Cationic Oxazaborolidine Catalyzed Enantioselective Diels-Alder Reaction of αβ-Unsaturated Acetylenic Ketones

摘要

Cationic oxazaborolidine >1 affords Diels-Alder adducts of α,β-unsaturated acteylenic ketones with cyclic and acyclic dienes in excellent yields and enantioselectivities. Importantly, dienophiles lacking the typical hydrogen bonding motif as required for other oxazaborolidinium mediated reactions also provide uniformly high levels of asymmetric induction. Subsequently, mechanistic studies of this reaction were undertaken through X-ray crystallographic and computational studies of BF3-acetylene complexes which both indicate a strong preference for syn-coordination geometry of the Lewis acid relative to the alkyne. Thus, we postulate syn-coordination of the acetylene by >1 is operative, which precludes the necessity for hydrogen bonding and maintains close proximity of the reactive sp carbon centers to the chiral environment of the catalyst, ensuring high enantioselectivities.