Field of the Invention This invention, in general, relates to a process for the preparation of beta-thymidine. In particular, the present invention provides an improved process for the preparation of pure beta-thymidine from 5-methyl uridine.
Background of the Invention
Beta-thymidine (1 -((2R,4S,5R)-4-hydroxy-5(hydroxymethyl)oxolan-2-yl)-5-methyl-
pyrimidine-2,4-dione is an important intermediate for the preparation of dideoxynucleosides. Beta-thymidine is of interest in the synthesis of active pharmaceutical ingredients, more specifically for the synthesis of active pharmaceutical ingredients such as AZT, Stavudine etc, which are useful in the treatment of patients suffering from acquired immune deficiency syndrome (AIDS).
A common way of producing beta-thymidine involves a coupling reaction between protected deoxyribose and a protected thymine as disclosed in A.J.Hubbard, et al. Nucleic Acids Research, 1984, 12, 6827 et seq.; U. Nieballa, et al. J. Org Chem. 1974. 39, 3654 et. seq. and U.S. Pat. No. 3,748,320. This results in the formation of mixed alpha- and beta-anomers of the protected thymidine. The alpha anomer prepared by the coupling reaction has very little use in the synthesis of pharmaceutically useful compounds. Moreover the separation of alpha and beta-anomer is a tedious process thereby reducing the process efficiency and also results in loss of raw materials due to formation of the alpha anomer.
US Pat No. 4,914,233 discloses a process for the production of the beta-anomer of thymidine using D-ribose and thymine as raw materials. The process steps involve coupling reaction between protected D-ribose (mixture of alpha- and beta- tetra-o-acyl ribofuranose) and protected thymine in presence of Lewis acid or Friedal-crafts catalyzed condensation to form tri-0-acyl ribofuranose, based-catalyzed hydrolysis to form 5-ribothymidine (5-methyl uridine) followed by cyclization reaction to form 2,2'-anyhydrothymidine by using low boiling, large volume of highly flammable, costly solvent such as ether for quenching the formed reaction mass. 2,2'-anyhydrothymidine is subjected to halohydrogenation to form 2'-halo-2'-deoxy-5-methyluridine and subsequent reduction to form beta-thymidine. The mixture of alpha- and beta- tetra-o-acyl ribofuranose formed during reaction process and

instability of intermediate 2'-halo-2'-deoxy-5-methyluridine formed during the process leads to unwanted by-products, thereby reducing the efficiency of the process.
US Pat No. 5,466,787 discloses a process for the preparation of beta-thymidine from 5-methyluridine wherein the process involves cumbersome steps for converting of 2, 2'-anyhydrothymidine to beta-thymidine in order to avoid degradation of the unstable intermediates formed during the reaction.
US Pat No. 5,596,087 discloses a process of preparation of beta-thymidine wherein the process includes the step of conversion of 5-methyl uridine to 2, 2'-anyhydrothymidine. The conversion of 5-methyl uridine to 2, 2'-anyhydrothymidine involves the use of low boiling, highly flammable and toxic organic solvent such as benzene in high volumes for quenching the formed reaction mass.
The processes disclosed in the prior art above use toxic or corrosive reagents and involve tedious, time-consuming, expensive and cumbersome steps to prepare and purify or isolate the products. Therefore, there is a need for an improved process for the preparation of beta-thymidine that overcomes the limitations associated with the existing prior art.
Summary of the Invention It is an object of the present invention to provide an improved process for producing beta-thymidine in a pure form employing 5-methyl uridine as a starting material.
It is another object of the present invention to provide an expeditious, efficient and economical process for producing beta-thymidine from 5-methyl uridine, wherein the process involves minimum steps and less time.
It is yet another object of the present invention to provide a process for producing beta-thymidine from 5-methyl uridine, wherein the process employs non-toxic and non-corrosive reagents.
The above and other objects of the present invention are further attained and supported by the following embodiments described herein. However, the scope of the invention is not restricted to the described embodiments herein after.

In accordance with one preferred embodiment of the present invention, there is provided a process for producing beta-thymidine, wherein the process comprises of cyclizing 5-methyl uridine to 2,2'-anyhydrothymidine, hydrohalogenating followed by reductive dehalogenation of the 2,2'-anyhydrothymidine to obtain crude beta-thymidine and purifying the resultant crude beta-thymidine to obtain pure beta-thymidine.
In accordance with another embodiment of the present invention, there is provided a process for producing beta-thymidine, wherein the step of cyclizing 5-methyl uridine to 2, 2'-anyhydrothymidine is carried out in presence of a solvent such as alkyl acetate for quenching the formed reaction mass, wherein the solvent is non-toxic and can be recovered easily.
In accordance with yet another embodiment of the present invention, there is provided a process for producing beta-thymidine, wherein the step of hydrohalogenation of 2, 2'-anyhydrothymidine to obtain 2'halo-2'deoxy-5-methyl uridine is carried out by reacting 2,2'-anyhydrothymidine with hydrogen bromide (HBr) in dimethyl formamide (DMF) preferably at a concentration between 17-21 %.
In accordance with yet another embodiment of the present invention there is provided a process for producing beta-thymidine, wherein reaction mass containing crude 2'-halo-2'-deoxy-5-methyl uridine is neutralized by employing ammonia gas at low temperature.
In accordance with still another embodiment of the present invention, there is provided a process for producing beta-thymidine, wherein the step of reductive dehalogenation of 2'-halo-2'-deoxy-5-methyluridine is carried out in presence of a catalyst preferably raney nickel and DMF at a pH maintained by using a buffer preferably ammonium acetate.
Detailed Description of the Invention While this specification concludes with claims particularly pointing out and distinctly claiming that, which is regarded as the invention, it is anticipated that the invention can be more readily understood through reading the following detailed description of the invention and study of the included examples.

The present invention discloses an improved process for producing beta-thymidine from 5-methyl uridine employing a simple and economical process.
The term "beta-thymidine" herein denotes beta anomer of thymidine or other technical equivalents but excluding the alpha anomer of thymidine.
The present invention provides a process comprising the steps of a) cyclizing 5-methyl uridine (I) to 2, 2'-anyhydrothymidine (II), b) hydrohalogenating 2, 2'-anyhydrothymidine (II) to obtain the halo derivative (III), followed by reductive dehalogenation to obtain crude beta-thymidine c) purifying crude beta-thymidine to obtain pure beta-thymidine (IV).
5-Methyl uridine (I) is used as a starting material for the present invention. 5-Methyl uridine is commercially available at a low cost than D-Ribose or D-xylose used in prior art for the preparation of beta-thymidine.
According to the invention, the cyclization step involves loss of water from 5-methyl uridine (I) to obtain 2, 2'-anhydrothymidine (II). The step of cyclizing involves heating 5-methyluridine (I) with dialkyl or diaryl carbonate in presence of dimethyl formamide (DMF). Resulting reaction mass was quenched with 2 volumes of methanol to 13 volumes of alkyl acetate, preferably ethyl acetate in place of large volumes of costly low boiling ether used in prior art. Further, easy handling of ethyl acetate favors larger recovery of the solvent. Low cost and easy recovery of ethyl acetate reduces the production cost of beta-thymidine.
2'-halo-2'deoxy-5-methyl uridine (III) preferably bromo derivative, is formed by hydrohalogenating 2, 2'-anyhydrothymidine (II) with hydrogen bromide (HBr) in DMF. The intermediate so formed is highly unstable in presence of excess of HBr resulting in formation of lot of impurities. The present invention discloses use of HBr at a standard concentration in DMF, preferably between 17-21% to stabilize the intermediate. Further, 2, 2'-anyhydrothymidine (II) is added in batches to HBr in DMF to avoid degradation of formed bromo product. The concentration of HBr in DMF is critical to get pure intermediate of 2'-bromo-2' deoxy-5 -methyluridine.

The stability of the intermediate is further enhanced by neutralizing the reaction mass using a weak base, preferably ammonia gas at a lower temperature, more preferably by passing ammonia in methanolic solution of the intermediate after cooling to 10 to 15°C.
According to the process, reductive dehalogenation of 2'-bromo, 2'deoxy-5-methyl uridine (III) to obtain beta-thymidine (IV) is carried out in presence of raney nickel in a mixture of methanol and DMF, the mixture being maintained between a specific pH range, preferably between 6 and 9. The specific pH range is maintained by addition of one or more buffer, preferably ammonium acetate.
In a typical procedure of the invention reductive dehalogenation of the intermediate to obtain crude beta-thymidine is carried out by using 5% raney nickel at 2-4 kg/cm hydrogen pressure at a temperature of 40°C for about 8-10 hrs in presence of ammonium acetate buffer. Yield of crude beta-thymidine so obtained is about 85% or more. The reductive dehalogenation in the process of the present invention may be practiced by maintaining the pH suitably for the reaction.
The purification of crude beta-thymidine (IV) involves removal of the salts from the crude product by pulping in methanol, avoiding the use of resins. The process of the present invention further, ensures the stability of the intermediate (III) by avoiding the use of Amberlyst 21 resin, since the intermediate is comparatively stable in acidic condition.


The following is the schematic representation of the reaction scheme
o


"N If
HO 1 N
HO OH

The following non-limiting examples illustrate specific embodiments of the present invention. They are, not intended to be limiting the scope of present invention in any way.

EXAMPLE -1
Cvclization of 5-methvl uridine (Ribothymidine) (1) to 2,2'-Anhvdrothvmidine dl) A lOL round bottom flask equipped with magnetic stir bar, reflux condenser, and nitrogen inlet was charged with 353g (1.366 mole) of ribothymidine, 352 g (leq) of biphenyl carbonate and 11.76 g of sodium bicarbonate in 706 mL of DMF. The mixture was gradually heated to HO^C and temperature maintained for 30 minutes. Subsequently, the reaction mixture was cooled to 30°C. The clear solution so obtained was slowly poured into 4.7 Liters of ethyl acetate and stirred for 3 to 4 hours to obtain a precipitate. The precipitate was filtered and dried to yield 300g (85%) of anhydrothymidine.
EXAMPLE -2
Hydrohalogenation of 2,2'-Anhydrothymidine (II ) to 2'-bromo 2'-Deoxy-5-Methvluridine (
m
a) Preparation of HBr in DMF
600 ml of DMF was taken in a round bottom flask at 30 °C and the reaction mass was cooled to 0 to 5 *^C. Hydrobromic acid in acetic acid was charged into another round bottom flask and heated slowly to generate HBr gas (anhydrous). The generated HBr gas was purged into cooled DMF in another round bottom flask till the percentage of HBr in DMF reaches to 21%.
b) Anhydrothymidine (300g) was added in batches to the aboye prepared HBr in DMF (21%,
1 Eqnt) mixture at room temperature and gradually heated to 85-90°C and maintained for 1
hr. The reaction mass containing 2'-bromo-2'-Deoxy-5-Methyluridine ( III)_was gradually
cooled to room temperature.
EXAMPLE -3
Reductive dehalogenation of 2'-bromo-2'-deoxv-5-methvluridine (III) to beta-thvmidine. Methanol (2.4 liters) was added to the above reaction mass containing 2'-bromo-2'-deoxy-5-methyl uridine and cooled to 10 tol5°C followed by neutralization with ammonia gas at 10 to 15 '^C to obtain a pH between 7.5 to 8. The neutralized reaction mass was taken in a hydrogenator followed by addition of ammonium acetate 96.12g (1.24 mole) and raney nickel (20g) and subsequently hydrogen was applied at a pressure of about 2-4 kg/cm at 40 C and maintained for 8-10 hours. The raney nickel catalyst was filtered and the filtrate was further filtered through hyflo. DMF and methanol are distilled completely under vacuum at a temperature below 80°C to obtain a residue.

EXAMPLE -4
Purification of crude beta-thvmidine
Methanol (900ml) was added to the residue obtained above and heated to get a clear solution followed by addition of charcoal (30g) at 40-45°C and maintained for 2 hours between 55 to 60 C. The reaction mass was filtered through hyflo and washed with hot methanol, concentrated to residual one volume, cooled to 0 to 5°C, maintained for 8 to 10 hours and the solid mass was filtered and washed with acetone and dried by suction to get 300g of beta-thymidine having purity 95%.
To 600 ml water, beta-thymidine (300gm) was added and the reaction mass was heated to obtain a clear solution at 55-60*^C. Charcoal (30 gm) was added to the solution, maintained for 2 hours and filtered through celite. The filtrate was cooled and the temperature maintained at 0-5°C for 8-10 hours. The resultant product was filtered and dried at 60 °C to yield pure beta-thymidine (250gm) having purity 99%.
Mother liquor was concentrated and methanol was added followed by filtration of salts at 45°C. The filtrate was concentrated and dissolved in 1.5 volume water, charcolised, filtered through celite, chilled and maintained for 16 hrs at 5°C .Filtered the product and dried at 60°C.Yield 26g (purity 99%).
While this invention has been described in detail with reference to certain preferred embodiments, it should be appreciated that the present invention is not limited to those precise embodiments rather, in view of the present disclosure, which describes the current best mode for practicing the invention, many modifications and variations, would present themselves to those skilled in the art without departing from the scope and spirit of this invention. This invention is susceptible to considerable variation in its practice within the spirit and scope of the appended claims.

We Claim:
1. A process for producing beta-thymidine, the process comprising the steps of:
a) cychzing 5-methyl uridine employing alkyl acetate to obtain 2, 2'-
anyhydrothymidine;
b) converting the resultant 2, 2'-anyhydrothymidine to crude beta-thymidine; and
c) purifying the crude beta-thymidine.

2. The process according to claim 1, wherein said alkyl acetate is preferably ethyl acetate.
3. The process according to claim 1, wherein the alkyl acetate is preferably used in about 13 volumes.
4. The process according to claim 1, wherein the step of converting 2, 2'-anyhydrothymidine to crude beta-thymidine comprising:

a) hydrohalogenating the 2, 2'-anyhydrothymidine to 2'halo-2'deoxy-5-methyl uridine; and
b) reductively dehalogenating the 2'halo-2'deoxy-5-methyl uridine to beta-thymidine in presence of raney nickel.

5. The process according to claim 4, wherein hydrohalogenation is carried out in the presence of hydrogen bromide (HBr) in dimethyl formamide (DMF).
6. The process according to claim 5, wherein a concentration of the hydrogen bromide (HBr) in dimethyl formamide (DMF) is in between 17 to 21 %.
7. The process according to claim 4, wherein the reductive dehalogenation is carried out at a pH range from 6 to 9.
8. The process according to claim 4, wherein the raney nickel is employed as a catalyst in non-aqueous slurry and wherein the non aqueous slurry preferably comprising a mixture of methanol and dimethyl formamide (DMF).

9. The process according to claim 4, wherein the hydrohalogenation is followed
by neutralizing the reaction mass, employing ammonia gas at low temperature.
10. The process according to claim 1, wherein the purification of crude beta-
thymidine is carried out by employing methanol and water.
11. The process according to claim 1, wherein the beta-thymidine is preferably
99% pure.