Lewis Acid-Promoted Enantioselective Dearomative Spirocyclizations of Allenes

A chiral oxazaborolidine combined with SnCl4 has been found to promote the dearomative spirocyclization of electron-rich benzyl allenyl ketones. The reaction outcome is sensitive to the nature of activating acid, which was rationalized using hard–soft acid–base (HSAB) theory. The spirocyclic product was obtained with up to 72% ee, which is the best result reported to date for these substrates. The formation of cross-conjugated or conjugated products is readily controlled by changing the oxygen-protecting groups.