FORM 2
THE PATENTS ACT 1970
(39 of 1970)
AND
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rulel3)
1. TITLE OF THE INVENTION:
"A NEW PROCESS FOR PREPARATION OF ATOVAQUONE AND NOVEL
INTERMEDIATES THEREOF"
2. APPLICANT(S):
(a) NAME: IPCA LABORATORIES LIMITED


(b)NATIONALITY: Indian Company incorporated under the Indian Companies Act, 1956
(c) ADDRESS: 48, Kandivali Industrial Estate, Charkop, Kandivali (West), Mumbai-400 067, Maharashtra, India.
3. PREAMBLE TO THE DESCRD7TION:
The following specification describes the nature of this invention and the manner in which it is to be performed:


Related Application:
This application is related to our main application number 760/MUM/2007 filed on March 17, 2008 which claims priority from provisional application filed on March 19, 2007
Technical field of invention:
The present invention relates to an improvement of our main invention filed as application number 760/MUM/2007. The improvement includes conversion of novel intermediates of Formula II and Formula III into isomeric mixture of atovaquone. The improvement further includes epimerization of isomeric mixture of atovaquone to the desired trans isomer.
Background of invention:
Atovaquone, chemical name being trarcs-2-[4-(4-chlorophenyl)cyclohexyl]-3-hydroxy-1,4-naphthoquinone, is a hydroxy-1,4-naphtoquinone, an analog of ubiquinone, with antipneumocystic activity. Atovaquone is potently active (in animals and in vitro) against Pneumocystis carinii, Plasmodia, and tachyzoite and cyst forms of Toxoplasma gondii. Due to its inhibitory effect in sensitive parasites, atovaquone can act by selectively affecting mitochondrial electron transport and parallel processes such as ATP and pyrimidine biosynthesis, thus has got great pharmaceutical interest/importance.
Atovaquone is the trans-'isomer of 2-[4-(4-chlorophenyl)cyclohexyl]-3-hydroxy-l,4-naphthoquinone whose synthesis, activity and uses are disclosed in the patent Nos. US 5053432 and EP 0362996.
There are only few reports available for the preparation of Atovaquone exploring various synthetic alternatives., and is mainly based on a decarboxylative condensation process, as shown in scheme 1, that is practically used in these disclosures. These methods are analogous to the methods disclosed in Journal of American Chem. Society, 1948, yielding the condensation product in nearly 1 to 6% only [F. Fieser, J. American Chemical Society, vol. 70, 3174-3180 (1948)]. As reported, the production yield of Atovaquone using process of'432 patent is very poor, practically in the range of 3-5% only.


Thus a search for a manufacturing process for the preparation of Atovaquone resulting in a satisfactory yield / purity of final product remains undoubtedly of interest.
The overall yields of atovaquone obtained by the process disclosed in our main invention are in the range of 10-14 % which still have scope of improvement. Therefore, the object of this invention is to further improve the process for the preparation of atovaquone disclosed in our main patent application in order to obtain better yields and commercial acceptability.
Summary of the invention:
The present inventors had discovered that the prior art processes present substantial difficulties in producing Atovaquone in a consistent and reliable manner in satisfactory yields. The invention, therefore, aims to provide a new process for making Atovaquone as an improvement of our main invention filed as application number 760/MUM/2007.
In accordance with one aspect, the invention provides a process for preparation of Atovaquone, which process includes reacting 1,4-naphthoquinone with trans-4-(4-chlorophenyl)cyclohexane carboxylic acid followed by halogenation, dehydrohalogenation, and hydrolysis according to scheme 2.



Scheme 2
In one embodiment, the intermediate II obtained as a mixture of its cis and trans-isomers, is directly subjected to halogenation, dehydrohalogenaion, and hydrolysis to obtain an isomeric mixture of atovaquone, followed by optionally epimerizing cis-isomer to trans-isomer and recovering atovaquone.
Detailed description of the invention:
Unless specified otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art, to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. To describe the invention, certain terms are defined herein specifically as follows.
Unless stated to the contrary, any of the words "including," "includes," "comprising," and "comprises" mean "including without limitation" and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it. Embodiments of the invention are not mutually exclusive, but may be
4


implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth the appended claims.
The term "isolating" is used to indicate separation or collection or recovery of the compound of the invention being isolated in the specified form. The term "separating from a solvent" with respect to the solids described herein means obtaining a solid of specified characteristics from a solution or a partial solution.
The term "treating" means adding or combining or mixing the stated reagent or materials to the things being treated. The term "forming a solution" means obtaining a solution of a substance in a solvent in any manner. It encompasses partial solutions. The term 'stable' as used herein, refers to the tendency to remain substantially in the same physical form for at least a month, preferably at least 6 months, more preferably at least a year, still more preferably at least 2 years, when stored under ambient conditions (20 °C/60% RH) without external treatment. Substantially the same physical form in this context means that at least 50%, preferably at least 80% and more preferably at least 90% of the crystalline form remains.
"Atovaquone" is a free species of trans-2-(4-chlorophenyl)-cyclohexyl)-3 -hydroxy-1,4-naphthoquinone, which has the trans geometry. It has the following Formula:

The inventors of the present invention had found that the use of 2-chloro-l,4-naphthoquinone in the condensation reaction with trans-4-(4-chlorophenyl)cyclohexane carboxylic acid does not provide a reliable, consistent methodology to prepare atovaquone due to poor selectivity, and yield reaches only in the range of 3-5%. The intermediate trans-isomer of compound of Formula IV (scheme 1), according to reported processes, is obtained in only 5-7% yield, and that too is present in a large junk of
5


impurities and its corresponding cw-isomer (Formula IV-cw-geometry) making it difficult to purify. The present inventors, on exploring various process alternatives, for a reliable process solution have found that the use of unsubstituted 1,4-naphthoquinone in the condensation reaction provides a significantly better yield in the condensation reaction and permits reliable isolation of trans-isomer of compound of Formula II, which can be further converted to Atovaquone. The use of 2,3-unsubstituted 1,4-naphthoquinone for the preparation of atovaquone and its intermediates was the subject of invention filed in our main patent Application No 760/MUM/2007. The present invention provides an improvement of the main invention wherein the novel intermediates of Formula II and Formula III are converted into atovaquone without separating any of the intermediate into its cis or trans isomer. Atovaquone thus obtained is present as a mixture of its cis and trans form. The mixture of isomers of atovaquone obtained is subjected to epimerization to convert the cis isomer into the trans isomer. Trans-atovaquone is then isolated and purified from the reaction mass thereof. The use of novel intermediates for the preparation of atovaquone as provided in the main invention led to significant increase in the overall yield of the reaction. As presented, the improvement comprising epimerization in the final step of the reaction has further increased the production yield significantly. Present improvement leads to about two fold increase in the overall yields of atovaquone. The process of the present invention is represented in scheme 2 given before.
Thus, according to the present invention, there is provided a process for preparation of Atovaquone of Formula I as claimed in our main patent Application No 760/MUM/2007, wherein the improved process comprises:

Atovaquone Formula I
a. Subjecting compound of Formula III to dehydrohalogenation reaction to obtain a compound of Formula IV as its cis or trans isomers or isomeric mixture thereof; and



b. Hydrolysis of compound of Formula IV into an atovaquone isomer or its
isomeric mixture;
c. Optionally epimerizing the cw-isomer or cis/trans-isomcric mixture into
atovaquone; and
d. recovering atovaquone from the reaction mixture.
In the process of step (a), 4-(4-chlorophenyl)cyclohexane carboxylic acid used in the process is preferably in trans-isomer form. The reaction is conducted in presence of a metal nitrate and a persulphate reagent, preferably the metal nitrate is silver nitrate and the persulphate reagent is ammonium persulphate. The reaction step (a) may be performed in an aqueous and/or organic solvent. The solvent for the reaction can be chosen from any inert solvent, preferably selected from polar protic and polar aprotic solvent. The aqueous solvent is water or water containing organic solvents. Preferably it is a mixture of water and acetonitrile. The reaction can be carried out optionally in presence of a catalyst, for example, a metal iodide or a phase-transfer catalyst.
As the phase transfer catalyst, mention can be made of, for example, alogen, quaternary
ammonium salts substituted with a residue selected from the group consisting of straight
or branched chain alkyl group having 1-18 carbon atoms, phenyl lower alkyl group and
phenyl group, such as tetrabutylammonium chloride, tetrabutylammonium bromide,
tetrabutylammonium fluoride, tetrabutylammonium iodide, tetrabutylammonium
hydroxide, tetrabutylammonium hydrogen sulfate, tributylmethylammonium chloride,
tributylbenzylammonium chloride, tetrapentylammonium chloride, tetrapentylammonium
bromide, tetrahexylammonium chloride, benzyldimethyloctylammonium chloride,
methyltrihexylammonium chloride, benzylmethyloctadecanylammonium chloride,
methyltridecanylammonium chloride, benzyltripropylammonium chloride,
benzyltriethylammonium chloride, phenyltriethylammonium chloride,
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tetraethylammonium chloride, tetramethylammonium chloride and the like; phosphonium salts substituted with a residue selected from the group consisting of straight or branched chain alkyl groups having 1-18 carbon atoms such as tetrabutylphosphonium chloride and the like; and pyridinium salts substituted with a straight or branched chain alkyl group having 1-18 carbon atoms such as 1-dodecanylpyridinium chloride and the like.
Among these phase transfer catalysts, quaternary ammonium salts substituted with a straight or branched chain alkyl group having 1-18 carbon atoms such as tetrabutylammonium bromide or alogen are particularly preferred. The reaction is carried out usually at a temperature not lower than ambient temperature and between the reflux temperature of the solvent, and preferably at a temperature of 50-110[deg.] C. The reaction time is usually from about 1 hour to about 10 hours. It is recommended to use the naphthoquinone in excess, usually in an amount of about 1.1 to 2 mol per mol of the 4-(4-chlorophenyl)cyclohexane carboxylic acid. Also to use the silver nitrate usually in an amount of 0.1 to 0.9 moles, preferably 0.2 to 0.5 moles and persulphate reagent in an amount of 1.5 to 5 moles, preferably 1.5 to 2.5 mol per mol of 4-(4-chlorophenyl)cyclohexane carboxylic acid, respectively. And to use, optionally, the phase transfer catalyst usually in an amount of 0.1-1 mol and preferably 0.1-0.25 mol per mol of the naphthoquinone, and to use the metal iodide in an amount of 0.1 to 1.0 mole and preferably in 0.1 to 0.3 mole per mol of the naphthoquinone.
The compound of Formula (II) formed during the above-mentioned reaction can easily be isolated by the conventional separating means. As said separating means, mention can be made of, for example, distillation of solvent and excess reactants followed by crystallization, extraction method using a solvent, dilution method, recrystallization method, column chromatography, preparative thin layer chromatography, etc. After the completion of step (a), the compound of Formula II is isolated in its cis isomer or trans isomer or as a mixture of its cis and trans isomer. Preferably the intermediate of formula II is not separated and carried forward for further reaction. In the process of step b), the compound of Formula II obtained in step a) is subjected to halogenation in presence of a suitable halogenating agent, preferably the halogenating agent is chlorine gas. Halogenation can be conveniently carried out in presence of glacial acetic acid or in any other conventional solvent to obtain compound of Formula III.

Preferably the compound of formula III is not purified and carried forward for further
reaction.
In step c), the dehydrohalogenation of compound III is carried out by treating it with an
acid or base in a suitable medium. Preferably, dehydrohalogenation is carried out by
treating it with sodium acetate in glacial acetic acid to obtain a compound of Formula IV.
The hydrolysis of compound of Formula IV into atovaquone is carried out in a conventional manner by reacting it with a base like potassium hydroxide or sodium hydroxide in a solvent, which is preferably an alcohol. Atovaquone thus obtained as an isomeric mixture of cis and trans isomer, may be further epimerized to afford the desired trans isomer. The epimerization of the isomeric mixture may be performed by techniques known in the art like treating the isomeric mixture with an acid. Preferably the acid used is sulfuric acid having strength of about 50-95 %. The reaction can be carried out at temperature ranging from 15 °C to 50 °C preferably between 25 to 35 °C. Reaction may get completed in about 1-6 hours depending upon the reaction conditions.
After the completion of the reaction, the reaction mass is worked up to obtain desired product. Work-up of the reaction mass can be done by techniques known in the art like quenching the reaction mass into crushed ice and separating the solid obtained. Separation can be conveniently carried out using conventional techniques like filtration, centrifugation, sedimentation or decantation. Atovaquone thus obtained can be further purified by the techniques known in the art. Purification can be conveniently carried out by crystallization. The solvents for crystallization can be selected from the group of alcohols, chlorinated solvents, ketones, ethers and nitriles or mixtures thereof. The preferred solvent system is acetonitrile with methylenedichloride. Epimerization of the undesired cis isomer of atovaquone into the desired trans isomer leads to significant increase in the yields of atovaquone. In fact, the inventors have recorded about two-fold increase in the overall yield of the reaction.
The following examples are presented to illustrate the working of the present invention, but are not limiting the scope of the individual embodiments presented.


Example 1: Preparation of atovaquone:
To a solution of Silver nitrate 14.17gm (0.0838 moles ) dissolved in 200 ml water, lOOgm (0.419moles) /ram-4-(4-chlorophenyl)cyclohexane carboxylic acid and acetonitrile (500ml) was added. The solution was heated to reflux followed by addition of 1,4-naphthoquinone (80 gm; 0.506moles). A solution of Ammonium persulfate (239 g; 1.048 moles) in water (600ml) was added dropwise to the above solution and continued reflux for half an hour. The reaction mass then cooled to 30-32°C and extracted with methylene chloride. The organic layer was first washed with water followed by 10% sodium carbonate aqueous solution, further with water till the pH neutral. The organic layer was concentrated to afford compound of Formula II 2-[4-(4-chlorophenyl)cyclohexyl]-l,4-naphthoquinone (149 gm). Formula II obtained was added to glacial acetic acid (745 ml) and to this solution chlorine gas was passed at about 20°C. After the complete disappearance of starting material, nitrogen gas was purged to remove excess chlorine from the reaction mass. Anhydrous sodium acetate (52.3 gm) was added and the reaction mass was heated to reflux for 90 minutes. The reaction mass was cooled and quenched into a mixture of methylenechloride and water, the methylenechloride is washed with water till neutral pH. The solvent is concentrated to obtain residue containing Formula IV (155 gm). The residue obtained was suspended in 4.5 liter methanol and a solution of potassium hydroxide (225 gm) in water (1550 ml) was added drop-wise under heating over a period of one hour. The reaction mass was refluxed for 2 hours and cooled to 30-32 °C and filtered. The filtrate was neutralized with 50% aqueous hydrochloric acid to precipitate atovaquone as cis/trans-isomeric mixture (116 gm).
Example 2: Epimerization:
2-[4-(4-Chlorophenyl)cyclohexyl]-3-hydroxy-l,4-naphthoquinone isomeric mixture (116 gm) obtained in example 1 was added to 1100 ml of 90 % sulphuric acid and stirred at 28-30 °C , the reaction was monitored by HPLC , when the cis isomer was less than 0.5% the reaction mixture was quenched into ice-water, filtered .washed with water till neutral pH. The crude product was purified in acetonitrile and methylenedichloride to obtain trans-atovaquone (44 gm). Yield: 28.5 % HPLC purity: 99.8%

We Claim,

wherein, the process comprises:
a) Subjecting compound of Formula III

1. A process for preparation of Atovaquone of Formula I as claimed in our main patent Application No 760/MUM/2007, wherein the improvement comprises,
to dehydrohalogenation reaction to obtain a compound of Formula IV as its cis or trans isomers or isomeric mixture thereof; and

b) Hydrolysis of compound of Formula IV into an atovaquone isomer or its isomeric mixture; and,
c) Optionally epimerizing the cw-isomer or cis/trans-isomeric mixture into atovaquone.

2. The process according to claim 1, wherein dehydrohalogenation is carried out in presence of sodium acetate.
3. The process according to claim 1, wherein in step b) comprises hydrolysis of compound of Formula IV is in presence of a base.


4. The process according to claim 1, wherein the epimerization in step c) is in presence of an acid.
5. The process according to claim 4, wherein in step c) the acid is sulfuric acid.
6. The process according to claim 1, wherein the epimerization in step c) is in presence of sulfuric acid.
7. A process according to claim 1, wherein the intermediate III is prepared by a process comprising:
a) reaction of a compound of Formula V with a compound of Formula VI to obtain an isomer or isomeric mixture of compound of Formula II; and

b) reacting the compound of Formula II with a halogenating agent.
8. The process according to claim 7, wherein the step a) is carried out in presence of a metal nitrate and persulphate compound.
9. The process according to claim 7, wherein the step a) is carried out in presence of silver nitrate and ammonium persulphate.
10. The process according to claim 7, wherein the halogenating agent is chlorine.