To a mixture of diethyl 2-methyl-4-(2-nitrophenyl)-6-hydroxymethyl-1,4-dihydropyridine-3,5-dicarboxylate (2.35 g) in pyridine (30 ml), as dropwise added a solution of acetyl chloride (942 mg) in methylene chloride (5 ml) under stirring and ice-cooling over the period of 7 minutes. The mixture was further stirred for 70 minutes at room temperature. The pyridine was distilled off under reduced pressure and ethyl acetate was added to the residue. The resultant mixture was washed twice with water and adjusted to pH 4 with hydrochloric acid and then an aqueous solution of sodium chloride, dried and concentrated under reduced pressure to give an oily substance. This oily substance was crystallized by treating with a mixture of diethyl ether and a small amount of n-hexane, and collected crude crystals (2.5 g) were washed with n-hexane and thus obtained crystals were recrystallized from diethyl ether to give pure crystals (1.78 g) of diethyl 2-methyl-4-(2-nitrophenyl)-6-acetoxymethyl-1,4-dihydropyridine-3,5-dicarboxylate, m.p. 89° to 90° C.
