The problem to be solved is to provide a method forproducing a compound having excellent activity as an .alpha.2.delta.ligand. The solution thereto is a method for producing acompound represented by the general formula (I) or a saltthereof by optical resolution:(see formula I)[in the formula, R1: a hydrogen atom or a C1-C6 alkylgroup, and R2: a hydrogen atom or a protective group forthe carboxy group]