters and acyclovir. The amino acid esters are prepared by conventional esterification methods, including reacting acyclovir with an N-carboxy-protected amino acid or an acid halide or acid anhydride of the amino acid, in a solvent such as pyridine or dimethylformamide, optionally in the presence of a catalytic base. PCT Patent Application WO 94/29311 discloses a process for the preparation of amino acid esters of a nucleoside analogue, including acyclovir and ganciclovir. This process comprises reacting a nucleoside analogue having an esterifiable hydroxy group in its linear or cyclic ether moiety, with a 2-oxa-4-aza-cycloalkane-l,3- dione of the formula wherein R may represent hydrogen, Cχ-4 alkyl or alkenyl group or other ammo acid side chains, and R 2 may represent hydrogen or a group COOR 3 where R3 is a benzyl, t-butyl, fluorenylmethyl or an optionally halo substituted linear or branched Ci-8 alkyl group. Preferred R groups include hydrogen, methyl, iso-propyl and isobutyl, yielding respectively the glycine, alanine, valine and isoleucine esters of acyclovir or ganciclovir. Examples 1-3 of PCT Patent Application W094/29311 discloses only the condensation of acyclovir with the valine-substituted 2-oxa-4-aza-cycloalkane-l,3-dione (Z-valine-N- carboxyanhydride) by conventional procedures. While the amino acid esters of the PCT application include both the acyclovir and ganciclovir (DHPG) esters, the application does not disclose how to prepare the ganciclovir esters, much less the mono-esters of ganciclovir. The L-monovaline ester derived from 2- (2-amino-l, 6- dihydro-6-oxo-purin-9-yl)methoxy-1, 3-propane-diol and its pharmaceutically acceptable salts are potent antiviral agents and are described in European Patent Application EP 694 547 A. These compounds have been found to have improved oral absorption and low toxicity. This patent application also discloses certain processes for preparing these esters, different from those described herein. The present invention relates to an improved process whereby ganciclovir is reacted with a trityl compound to provide a ganciclovir intermediate with trityl protection at one of the aliphatic hydroxyl groups and at the 2-amino moiety of the guanine group. This intermediate is isolated in high yields and with high purity and allows for mono-esterification by an L-valine derivative to produce the L-monovaline ester of ganciclovir. SUMMARY OF THE INVENTION This invention provides a process for preparing the compound of the formula I: and the pharmaceutically acceptable salts thereof, which compound is named hereinafter 2- (2-amino-l, 6-dihydro-6- - 7 - oxo-purin-9-yl) -methoxy-3-hydroxy-l-propyl-L-valinate or mono-L-valine ganciclovir. This process involves the reaction of ganciclovir with a trityl compound to provide a ganciclovir intermediate with trityl protection at one of the aliphatic hydroxyl groups and at the 2-amino moiety of the guanine group of ganciclovir. This intermediate, in turn, allows for mono-esterification by an L-valine derivative, to provide an N,0-bistrityl-monovaline ester of ganciclovir, followed by removal of the protecting groups to yield the prodrug of Formula I. Optionally, the process can also include the formation of salts of the prodrug of Formula I, the conversion of an acid addition salt of the prodrug of Formula I into a non-salt form, the optical resolution of a prodrug of Formula I or the preparation of the prodrugs of Formula I in crystalline form. Details of the process are described below. DETAILED DESCRIPTION OF THE INVENTION Definitions Unless otherwise stated, the following terms used in the specification and claims have the meanings given below: „Alkyl" means a straight or branched saturated hydrocarbon radical having from one to the number of carbon atoms designated. For example, C1-7 alkyl is alkyl having at least one but no more than seven carbon atoms, e.g. methyl, ethyl, i-propyl, n-propyl, n-butyl,n-pentyl, n-heptyl and the like. „Lower alkyl" means an alkyl of one to six carbon atoms. „Aryl" means an organic radical derived from an aromatic hydrocarbon by the removal of one hydrogen atom. Preferred aryl radicals are aromatic carbocyclic radicals having a single ring (e.g., phenyl) or two condensed rings (e.g., naphthyl) . Most preferred is the phenyl radical C 6 H 5 -. „Aralkyl" means an organic radical derived from an alkyl in which a hydrogen atom is replaced by an above- defined aryl group, e.g., benzyl, phenylethyl, phenyl- propyl, and the like. „Acyl" means an organic radical derived from an organic acid by the removal of the hydroxyl group; e.g., CH3CO- is the acyl radical of CH3COOH, or acetyl. Other examples for such acyl groups are propionyl, or benzoyl, etc. The term „acyl" includes the term „alkanoyl" which is the organic radical RCO- in which R is an alkyl group as defined above. .Lower alkoxy", „ (lower alkyl)amino" , „di (lower alkyl) amino" , „ {lower alkanoyl)amino" , and similar terms mean alkoxy, alkylamino, dialkylamino, alkanoylamino, etc. in which the or each alkyl radical is a „lower alkyl" as described above. „Halogen" or „halo" means fluorine, chlorine, bromine, or iodine. „Derivative" of a compound means a compound obtainable from the original compound by a simple chemical process. ^ .Activated derivative" of a compound means a reactive form of the original compound which renders the compound active in a desired chemical reaction, in which the original compound is only moderately reactive or non- reactive. Activation is achieved by formation of a derivative or a chemical grouping within the molecule with a higher free energy content than that of the original compound, which renders the activated form more susceptible to react with another reagent. In the context of the present invention activation of the carboxy group is of particular importance and corresponding activating agents or groupings which activate the carboxy group are described in more detail below. An example of an activated derivative of L-valine is the compound of Formula III 3 wherein P is an ammo-protecting group and A is a carboxy-acti ating group, for example, halo, a lower acyloxy group, a carbodiimide group, such as 1-ethyl-3- (3- dimethylaminopropyl)carbodiimide (EDAC) , an isobutyrate group, and the like. Of particular interest for the present invention is an amino acid anhydride which is an activated form of an amino acid which renders the amino acid (especially L- valine) susceptible to esterification. Amino acid anhydrides are included in the compounds of Formula III, above. Especially useful for the present invention are the cyclic amino acid anhydrides of L-valine, described in PCT Patent Application WO 94/29311, such as 2-oxa-4-aza-5- isopropyl-cycloalkane-1,3-dione of formula Ilia: in which P is an amino protecting group. Other examples of the cyclic amino acid anhydrides are protected amino acid N-carboxy anhydrides (NCA's) described in more detail below. „ rotecting group" means a chemical group that (a) preserves a reactive group from particisopropyl alcoholting in an undesirable chemical reaction; and (b) can be easily removed after protection of the reactive group is no longer required. For example, the benzyl group is a protecting group for a primary hydroxyl function. „Amino-protecting group" means a protecting group that preserves a reactive amino group that otherwise would be modified by certain chemical reactions. The definition includes the trityl group -C(Ph)3 (i.e., triphenyl¬ methyl), or substituted trityl groups, such as the monomethoxytrityl group, dimethoxytrityl groups such as the 4, 4' -dimethoxytrityl or 4, 4' -dimethoxytriphenylmethyl group, the formyl group or lower alkanoyl groups with 2 to 4 carbon atoms, in particular the acetyl or propionyl group, the trichloroacetyl group, the trifluoroacetyl group, and the N- (9-fluorenyl ethoxycarbonyl) or „FMOC" group, the allyloxycarbonyl group or other protecting groups derived from halocarbonates such as (C6-Cχ2)aryl lower alkyl carbonates (such as the N-benzyloxycarbonyl group derived from benzylchlorocarbonate) , or derived from biphenylalkyl halo carbonates, or tertiary alkyl halo carbonates, such as tertiary butylhalocarbonates, in particular tertiary butylchloro-carbonate, or di (lower) - alkyldicarbonates, in particular di (t-butyl)dicarbonate, the phthalyl group and triphenylmethyl halides such as triphenylmethyl chloride, and trifluoroacetic anhydride. In the context of the present invention, an amino- protecting group may be present at the 2-amino moiety of the guanine group of ganciclovir, or at the amino group of the L-valine derivative. Of particular interest for the present invention are the trityl and substituted trityl groups as amino-protecting groups for the 2-amino moiety of the guanine group of ganciclovir. „Trt" is a trityl or substituted trityl group, as defined above. „Hydroxy-protecting group" means a protecting group that preserves a hydroxy group that otherwise would be modified by certain chemical reactions. In the context of the present invention, the hydroxy protecting group is a trityl group -C(Ph)3 (i.e., triphenylmethyl), or a substituted trityl group, as defined above. The trityl protecting groups, as amino- or hydroxy- protecting groups, are derived from the reaction of an optionally substituted trityl compound with the 2-amino group on the guanine group of ganciclovir or with the side-chain hydroxy moiety of ganciclovir. Trityl compounds are well-known in the art; generally they are of the formula XC(Ph)3 , wherein X is chloro or bromo. Substituted trityl compounds will yield the substituted trityl groups as defined above, such as the monomethoxytrityl group, dimethoxytrityl groups such as the 4, 4'-dimethoxytrityl or ,4 x -dimethoxytriphenylmethyl group. ^ Leaving group" means a labile group that is replaced in a chemical reaction by another group. Examples of leaving groups are halogen, the optionally substituted benzyloxy group, the isopropyloxy group, the mesyloxy group, the tosyloxy group or the acyloxy group. All the activating and protecting agents employed in the preparation of the compound of Formula I must meet the following qualifications: (1) their introduction should proceed quantitatively and without racemization of the L- valine component; (2) the protecting group present during the desired reaction should be stable to the reaction conditions to be employed; and (3) the group must be readily removed under conditions in which the ester bond is stable and under which racemization of the L-valine component of the ester does not occur. The process of the invention may also include the optical resolution of a prodrug of Formula I. Terminology relating to the stereochemistry and optical resolution of these compounds is described in European Patent Application EP 694 547 A. „Optional" or „optionally" means that a described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances in which it does not. For example, ^ optionally substituted phenyl" means that the phenyl may or may not be substituted and that the description includes both unsubstituted phenyl and phenyl wherein there is substitution; „optionally followed by converting the free base to the acid addition salt" means that said conversion may or may not be carried out in order for the process described to fall within the invention, and the invention includes those processes wherein the free base is converted to the acid addition salt and those processes in which it is not. „Pharmaceutically acceptable" means that which is useful in preparing a pharmaceutical composition that is generally safe and non-toxic and includes that which is acceptable for veterinary use as well as human pharmaceutical use. pharmaceutically acceptable salts" means salts which possess the desired pharmacological activity and which are neither biologically nor otherwise undesirable. Such salts include acid addition salts formed with inorganic acids such as hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, phosphoric acid, and the like; or with organic acids such as acetic acid, propionic acid, hexanoic acid, heptanoic acid, cyclopentane-propionic acid, glycolic acid, pyruvic acid, lactic acid, malonic acid, succinic acid, malic acid, maleic acid, fumaric acid, tartaric acid, citric acid, benzoic acid, o- (4-hydroxy-benzoyl) -benzoic acid, cinnamic acid, mandelic acid, methanesulfonic acid, ethanesulfonic acid, 1,2-ethane-disulfonic acid, 2-hydroxyethane-sulfonic acid, benzenesulfonic acid, p-chlorobenzenesulfonic acid, 2-naphthalenesulfonic acid, p-toluenesulfonic acid, camphorsulfonic acid, 4-methyl-bicyclo [2 . 2 . 2 ] oct-2-ene-l- carboxylic acid, gluco-heptonic acid, 4, 4'-methylenebis (3- hydroxy-2-naphthoic) acid, 3-phenylpropionic acid, trimethyl-acetic acid, tertiary butylacetic acid, lauryl sulfuric acid, gluconic acid, glutamic acid, hydroxy- naphthoic acids, salicylic acid, stearic acid, muconic acid, and the like. Preferred pharmaceutically acceptable salts are those formed with hydrochloric, sulfuric, phosphoric acid, acetic or methanesulfonic acid, ethanesulfonic acid, 1, 2-ethanedisulfonic acid, 2-hydroxyethanesulfonic acid, benzene-sulfonic acid, p-chlorobenzenesulfonic acid, and 2-naphthalenesulfonic acid, p-toluenesulfonic acid, camphorsulfonic acid. Synthetic Reaction Parameters Unless specified to the contrary, the reactions described herein take place at atmospheric pressure within a temperature range from 5°C to 170°C (preferably from 10°C to 50°C; most preferably at „room" or „ambient" temperature, e.g., 20° - 30°C) . However, there are clearly some reactions where the temperature range used in the chemical reaction will be above or below these temperature ranges. Further, unless otherwise specified, the reaction times and conditions are intended to be approximate, e.g., taking place at about atmospheric pressure within a temperature range of about 5°C to about 100°C (preferably from about 10°C to about 50°C; most preferably about 20°C) over a period of about 1 to about 100 hours (preferably about 5 to 60 hours) . Parameters given in the Examples are intended to be specific, not approximate. Isolation and purification of the compounds and intermediates described herein can be effected, if desired, by any suitable separation or purification procedure such as, for example, filtration, extraction, crystallization, column chromatography, thin-layer chromatography or thick-layer chromatography, or a combination of these procedures. Specific illustrations of suitable separation and isolation procedures can be had by reference to the examples hereinbelow. However, other equivalent separation or isolation procedures can, of course, also be used. Presently Preferred Embodiments While the broadest definition of this invention is set forth in the Summary of the Invention as a process for preparing the compound of Formula I and its pharmaceutically acceptable salts, the (R,S) mixture and certain salts are preferred. The following acids are preferred to form pharma- ceutically acceptable salts with the compound of Formula I: hydrochloric, sulfuric, phosphoric acid, acetic, methanesulfonic, ethanesulfonic, 1,2-ethanedisulfonic, 2-hydroxyethanesulfonic, benzenesulfonic, p-chlorobenzene¬ sulfonic, 2-naphthalenesulfonic, p-toluenesulfonic and camphorsulfonic acid. Most preferred are strong inorganic acids, such as hydrochloric, sulfuric or phosphoric acid. The most preferred compounds are 2- (2-amino-l, 6- dihydro-6-oxo-purin-9-yl) -methoxy-3-hydroxy-1-propyl L- valinate hydrochloride and acetate. These compounds can be prepared as crystalline materials and therefore can be easily manufactured into stable oral formulations. In any of the processes described herein, a reference to Formula I, II, III, Ilia, IV or V refers to such Formulae wherein P 1, P2, P3 , Trt and A are as defined m their broadest definitions set forth in the Summary of the Invention, with the processes applying particularly to the presently preferred embodiments. Details of the Synthetic Processes The process of the present invention is depicted in the Reaction Sequence shown below: wherein P 1 is hydrogen or an amino-protecting group, P2 is an optionally substituted trityl group or an amino- 3 protecting group, P is an ammo-protecting group and Trt is an optionally substituted trityl group. 2- (2-amino-l, 6-dihydro-6-oxo-purin-9-yl) -methoxy-1, 3- propanediol (ganciclovir) is reacted with an optionally substituted trityl compound as defined above to yield the N,0-bis trityl ganciclovir intermediate with trityl protection at one of the aliphatic hydroxy groups and at the 2-amino function (Formula II) . The product of this reaction is esterified with an L-valine derivative of Formula III or Ilia to form an N,0-bistrityl-monovaline ester of ganciclovir (Formula IV) , followed by removal of the trityl protecting groups, and any other protecting groups to afford a compound of Formula I. This process of this invention provides a quick and easy method for preparing the N,O-bistrityl ganciclovir intermediate of Formula II. The bis-trityl intermediate can be recovered with high purity and in yields of greater than 60%, and can be easily separated from the mono- and tri-tritylated impurities by filtration and simple extraction. This intermediate then allows for mono¬ esterification with an L-valine derivative to provide the compounds of Formula I. Compounds of Formula I can optionally be converted into a pharmaceutically acceptable salt thereof. The process can also include the conversion of an acid addition salt of the prodrug of Formula I into a non-salt form, the optical resolution of a compound of Formula I or the preparation of the compound of Formula I in crystalline form. The process for producing the compound of the Formula I may or may not involve protection of the amino group in the 2-position of the guanine base, though the preferred process is carried out without a protected amino group. For the case when the ganciclovir starting material does have a protected 2-amino group, the protecting group may be removed by conventional procedures, well-known in the art. " For example, if the amino-protecting group is a lower alkanoyl group basic conditions (pH between 8 to 11) are employed to remove the protecting group. For example, 2-N-acetyl ganciclovir is treated with an alkaline reagent such as ammonium hydroxide, sodium or potassium carbonate or sodium or potassium hydroxide until the removal of the acetyl group is complete. In general, this reaction will be conducted in the presence of a suitable solvent such as a lower alkanol. Preferably the starting material is dissolved in methanol and a stoichiometric excess of ammonium hydroxide is added. The reaction temperature is kept between 0° to 50°C, preferably at room temperature. After the reaction is complete (which can be determined by TLC) , another solvent may be added to facilitate isolation of the de-protected product, such as ethyl ether which leads to precipitation of the de-acylated product which can be filtered off and isolated using conventional separation methods. Starting Materials All starting materials employed to make the compound of Formula I are known, such as ganciclovir, the trityl compounds, and the protecting and carboxylic-group- activating reagents. Prior to carrying out Step II (esterification step) , the amino group of the L-valine derivative must be protected to avoid its interference with the esterification by undesirable amide formation. The various amino-protected L-valine derivatives useful in this invention, such as N-benzyloxycarbonyl-L-valine, BOC- L-valine and FMOC-L-valine, N-formyl-L-valine and N- benzyloxycarbonyl-N-carboxy-L-valine anhydride, are all commercially available (SNPE Inc., Princeton, NJ, Aldrich Chemical Co., Milwaukee, WI and Sigma Chemical Co., St. Louis, MO.), or are described in the literature, such as N-allyloxycarbonyl-L-valine. Cyclic amino-protected L- valine derivatives are also described in the literature, as noted above. Of particular interest for the present invention is the benzyloxycarbonyl valine-substituted 2- oxa-4-aza-cycloalkane-l, 3-dione (Z-valine-N- carboxyanhydride, or Z-Valine-NCA) , and Boc-L-valine, which are also commercially available (SNPE Inc., Princeton, NJ) . Alternatively, the protecting step may be carried out by conventional methods. A preferred ganciclovir starting material for the preparation of the compound of the invention is the unprotected ganciclovir (2- (2-amino-l, 6-dihydro-6-oxo- purin-9-yl) -methoxy-1, 3-propanediol) which is described in US Patent No. 4,355,032. Other ganciclovir starting materials may have protection at the 2-amino group, such as 2- (2-acyl-amino-1, 6-dihydro-6-oxo-purin-9-yl)methoxy- 1,3-propanediol Preferred trityl compounds are trityl chloride and trityl bromide. Preparation of Activated derivative of L-valine: Prior to carrying out Step II (esterification step) , L-valine must also be activated. At least 1 equivalent of the protected amino acid and 1 equivalent of a suitable coupling agent or dehydrating agent, for example 1,3- dicyclohexylcarbodiimide or salts of such diimides with basic groups should be employed from the start. Other carbodiimides such as N,N V -carbonyldiimidazole may also be used. Further useful dehydrating agents are trifluoro¬ acetic anhydride, mixed anhydrides, acid chlorides, 1- benzo-triazolyloxy-tris (dimethylamino)phosphonium hexafluorophosphate, benzotriazole-lyl-oxy-trispyrroli- dinophosphonium hexafluorophosphate, 1-hydroxybenzo- triazole, l-hydroxy-4-azabenzotriazole, l-hydroxy-7- azabenzotriazole, N-ethyl-N'-(3- (dimethylamino) - propyl)carbodiimide hydrochloride, 3-hydroxy-3, -dihydro- 4-oxo-l,2, 3-benzotriazine, 0- (benzotriazol-1-yl) -1,1,3,3- tetramethyluronium hexafluorophosphate, 0- (7-azabenzo- triazol-1-yl) -1,1,3, 3-tetramethyluronium hexafluoro- phosphate, 0- (7-azabenzotriazol-l-yl) -1,1,3, 3-tetramethyl- uronium tetrafluoroborate, 0- (lH-benzotriazol-1-yl) - 1,1,3,3-bis (tetramethylene) -uronium hexafluorophosphate or 0- (7-azabenzotriazol-l-yl) -1,1,3, 3-bis- (tetramethylene) - uronium hexafluorophosphate. A description of these coupling agents by L. A. Carpino can be found in J " . Am. Chem. Soc . 1993, 115, p. 4397-4398. Also useful for this purpose are urethane-protected amino acid N-carboxy anhydrides (UNCA's) which are an activated form of an amino acid; these have been described by William D. Fuller et.al., J. Am. Chem. Soc . 1990, 112, 7414-7416, which is incorporated herein by reference. Other protected amino acid N-carboxy anhydrides are described in PCT Patent Application WO 94/29311 discussed above. In summary, any other reagent that produces an anhydride or another activated derivative of the protected amino acid under mild conditions can be used as the coupling agent. The amino-protected amino acid is dissolved in an inert solvent such as a halogenated lower alkane, preferably dichloromethane under an inert atmosphere, for example nitrogen, and the coupling agent is added (preferably 1, 3-dicyclohexylcarbodiimide) . The reaction mixture is stirred at temperatures between 0° and 50°C preferably at about room temperature. The reaction mixture is filtered and the reaction product (the anhydride of the protected amino acid) isolated. The resulting product is dissolved in a dry inert solvent such as dry dimethyl¬ formamide and placed under nitrogen. Preparation of Mono-L-valine Ganciclovir Step I: Ganciclovir is reacted with a trityl compound to give the ganciclovir intermediate with trityl protection at one of the aliphatic hydroxy groups and the 2-amino group of the guanine ring system of ganciclovir (Formula II) . For example, a solution of 2-3 equivalents of trityl chloride is added to a suspension of ganciclovir in an inert solvent, preferably an aprotic polar solvent, such as dimethylformamide, at a temperature of 0°-100 °C. preferably at 40°-60 °C. This yields a mixture of N- trityl-ganciclovir, O-trityl-ganciclovir, N,O-bistrityl- ganciclovir and tris-trityl-ganciclovir. The tris-trityl- ganciclovir is insoluble and is filtered off; the mono- and bis-tritylated-ganciclovir are precipitated from the solvent solution, which provides some purification of bis- tritylated compound. Further purification is achieved by washing with an organic solvent such as ethyl acetate, and any bis-trityl dissolved may be recovered by crystallization from this ethyl acetate wash. Step II: In this step an activated derivative of amino- protected L-valine of the Formula IV or IVa is esterified with the N,0-bistrityl-ganciclovir intermediate obtained in Step I. Suitable amino-protecting groups for the L- valine derivative are the N-benzyloxycarbonyl group, the phthalyl group, the tertiary butyloxycarbonyl group and the N-(9-fluorenylmethoxycarbonyl) or „FM0C" group. A solution of the N,0-bistrityl-ganciclovir product of Step I in an inert solvent is added to about 1-3 equivalents of the activated L-valine derivative. preferably Z-valine-N-carboxyanhydride or Boc-L-valine, in the presence of an organic base, preferably triethyl amine (TEA) . The reaction mixture is stirred at 10°-40°C, preferably at ambient temperature for 10-90 hours, preferably about 24 hours. The product, such as N,0- bistrityl-O- (N -Z-L-valine) -ganciclovir is collected by filtration, washed and dried under vacuum at 10°-70 °C. Step III: The trityl protecting groups can be removed after completion of the previous steps by dissolving the product of Step II in a polar organic solvent such as trifluoroethanol (TFE) , followed by the addition of anhydrous alkanoic acid such as trifluoroacetic acid (TFA) . The reaction mixture is stirred for 30 minutes to 10 hours at ambient temperature until the reaction is complete by TLC. Solid material is removed by filtration. Most of the trifluoroethanol/trifluoroacetic acid is removed by distillation. The reaction mixture is then slowly added to a mixture of organic solvents, preferably tertbutyl methyl ether (MTBE) :hexane (1:1). The product precipitates out of solution and is collected by filtration, washed and dried under vacuum 10-24 hours. The product can be further purified by crystallization. Step IV: The valine protecting group of the product of Step III is removed by a de-protection reaction, preferably in an acidic medium or solvent, most preferably by hydrogenolysis. De-protection under acidic conditions is preferred, as this will ensure that the amino group liberated in the de-protection reaction will be protonated; that is, that the base of Formula I as it is formed in the de-protection reaction will be captured by an at least stoichiometric amount of acid present. Isolating the compound of Formula I as an acid addition salt will protect the desired stereoconfiguration of the compound of Formula I. Therefore, those examples given below that show the de-protection step also show the concomitant salt formation step. The de-protection reaction is carried out by dissolving the product of the previous step in an inert solvent, preferably in an acidic solvent, using a hydrogenation catalyst, such as platinum, or palladium hydroxide on carbon or palladium on carbon, using elevated hydrogen pressure between 1 and 2000 psi (0.07-140 at ) , preferably 50 to 200 psi (3.5-14 atm) , most preferably 5 to 20 psi (0.35-1.4 atm) . The completion of the reaction can be monitored using conventional TLC analysis. The hydrogenolysis is continued until the conversion is complete, if required with addition of further hydrogenation catalyst. The catalyst is removed and washed. The combined filtrates from filtration and the washings are concentrated and lyophilized to isolate ganciclovir L-valine ester. The purification of the product and the isolation of a crystalline ester is carried out by recrystallization or other purification techniques, such as liquid chromatographic techniques. If present, any protecting group at the 2-amino group of the guanine group may be removed by conventional procedures, as described above. If the tertiary butyloxycarbonyl group is being used as amino-protecting group, its removal is effected with acid, such as HCl and isopropanol as a solvent or with trifluoroacetic acid. Preparation of Salts One of ordinary skill in the art will also recognize that the compound of Formula I may be prepared as an acid addition salt or as the corresponding free base. If prepared as an acid addition salt, the compound can be converted to the free base by treatment with a suitable base such as ammonium hydroxide solution, sodium hydroxide, potassium hydroxide or the like. However, it is important to point out that the free base of Formula I is more difficult to characterize than its acid addition salts. When converting the free base to an acid addition salt, the compound is reacted with a suitable organic or inorganic acid (described earlier) . These reactions are effected by treatment with an at least stoichiometric amount of an appropriate acid (in case of the preparation of an acid addition salt) or base (in case of liberation of the free compound of Formula I) . In the salt-forming step of this invention, typically the free base is dissolved in a polar solvent such as water or a lower alkanol (preferably isopropanol) and mixtures thereof and the acid is added in the required amount in water or in lower alkanol. The reaction temperature is usually kept at about 0° to 50° C, preferably at about room temperature. The corresponding salt precipitates spontaneously or can be brought out of the solution by the addition of a less polar solvent, removal of the solvent by evaporation or in a vacuum, or by cooling the solution. Isolation of Stereoisomers and the Manufacture of Crystalline 2- (2-Amino-l, 6-dihydro-6-oxo-purin-9-yl) - methoxy-3-hydroxy-1-propyl-L- alinate From the Formula I it is apparent that the compound of the invention has one asymmetric carbon atom (chiral center) in the propyl chain, in addition to the asymmetric carbon atom in L-valine. Therefore, two diastereomeric forms exist, the (R)- and (S)- form as determined by the rules of Cahn et al. Suitable methods for the separation of the diastereomers are described in European Patent Application EP 694 547 A. The compounds of Formula I may also be prepared in crystalline form, which has many well-known advantages over the non-crystalline form. Suitable methods for the preparation of the compounds of the invention in crystalline form are also described in European Patent Application EP 694 547 A. The following preparations and examples are given to enable those skilled in the art to more clearly understand and to practice the present invention. They should not be considered as limiting the scope of the invention, but merely as being illustrative and representative thereof. EXAMPLE 1 Preparation of N-Trityl-2- (2-amino-l, β-dihydro-6-oxo- purin-9-yl) -methoxy-3-trityloxy-propan-1-ol 2- {2-Amino-1, 6-dihydro-6-oxo-purin-9-yl)methoxy-l, 3- propanediol (ganciclovir) was bis-tritylated at the N and 0 functionalities as follows: to a slurry of 4.1 kg ganciclovir, 9.5 g DMAP, and 8.65 kg TEA in dimethyl¬ formamide (38.3 kg ) at 50°C was added a solution of 11.9 kg trityl-chloride in 38.2 kg dimethylformamide over 25 minutes. The reaction mixture was heated and stirred at 50°C overnight. The following morning the reaction was complete by HPLC. This yields a mixture of N-trityl- ganciclovir, O-trityl-ganciclovir, N,0-bis-trityl- ganciclovir, and tris-trityl-ganciclovir in a approximate ratio of mono to bis to tri-substituted ganciclovir of 1:2:1. The tris-trityl-ganciclovir which is dimethyl- formamide insoluble is then filtered from the dimethylformamide mixture by cooling the reaction mixture to 10°C and filtering out the tris-trityl-ganciclovir. The tris-tritylated-ganciclovir was washed with 10 kg dimethylformamide, and the transfer lines were washed with 2.3 kg dimethylformamide. The mono- and bis-tritylated- ganciclovir were then precipitated from this dimethyl¬ formamide solution by warming the bistrityl-ganciclovir/ monotrityl-ganciclovir/ dimethylformamide solution to 20°C. and adding 90 kg water over 1 hour. An additional 60 kg water was then added. Some purification of the bistrityl-ganciclovir from the monotrityl-ganciclovir occurs at this point. The temperature of the above mixture rose to 32°C. The mixture was seeded with 0.5 g of N,O-bistrityl-ganciclovir and allowed to age overnight at room temperature. The following morning crystals had formed. The solid was filtered out. Further purification of the bis-trityl-ganciclovir was achieved by washing the solid with 50 kg of water and then with 21 kg of ethyl acetate. The solid was dried under a stream of nitrogen overnight and then in a vacuum oven (-50°C, ~650 millitorr, nitrogen sweep) for 4 days. Yield: 63.2%. Weight: 7.51 kg. HPLC: greater than 90 % N,O-bistrityl- ganciclovir, impurities 1.8%. EXAMPLE 2 2A. Preparation of N-Trityl-2- (2-amino-l, 6-dihydro-6-oxo- purin-9-yl) -methoxy-3-trityloxy-1-propy1-N- (benzyloxycarbonyl) -L-valinate N,O-bistrityl-ganciclovir was coupled with 1.2 equivalents of Z-valine-NCA (N -benzyloxycarbonyl-L- valine-N-carboxy anhydride) as follows: 3.7 kg of N,0- bistrityl-ganciclovir was dissolved in 17.7 kg dimethylformamide and 1.85 1 toluene. The toluene was distilled off (29 ins [736.6 mm] of Hg, to 50°C) for 2.5 hours. The dimethylformamide/ N,O-bistrityl-ganciclovir solution was cooled to 22°C, and then 0.7 1 TEA / 0.2 1 dimethylformamide was added. To this mixture was added a solution of 1.67 kg Z-valine-NCA dissolved in 4.2 1 dimethylformamide over 0.5 hours at 22°C. The reaction was checked by TLC (10% methanol in methylene chloride, silica) and after 1 hour, the reaction was about 90% complete. The reaction was left mixing overnight. The following morning, the reaction was complete by HPLC. The coupled reaction mixture was added to 23 kg of water over 20 minutes, and the pot temperature rose from 22°C to 31°C. This mixture was cooled to 26°C and aged for 45 minutes. Then the mixture was cooled to 10°C over 2 hours and 15 minutes. The solid was filtered out and dried in a vacuum oven (50°C, ~28 ins [711.2 mm] of Hg, nitrogen sweep) for 4 days. Weight: 4.8 kg. HPLC: 93.5% N,0- bistrityl-O- (N -Z-L-valine) -ganciclovir. 2B. Preparation of N-Trityl-2- (2-amino-l, 6-dihydro-6-oxo- purin-9-yl)- methoxy-3-trityloxy-l-propyl-N- (butyloxycarbonyl) -L-valinate The symmetric anhydride of Boc-L-valine was formed by adding 12.6 g 1, 3-dicyclohexylcarbodiirrd.de (DCC) in 80 ml dichloromethane to 26.5 g Boc-L-valine dissolved in 100 ml dichloromethane at 0°-2°C. The temperature rose to about 10°C during the DCC addition. The solution was stirred for 30 minutes at 0° to 3°C and then the 1,3-dicyclohexyl- urea (DCU) was filtered out of solution. This solution was added to 36.5 g N,O-bistrityl-ganciclovir in 180 ml dimethylformamide in the presence of 63.6 mg of DMAP and 8.5 ml TEA at 11°C. The coupling mixture was stirred at room temperature for 16.5 hours. The reaction was judged complete by HPLC (91% N,0-bistrityl-O- (N α -Boc-L-valine) - ganciclovir, 0.16% N,O-bistrityl-ganciclovir. ) 380 ml of water was added and the reaction mixture was stirred for 1 hour. The dichloromethane layer was separated and the solvent was evaporated to obtain a residue. The aqueous layer was extracted with 750 ml ethyl acetate. The ethyl acetate was separated and then combined with the dichloromethane residue. 750 ml of toluene was added to this ethyl acetate layer. The organic layer was washed once with 250 ml water, once with 250 ml 5% NaHC03, and twice with 250 ml water. The organic layer was dried over MgS04, then the MgS04 filtered out. The solvents were evaporated off to 50°C. A residue was obtained. Weight: 53.64g (some solvent still present.) 2C. Preparation of N-Trityl-2- (2-amino-l, 6-dihydro-6-oxo- purin-9-yl)- methoxy-3-trityloxy-l-propyl-N- (butyloxycarbonyl) -L-valinate To 1 g of N,O-bistrityl-ganciclovir in 5 ml dimethylformamide was added 460 mg of Boc-L-valine-NCA, and 0.2 ml TEA (stirred overnight.) The reaction was checked and judged complete by TLC (90 % N,O-bistrityl-O- (N -Boc-L-valine) -ganciclovir by HPLC.) 5 ml of water was added to this solution to precipitate out the product. The solid was filtered out and washed with water. The solid was dried in a vacuum oven (about 50°C, 25 ins [635 mm] of Hg, nitrogen sweep) overnight. Weight: 1.12g. HPLC: 92% N,O-bistrityl-0- (N α -Boc-L-valine) -ganciclovir, 3.6% N,O-bistrityl-ganciclovir. EXAMPLE 3 Preparation of 2- (2-Amino-l, 6-dihydro-6-oxo-purin-9-yl) - methoxy-3-hydroxy-1-propyl-N- (benzyloxycarbonyl) -L- valinate The trityl groups of N,0-bistrityl- (N α -Z-valine) - ganciclovir were removed as follows: To 4.8 kg of N,0- bistrityl-O- (N α -Z-L-valine) ganciclovir in 17.4 kg TFE was added a solution of 1.7 1 TFA in 17.3 kg TFE over 15 minutes. The reaction temperature rose from 18° to 20°C. After 4 hours of mixing the reaction was 85% to 95% complete by TLC (10% methanol in methylene chloride, silica) . After 5 hours of mixing the solid was filtered out. The solid was washed with 5.6 kg of TFE. TFE/TFA was vacuum distilled off (pot temperature to 45°C, vacuum to 26 ins of Hg) over 1 hour and 20 minutes. After distillation of about 80% of the TFE by volume, the pot residue was added to 37 kg MTBE and 32.4 kg of hexane and with vigorous mixing. The precipitant was then filtered out, and the solid was washed with a solution of 7.4 kg MTBE / 6.6 kg hexane. The solid was dried under a stream of nitrogen. This 0-(N GC-Z-L-valme) -ganciclovir was dissolved in 38.8 kg methanol at 41°C; 59 kg of water was added while distilling off the methanol to a jacket temperature of 58°C, about 28 ins [660.4 mm] of Hg. The pot temperature ranged from 16°C to 26°C. The solid was filtered out and washed with 13.2 kg water. The solid was dried under a stream of nitrogen; 1.2% water was determined by the Karl Fisher test. Weight: 1.78 kg. HPLC: greater than 90% Z-valine-ganciclovir, less than 0.5% ganciclovir. EXAMPLE 4 Preparation of 2-(2-Amino-l, 6-dihydro-6-oxo-purin-9-yl) - methoxy-3-hydroxy-1-propyl) -L-valinate hydrochloride 4A. 10 g of N,0-Bistrityl-0-(N α -Boc-L-valine) ganciclovir was dissolved in 20 ml dichloromethane. To this was added 80 ml 50% TFA in dichloromethane, and the reaction mixture was stirred for 1 hour. Cleavage of the trityl groups and the valine protecting groups were judged complete by TLC. The reaction solution was added dropwise to 1 1 of (1:1) MTBE: hexane over 30 minutes. The precipitate was filtered out and was washed with 500 ml (1:1) MTBE: hexane. The solid was dried in a vacuum oven (about 50°C, 25 ins [635 mm] of Hg, nitrogen sweep) overnight. Crude weight of precipitate: 4.31 g. The solid was dissolved in 200 ml acetone containing 2 to 3 ml acetic acid. To this solution was added 4 ml of 4.2 M HCl in ethyl acetate, forming a solid. This was followed by stirring for 45 minutes and then an additional ml of 4.2 M HCl in ethyl acetate was added. This was stirred 30 minutes and then the solid was filtered out. The solid was washed with 50 ml acetone and then dried in a vacuum oven (about 50°C, 25 ins [635 mm] of Hg, nitrogen sweep) ovemight. Weight: 3.82 g, HPLC 97.4% L-valine- ganciclovir, 2.1% ganciclovir, 0.5% guanine. 4B. Alternatively, 2- (2-amino-l, 6-dihydro-6-oxo- purin-9-yl) -methoxy-3-hydroxy-1-propyl) -L-valinate may be prepared directly from the N,0-bistrityl-0- (N -Boc-L- valine) -ganciclovir as follows: To 471 mg of N,0- bistrityl-O- (N α -Boc-L-valine) -ganciclovir was added 10 ml of 2.1 M HCl in ethyl acetate. This reaction mixture was stirred for 15 minutes, and the reaction judged complete by TLC. The solid was filtered out and washed with ethyl acetate. The solid was dried in a vacuum oven (about 50°C, 25 ins [635 mm] of Hg, nitrogen sweep.) Weight: 165 mg, HPLC: 91% L-valine-ganciclovir, 5.9 % guanine, 2.3 % ganciclovir. EXAMPLE 5 Preparation of 2- (2-Amino-l, 6-dihydro-6-oxo-purin-9-yl) - methoxy-3-hydroxy-l-propyl-L-valinate hydrochloride The Z (benzyloxycarbonyl) group was removed from 0- (N -Z-valine) -ganciclovir prepared in Example 3 as follows: To 174 g of 10% Pd on carbon and 1.74 kg of Z- valine-ganciclovir was added 14 kg of methanol mixed with 357 ml 12 M HCl. This reaction mixture was placed under 5 to 10 psi [0.35-0.7 atm] hydrogen. After 2.5 hours of agitation an aliquot was removed from this reaction mixture. The reaction was complete by TLC ( 10 ml acetonitrile, 1 ml water, 1 ml acetic acid, silica) . After 3.5 hours of agitation the catalyst was removed by filtration of the reaction mixture through a Solka Floe bed. The Solka Floe bed was washed with 13.7 kg of methanol. The methanol solutions were combined. Methanol was removed by rotary evaporation to a 60°C bath temperature, and a final vacuum of about 28 ins [711.2 mm] of Hg. The residue was then dissolved in 1.4 1 sterile water at 50° to 60°C. This solution was washed with 1 1 ethyl acetate, and then the ethyl acetate layer was separated. Any residual solvent was removed from this aqueous solution by evaporation. 5.6 1 of approximately 50°C isopropyl alcohol was added dropwise to this aqueous solution at 50°C. The reaction mixture was seeded with a previous batch of 2-(2-amino-l, 6-dihydro-6-oxo-purin-9- yl) -methoxy-3-hydroxy-l-propyl-L-valinate hydrochloride. This crystallization mixture was allowed to cool slowly to 30°C, after which an additional 4.2 1 of isopropyl alcohol was added dropwise at 37°C to 30°C. This mixture was allowed to cool slowly to room temperature with agitation overnight. The following morning the crystallization mixture was cooled to -5°C, and aged for 2 hours. The solid was filtered out and then washed with 2 1 cold 5% water in isopropyl alcohol and then 4 1 cold isopropyl alcohol. The solid was dried in a vacuum oven (675 millitorr, 50°C, nitrogen sweep) for 50 hours. Mass of solid 1.05 kg. Overall yield from ganciclovir starting material: 34%. HPLC: 98.4% 2- (2-amino-l, 6-dihydro-6- oxo-purin-9-y1) -methoxy-3-hydroxy-1-propyl-L-valinate hydrochloride; MS: 355 (MH) + .

Claims: Claims 1. A process for preparing the compound 2-(2-amino- 1, 6-dihydro-6-oxo-purin-9-y1) -methoxy-3-hydroxy-1-propyl- L-valinate or a pharmaceutically acceptable salt or diastereomers thereof which process comprises: (a) reacting 2- (2-amino-l, 6-dihydro-6-oxo-purin-9-yl) - methoxy-1,3-propanediol (ganciclovir) or a salt thereof, with an optionally substituted trityl compound; (b) esterifying N-trityl 2- (2-amino-l, 6-dihydro-6-oxo- purin-9-yl) -methoxy-3-trityloxy-propan-l-ol or a salt thereof, with an activated derivative of L- valine; (c) removing any trityl groups and optionally any amino-protecting group from a compound with the formula wherein P is an optionally substituted trityl group or an amino-protecting group, P is an amino-protecting group and Trt is an optionally substituted trityl group to afford the compound 2- (2-amino-l, 6-dihydro-6-oxo-purin-9- yl) -methoxy-3-hydroxy-1-propyl-L-valinate or a pharma¬ ceutically acceptable salt thereof; (d) optionally converting the compound 2- (2-amino-l, 6- dihydro-6-oxo-purin-9-yl) -methoxy-3-hydroxy-1- propyl-L-valinate into a pharmaceutically acceptable salt thereof; (e) optionally converting the acid addition salt of the compound 2- (2-amino-1, 6-dihydro-6-oxo-purin-9-yl) - methoxy-3-hydroxy-1-propyl-L-valinate to a non-salt form; or (f) optionally diastereoisomerically separating 2- (2- amino-1, 6-dihydro-6-oxo-purin-9-yl) -methoxy-3- hydroxy-1-propyl-L-valinate into its (R) and (S) diastereoisomers. 2. The process of Claim 1 wherein the trityl compound is trityl chloride or trityl bromide. 3. The process of Claim 1 wherein the activated derivative of L-valine is Z-valine-N-carboxyanhydride or butyloxycarbonyl-L-valine. 4. The process of Claim 1 wherein the trityl groups are removed with a polar organic solvent, preferably with trifluoroethanol, or dichloromethane/trifluoroacetic acid.

Inventors: ARZENO HUMBERTO B, HUMPHREYS ERIC R, ARZENO, HUMBERTO, B., HUMPHREYS, ERIC, R.
Applicants: HOFFMANN LA ROCHE, F. HOFFMANN-LA ROCHE AG
Application year: 1997
Main classification: C07D473
IPC codes: C07D473