"Process for the large scale production of Stavudine"

The present invention relates to an improved process for large-scale production of Stavudine through novel solvates of Stavudine

Background of the invention:

2',3'-didehydro-3'-deoxythymidine (Stavudine) has the formula given below.

Stavudine is also known as d ₄ T which is approved by U.S. FDA for the therapeutic treatment of patients infected with retroviruses.

Synthesis of Stavudine is first reported by J.P. Horwitz et al (J.Org. Chem. (1996) 31, 205) starting from 3'4'-dimesylthymidine as shown in Scheme- 1.

Scheme- 1

In the above synthesis, 3',5'-di-O-mesylthymidine is first treated with sodium hydroxide in refluxing water for 2 hours and the resultant 3,4-anhydrothymidine is treated with potassium t-butoxide in dimethyl sulfoxide (DMSO) at room temperature for 2 hrs. The reaction mixture is neutralized, evaporated to dryness and after a series of manipulations which included extraction, declourization, precipitation and recrystallization, gave Stavudine in 79% yield and an overall yield of 56%.

The above procedure is modified by Mansuri et al (J.Med. Chem. (1989) 32, 461) where the potassium t-butoxide / DMSO mixture is poured in to 30 volumes of toluene whereby the Stavudine precipitates as the potassium salt along with excess potassium tert- butoxide. This modification avoided the distillation of DMSO which caused decomposition of Stavudine. The obtained salt is neutralized in water and the precipitated product is extracted with acetone and then evaporated to dryness to give an off-white solid in 57% yield.

Adachi et al. [Carbohydrate Research (1979) 113] over-came some of the decomposition problems by employing sodium hydroxide in hexamethyl phosphorictriamide (HMPA). HMPA is removed by forming a chloroform complex in an aqueous mixture and Stavudine is isolated from the aqueous phase.

U.S. Pat. No. 5,539,099 discloses a modified route for producing Stavudine as shown in Scheme-2.

However, all the above discussed processes have one major draw back in common in that the final purity of Stavudine is less than the desired 99.5%. Known impurities which are difficult by using conventional techniques are still present, some of which were thymine, thymidine, threo-thymidine, 3,5-anhydrothymidine and 5'-O-[stavudin-5"-yl]-thero- thymidine being particularly difficult to remove.

Further U.S. Pat. No. 5,539,099 teaches the use of only N-methyl-2-pyrrolidinone as suitable to form a solvate of Stavudine whose recovery requires the use of an aprotic ester, amide or ketone solvent. No consideration is given for their use to form a solvate.

U.S. Pat. No. 6,635,753 discloses the novel Stavudine solvates of N,N-dimethyl acetamide, N,N-dimethylacrylamide, N,N-dimethylpropionamide etc. and recovering substantially pure Stavudine by breaking the solvate produced.

It is therefore an object of this invention to provide an improved process which is more efficient, using less toxic materials (for example, using solvents which are less toxic than N-Methyl-2-pyrrolidinone, DMA, DMAC and DMP) and providing pure product before recrystallization steps are carried out.

Summary of the Invention:

The main object of the present invention is to provide an improved process for producing substantially pure Stavudine on commercial scale.

Another object of the invention is to provide a process for preparing novel solvates of Stavudine by reacting Stavudine with the selected solvent.

Yet another object of the invention is to provide a process or producing substantially pure Stavudine by desolvation of novel solvate of Stavudine.

Brief description of the drawings:

Fig: 1 shows the X-ray diffraction pattern of Stavudine obtained as per example-2

Fig: 2 shows the X-ray diffraction pattern of crystalline Stavudine form-I

Detailed description of the invention:

Thus in accordance with the present invention process for preparation of substantially pure Stavudine comprising steps of:

• Converting 3',5'-anhydrothymidine to crude Stavudine • Converting crude Stavudine to stable solvates of Stavudine

• Desolvation of the solvates to give pure Stavudine

The reaction scheme can be expressed as follows.

iliera/Pi-outi..tiecrit 1..bUuUtioUxAiidUeC DeSOlVation

DMPU solvate Stavudine

In a specific embodiment, the present invention provides a process for the preparation of Stavudine, which involves

a) Heating a mixture of 3',5'-anhydrothymidine and potassium tert-butoxide, in acetonitrile to 40° - 80 ⁰ C preferably 60°-70°C; b) Maintaining the reaction mass for about 1 to about 4 hours preferably 2-3 hours c) Cooling the reaction mass to about 5° - 10 ⁰ C d) Dissolving the reaction mass in methanol e) Neutralizing the reaction mass with an organic acid preferably acetic acid or an inorganic acid preferably Hydrochloric acid, f) Filtering the reaction mass to remove the salts g) Evaporating the filtered solution to get crude Stavudine

Further crude Stavudine is converted to its solvates by a) Charging the crude Stavudine in an organic solvent which is selected from an alcohol wherein the alcohol is selected from the group consisting of methanol, ethanol, n -propanol, 2-propanol, 1-butanol and iso butanol, ketone wherein the ketone is selected from the group consisting of acetone, methyl ethyl ketone and ' methyl isobutyl ketone and heating to reflux b) Filtering the solution while hot condition through hyflow c) Treating the filtered solution with a compound selected from N ₅ N- Dimethylpropyleneurea, N, N-Dimethylethylenurea and their mixtures thereof in an organic solvent d) Precipitating the solvated form of Stavudine and a solvent selected from N, N- Dimethypropyleneurea, N, N-Dimethylethyleneurea and their mixtures thereof e) Filtering the product of step d) to obtain the solvated form of Stavudine and a solvent selected from N,N-Dimethypropyleneurea, N,N-Dimethyl ethyleneurea and their mixtures thereof

The prepared Stavudine DMPU solvate and Stavudine DMEU solvate are novel. They are identified and characterized by chemical analysis, NMR & Mass spectral data. Stavudine solvates are further converted to Stavudine by

a) Suspending Stavudine solvate in a solvent which is selected from water, alcohol wherein the alcohol is selected from the group consisting of methanol, ethanol, 1- propanol, 2-propanol, 1-butanol and iso butanol, ketone wherein the ketone is selected from the group consisting of acetone, methyl ethyl ketone and methyl isobutyl ketone and an amide other than those which form solvates

b) Heating the suspension to 40 ⁰ C to reflux temperature preferably to 50°-60°C for dissolution

c) Treating with activated carbon

d) Filtering the mass while hot condition preferably through hyflow e) Cooling the clear filtrate to O ⁰ C to room temperature preferably O ⁰ C to 5 ⁰ C f) Isolating the precipitated product g) Drying the product to give pure Stavudine having purity more than 99.5%

The obtained Stavudine is crystalline in nature and the XRD resembles polymorphic form I both in wet and dry product. The XRD of the obtained product is mentioned in figure- 1

The % purity of the product obtained by this process generally exceeds 99.5% and in some instances exceeds 99.80%.

Several advantages in carrying out embodiments of the invention include but are not limited to the following:

When the crystallization solvent is essentially DMPU, the impurity levels are very low which is especially important for thymine and stavudinyl-threo-thymidine impurities since they are particularly difficult to remove using conventional techniques.

N,N-Dimethylpropyleneurea is a more common, more readily available, less hazardous, and less toxic solvent than N-Methyl-2-pyrrolidmone (NMPO), N,N-Dimethylacetamide and other solvents disclosed in the prior art.

In sum the new improved process for producing Stavudine and related analogs is amenable to large-scale use by virtue of its selection of regents, reaction conditions, and separation/purification features that result in an efficient process minimizing troublesome impurities and product degradation as well as providing a high yield and purity of product without generation of its that are either toxic or produced in large scale.

Example-l: Preparation of 2',3'-Didehydro-3'-deoxythymidine-N,N-dimethyl propylene urea solvate (Stavudine DMPU solvate)

3',4'-anhydrothymidine (25 g, 0.116 mole) is suspended in acetonitrile (300 ml). The slurry is stirred and potassium tert-butoxide (25 g, 0.2232 mole) is added. The resulting

solution is heated to 60-65°C for 2 hrs. The solution is cooled to 5-10°C, separated salt is filtered under nitrogen atmosphere and immediately dissolved in methanol (200 ml). pH of the solution is adjusted to 6.5 with cone. HCl and cooled to 2O ⁰ C. The precipitated KCl is filtered and washed with methanol (25 ml). The combined filtrate and wash are concentrated under vacuum. To the residue acetone (250ml), is added and heated to reflux for 0.5 hrs. Reaction mass is filtered while hot and washed with hot acetone (25 ml). Combined filtrate and wash are mixed with N, N-dimethylpropyleneurea (27 ml) and concentrated until about 100 ml of acetone remained. The solution is allowed to cool to 0-5°C, filtered, washed with acetone (25 ml) to give a total of 27.6 g (70.2%) of the title compound.

Example-2: Preparation of 2',3'-didehydro-3'-deoxythymidine (Stavudine)

Stavudine DMPU solvate (25 g, 0.060 mole) is dissolved in isopropanol (250 ml), and treated with activated carbon anti-eno crowns (2.5 g) at reflux temperature. Reaction mass is filtered while hot and washed with hot isopropanol (25 ml). The combined filtrate and wash are concentrated to 100 ml and allowed the mass to cool to room temperature slowly. Cooled the mass to 0-5 ⁰ C and the precipitated product is filtered. washed the product with chilled isopropanol (10 ml) and air dried to give 13.2 g (83%) of the title compound which purity greater than 99%.XRD data resembles crystalline form-I

General procedure

Stavudine-N,N-DimethyI propylene urea (DMPU) solvate from crude Stavudine

Method A:

Crude Stavudine (25 g, 95.79% pure, 0.116 mole) is dissolved in N,N-Dimethyl propylene urea (30.0 g, 1.2 parts) at 85-9O ⁰ C. The solution is allowed to cool to room temperature. The mixture is further cooled to 6-8°C and maintained for 2 hrs. The crystals are filtered, washed with acetone (2 x 12 ml) and dried to give white Stavudine.DMPU solvate (33.5 g, 84.9%) with a chromatographic purity of 99.87%.

MP: 120° and SOR [α] ^2S = -27.5 (c=l; water)

The Bruker Avance 300 MHz IH NMR spectrum in DMSO d6 showed δ 1.72 (s,3H, Stavudine, CH3), 1.86 (m,2H); 2.75 (s,2x 3H, N(CH3)2); 3.2 (m,4H, 2 x CH2); 3.60 (m,2H, 2 x H-5'); 4.77 (s,lH,H-4); 5.02 (t,-OH); 5.91 (dd, IH, H-2'); 6.39 (dl, IH, H-3'); 6.82 (d,lH,H-l'); 7.64 (s,lH,H-6); 11.30 (s, IH ₅ NH). The Stavudine position of the above spectrum agrees with the literature values (J.Med. Chem. 32 461 (1989).

Method B:

Crude Stavudine (25 g, 96.7% pure, 0.1116 mole), N,N,-dimethyl propylene urea (37.5 g, 1.5 parts) are added to isopropanol (50 ml, 2 volumes) and heated to 70-75 ⁰ C for dissolution. The obtained solution is stirred for 15 min. at 75°C and cooled to 6-8 ⁰ C. The precipitated product is filtered, washed with IPA and dried to give 34.1 g of Stavudine DMPU solvate (86.7%) with a chromatographic purity of 99.91%

Method C:

Crude Stavudine (25 g, 97.7% pure, 0.1116 mole), DMPU (37.5 g, 1.5 parts) are added to acetone (50 ml) and heated to 56°C for dissolution. The obtained solution is stirred for 15 min. at 56°C and cooled to 6-8 ⁰ C. The precipitated product is filtered, washed with acetone and dried to give 34.4 g of Stavudine DMPU solvate (87.5%) with a chromatographic purity 99.93%.

The following Table 1 summarizes the improvements in chromatographic purity for the above examples.

TABLE 1

The above three methods were presented to illustrate different ways of obtaining the Stavudine DMPU solvate. Method A showed Stavudine and the solvent being combined, heated to dissolve, cooled to crystallize and then isolated. Methods B and C illustrated the use of protic and aprotic solvents such as isopropanol and acetone to aid in the filtration (prevents premature crystallization during filtration) of the solution to remove inorganic salts.

Isolation of pure Stavudine from Stavudine DMPU solvate:

Method D: Stavudine DMPU solvate (20 g, 0.057 mole) is dissolved in water (20 ml) at 55°C and the solution is allowed cooling on its own for an hour to crystallize. The mixture is further cooled on 0-5 ⁰ C for 2 hrs. The precipitated product is filtered, washed with acetone and dried to give 8.5 g (66.82%) of pure white crystalline Stavudine. HPLC results are given in Table-2

Method E:

Stavudine DMPU solvate (20 g, 0.057 moles), water (3.6 ml) and acetone (289 ml) and heated to 55 ⁰ C and the hot solution is filtered. The clear filtrate is distilled until about 70 ml of acetone remained. The solution is allowed to cool on its own and at about 40°C it is seeded with pure Stavudine. Cooled the mass to 0-5 ⁰ C and stirred for 1 hr. The precipitated product is filtered, washed with chilled acetone (2 x 12 ml) and dried to give 10.68 g (84 %) pure product. The HPLC results are given in Table-2.

Method F:

Stavudine-DMPU solvate (25 g, 0.07 moles), water (5 g) and isopropanol (250 ml) are heated to about 82 ⁰ C to get clear colorless solution. Heating is continued so as to distill the solution at atmospheric pressure until about 80 ml of isopropanol remained in side. The solution is allowed to cool on its own. The mixture is further cooled to 0-5 ⁰ C for 1.5 hrs. The precipitated product is filtered, washed with isopropanol (2 x 10 ml) and air

dried to give 13.9 g (87.2%) of pure white crystalline Stavudine. The HPLC results are given in Table 2.

TABLE 2

The above three examples illustrate the use of three different solvent systems for breaking the solvate and isolating pure Stavudine.

The DMEU solvate can be prepared and desolvated similarly as DMPU solvate to get pure Stavudine.

As many changes can be made to the embodiments without departing from the scope of the invention, it is intended that all material contained herein be illustrative of the invention and not be interpreted in a limiting sense