Cationic‐Oxazaborolidine‐Catalyzed Enantioselective Diels–Alder Reaction of α,β‐Unsaturated Acetylenic Ketones

Yne also a good dienophile: The cationic oxazaborolidine 1 promoted the formation of Diels–Alder adducts between acetylenic ketones and both cyclic and acyclic dienes in excellent yield with 99 % ee (see scheme; Tf=trifluoromethanesulfonyl, TMS=trimethylsilyl). Importantly, high levels of asymmetric induction were also observed with dienophiles that lacked the typical hydrogen-bonding motif required for other oxazaborolidinium-mediated reactions.