For example, the compound (II) in which R1 is hydrogen, n is an integer of 2 and R4 is trityl can be readily separated into each of the optical isomers by reacting the racemic modification of the compound (II) with L- or D-phenylalanine methyl ester in a solvent (e.g., a mixture of methanol and dioxane) to form the diastereoisomeric salts thereof, and separating said diastereoisomers into each com