Photosensitized 2 + 2 cycloaddition reaction using homochirality generated by spontaneous crystallization.

A photosensitized reaction was performed using molecular chirality, which was generated and amplified by the spontaneous crystallization of achiral coumarincarboxamide. The molecular chirality was retained in a cold solution caused by slow racemization and was transferred to optically active products by a photosensitized 2 + 2 cycloaddition reaction with high enantiomeric excesses.