A stirred ethanolic solution (200 ml) of an alkaline alkoxide, for example sodium ethoxide (20 g), is saturated, at +10° C., with dry hydrogen sulfide, cooled to -15° C., and added with a solution of 2-chloromethyl-3,5-dicarboethoxy-4-(m-nitrophenyl) -6-methyl-1,4-dihydropyridine (25 g) in DME (70 ml). After one hour, the reaction mixture is warmed to 0° C., acidified with H2SO4 (10% water solution; 100 ml) and diluted with water (1400 ml). The precipitate is filtered off, dried in vacuum, and recrystallized from Et2O to give 23 g of 2-mercaptomethyl-3,5-dicarboethoxy-4-(m-nitrophenyl) -6-methyl-1,4-dihydropyridine, m.p. 145°-147° C.
