WO2008068600A2

WO2008068600A2 - An improved process for the preparation of zaleplo

Info

Publication number: WO2008068600A2
Application number: PCT/IB2007/003787
Authority: WO
Inventors: Mahender Rao Siripragada; Sampath Kumar Upparapalli; Thirumurugan Kunchithapatham; Mannathan Subramaniyan
Original assignee: Orchid Chemicals and Pharmaceuticals Ltd
Current assignee: Orchid Pharma Ltd
Priority date: 2006-12-06
Filing date: 2007-12-06
Publication date: 2008-06-12
Anticipated expiration: 2009-06-06
Also published as: WO2008068600A3

Abstract

The present invention relates to an improved process for the preparation of N-[3-(3-cyanopyrazolo[ 1,5-a]pyrimidin-7-yl)phenyl]-N-ethylacetamide (Zaleplon) of formula (I), more particularly the present invention relates to a method for the purification of Zaleplon of formula (I), which is useful in medicine as an anxiolytic, sedative and skeletal muscle relaxing agent. Formula (I)

Description

ANIMPROVED PROCESS FORTHE PREPARATION OF ZALEPLON

Field of the Invention

The present invention relates to an improved process for the preparation of

N-[3-(3-cyanopyrazolo[l,5-a]pyrimidin-7-yl)phenyl]-N-ethylacetamide (Zaleplon) of formula (I), more particularly the present invention relates to a method for the purification of Zaleplon of formula (I), which is useful in medicine as an anxiolytic, sedative and skeletal muscle relaxing agent.

Background of the Invention

Zaleplon is chemically known as N-[3-(3-cyanopyrazolo[l,5-a]pyrimidin-7- yl)phenyl]-N-ethylacetamide is a non-benzodiazepine hypnotic from the pyrazolopyrimidine class and has the following structural formula:

(I)

Zaleplon is useful as a sedative and hypnotics agent, and it is marketed as

Sonata^® by Wyeth Pharmaceuticals.

Zaleplon was first reported in US Patent No. 4,626,538 (henceforth '538). According to the teachings of '538 patent, Zaleplon is prepared by the reaction of 3-dimethylamino- l-(3-N-ethyl-N-acetylaminophenyl)-2-propen- 1 -one with 3-amino- pyrazole-4-carbonitrile in glacial acetic acid at a reflux temperature. The method described in '538 patent solvent glacial acetic acid is removed by distillation, which leads to the degradation of product, formation of impurities and reduction in yield moreover it involves additional step and longer reaction time.

US Patent No. 5,714,607 (henceforth '607) describes a process for the preparation of Zaleplon by reaction of 3-dimethylamino-l-(3-N-ethyl-N- acetylaminophenyl)-2-propen-l-one with 3-amino-pyrazole-4-carbonitrile in a mixture of water and acetic acid. As reported in US 7,057,041 reaction carried out under the condition described in '607 resulted in Zaleplon contaminated with a side product, N- [3-(3-cyanopyrazolol[ 1 ,5-a]pyrimidine-5-yl)phenyl]-N-ethylacetamide. The formation of this by-product creates a serious technological problem in the industrial scale production of Zaleplon intended for use as an active ingredient in pharmaceutical formulations.

The PCT Publication No. WO 2005/073235 A2 describes a process for the preparation of Zaleplon containing less than 0.2% impurities by reaction of 3-amino-4- pyrazol-carbonitrile with equi-molar amount of N-{3-[3-(dimethylamino)-l-oxo-2- propenyl]phenyl}-N-ethyl-acetamide hydrochloride in the presence of aliphatic alcohols or a mixture of water and aliphatic alcohols at a pH of 3.0-3. 5.

While certain processes of Zaleplon preparation are known, but still there is a continuing need for simple and improved processes for the preparation of Zaleplon. We focused our research to develop a process, which reduces the formation of impurity / by-product, improves the quality and yield of product. The proposed process has advantages of simple operations, high yield and suitable for industrial production over the processes described in the related prior arts.

Objectives of the Invention

The main objective of the present invention is to provide an improved process for the preparation of a compound of formula (I) in good yield and high chemical purity. Another objective of the present invention is to provide a process for the preparation of a compound of formula (I), which minimizes the steps and would be easy to implement on commercial scale.

Still another objective of the present invention is to provide a method for the purification of a compound of formula (I), which would result higher chemical purity of a compound of formula (I).

Summary of the Invention

Accordingly, the present invention provides an improved process for the preparation of Zaleplon (I), comprising the steps of;

a) condensing 3-amino-4-cyanopyrazole of formula (III) with 3-dimethylamino-l- (3-N-ethyl-N-acetylaminophenyl)-2-propen-l-one of formula (II) in glacial acetic acid;

b) mixing the reaction mixture with an organic solvent without distillation of acetic acid; c) washing the organic layer with water; d) precipitating with an anti-solvent to get Zaleplon of formula (I); and e) optionally purifying Zaleplon by dissolving in an organic solvent and precipitating by adding an anti-solvent to obtain pure Zaleplon of formula (I).

Alternatively the present invention further provides another improved process for the preparation of Zaleplon (I), comprising the steps of;

a) condensing 3-amino-4-cyanopyrazole of formula (III) with 3-dimethylamino- 1 -(3 -N-ethyl-N-acetylaminophenyO-Z-propen- 1 -one of formula (II) in glacial acetic acid;

b) optionally cooling the reaction mixture; c) adding water to the reaction mixture; d) isolating Zaleplon by filtration; e) treating an alcoholic solution of Zaleplon with an aliphatic amine; f) optionally charcoalizing the solution obtained from step (e); g) cooling and isolating the Zaleplon of formula (I); and h) optionally purifying Zaleplon of formula (I) by dissolving in an organic solvent and precipitating by adding an anti-solvent to obtain pure Zaleplon of formula

(I)- The process is shown in the scheme given below:

Before Purification

PURIFICATION

(D (Purified Zaleplon)

After Purification

Description of the Invention

Accordingly, the present invention provides a process by which Zaleplon is prepared from 3-amino-4-cyanopyrazole and 3-dimethylamino-l-(3-N-ethyl-N- acetylaminophenyl)-2-propen-l-one in glacial acetic acid without distilling out the acetic acid.

In an embodiment of the present invention, organic solvent employed for dissolving Zaleplon is selected from the group consisting of dichloromethane, chloroform, ethylene dichloride, carbon tetrachloride and the like, most preferably dichloromethane.

In another embodiment of the present invention, anti-solvent solvent employed for precipitating Zaleplon is selected form straight, branched or cyclo alkane, which is selected from hexane, n-heptane or cyclo hexane and the like, most preferably n- heptane. In an embodiment of the present invention, the present invention provides a process for purification of Zaleplon. Accordingly the Zaleplon dissolved using alcoholic solvent and the resultant solution treated with organic amine. The process of reacting the alcoholic solution with organic amine, preferably aliphatic primary amine removes the undesired by-products and pure Zaleplon is obtained by cooling the reaction mass to below 15° C.

In another embodiment of the present invention, aliphatic primary amine used is preferably methylamine, ethyl amine and n-propyl amine; most preferably aqueous methylamine; and alcoholic solvent used is selected from the group comprising of methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, tertiary butanol, and mixtures thereof; most preferably methanol.

In another embodiment of the present invention, all the steps are preferably performed at a temperature in the range of (-) 10⁰C to reflux temperature of the solvent used.

In the present invention the starting materials are prepared according to the literature available in the prior art.

The present invention is illustrated with the following examples, which should not be construed for limiting the scope of the invention.

Example (1)

In a reaction vessel, glacial acetic acid (75 mL) and 3-amino-4-cyanopyrazole (15.56 g) were taken, the contents heated to 80⁰C to 85⁰C and 3-dimethylamino-l-(3-N- ethyl-N-acetylaminophenyl)-2-propen-l-one (25 g) was added. The reaction mass was stirred at the same temperature till the completion of the reaction. After completion of the reaction the reaction mass was cooled to 25⁰C to 35⁰C, dichloromethane added and stirred. The reaction mass was washed with water followed by sodium carbonate solution. To the organic layer n-heptane (150 mL) was added and stirred for 30 to 45 min. The solid was filtered and suck dried to obtain of crude Zaleplon as a wet cake (20 g)-

Crude Zaleplon (20 g) was dissolved in six volume of dichloromethane in a reaction vessel at 25⁰C to 30⁰C. n-Heptane (120 mL) was added to the solution of dichloromethane and stirred for 45 to 60 min. The solid was filtered and dried under vaccum to obtain pure Zaleplon (H g; purity by HPLC not less than 99.72%).

Example (2) In a reaction vessel, glacial acetic acid (150 mL) and 3-amino-4-cyanopyrazole

(27 g) were taken, and the contents heated to 80⁰C to 90⁰C. A solution of 3-dimethylamino-l-(3-N-ethyl-N-acetylaminophenyl)-2-propen-l-one (50 g) in glacial acetic acid (100 mL) was added, maintained at 8O⁰C to 90⁰C for 3 to 5 hrs and then cooled to 50⁰C to 60⁰C. After completion of the reaction water was added, material filtered and washed with water. The material was dried under vacuum to obtain crude Zaleplon (43 g).

Example (3)

In a reaction vessel, crude Zaleplon (50 g) obtained according to the example 2, was added to methanol (1000 mL) and the contents heated at 45°C to 5O⁰C until a clear solution was obtained. Aqueous methylamine solution was added to the solution and maintained at 45°C to 50⁰C for 5 to 6 hrs. The solution was charcoalized, and the filrate concentrated under vacuum until it reached 6 volumes. The reaction mass was cooled and maintained at 5 to 10⁰C for 30 to 45 min, filtered, washed with cold methanol and dried to obtain pure Zaleplon (42 g; purity by HPLC not less than 99.3).

Example (4)

In a reaction vessel, Zaleplon (38 g) obtained according to the example 3, was dissolved in dichloromethane (190 mL) under stirring to get a clear solution. The solution was charcoalized, and the filtrate concentrated up to 5 volumes at 4O⁰C. The concentrated mass was cooled to 25 to 30⁰C and n-heptane (247 mL) added and stirred for 60 to 90 mins. Filtration followed by washing with n-heptane (76 mL) and drying under vacuum afforded pure Zaleplon (30 g; purity by HPLC not less than 99.7%).

Claims

We Claim:

(I) A process for the preparation of Zaleplon of formula (I), comprising the steps of;

a) condensing 3-amino-4-cyanopyrazole of formula (III) with 3-dimethylamino- 1 -(3-N-ethyl-N-acetylaminophenyl)-2-propen- 1 -one of formula (II) in glacial acetic acid;

b) optionally cooling the reaction mixture; c) adding water to the reaction mixture; d) isolating Zaleplon by filtration; e) treating an alcoholic solution of Zaleplon with an aliphatic amine; f) optionally charcoalizing the solution obtained from step (e); and g) isolating the Zaleplon of formula (I).

(2) A process for the purification of Zaleplon of formula (I), comprising the steps of; a) treating an alcoholic solution of Zaleplon with an aliphatic amine; b) optionally charcoalizing the solution obtained from step (a); and c) isolating the Zaleplon of formula (I).

(3) A process according to claim no. 1 or 2 wherein the said alcoholic solution of Zaleplon is prepared by dissolving Zaleplon in an alcoholic solvent selected from the group consisting of methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n- butanol isobutanol, tertiary butanol, or mixtures thereof; most preferably methanol.

(4) A process according to claim 1 or 2, wherein the said aliphatic amine is primary aliphatic amine.

(5) A process according to claim 4, wherein the said primary aliphatic amine is selected from methylamine, ethylamine, n-butyl amine or n-propyl amine; most preferably aqueous methylamine.

(6) A process according to claim 1 or 2, wherein the Zaleplon is isolated from reaction mass by cooling the reaction mass.

(7) A process for the preparation of Zaleplon of formula (I), comprising the steps of;

(I) a) condensing 3-amino-4-cyanopyrazole of formula (III) with 3-dimethylamino-l- (3-N-ethyl-N-acetylaminophenyl)-2-propen-l-one of formula (II) in glacial acetic acid;

(II) (III)

b) mixing the reaction mixture with an organic solvent; c) washing the organic layer with water; and d) precipitating with an anti-solvent to get Zaleplon of formula (I); wherein the improvement consists of treating the reaction mixture with organic solvent without distillation of acetic acid.

(8) A process according to claim no. 7, wherein the said organic solvent used in step (d) is selected from the group consisting of dichloromethane, chloroform, ethylene dichloride, carbon tetrachloride or mixtures thereof.

(9) A process according to claim no. 7, wherein the said anti-solvent used in step (d) is selected from hexane, n-heptane or cyclohexane or mixtures thereof.