Steric Hindrance Tuned [4 + 1] Annulation of α-Substituted Conjugated Enones with Ylides for Dihydrofuran Synthesis

In sharp contrast to the intuitive modulation by the electronic effect, the [4 + 1] annulation reaction between α-substituted conjugated enones and ylides has been reliably controlled by the match of the α-substituent and the ylide in steric hindrance, providing a straightforward and efficient method for valuable 4-cyano, 4-alkynyl, 4-vinyl, and 4-aryl 2,3-dihydrofurans.