The object of the invention is to provide a process for the synthesis of cyclopentenenitrile derivatives through conjugate addition of a nitrile derivative to an +-,-unsaturated ketone whereby cyclopentenenitrile derivatives, particularly those capable of optical resolution, can be obtained in high yield without expensive reagents or compounds in a decreased number of steps. A novel process for the production of cyclopentenenitrile derivatives in high yield, which comprises trapping a compound obtained by conjugate addition of a 4-alkoxy-3-butenenitrile which may have alkyl or allyl at the +--position (e.g., 4-methoxy-3-butenenitrile) to an +-,-unsaturated ketone as an enol ester with acetic anhydride, chlorocarbonic ester, or the like and treating the enol ester with a strong acid or the like to form a five-membered ring. |