CHEMICAL PROCESS

Abstract: A process for preparing 3-[4-(4-oxo-1,4-dihydro­ pyridin-1-yl)benzoyl]butanoic acid is described which comprises reacting 3-(4-fluorobenzoyl)butanoic acid with 4-hydroxypyridine characterised in that the process is performed under aqueous conditions in the presence of a suitable base.

Description: The present invention relates to a process for preparing N-phenylpyridone derivatives. Specifically this invention relates to a process for preparing the compound of the formula (1) which is a useful intermediate in the preparation of the compound of the formula (2) : The compound of the formula (2) is a selective phosphodiesterase type III inhibitor with positive inotropic and vasodilator activity as described in European Patent Application Publication No 220 044. A suitable preparation of the compound of the formula (1) is to treat a compound of the formula (3) with 4-hydroxypyridine. In the literature similar nucleophilic aromatic substitutions have been performed in the presence of strong bases in dipolar aprotic solvents under anhydrous conditions. For example in J. Med. Chem. 1985 28 , 1405-1413 a compound of the formula (4) is prepared by reacting a compound of the formula (5) with imidazole in the presence of 2 molar equivalents of sodium hydride in a mixture of dimethylsulphoxide and toluene at 100-110°C for 18 hours. We have prepared the compound of the formula (1) by reacting the sodium derivative of a compound of the formula (3) with the sodium derivative of 4-hydroxypyridine in N-methylpyrrolidin-2-one at 105°C for 4½ hours. The sodium derivatives were separately formed by treatment with sodium hydride in N-methylpyrrolidin-2-one. In order to isolate the product the cooled reaction mixture was poured into water, the resultant mixture was acidified to pH 4 with dilute hydrochloric acid and then extracted with dichloromethane. Evaporation of the organic extract gave the compound of the formula (1) in 40% yield. Extraction of the remaining aqueous mixture with n-butanol and evaporation of the organic extract gave less pure product in 26% yield. Although this process affords the compound of the formula (1) in an overall yield of 66% the product required further purification and the inefficient extraction procedure involving n-butanol is unattractive on a commercial scale. We have attempted to avoid the use of dipolar aprotic solvents and to prepare the compound of the formula (1) by reacting a compound of the formula (3) with 4-hydroxypyridine in ethanol under reflux in the presence of sodium ethoxide. Under these conditions the reaction is slow and leads to the formation of compounds of the formulae (1) and (6) in approximately equal amounts. Surprisingly, we have now found that a compound of the formula (1) can be prepared from a compound of the formula (3) in very good yield under conditions which are particularly suitable for commercial manufacture on the plant scale. Accordingly the present invention provides a process for preparing a compound of the formula (1) which comprises reacting a compound of the formula (3) with 4-hydroxypyridine characterised in that the process is performed under aqueous conditions in the presence of a suitable base. Suitably an excess of 4-hydroxypyridine is used in the reaction. preferably between 1 to 4 moles, particularly 1.5 to 3 moles. of 4-hydroxypyridine are used for each mole of a compound of the formula (3). By a suitable base we mean a base which will abstract the acidic protons of 4-hydroxypyridine and a compound of the formula (3) under aqueous conditions. Examples of such bases include alkali metal hydroxides such as sodium and potassium hydroxide, alkaline earth metal hydroxides such as calcium hydroxide or organic bases such as tetramethylguanidine or 1,8-diazabicyclo[5.4.0]undec-7-ene. Preferably a sufficient quantity of an alkali metal hydroxide is used to form the anions of 4-hydroxypyridine and the compound of the formula (3). Thus if 1 mole of a compound of the formula (3) is used with 2 moles of 4-hydroxypyridine, then preferably at least 3 moles of sodium hydroxide are used. Suitably an excess of an alkali metal hydroxide is used in the reacton. Suitably the reaction is performed within a pH range of 9 to 14. Suitably the reaction is performed at a elevated temperature, for example at the reflux temperature of the reaction mixture. Suitably the reaction is performed in an autoclave within the temperature range 100-200°C, for example at about 140°C. Preferably the reaction is performed at a reasonable concentration, for example 2 moles of a compound of formula (3) per litre of solvent. The product is conveniently precipitated in high yield and high purity from the cooled reaction mixture on acidification to pH 3-4 with a mineral acid such as hydrochloric acid. This process not only avoids the use of expensive and inflammable organic solvents and bases such as sodium hydride but has the advantage that the product can be readily isolated from the reaction mixture on acidification without recourse to a laborious and inefficient solvent extraction process. Organic solvents such as alcohols e.g. isopropanol or n-butanol can be added to the reaction mixture, however since isolation of the product would be more complicated involving evaporation of the organic solvent followed by acidification of the residue, preferably the reaction is performed in the absence of an organic co-solvent. This invention is illustrated by the following examples. Example 1 A solution of 3-(4-fluorobenzoyl)butanoic acid (8 g), 4-hydroxypyridine (8 g) and sodium hydroxide (4.6 g) in water (80 ml) was heated in an autoclave at 140°C for 20 hours. The reaction mixture was cooled and was acidified to pH 3-4 with dilute hydrochloric acid to afford as a white crystalline precipitate 3-[4-(4-oxo-1,4-dihydropyridin-1-yl)benzoyl]butanoic acid, 9.45 g (87%), m.p. 250-252°C. Example 2 A solution of 3-(4-fluorobenzoyl)butanoic acid (885 g), 4-hydroxypyridine (663 g) and sodium hydroxide (453 g) in water (2.2 litres) was stirred under reflux for 24 hours. The reaction mixture was cooled, diluted with water (10 litres) and filtered through diatomaceous earth. The filtrate was acidified to pH 3 with concentrated hydrochloric acid to afford as a cream solid 3-[4-(4-oxo-1,4-dihydropyridin-1-yl)benzoyl]butanoic acid, 1127 g (94%), m.p. 251-254°C.

Claims: A process for preparing a compound of the formula (1) : which comprises reacting a compound of the formula (3) : with 4-hydroxypyridine characterised in that the process is performed under aqueous conditions in the presence of a suitable base. A process according to claim 1 wherein from 1 to 4 mole equivalents of 4-hydroxypyridine are used for each mole equivalent of a compound of the formula (3). A process according to claim 1 or 2 wherein a suitable base is an alkali metal hydroxide or alkaline earth metal hydroxide. A process according to any one of claims 1 to 3 wherein sufficient alkali metal hydroxide is used to form the anions of a compound of the formula (3) and 4-hydroxypyridine. A process according to claim 4 wherein the alkali metal hydroxide is sodium hydroxide. A process according to any one of claims 1 to 5 wherein the reaction is performed in the absence of an organic co-solvent. A process according to claim 6 wherein the product is isolated by adjustment to pH 3-4 with a mineral acid. A process according to claim 7 wherein the mineral acid is hydrochloric acid.

Inventors: MITCHELL, MICHAEL BARRY, PHANCHOLI, KIRITKANT DAHYABHAI
Applicants: SMITH KLINE & FRENCH LABORATORIES LIMITED
Application year: 1987
Main classification: C07D213
IPC codes: C07D213