A new cyclopentene annulation method based on a conjugate additionreaction with 4-methoxybut-3-enenitrile was developed. Treatment of a cyclic enone with the potassium carbanion of the nitrile followed by acetic anhydride afforded an enol acetate, which underwent an HCl-mediated intramolecular cyclization reaction to yield a bicyclo[n.3.0]alkenone derivative in good yield. A lipase-mediated optical