Title of Invention

"PROCESS FOR THE ESTERIFICATION OF A CARBOTHIOIC ACID"

Abstract

A process for preparing compounds of formula: (II) by esterification of the C-17 hydroxyl group of 6α,9α-difluoro-llß, 17α-dihydroxy-16α-methyl-3-oxoandrosta-l, 4-diene-17β-carbothioic acid, the compound of formula: (I) comprises treating compound (I) with a slight excess of an acyl chloride of general formula R-COCL, where R represents -CH2CH3, -CH2CH2CH3 or -CH (CH3)2, in an inert solvent, in the presence of a tertiary amine. Preferably the process is carried out using pyridine in the presence of acetone at a temperature of from 5 °C to -20°C z.

Full Text

The present invention relates generally to a process for the esterification of a carbothioic acid, particularly but not exclusively in the 'preparation of fluticasone propionate; and to the use of certain intermediates, In one aspect, the present invention relates to an improved process for the esterification of the C-17 hydroxyl group of 6a,9a-difluoro-11}J,17a-dihydroxy-16a-methyl-3-oxoandrosta-1,4-diene-17p-carbothioic acid, compound of formula [ 1 ], comprising treating this intermediate with a slight excess of an acyl chloride of general formula R-COCI, where R represents, -CH2CH3l -CH2CH2CH3 or -CH{CH3)2, in the presence of an appropriate tertiary amine, in an inert solvent, at a temperature between 5°C and -20°C, to obtain selectively the 17a-acy! derivative of formula [ II ]. (Figure Removed) This invention provides an esterification process which is simpler and more economically efficient than those disclosed in the prior art because it suppresses one of the two chemical reactions described in those processes. Another feature of the process of this invention is that it yields 17 More particularly, this invention relates to an improved process for the preparation of 6a,9a-difluoro-11 p-hydroxy-16a-methyl-17a-propionyloxy-3-oxoandrosta-1,4-diene-17p-carbothioic acid, compound of formula [ III } which is an intermediate in the synthesis of fluticasone propionate, an active ingredient used as an anti-inflammatory steroid, effective for the treatment of inflammatory diseases such as asthma and chronic obstructive pulmonary disease. (Figure Removed) US patent 4,335,121, British patents GB 2,088,877, GB 2,137,206, US patent 4,578,221 and J. Med. Chem., 1994, 37, 3717-3729, describe the 17a-propionylation of compound [ I ], to obtain the fluticasone propionate intermediate [ 111 ], through the two chemical steps illustrated below, via the mixed anhydride compound [ IV ]. (Figure Removed) In the first step (a), the mixed anhydride [ IV ] is prepared with an excess of at least 2 moles of propionyl chloride per mol of compound [ I ], in the presence of triethylamine and using dichloromethane as solvent. Upon conclusion of the propionylation reaction, the reaction mixture is worked-up and a buff solid is obtained. In the second step (b), this solid is dissolved in acetone and treated with diethylamine, to convert the mixed anhydride to compound [ ill ]. Once the aminolysis reaction is complete, the reaction mixture is worked up to isolate compound [ III ]. International patent application WO 03/066654 claims the preparation of intermediate [ III ] by: (a) reacting compound [ I ] with at least 1.3 moles of an activated derivative of propionic acid per mol of compound [! ], and removal of the sulphur linked moiety from any compound of formula [ IV ] with an organic primary or secondary amine such as diethanolamine or N-methylpiperazine. Patent application WO 01/62722 discloses the 17a-esterification of the hydroxyacid compound [ V ] with an alkanoyl halide, in presence of a base, and particularly describes the preparation of the 17a-propionate compound of formula {VI3 (Figure Removed) by: (a) reacting the hydroxyacid of formula [ V j with 2.3 moles of propionyl chloride per rnol of compound [ V ], using triethylamine as base, and (b)./n situ reacting the compound obtained in (a) with diethylamine. All of the 17a-acylation procedures described in the prior art, either for the carbothioic acid [ I ] or the related carboxylic acid [ V ], use an excess of the acylating agent to ensure completion of the 17a-acylation, thus requiring aminolysis, with an adequate primary or secondary amine, of any mixed anhydride formed. We have now found that the transformation of the carbothioic acid compound [ I ] into the compounds of general formula [ II ], can selectively take place directly, with negligible formation of the correspondent mixed anhydride, under conditions herein described below. By following the process of the present invention, the intermediate 6a,9a-difluoro-11p,17a-dihydroxy-16a-methyl-3-oxoandrosta-1,4-diene-17p-carbothioic acid, compound [ I ], is directly converted to compounds of general formula [ II ] (Figure Removed) by reaction with a slight excess of the corresponding acyi chloride, in an inert solvent, in the presence of an appropriate tertiary amine. The particular advantage of the present invention over those of the prior art is that compounds of general formula [ II ] are obtained directly from compound [ I ], without the need to perform the arninolysis of the corresponding mixed anhydride. Additionally, under the arninolysis conditions described in the prior art, the chemical stability of carbothioic acid derivatives such as compound [ HI ] is limited therefore, the simplified process of this invention, by eliminating the arninolysis reaction, affords 17a-esters of higher purity. According to the present invention, there is provided a process for preparing compounds of formula [ II.] (Figure Removed) by esterification of the C-17 hydroxyl group of 6a,9a-difluoro-11p,17a-dihydroxy-16a-methyi-3-oxoandrosta-1,4-diene-17p-carbothioic acid, compound of formula [ I ], which process comprises treating compound [ I ] with a slight excess of an acyl chloride of general formula R-COCI, where R represents -Ch^CHa, -CH2CH2CH3 or -CH(CH3)2, in an inert solvent, in the presence of a tertiary amine. Preferably, the process is carried out at a temperature of from 5°C to -20°C. The invention also provides the use of compounds of formula [ II ] when made according to the process of the invention for the preparation of therapeutically useful medicaments. The present invention provides an improved and simplified process for the selective 17a-esterification of the compound [ I ], forming negligible amounts of mixed anhydrides of general formula [ VII ] (Figure Removed) in which R represents -CH2CH3, -CH2CH2CH3 or -CH(CH3)2, without the need to perform the aminolysis reaction of the corresponding mixed anhydrides. The process comprises the reaction of compound [ i ], 6a,Sa-difluoro-11|3,17a-dihydroxy-16a-methyl-3-oxoandrosta-1,4-diene-17p-carbothioic acid, with a slight excess of an acy! chloride of general formula R-COCI, in which R represents -CH^CHa, -CH2CH2CH3 or -CH{CH3)2, in the presence of an appropriate organic tertiary amine, in an inert solvent, to yield the compounds of general formula [ II ] (Figure Removed) in which R represents -CH2CH3, -CH2CH2CH3 or -CH(CH3)2. A particularly preferred embodiment of the present invention is to provide an improved process for the preparation of 6a,9a-difluoro-11p-hydroxy-16a-methyl-17a-propionyloxy-3-oxoandrosta-1,4-diene-17p-carbothioic acid, compound of formula [ III ], comprising of reacting 6a,9a-difiuoro-11p,17a-dihydroxy~16a-methyl-3-oxoandrosta-1,4-diene-17f}-carbothioic acid [ II ] with a slight excess of propionyl chloride, in the presence of an appropriate tertiary amine, in an inert solvent. The compound of formula [ III ] is a known intermediate useful in the preparation of anti-inflamatory steroids such as fluticasone propionate of formula [ A] (described in US 4 335 121), highly effective in the treatment of inflammatory diseases like asthma and chronic obstructive pulmonary disease (COPD), and in the preparation of the related 6a,9a-difluoro-11 p-hydroxy-16a-methyl-3-oxo-17a-propionyloxy-androsta-1,4-diene-17p-carbothioic acid S-(2-oxo-tetrahydro-furan-3-yl) ester of formula [ B ] (described in the application WO 97/24365), possessing a useful anti-inflammatory activity and having little or no systemic activity. (Figure Removed) Accordingly, the invention also provides a process for preparing fluticasone propionate which process comprises preparing 6a,9a-difluoro-11^-hydroxy-16a-methyl-17a-propionyloxy-3-oxoandrosta-1,4-diene-17^-carbothioic acid according to the process of the invention and converting the said compound to fluticasone propionate. There is also provided a process for the preparation of 6a -9a ~difluoro-11p-hydroxy-16a -methyi-3-oxo-17a-propionyloxy-androsta-1,4-diene-17p-carbothioic acid S-{2-oxo-tetrahydrofuran-3-yl) ester which process comprises preparing 6a, 9a-difiuoro -11£S- hydroxy-16a - methyl -17a - propionyloxy - 3 - oxoandrosta - 1,4-diene -17(3 -carbothioic acid according to the process of the invention and converting said compound to the said S-estsr. The present invention provides an advantageous process for the preparation of 6ct,9a-difluoro-11 p-hydroxy-16a-methyl-17a-propionyloxy-3-oxoandrosta-1,4-diene- 17p-carbothioic acid, compound of formula [ III.] comprising of treating the compound [ I ], 6a,9a-difluoro-11p,17a-dihydroxy-16a-methyl-3-oxoandrosta-1,4-diene-17(3-carbothioic acid, with a slight excess of propionyl chloride, in the presence of an appropriate tertiary amine, in an inert solvent, at a temperature between 5°C and -20°C. The acyl chloride of general formula R-COCI is preferably used in the process of the present invention in a molar ratio of from 1.0 to 1.2 moles of acyl chloride per mol of starting compound [ I ], preferably in an amount within this range. Inert solvents for the 17a-acylation process of the present invention include acetone, tetrahydrofuran, dimethylacetamide, dichloromethane, ethyl acetate, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone and 2-butanone. Acetone is the preferred solvent. Appropriate tertiary amines to carry out the selective 17a-propionylation include pyridine, 2-picoline 3-picoline, 4-picoline, N-methylimidazole, and N-methylpyrrolidine. The 17a-acylation reaction of the invention is preferably performed at a temperature offrom5°Cto-20°C. The use of the tertiary amines defined above as adequate for the process of the patent, present advantages over other bases previously described in the prior art such as triethylamine, tripropylamine, ethyldiisopropylamine, N.N-dimethylaniline, N.N-dimethyicyclohexylamine, inorganic carbonates such as e.g. sodium hydrogen carbonate, potassium carbonate and sodium carbonate. When using these bases the formation of high levels of the mixed anhydride compound [ IV ] or incomplete consumption of compound [ I ] or the formation of high levels of impurities as is the case when sodium hydrogen carbonate is avoided. ' Under the preferred conditions of the process of the present invention, 17tx-esterificaiion goes to completion with 1.0 to 1.2 moles of propionyl chloride per mole of compound [ I ], in the presence of pyridine, and the levels of mixed anhydride formed during the reaction are below 3% (in area, by HPLC). With these low levels of mixed anhydride, the aminolysis reaction is not required and compound [ 111 ] is isolated with a high degree of purity, on work-up of the propionylation reaction. Under the conditions described in the prior art when compound [ III ] is prepared via the mixed anhydride, partial degradation of compound [ III ] may occur during the aminolysis reaction. This degradation is especially problematic when prolonged reaction times take place, which is to be expected on an industrial scale. Under the conditions disclosed in the present invention this in situ degradation of compound [ III ] is avoided. We have found that by following the prior art a precursor (G precursor) to an impurity (impurity G) is formed. (Figure Removed) With the process of the present invention, dimer G, which is difficult to remove from the final product, fluticasone propionate, by recrystallization when at levels higher than 0.5%, is formed at levels be ow 0.3%. Hence, repeated recrystallizations to obtain material of the required purity, which if carried out significantly reduces the overall yield, are avoided. Another embodiment of this invention is to provide an improved process for the preparation of the compounds 6a,9a-difluoro-11p-hydroxy-16a-methyl-17a-butyiy!oxy-3-oxoandrosta-1,4-diene-17p-carbothioic acid, compound of formula [ Vlli ] and 6a,9a-difluoro-11p-hydroxy-16a-methyl-17a-isobutyryioxy-3-oxoandrosta-1,4-diene-17p-carbothioic acid of formula [ IX ]. (Figure Removed) Compounds of formula [ VIII ] and [ IX ] are directly prepared from compound [ I ] by reaction with a slight excess of the corresponding acyl chloride, without the need to perform sn aminolysis reaction of the corresponding mixed anhydrides. According to a preferred aspect of to the pment invention, esterification of compound [ I ] to obtain compound [ VIII ], is performed with 1.0 to 1.2 moles of butyryl chloride, in acetone, in the presence of pyridine, at a temperature between 5°C and -20°C, preferably between 5°C and 0°C. The esterification of compound [ I j to obtain compound [ IX ] is preferably performed with 1.0 to 1.2 moles of isobutyryl chloride, in acetone, in the presence of pyridine, at a temperature between 5°C and -20°C, preferably between 5°C and 0°C. The following examples illustrate the invention and certain preferred embodiments and are exempt of limitative character of the scope of the invention. Example 1: Preparation of 6a,9a-difluoro-11p-hydroxy-16a-methyl-17 A suspension of compound [ I ] (10 g, 0.024 moles) in acetone (175 ml) is cooled to -20°C and treated with pyridine (30 ml) and propionyl chloride (2.3 ml, 0.026 moles), while maintaining the temperature between -20°C and -15°C. The mixture is stirred at -20°C/-15°C until completion of reaction. A second addition of propionyl chloride (0.02 ml) can be carried out if necessary to complete the reaction. Compound [ If I ] is precipitated by addition of water (240 ml) and concentrated hydrochloric acid (30 ml). The suspension is stirred for 2 hours at a temperature between 0°C and 5°C, filtered, and the wet cake washed with cold water until neutral pH. The solid is dried at a temperature below 40°C, under vacuum, to give the title compound as a,,white to off white .solid (10.04 g; 88%; Purity, area % by HPLC: 96.1 %). Example 2: Preparation of 6a,9a-difluoro-11j3-hydroxy-16a-methyl-17a-isobutyryloxy-3-oxoandrosta-1,4-diene-17p-carbothioic acid Isobutyryl chloride (0.28 ml, 0.0027 moles) is slowly added to a mixture of acetone (17.5 ml), compound [ I ] (1 g, 0.0024 moles) and pyridine (3 ml), at temperatures between 5°C and 0°C, and the mixture is stirred at that temperature range until consumption of compound [ I ]. Upon completion of the reaction the title compound is precipitated by addition of water (24 ml) and concentrated hydrochloric acid (3 ml). The suspension is stirred for 1 to 2 hours, at a temperature between 0°C and 5°C, filtered, and the wet cake washed with cold water until neutral pH. The wet solid is dried at a temperature lower than 40°C, under vacuum, to yield the title compound as a white to off white solid (1.09 g; 93%; Purity, area % by HPLC: 94.8%). Example 3: Preparation of 6a,9a-difluoro-11f5-hydroxy-1€a-methyM7a-propionyIoxy-3-oxoandrosta-1,4-diene-17p-carbothioic acid A suspension of compound [ I ] (1 g, 0.0024 moles) in acetone (17 ml) is cooled to a temperature between -20°C and -15°C. N-methylimidazole (2.9 rnl) and propionyl chloride (0.23 ml, 0.0026 moles) are sequentially added to the solution, maintaining the temperature between -20°C and ->15°C. Upon completion of the reaction, compound { HI ] is precipitated by addition of water (24 ml) and concentrated hydrochloric acid (3 ml). The suspension is stirred for 2 hours at ca. 0°C, filtered, and the wet cake washed with cold water until neutral pH. The wet solid is dried at a temperature below 40°C, under vacuum, to give the title compound as a white to off white solid (1.0 g; 88%; Purity, area % by HPLC: 96.8%). We Claim: 1. A process for preparing esters of C-17 hydroxyl group of 6a, 9a-difluoro-l Ip, 17aclihydroxy- 16a-methyl-3-oxoandrosta-l, 4-diene-17 P-carbothioic acid of formula [II] O SH WO >,_.-'sJ,^f*'^ ^ [II] by ester ification of the C-17 hydroxyl group of 6a, 9a-difluoro-lip, 17a-dihydroxy-16amethyl- 3-oxoandrosta-l,4-diene-17p-carbothioic acid, the compound of formula [I], MO 4„^,,,,^|/.0« [I] which process comprises treating compound [I] with from 1.0 mol to 1.2 moles of an acyl chloride of general formula R-COCl, where R represents -CH2CH3-, -CH2CH2CH3 or - CH(CH3)2 per mol of compound [I], in an inert solvent, in the presence of a tertiary amine, which is pyridine, 2-picoline, 3-picoline, 4-picoline, N-methylimidazole, or Nmethylpyrrolidine, preferably pyridine, 3-picoline, 4-picoline, N-methylimidazole, or Nmethylpyrrolidine. 2. A process as claimed in claim 1 wherein the process is carried out at a temperature of from 5°C to -20'^C. 3. A process as claimed in claim 1 or 2 where the acyl chloride of general formula R-COCl is propionyl chloride, R representing -CH2CH3. 4. A process as claimed in any preceding claim wherein the tertiary amine is pyridine, or Nmethylimidazole. 5. A process as claimed in any preceding claim wherein the tertiary amine is pyridine. 4 6. A process as claimed in any preceding claim wherein the inert solvent is acetone. 7. A process as claimed in any one of claims 1 to 5 wherein pyridine is used as tertiary amine and the inert solvent is acetone, tetrahydrofuran, dimethylacetamide, dichloromethane, ethyl acetate, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone or 2- butanone, preferably acetone. 8. A process as claimed in any preceding claim wherein the reaction is performed at a temperature of from -1 S^'C to -20°C. 9. A process as claimed in any preceding claim wherein the acyl chloride of general formula R-COCl is butyryl chloride, R representing -CH2CH2CH3, wherein compound [I] is treated with 1.0 to 1.2 moles of butyryl chloride per mol of compound [I], in the presence of pyridine, in acetone, at a temperature of from S^^C to -20°C, preferably from S'^C to OV. 10. A process as claimed in any one of claims 1 to 8 wherein the acyl chloride of general formula R-COCl is isobutyryl chloride, R representing -CH(CH3)2, wherein compound [I] is treated with 1.0 to 1.2 moles of isobutyryl chloride per mol of compound [I], in the presence of pyridine, in acetone, at a temperature of from 5°C to -20'C, preferably from S'^C to 0°C. •- 11. A process for preparing fluticasone propionate which process comprises preparing 6a,9adifluoro- 11 p-hydroxy-16a-methyl-17a-propionyloxy-3-oxoandrosta-1,4-diene-17pcarbothioic acid as claimed in the process of any preceding claim, and converting said compound to fluticasone propionate. 12. A process for the preparation of 6a,9a-difluoro-11 P-hydroxy-16a-methyl-3-oxo-l 7apropionyloxy- androsta-l,4-diene-17P-carbothioic acid S-(2-oxo-tetrahydrofuran-3-yl) ester which process comprises preparing 6a, 9a-difluoro-11 P-hydroxy-16a-methyl-3- oxo-17a-propionyloxy-3-oxoandrosta-l,4-diene-17p-carbothioic acid as claimed in the process of any preceding claim, and converting said compound to the said S-ester. r t 13. A process for the preparation of therapeutically useful medicaments, which process comprises preparing a compound of formula [II] as claimed in the process of any one of claims 1 to 11 and then preparing said medicaments therefrom. \ n^ Dated this 27* day of April, 2007 \J J (/^H^—' DEEPA KACHRC pXIKU (Dr.) OF K & » PARTNERS ATTORNEY FOR THE APPLICANT(S)

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