Asymmetric Organocatalytic [2 + 2] Cycloaddition Affording Bicyclo[n.2.0]alkenes

The development of an organocatalytic asymmetric [2 + 2] cycloaddition to synthesize optically active bicyclo[n.2.0]alkene is described. Chiral imidodiphosphorimidate (IDPi)-based asymmetric counter anion-directed silylium catalysis enables readily accessible enol silyl ether to be combined with commercial propiolate with an asymmetric manner through an intermolecular Michael addition followed by an intramolecular aldol cyclization sequence. Good yields and enantiomeric excesses were obtained with specific substrates, and an effective structural motif of the catalyst was identified. The possibility that the reaction outcome was highly differentiated by the ring size of enol silyl ether and the catalyst acidity was also indicated.