Process for producing optically pure 1,4-dihydropyridines

Process for producing a 1,4-dihydropyridine of formula Ia or Ib ##STR10## in which R.sup.1 is a C.sub.1 -C.sub.4 alkyl group, and one of the groups X.sub.1 and X.sub.2 represents a nitro, fluoro, chloro or trifiuoromethyl group and the other represents a hydrogen atom, or X.sub.1 and X.sub.2 both represent a chloro group, which comprises the crystallization of a calcium salt of a lactic acid esterified with a racemic 1,4-dihydropyridine, of formula ##STR11## so as to separate out calcium salt crystals of formula:

• ##STR12## and hydrolysis of the crystallized salt IIIa or IIIb in order to obtain a 1,4-dihydropyridine of formula Ia or Ib respectively.