A NOVEL PROCESS FOR THE PREPARATION OF DIDANOSINE USING

NOVEL INTERMEDIATES

FIELD OF THE INVENTION The present invention provides a novel process for preparation of didanosine in high yield and purity using novel intermediates. The present invention also relates to novel crystalline alkali metal and alkaline earth metal salts of 2',3'-dideoxy-2',3'-didehydroinosine.

BACKGROUND OF THE INVENTION European Patent No. EP 206497 disclosed certain 2',3'-dideoxy- nucleosides and pharmaceutically acceptable derivatives thereof. These compounds are antiviral agents. Among them didanosine, chemically 2', 3'- dideoxyinosine is a selective HIV-I inhibitor and can be used as medicaments in the treatment and prophylaxis of viral, especially retroviral infections. Didanosine is represented by the following structure:

U.S. Patent No. 5,451 ,671 described purification of crude didanosine by extracting didanosine with a solvent such as an alcohol from a basic solution having a pH of not less than 13 of crude didanosine. U.S. Patent No. 5,451 ,671 also described purification method of didanosine by passing a resin containing column at a pH of not less than 11.

U.S. Patent No. 6,207,650 disclosed that pure didanosine can be obtained by isolating didanosine as alkali metal or alkaline earth metal salt of didanosine as a crystalline solid and then converting the salt to didanosine. Even though, the process described (above) in U.S. Patent No.

6,207,650 yields pure didanosine, the yields obtained by the crystallization as alkali metal or alkaline earth metal salt of didanosine are very low and so, there

needs a process for purification of didanosine that can yield pure didanosine in high purity and in high yield.

U.S. Patent No. 5,290,927 (column 11 , lines 24 - 29) indicated that water, if contained during the catalytic reduction of Z.S'-didehydro^'.S ¹ - dideoxyinosine to 2',3'-dideoxyinosine greatly accelerates the decomposition of substrate and indicated, so, that the reaction is preferably conducted in an nonaqueous medium.

We have surprisingly found that even though hydrogenation of 2', 3'- dideoxy-2',3'-didehydroinosine can not be hydrogenated in aqueous medium, their alkali metal or alkaline earth metal salts can be hydrogenated smoothly and completely to give 2',3'-dideoxyinosine alkali metal or alkaline earth metal salts. The salts thus obtained can be converted to didanosine by known methods.

It has also been found that didanosine or didanosine alkali metal or alkaline earth metal salt; which is obtained by hydrogenation of alkali metal or alkaline earth metal salt of 2',3'-dideoxy-2',3'-didehydroinosine has very purity and the product is obtained in high yield.

It has also been found that the alkali metal or alkaline earth metal salt of

2',3'-dideoxy-2',3 ^l -didehydroinosine can be isolated from a solution or reaction mass as a crystalline solid. The crystallization of the alkali metal or alkaline earth metal salt of 2',3'-dideoxy-2',3 ^l -didehydroinosine provides a purification method for preparing finally didanosine.

Thus, the crystalline alkali metal or alkaline earth metal salt of 2', 3'- dideoxy-2',3'-didehydroinosine provides a valuable intermediate for preparation of didanosine or a alkali metal or alkaline earth metal salt of didanosine.

U.S. Patent No. 6,207,650, which is incorporated herein by reference, discloses crystalline alkali metal or alkaline earth metal salt of didanosine and its use in the pharmaceutical formulation and the use as an intermediate for preparing didanosine. The present invention also provides a process for preparing sodium salt of didanosine using novel intermediate.

One object of the present invention is to provide a novel process for preparing didanosine in high yield and purity using novel intermediates.

Another object of the present invention is to provide novel crystalline alkali metal and alkaline earth metal salts of 2',3 ^l -dideoxy-2',3'-didehydroinosine and processes for preparing them.

DETAILED DESCRIPTION OF THE INVENTION According to one aspect of the present invention, there is provided a novel crystalline alkali metal or alkaline earth metal salt of 2',3'-dideoxy-2',3'- didehydroinosine of formula II:

wherein M is an alkali metal ion such as Na ⁺ , K ⁺ or Li ⁺ ; or an alkaline earth metal ion such as Ca ²⁺ or Mg ²⁺ .

According to another aspect of the present invention, a process is provided for preparation of crystalline alkali metal or alkaline earth metal salt of 2',3'-dideoxy-2',3 ¹ -didehydroinosine, which comprises crystallizing the alkali metal or alkaline earth metal salt of Z^'-dideoxy^'^'-didehydroinosine from a solution of alkali metal or alkaline earth metal salt of 2\3'-dideoxy-2',3'- didehydroinosine in a solvent selected from alcoholic solvents such as methanol, ethanol, isopropyl alcohol, tert-butyl alcohol and n-butyl alcohol; water; ethyl acetate; acetonitrile; and a mixture thereof.

Preferable solvent is selected from the group consisting of methanol, ethanol, water and a mixture thereof.

Preferable alkali metal ion is Na ⁺ or K ⁺ and more preferable alkali metal ion being Na ⁺ . Preferable alkaline earth metal ion is Mg ²⁺ .

Preferably, the invention particularly relates to a novel crystalline 2', 3'- dideoxy-2',3'-didehydroinosine sodium salt of formula Ma:

Crystalline 2 ¹ ,3'-Dideoxy-2',3 ^l -didehyciroinosine sodium salt is characterized by an x-ray powder diffraction spectrum having peaks expressed as 2θ at about 15.6, 17.3, 18.0, 21.5, 23.4, 24.7, 27.2 and 29.8 degrees. Figure 1 shows typical X-ray powder diffraction spectrum of crystalline 2',3'-dideoxy- 2',3'-didehydroinosine sodium salt.

According to another aspect of the present invention, there is provided a novel process for preparing didanosine of formula I.

which comprises: a) hydrogenating an aqueous solution of an alkali metal or alkaline earth metal salt of 2',3'-dideoxy-2',3'-didehydroinosine of formula II:

wherein M is an alkali metal ion such as Na ⁺ , K ⁺ or Li ⁺ ; or an alkaline earth metal ion such as Ca ²⁺ or Mg ²⁺ ;

by using an hydrogenating metal catalyst; and b) neutralizing with an acid to obtain didanosine of formula I.

Any of the conventionally used hydrogenating metal catalysts may be , used. Preferably, hydrogenation may be carried out by using raney nickel, palladium, platinum, rhodium. These catalysts may be used as their metal form or in the form of their compounds such as oxides. The catalyst used may be supported on an inert support. Some examples of the catalysts that may be used are raney nickel, palladium, platinum, palladium carbonate, platinum oxide, palladium on carbon, palladium on alumina and palladium on CaCO ₃ . The hydrogenation reaction is preferably carried out at the pressure below about 5 Kg/cm ² , more preferably at below about 3 Kg/cm ² and still more preferably at about 1 - 2 kg/cm ² .

Preferably the hydrogenation reaction is carried out at about 5 - 50 ⁰ C, more preferably at about 10 - 30 ⁰ C and still more preferably at about 10 - 20 ⁰ C. The hydrogenation may also be carried out in the presence of water plus any other solvent. The solvent, which can be used along with water, may be selected from alcohols such as methanol, ethanol, isopropyl alcohol, tert-butyl alcohol, n-butyl alcohol and a mixture thereof.

The choice of the acid in step-(b) is not critical and any inorganic or organic acid may be used. Examples of inorganic acids are hydrochloric acid, sulfuric acid and phosphoric acid. Examples of organic acids are carboxylic acids such as formic acid and acetic acid; sulfonic acids such as methanesulfonic acid and toluenesulfonic acid.

The Didanosine obtained above may be worked up by conventional means or by a known method to obtain didanosine as a solid.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows typical X-ray powder diffraction spectrum of crystalline 2',3'- dideoxy-2',3'-didehydroinosine sodium salt. X-Ray powder diffraction spectrum was measured on a Bruker axs D8 advance x-ray powder diffractometer having a Copper-Kα radiation. Approximately 1 gm of sample was gently flattened on a sample holder and scanned from 2 to 50 degrees two-theta, at 0.03 degrees two-theta per step and a step time of 0.5 seconds. The sample was simply placed on the sample

holder. The sample was rotated at 30 rpm at a voltage 40 KV and current 35 mA.

The invention will now be further described by the following examples, which are illustrative rather than limiting.

Example 1 δ'-O-Acetyl^'.S'-dideoxy^'.S'-didehydroinosine (100 gm) is added to 25% monomethyl amine (360 ml) at 25 - 35 ⁰ C and stirred for 4 - 5 hours at 25 - 35 ⁰ C. Filtered the mass on celite bed, washed with methanol (50 ml) and distilled off the solvent completely under reduced pressure at below 70 ⁰ C. Methanol (25 ml) is added to the filtrate, distilled off the solvent completely, again methanol (25 ml) is added and co-distilled. To the mass, acetone (25 ml) is added, co-distilled, again co-distilled with 25 ml of acetone at below 7O ⁰ C and acetone (400 ml) is added. The contents are cooled to 25 - 35 ⁰ C, stirred for 2 hours to 2 hours 30 minutes at 25 - 35 ⁰ C. Then the reaction mass is cooled to 0 - 1O ⁰ C and stirred for 1 hour to 1 hour 30 minutes. Filtered the material, washed with acetone (80 ml) and dried at 55 - 65 ⁰ C to give 62.5 gm of 2',3'-dideoxy- 2',3'-didehydroinosine (HPLC Purity: 98.14%).

Example 2

2',3'-Dideoxy-2',3'-didehydroinosine (55 gm) is added to ethanol (330 ml) and then sodium hydroxide flakes (7 gm) are added during 30 minutes to 1 hour at below 50 ⁰ C. To the contents 40% sodium hydroxide solution (35 ml) is added at below 50 ⁰ C and stirred for 30 minutes at 40 - 5O ⁰ C. Water (165 ml) is added to the reaction mass and stirred for 1 1 - 12 hours at 25 - 3O ⁰ C. The reaction mass is cooled to 5 - 15 ⁰ C and stirred for 1 hour. Filtered the solid, washed with ethanol (45 ml) and dried at 50 - 55 ⁰ C to give 57.2 gm of Z.S'-dideoxy^'.S ¹ - didehydroinosine sodium salt (HPLC Purity: 99.63%).

Example 3

2',3'-Dideoxy-2',3 ¹ -didehydroinosine sodium salt (50 gm) is added to water (180 ml) and stirred for 20 minutes at 25 - 35 ⁰ C. Then the contents are hydrogenated under nitrogen atmosphere at 1.5 - 2.0 kg/cm ² hydrogen pressure at 15 - 2O ⁰ C using raney nickel catalyst for 7 - 8 hours. Filtered the reaction

mass on celite bed and washed with water (25 ml). To the filtrate activated carbon (2 gm) is added and stirred for 20 minutes. Filtered the mass on celite bed and washed with water (25 ml). The filtrate is cooled to 5 - 15 ⁰ C, the pH is adjusted to 7.0 - 8.0 with dilute hydrochloric acid (13 ml water + 13 ml cone. HCI) at 5 - 15 ⁰ C and stirred for 2 hours 30 minutes. Filtered the mass, washed with water (40 ml). The resulting wet cake is added to water (120 ml), the temperature is raised to 65 - 75 ⁰ C and stirred for 20 minutes at the same temperature. The resulting solution is cooled to 25 - 35 ⁰ C and stirred for 1 hour 30 minutes. The reaction mass is then cooled to 10 - 15 ⁰ C and stirred for 1 hour 30 minutes. Filtered the solid, washed with the 40 ml of water and dried at 55 - 6O ⁰ C to give 30.5 gm of didanosine (HPLC Purity: 99.85%).