Facile Synthesis of Spirooxindole‐Cyclohexenes via Phosphine‐Catalyzed [4 + 2] Annulation of α‐Substituted Allenoates

A phosphine-catalyzed [4+2] annulation of -substituted allenoate with exocyclic alkene moiety of oxindoles or indan-1,3-diones has been developed. Thus, under the catalysis of PPh3 (20mol%), a series of spirooxindole- or spiroindan-1,3-dione-cyclohexenes have been obtained in moderate to excellent yields and regioselectivity from the annulations of -methyl allenoates with 3-methyleneoxindoles or 2-methyleneindan-1,3-diones. This method offers an easy access to structurally novel spirocyclohexenes.