Lewis-Acid Promoted Enantioselective Dearomative Spirocyclizations of Allenes

摘要

A chiral oxazaborolidine combined with SnCl4 has been found that promotes the dearomative spirocyclization of electron rich benzyl allenyl ketones. The reaction outcome is sensitive to the nature of activating acid which was rationalized using HSAB theory. The spirocyclic product was obtained with up to 72% ee, 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.