FORM 2
THE PATENTS ACT 1970
(Act 39 of 1970)
&
THE PATENTS RULE, 2003
COMPLETE SPECIFICATION
(SECTION 10 and Rule 13)
TITLE OF THE INVENTION "AN IMPROVED PROCESS FOR PREPARATION OF
TREOSULFAN"
Emcure Pharmaceuticals Limited,
an Indian company, registered under the Indian Company's Act 1957
and having its registered office at
Emcure House, T-184, M.I.D.C, Bhosari, Pune411026, India.
THE FOLLOWING SPECIFICATION DESCRIBES THE NATURE OF THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

FIELD OF THE INVENTION
The present invention relates to a cost-effective and convenient process for the preparation of Treosulfan conforming to regulatory specifications. The invention specifically relates to the reduction of dimethyl-2,3-0-isopropylidene-L-tartrate of formula (II) with sodium-bis(2-methoxyethoxy) aluminium hydride, followed by methylsulfonation of the resulting diol of formula (III) and subsequent deprotection of the isopropylidene moiety to give Treosulfan (I).
BACKGROUND OF THE INVENTION
Treosulfan (I), chemically known as (2S,3S)-2,3-Dihydroxy-4-memylsidfonyIoxybutylj methanesulfonate is a drug commonly used for treating ovarian cancer. It belongs to the family of anti-cancer medicines called the alkylating agents, which prevent the growth and division of cancerous cells. Treosulfan has been used for bone-marrow ablation before stem-cell transplantation and in the treatment of malignant melanoma and breast cancer.

US 3,155,702 discloses synthesis of Treosulfan by replacement of the halogen function in L-Threitol-l,4-dibromobutane-2,3-diol, by treating with a large excess of an expensive reagent like silver methanesulfonate. Further, the presence of unprotected hydroxyl groups in the starting material inevitably leads to the formation of undesired impurities, which requires additional purification steps for removal of impurities as well for lowering the level of free silver in the active ingredient as per ICH guidelines, which results in lower yields and increases the costs substantially.
Another method reported in US 3,246,012 involves acetal formation of diethyl-L-tartrate with acetone to obtain 2,3-O-isopropylidene-diethyl-L-tartrate, which, when reduced with lithium aluminium hydride gives 2,3-0-methylene-L-threitol. The obtained alcohol was treated with methanesulfonyl chloride to yield the penultimate Treosulfan intermediate, 2,3-O-methylene-L-threitol-1,4-di-(methanesulfonate).

A similar approach which employs tartrate esters in the synthesis of Treosulfan, is disclosed in Organic Syntheses, (1993), Vol.8, p. 155 and Organic .Syntheses, (2004), Coll.Vol.10, p.297. L-tartaric acid is reacted with 2,2-dimethoxypropane in presence of methanol. The resulting methyl ester, dimethyl 2,3-O-isopropylidene-L-tartrate is reduced with lithium aluminium hydride to obtain 2,3-di-O-isopropylidene-L-threitol, which, upon reaction with methanesulfonyl chloride, followed by treatment with methanesulfonic acid yields Treosulfan.
Although these routes involve protection of the diol group and avoid impurities arising out of substitution at those alcohol functionalities, use of a highly pyrophoric, hazardous reagent such as lithium aluminium hydride severely limits their synthetic applicability, especially on commercial scale. Further, the final step involves reaction of 2,3-di-O-isopropylidene-L-threitol with methanesulfonic acid, which is quite sluggish and causes considerable rise in the total number of impurities due to long reaction time.
Thus, there is a need for a convenient, economical process for a commercial scale synthesis of Treosulfan (I), which overcomes the shortcomings of the prior art, does not involve use of hazardous, pyrophoric reagents and yields Treosulfan conforming to regulatory specifications.
The present inventors have developed a novel process for preparation of (2S,3S)-2,3-Dihydroxy-4-methylsulfonyloxybutyl] methanesulfonate (I). The scheme for synthesis comprises reaction of dimethyl 2,3-O-isopropylidene-L-tartrate of formula (II) with sodium-bis(2-methoxyethoxy) aluminum hydride to give the protected diol, 2,3-0-isopropylidene-L-threitoI (III), which on further treatment with methanesulfonyl chloride, followed by reaction of the resultant ester, 2,3-O-isopropyliden-L-threitol 1,4 bismethanesulfonate (IV) with formic acid, yields Treosulfan (I) having desired purity and with impurity levels conforming to ICH guidelines.

OBJECT OF THE INVENTION
An objective of the present invention is to provide a safe, industrially applicable process for synthesis of Treosulfan which avoids use of toxic and environmentally hazardous reagents such as lithium aluminium hydride
Another object of the invention is to provide a convenient method for synthesis of Treosulfan which involves safe, industrially viable reduction of dimethyl-2,3-0-isopropylidene-L-tartrate, which, after reaction with methanesulfonyl chloride, followed by facile deprotection of the resultant diol yields Treosulfan having purity and impurity levels conforming to ICH guidelines.
SUMMARY OF THE INVENTION
The present invention relates to an improved for the preparation of Treosulfan (I) by overcoming the problems faced in the prior art.
The invention relates to preparation of (2S,3S)-2,3-Dihydroxy-4-methylsulfonyloxybutyl] methanesulfonate (I) comprising reaction of dimethyl 2,3-O-isopropylidene-L-tartrate of formula (II) with sodium-bis(2-methoxyethoxy) aluminium hydride in solvent toluene to give 2,3-O-isopropylidene-L-threitol of formula (III), followed by reaction of compound (III) with methanesulfonyl chloride in dichloromethane to give 2,3-O-isopropylidene-L-threitol 1,4-bismethanesulfonate of formula (IV) and its further treatment with formic acid to give Treosulfan (I) having purity conforming to regulatory specifications.
The objectives of the present invention will become apparent from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors in their quest for development of an efficient, economical, industrially viable process for Treosulfan, during the course of experimentation tried various reducing agents for the reduction of dimethyl 2,3-O-isopropylidene-L-tartrate of formula (II). It was observed that use of most of the reducing agents was hampered by handling problems and associated issues such as formation of tarry mass during the

reaction, excessive formation of side products etc., leading to poor yields. It was surprisingly found out that the ester of formula (II) could be conveniently reduced to the corresponding alcohol of formula (III) using an easy-to-handle, convenient and efficient reagent like sodium-bis(2-methoxyethoxy) aluminium hydride. By utilizing this reagent, the need of using hazardous and pyrophoric reagents like lithium aluminium hydride was eliminated and formation of side product was substantially reduced, which, in turn made the process feasible on industrial scale.
The inventors observed that use of sodium-bis(2-methoxyethoxy) aluminium hydride had several advantages. The handling hazards and storage related problems associated with lithium aluminium hydride were easily circumvented due to the non-pyrophoric nature of the reducing agent. Further, the reaction was facile and the formation of impurities was controlled below the desired limits. Moreover, unlike lithium aluminium hydride, sodium-bis(2-methoxyethoxy) aluminium hydride is not sensitive to oxygen, which is of immense help in plant-scale handling of the reagent. All these facts together had a considerable impact during commercial scale operations of the reduction reaction. Further, since this reagent is soluble in a number of aromatic hydrocarbon solvents, the reaction could be carried out using toluene as a solvent, avoiding low boiling, and highly inflammable solvents such as diethyl ether.
2,3-O-isopropylidene-L-threitol of formula (III) was further treated with methanesulfonyl chloride, and the resulting acetonide, 2,3-0-isopropyliden-L-threitol-l,4 bismethanesulfonate of formula (IV) was easily deprotected using formic acid to yield the final product, Treosulfan


Scheme 1; Method embodied in the present invention for the preparation of Treosulfan (I)
In an embodiment, dimethyl 2,3 -O-isopropylidene-L-tartrate of formula (II) was treated with sodium-bis-(2-methoxyethoxy) aluminium hydride in presence of an organic solvent, and in the temperature range of 25 to 80°C, but preferably 60 to 75°C.
The organic solvent was selected from the group of toluene, xylenes, nitrobenzene, hexane, cyclohexane, heptane, N-methyl-2-pyrroIidone, ethers etc.
Upon completion of the reaction, as monitored by TLC, water was carefully added to the reaction mass and the mixture was extracted with a water immiscible organic solvent.
The organic solvent was selected from the group comprising of n-hexane, cyclohexane, heptane, methyl isobutyl ketone, 2-methyl tetrahydrofuran, cyclopentyl methyl ether etc.
The organic layer was separated and concentrated under reduced pressure to give 2,3-0-isopropylidene-L-threitol of formula (III) of desired purity.
It is pertinent to mention that the reaction was quite facile and the desired product was obtained with minimal formation of associated impurities and did not require any subsequent purification.

Further reaction of compound (III) with methanesulfonyl chloride was carried out at 25 to 35°C, in an organic solvent, in presence of an organic base.
The organic solvent was selected from the group comprising of chloroform, ethylene dichloride, dichloromethane, carbon tetrachloride etc., but preferably dichloromethane.
The organic base was selected from triethyl amine, tributyl amine and pyridine.
The reaction mixture was stirred at 25-35°C and after completion of the reaction as monitored by TLC, aqueous solution of sodium bicarbonate was added slowly to the reaction mass. The organic layer was separated, concentrated under reduced pressure and stirred with isopropyl alcohol to obtain the desired compound, 2,3-O-isopropylidene-L-threitol-l,4-bis(methanesulfonate) of formula (IV).
In a further embodiment, compound (TV) was hydrolyzed by treating with formic acid at 25 to 35°C based on TLC. After completion of the reaction, the reaction mass was concentrated and the product Treosulfan (I) was isolated by addition of isopropyl alcohol to the concentrated mass.
It is pertinent to mention that Organic Syntheses (2004), Coll.Vol. 10, p.297 discloses the hydrolysis reaction using methanesulfonic acid in ethanol at reflux temperature. However, the time taken for completion is about ten hours and the procedure is applicable only for laboratory scale reaction. The hydrolysis step disclosed in the present invention is easily scalable and so facile that it takes place at room temperature and within one to two hours. This reduces the time cycle for each batch run and also reduces the possibility of formation of undesired side products.
Dimethyl 2,3-O-isopropylidene-L-tartrate of formula (II) was prepared by the reaction of dimethyl -L-tartrate with acetone by following known synthetic procedures.

The following examples are meant to be illustrative of the present invention. These examples exemplify the invention and are not to be construed as limiting the scope of the invention.
EXAMPLES
Example 1: Synthesis of 2,3-O-isopropylidene-L-threitol (HI)
A solution of dimethyl-2,3-0-isopropylidene-L-tartrate (50.3 g) in toluene (50 ml) was gradually added to the stirred mixture of sodium-bis(2-methoxyethoxy) aluminum hydride (122.8 g) in toluene (50 ml) at 20-40°C. The reaction mixture was heated to 60-80°C, and the reaction was continued till completion, as monitored by TLC. When the reaction was complete, the mass was cooled to 25-3 5°C, quenched with careful addition of water (10ml) and concentrated. Treatment of the resulting residue with methyl tertiary butyl ether, followed by evaporation of the organic layer under reduced pressure afforded 2,3-0-isopropyliden -L-threitol ( III) as pale yellow oil. Yield: 29.8 g (81.2%) [α]D20 + 4.6.°(CHC13, c 5)
Example 2: Synthesis of 2,3-0-isopropylidene-L-threitol-l,4-bis(methanesulfonate)
(IV)
A stirred solution of 2,3-O-isopropylidene-L-threitol (100.2 g), methylene chloride (1250
ml) and pyridine (146.3 g) was cooled to 0-5°C and methanesulfonyl chloride (176.6 g)
was slowly added to it. Temperature of the reaction mixture was raised to 25-35°C and the
reaction was continued at the same temperature till completion of the reaction, as
monitored by HPLC. After completion of the reaction, aqueous sodium bicarbonate
solution was slowly added to the reaction mass and the organic layer was separated.
Aqueous layer from the reaction mixture was extracted with methylene chloride and the
organic layers were combined. Distillation of the organic solvent, optionally followed by
addition of isopropyl alcohol gave the product, 2,3-0-isopropylidene-L-threitol-l,4-
bis(methanesulfonate).
Yield: 160.7 g (79.7%)
[α]D20-21.6°(acetone,c2)

Example 3: Synthesis of Treosulfan (I)
A mixture of formic acid (98%, 1000 ml) and 2,3-0-isopropylidene-L-threitol-l,4-bis(methanesulfonate) (100.5 g) was stirred at room temperature until completion of the desired reaction, as monitored by TLC, When the reaction was complete, the reaction mass was concentrated under reduced pressure..
Treatment of the residue after evaporation with isopropanol yielded the final product Treosulfan, which was optionally subjected to further treatment with acetone and nexanes or petroleum ether, Yield: 74.3 g (85.0%) [α]D20 - 5.3°(acetone, c 2) Purity: > 99 %.

We claim,
1. A process for preparation of Treosulfan (I) comprising reaction of dimethyl 2,3-0-isopropylidene-L-tartrate of formula (II) with sodium-bis(2-methoxyethoxy) aluminium hydride in an organic solvent to give 2,3-O-isopropylidene-L-threitol of formula (III), followed by reaction of compound (III) with memanesulfonyl chloride in dichloromethane to give 2,3-0-isopropylidene-L-threitol-l,4-bis(methanesulfonate) of formula (IV), followed by further treatment of compound (IV) with an organic acid at 25 to 35°C and isolating Treosulfan (I)
2. A process as claimed in claim 1, wherein the organic solvent is selected from toluene, xylenes, but preferably toluene.
3. A process as claimed in claim 1, wherein the reaction with sodium-bis(2-methoxyethoxy)aluminum hydride is carried out in the temperature range of 60 to 80°C.
4. A process as claimed in claim 1, wherein the organic acid is formic acid.
5. A process as claimed in claim 1, wherein Treosulfan (I) is isolated from the reaction mixture by concentrating the reaction mass and optionally treating the obtained residue with isqprqpanol to give Treosulfan (I).