Process for the preparation of 1,4-dihydropyridine derivatives

Abstract: In the preparation of 1,4-dihydropyridine derivatives of formula (I), the improvement which comprises reacting a 2-haloethyl 2-benzylidene-3-oxobutanoate of formula (IV) with a 3-aminocrotonic acid ester of formula (V) in a reaction medium in the presence of a dehydrating agent. In the formulae R¹, R², R³ and X have the same meanings as defined in the claims.

Description: FIELD OF THE INVENTION This invention relates to a process for the preparation of 1,4-dihydropyridine derivatives having a 2-haloethoxycarbonyl group at the 3-position which are useful as an intermediate compound for the synthesis of 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylic acid diesters having valuable vasodilating and blood pressure lowering effects, etc. BACKGROUND OF THE INVENTION Diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate is known to obtain by reacting ethyl 2-benzylidene-3-oxobutanoate, ethyl 3-aminocrotonate or ethyl 3-oxobutanoate and ammonia, as reported in Ber. Deutsch Chem. Ges. 31 743 (1971). DOS 2117517 and 2117573 disclose that similar compounds can be used as coronary arteriodilating and antihypertensive agents, and inter alia dimethyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate disclosed therein has been used extensively under the name of Nifedipine. Since commercial success of Nifedipine, a large number of compounds having similar chemical structure have been developed and these compounds are disclosed in US Patents 3,574,843; 4,264,611; 3,799,934 4,239,893; 4,317,768; 4,044,141 and 4,258,042; EPO Appln. 0012180; and French Patent 2,182,98. We have developed 1,4-dihydropyridine derivatives wherein a heterocyclic group is linked to an ethylene group through an ester bond in an ester moiety at the 3-position of 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylic acid diesters and processes for the preparation thereof, which are disclosed in Japanese Patent LOP Publication Nos. 174070/1987 and 292781/1987. The 1,4-dihydropyridine derivatives having a 2-haloethoxycarbonyl group at the 3-position as their intermediates are synthesized by a simultaneous reaction of an aromatic aldehyde, a 2-haloethyl 3-oxobutanoate and a 3-aminocrotonic acid ester in an alcohol solvent (Japanese Patent LOP Publication Nos. 9083/1980 and 174070/1987) or by a stepwise reaction which comprises reacting an aromatic aldehyde with a 2-haloethyl 3-oxobutanoate to form a 2-haloethyl 2-benzylidene-3-oxobutanoate and then reacting 2-benzylidene-3-oxobutanoate with a 3-aminocrotonic acid ester (Japanese Patent LOP Publication Nos. 54318/1985 and 199874/1985). However, the former method of reacting in one step three materials of an aromatic aldehyde, a 2-haloethyl 3-oxobutanoate and a 3-aminocrotonic acid ester forms a large amount of by-products and is not practical. The latter method by the stepwise reaction is of the drawback that a water molecule forming during the reaction hydrolyses a 2-haloethyl group to a 2-hydroxyethyl group which results in reducing a yield of the aimed compound having a 2-haloethoxycarbonyl group at the 3-position of the 1,4-dihydropyridine ring. Thus a process for the production of these intermediate compounds in higher yield has been demanded. We have extensively studied the above-mentioned latter process by the stepwise reaction in an effort to overcome the drawbacks and found that the aimed 1,4-dihydropyridine derivatives containing a 2-haloethoxycarbonyl group are produced in high yield by carrying out the reaction of a 2-haloethyl 3-oxobutanoate and a 3-aminocrotonic acid ester in the presence of a dehydrating agent. Thus the present invention provides a process for the preparation of 1,4-dihydropyridine derivatives of formula I wherein R¹ and R² may be the same or different and each is hydrogen, halogen, trifluoromethyl or nitro but both are not hydrogen, X is halogen and R³ is C₁-C₆ alkyl or C₁-C₆ alkoxyalkyl which comprises reacting an aromatic aldehyde of formula II wherein R¹ and R² have the same meaning as defined above with a 2-haloethyl 3-oxobutanoate of formula III wherein X has the same meaning as defined above to form a 2-haloethyl 2-benzylidene-3-oxobutanoate of formula IV wherein R¹, R² and X have the same meaning as defined above and then reacting the 2-haloethyl 2-benzylidene-3-oxobutanoate with a 3-aminocrotonic acid ester of formula V in a reaction medium in the presence of a dehydrating agent. The aromatic aldehydes of formula II which are used in the process of the invention include 3-nitrobenzaldehyde, 2-nitrobenzaldehyde, 2,3-dichlorobenzaldehyde, 2-trifluoromethylbenzaldehyde, 4-nitrobenzaldehyde, 3-trifluoromethylbenzaldehyde, 4-trifluoromethylbenzaldehyde or the like. The 2-haloethyl 3-oxobutanoate of formula III include 2-chloroethyl 3-oxobutanoate, 2-bromoethyl 3-oxobutanoate, 2-iodoethyl 3-oxobutanoate, 2-fluoroethyl 3-oxobutanoate or the like. The 3-aminocrotonic acid esters of formula V include methyl 3-aminocrotonate, ethyl 3-aminocrotonate, isopropyl 3-aminocrotonate, n-propyl 3-aminocrotonate, isobutyl 3-aminocrotonate, n-butyl 3-aminocrotonate, sec-butyl 3-aminocrotonate, tert-butyl 3-aminocrotonate, 2-methoxyethyl 3-aminocrotonate, methoxypropyl 3-aminocrotonate, ethoxyethyl 3-aminocrotonate, or the like. In the process of the invention, the compound of formula II and the compound of formula III are used in a molar ratio of 1.0 : 0.1-10, especially preferably 1.0 : 0.8-1.2. The compound of formula IV and the compound of formula V are used in a molar ratio of 1.0 : 0.1-10, especially preferably 1.0 : 0.8-1.2. The reaction between the compound of formula II and the compound of formula III is usually carried out at a reaction temperature of 0° to 150°C and a reaction time of one hour to three days without any solvent or in a reaction medium such as an aromatic hydrocarbon solvent e.g. benzene, toluene, xylene, an aliphatic hydrocarbon solvent e.g. hexane, cyclohexane, ligroin, an ether solvent e.g. isopropyl ether, diethyl ether, tetrahydrofuran, dioxane, methyl cellosolve, a halogenated hydrocarbon solvent e.g. methylene chloride, chloroform, trichlene, an alcohol solvent e.g. methanol, ethanol, isopropyl alcohol in the presence of an acid catalyst such as hydrogen chloride, sulfuric acid, acetic acid or a basic catalyst such as diisopropylamine, pyridine, piperidine, triethylamine or a mixture of these acid catalysts or a mixture of these basic catalysts. A condensation reaction between the compound of formula IV and the compound of formula V is usually performed at a reaction temperature of 0°C to 150°C and a reaction time of one hour to three days in a reaction medium such as methanol, ethanol, isopropyl alcohol, n-butyl alcohol, toluene, benzene, cyclohexane alone or in combination. In the feature of the invention carrying out a condensation reaction between the compound of formula IV and the compound of formula V in the presence of a dehydrating agent, the dehydrating agents used in the reaction include anhydrous sodium sulfate, anhydrous magnesium sulfate, molecular sieve 4A, molecular sieve 3A or the like, molecular sieve 3A being especially preferred. The dehydrating agent is used in a proportion of 50-500 grams per mole of the compound of formula IV. The compound of formula IV prepared by reaction of the compound of formula II with the compound of formula III may be isolated and condensed with the compound of formula V, but may be subjected to a reaction with the compound of formula V without isolation. The compound of formula IV can be isolated by extraction, column chromatography, crystallization or the like. The compound of formula I prepared by a condensation reaction between the compound of formula IV and the compound of formula V can be isolated by extraction, column chromatography, crystallization or the like. According to the present process wherein 2-haloethyl 2-benzylidene-3-oxobutanoate prepared by reaction of aromatic aldehydes with 2-haloethyl 3-oxobutanoate are reacted with 3-aminocrotonic acid esters in the presence of the dehydrating agent, there can be prepared in good yield the desired 1,4-dihydropyridine derivatives having 2-haloethoxycarbonyl group at the 3-position. The invention is further illustrated by the following examples. EXAMPLE 1 2-Chloroethyl methyl 2,6-dimethyl-4-(3-nitropbenyl)-1,4-dihydropyridine-3,5-dicarboxylate 2-Chloroethyl 2-(3-nitrobensylidene)-3-oxobutanoate (8.00 g) and methyl 3-aminocrotonate (3.40 g) were charged into a 100 ml three necked flask and dissolved in 30 ml of isopropyl alcohol. A Soxhlet extractor containing 5 g of molecular sieve 3A and a reflux condenser were attached to the flask and a reaction mixture was heated under reflux for 3 hrs. After completion of the reaction, 2-propanol was distilled off, the reaction mixture was dissolved in chloroform and washed with diluted hydrochloric acid and water. It was dried over magnesium sulfate, separated and purified by column chromatography and crystallized from ethyl acetate/hexane. The crystals were filtered and dried under reduced pressure to afford 10.16 g (96% yield) of the title compound, m.p. 134-137°C. For comparison, the reaction was carried out without using molecular sieve in the above manner. The yield was 84% with the formation of a hydrolyzed compound, 2-hydroxyethyl methyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate (10% yield, m.p. 180-182°C) as by-product. EXAMPLE 2 2-Chloroethyl methyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate 2-Chloroethyl 2-(2-nitrobenzylidene)-3-oxobutanoate (8.00 g) and methyl 3-aminocrotonate (3.40 g) were reacted in a similar manner as in Example 1 to afford 8.44 g (80% yield) of the title compound, m.p. 117-122°C. For comparison, the reaction was carried out without using dehydrating agent in the above manner. The yield was 59% with the formation of a hydrolyzed compound, 2-hydroxyethyl methyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate (11% yield, m.p. 127-129°C) as by-product. EXAMPLE 3 2-Chloroethyl methyl 2,6-dimethyl-4-(2-trifluoromethylphenyl)-1,4-dihydropyridine-3,5-dicarboxylate 2-Chloroethyl 3-oxo-2-(2-trifluoromethylbenzylidene)butanoate (19.24 g) and methyl 3-aminocrotonate (7.60 g) were reacted in the presence of molecular sieve 3A (10 g) in a similar manner as in Example 1 to give 20.38 g (81% yield) of the title compound, m.p. 132-134°C. For comparison, the reaction was carried out without using dehydrating agent in the above manner. The yield was 57% with the formation of a hydrolyzed compound, 2-hydroxyethyl methyl 2,6-dimethyl-4-(2-trifluoromethylphenyl)-1,4-dihydropyridine-3,5-dicarboxylate (16% yield, m.p. 141-144°C) as by-product. 2-Chloroethyl 3-oxo-2-(2-trifluoromethylbenzylidene)butanoate used in this example was prepared in the following manner. To a cyclohexane (1 lit.) solution of 2-trifluoromethyl benzaldehyde (250 g) and 2-chloroethyl 3-oxobutanoate (260 g) was added sulfuric acid (4 ml), a reaction mixture was heated under reflux for 5 hrs. and an azeotropically producing water was removed with a water separator. After completion of the reaction, it was treated in a conventional way to afford 373 g of 2-chloroethyl 3-oxo-2-(2-trifluoromethylbenzylidene)butanoate. EXAMPLE 4 2-Chloroethyl methyl 2,6-dimethyl-4-(2,3-dichlorophenyl)-1,4-dihydropyridine-3,5-dicarboxylate 2-Chloroethyl 2-(2,3-dichlorobenzylidene)-3-oxobutanoate (8.64 g) and methyl 3-aminocrotonate (3.40 g) were reacted in a similar manner as in Example 1 to give 8.91 g (79% yield) of the title compound, m.p. 141-145°C. For comparison, the reaction was carried out without using dehydrating agent in the above manner. The yield was 64% with the formation of a hydrolyzed compound, 2-hydroxyethyl methyl 2,6-dimethyl-4-(2,3-dichioropbenyl)-1,4-dihydropyridine-3,5-dicarboxylate (11% yield, m.p. 113-118°C) as by-product. EXAMPLE 5 2-Chloroethyl methyl 2,6-dimethyl-4-(2-trifluoromethylphenyl)-1,4-dihydropyridine-3,5-dicarboxylate 2-Chloroethyl 3-oxo-2-(2-trifluoromethylbenzylidene)butanoate (19.24 g) and methyl 3-aminocrotonate (3.40 g) were reacted in the presence of anhydrous magnesium sulfate (7.3 g) in a similar manner as in Example 1 to give 16.62 g (66% yield) of the title compound, m.p. 132-134°C.

Claims: A process for the preparation of 1,4-dihydropyridine derivatives of formula I wherein R¹ and R² may be the same or different and each is hydrogen, halogen, trifluoromethyl or nitro but both are not hydrogen, X is halogen and R³ is C₁-C₆ alkyl or C₁-C₆ alkoxyalkyl characterized by reacting an aromatic aldehyde of formula II wherein R¹ and R² have the same meaning as defined above with a 2-haloethyl 3-oxobutanoate of formula III wherein X has the same meaning as defined above to form a 2-haloethyl 2-benzylidene-3-oxobutanoate of formula IV wherein R¹, R² and X have the same meaning as defined above and then reacting the 2-haloethyl 2-benzylidene-3-oxobutanoate with a 3-aminocrotonic acid ester of formula V in a reaction medium in the presence of a dehydrating agent. A process of Claim 1 wherein a molar ratio of the compound of formula II to that of formula III is 1.0 : 0.1-10, especially preferably 1.0 : 0.8-1.2. A process of Claim 1 or 2 wherein a molar ratio of the compound of formula IV to that of formula V is 1.0 : 0.1-10, especially preferably 1.0 : 0.8-1.2. A process of Claim 1, 2 or 3 wherein a condensation reaction between the compound of formula IV and that of formula V is usually performed at a reaction temperature of 0°C to 150°C and a reaction time of one hour to three days. A process of Claim 1, 2, 3 or 4 wherein the dehydrating agent includes anhydrous sodium sulfate, anhydrous magnesium sulfate, molecular sieve 4A or molecular sieve 3A. A process of Claim 1, 2, 3, 4 or 5 wherein the dehydrating agent is used in a proportion of 50-500 grams per mole of the compound of formula IV.

Inventors: HIMOTO SHIZUO, SATOH HIROAKI, HIMOTO, SHIZUO, SATOH, HIROAKI
Applicants: NISSHIN FLOUR MILLING CO, NISSHIN FLOUR MILLING CO., LTD.
Application year: 1989
Main classification: C07D211
IPC codes: A61K31, A61P9, C07D211