Abstract: A process is provided for producing diastereoisomer A or a pharmaceutically acceptable acid addition salt of 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-(1-benzylpyrrolidin-3-yl) ester 5-methyl ester which comprises subjecting a mixture of diastereoisomers A and B of 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-(1-benzylpyrrolidin-3-yl) ester 5-methyl ester to column chromatography using silica gel as a carrier and a mixture of ethyl acetate and acetic acid as an eluant, and then separating the acetate of diastereoisomer A from the eluate, or treating the acetate with a base, or further treating the resulting diastereoisomer A with a pharmaceutically acceptable acid.

Abstract: A dextro-rotatory optical isomer of diastereoisomer A of (.+-.)-2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-(1-benzylpyrrolidin-3-yl)ester 5-methyl ester is provided having the formula ##STR1## and wherein the melting point of the hydrochloride of the dextro-rotatory optical isomer is 223.degree. to 230.degree. C. (decomp.), or a pharmaceutically acceptable acid addition salt thereof.

Abstract: The diastereoisomer A of a dihydropyridine-3,5-dicarboxylic acid ester derivative is provided as well as its pharmaceutically acceptable acid addition salts. The invention further discloses a process for producing the compounds and their use as medicaments in imparting vasodilating activity.