CLIAMS:We Claim:

1. A process for the preparation of isolated O-methyl Atovaquone of formula II,

Formula II

which comprises reaction of chloroatovaquone with alkali metal cation in alcohol to provide isolated O-methyl atovaquone.

2. The process of claim 1, wherein said alkali metal cation is selected from sodium, potassium, lithium, Rubidium and Caesium.

3. The process of claim 1, wherein said alcoholic solvent is selected from the group of methanol, ethanol, isopropyl alcohol and n-butanol.

4. The process of claim 1, wherein said reaction is performed at reflux temperature.

5. The process of claim 1, wherein said reaction further comprises isolation of solid of formula II by using a nitrile solvent.

6. The process of claim 5, wherein said nitrile solvent is acetonitrile.

7. The process of claim 5, wherein said isolation of solid of formula II is performed by using recrystallization technique.
8. The process of claim 1, wherein said O-methyl atovaquone is having purity greater than 98% by HPLC.

Dated this 27th day of March 2013 For Wockhardt Limited

(Dr Mandar Kodgule)
Authorized Signatory
,TagSPECI:DESCRIPTION

The present invention provides a chemical process for preparing isolated impurity of Atovaquone, for example, O-methyl Atovaquone of Formula II, which is useful as reference marker.

Formula II

Atovaquone, chemically known as trans-2-[4-(4-chlorophenyl)cyclohexyl]-3-hydroxy-1,4-naphthalenedione of Formula I:

Formula I

Atovaquone is a widely used antiprotozoal and is potently active (in animals and in vitro) against Pneumocystis carinii, Plasmodia, and tachyzoite and cyst forms of Toxoplasma gondii. It is a highly lipophilic compound resembling ubiquinone and has a low aqueous solubility. This is the reason for the poor bioavailability of atovaquone after oral administration.

U.S. Patent No. 4,981,874 discloses the use of atovaquone against Pneumocystis carinii infection in a mammal. EP Patent No. 0 123 238 and U.S. Patent No. 5,053,432 disclose the use of atovaquone against Plasmodium falciparum and also against Eimeria species such as E. tenella and E. acervulina which are causative organisms of coccidiosis. Further, use of atovaquone against Toxoplasmosis and Cryptosporidiosis is disclosed in EP Patent No. 0 445 141 and 0 496 729 respectively.

Currently, atovaquone suspension marketed under trade name MEPRON is a formulation of micro-fine particles of atovaquone.

The skilled in the art of drug manufacturing research and development understand that a compound in a relatively pure state can be used as a "reference standard." A reference standard is similar to a reference marker, which is used for qualitative analysis only, but is used to quantify the amount of the compound of the reference standard in an unknown mixture, as well. A reference standard is an "external standard," when a solution of a known concentration of the reference standard and an unknown mixture are analyzed using the same technique. (Strobel p. 924, Snyder p. 549, Snyder, L. R.; Kirkland, J. J. Introduction to Modern Liquid Chromatography, 2nd ed. (John Wiley & Sons: New York 1979)). The amount of the compound in the mixture can be determined by comparing the magnitude of the detector response.

The reference standard can also be used to quantify the amount of another compound in the mixture if a "response factor," which compensates for differences in the sensitivity of the detector to the two compounds, has been predetermined. (Strobel p. 894). For this purpose, the reference standard is added directly to the mixture, and is known as an "internal standard." (Strobel p. 925, Snyder p. 552).

Use of a compound as a reference marker requires recourse to a sample of substantially pure compound. Therefore, there is a need to develop an efficient process to provide O-methyl atovaquone of Formula II, which can be used as reference marker to establish the level of the purity of Atovaquone.

The inventors of the present invention developed a conventional and industrially feasible process to provide isolated compound of O-methyl atovaquone of Formula II.

As used herein, the term "isolated" refers to a compound that is at least 95 % purity determined by HPLC.

In an aspect, the present invention provides a process for the preparation of isolated O-methyl Atovaquone of formula II. The process comprises reaction of chloroatovaquone with alkali metal cation in alcohol to provide isolated O-methyl atovaquone having purity greater than or equal to 98 %.

The starting material, Chloroatovaquone, is prepared by the known processes of the prior art, for example, U.S. Patent No. 5,053,432.

The alkali metal cation used includes but are not limited to sodium, potassium, lithium, Rubidium, Caesium and the like. The alcoholic solvent is selected from the group of methanol, ethanol, isopropyl alcohol, n-butanol and the like.

The reaction is performed at a reflux temperature and the reaction is maintained for a period of 3 to 4 hours or more to affect the conversion of chloroatovaquone completely to O-methyl atovaquone of formula II.

After completion of the reaction, the reaction mixture is cooled to 25 to 45 °C and subjected for concentration to provide crude of reaction mixture. The resultant reaction mixture may be subjected for purification of crude or isolation of solid by using suitable techniques.

The purification of crude of the present invention involves use of basic treatment such as solution of sodium hydroxide, solution of potassium hydroxide, solution of sodium bicarbonate, solution of potassium bicarbonate and the like. The obtained crude from the reaction may be subjected for layer separation using water and immiscible organic solvent and then the organic layer is treated with base followed by concentration to provide purified crude of O-methyl atovaquone of Formula II. The immiscible organic solvent, includes but are not limited to chlorinated solvent such as dichloromethane, chloroform and dichloroethane; ester solvent includes ethyl acetate, isopropyl acetate and 2-methoxyethyl acetate.

The present invention also provides isolation/precipitation of solid of formula II from the crude or reaction mass using nitrile solvent. The solid may be precipitated from the reaction mass or crude by using recrystallization technique, drastic cooling technique, concentrating certain portion of solution to effect the crystal formation and then cooling, and treating solution with carbon and concentrating completely and the like. The nitrile solvent for the precipitation is selected from acetonitrile and propionitrile.

For the precipitation of solid, the reaction mixture is dissolved in a solvent, filtered and subjected the filtrate for precipitation of solid of Isolated Formula II.

The solution obtained above may be treated with carbon and filtered to remove any undissolved particles, impurities especially color impurities, prior to further processing. The solution may be filtered by passing through paper (for example, Whatman filter paper), glass fiber, or other membrane material, or a bed of a clarifying agent such as celite. Depending upon the equipment used and the concentration and temperature of the solution, the filtration apparatus may need to be preheated to avoid premature crystallization.

The resultant filtrate is subjected for crystallization/precipitation of solid using concentration or/and cooling technique.

The purity of isolated O-Methyl atovaquone prepared by the process of the present invention is greater than or equal to 98% determined by HPLC method.

The present invention is further illustrated by the following example, which does not limit the scope of the invention. Certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present application.

EXAMPLE:

Example-1: Synthesis of O-Methyl atovaquone (USP)

Sodium (6.0 g, 0.26 mol) was dissolved in Methanol (400 ml) and treated with Chloroatovaquone (30.0 g, 0.08 mol). The reaction mixture was stirred at reflux for 3 hours. Sample analyzed in TLC to ensure absence of chloroatovaquone. The reaction mixture was cooled to 40 °C and recovered methanol under vacuum. The resultant solid, which remained was separated between water (150 ml) and chloroform (300 ml). The organic layer was washed with 1 N sodium hydroxide (100 ml), followed by water (100 ml). The resultant organic layer was concentrated under reduced pressure. The resultant yellow solid was recrystallised from acetonitrile (200 ml) to get 16.0 g pure o-Methyl-atovaquone Melting point, 129 – 131.2 oC (HPLC purity 98.05 %).
1H NMR, dppm (DMSO-d6): 1.48 – 2.05 (8H, m, 4 x CH2), 2.59 (1H, m, CH), 3.09 (1H, m, CH), 4.01 (3H, s, OCH3), 7.31 (4H, m, 4 x ArH), 7.80 – 7.82 (2H, m, 2 x ArH), 7.94 – 7.98 (2H, m, 2 x ArH).

13C NMR, dppm (DMSO-d6): 29.39, 33.88, 34.55, 42.54, 61.04, 125.55, 125.78, 128.09, 128.54, 130.28, 131.04, 131.60, 133.51, 134.02, 137.55, 146.11, 158.30, 181.01, 184.83.

Mass Spectrometry: The protonated molecular ion at m/z 381.2 (M+) confirms the mass, which corresponds to molecular formula of C23H21ClO3.