A method including resolution of a diastereomeric mixture represented by the formula ##STR1##

wherein R¹ and R² are the same or different and each is an optionally substituted hydrocarbon group or an optionally substituted heterocyclic group, only one of R¹ and R² contains one asymmetric carbon, and R^a is an optically active and optionally substituted hydrocarbon group or an optically active and optionally substituted heterocyclic group, or a salt thereof, to produce the diastereomer having a steric configuration of the asymmetric carbon for R¹ or R² of an R configuration or an S configuration, or a salt thereof.