Chiral Oxazaborolidine-Aluminum Bromide Complexes Are Unusually Powerful and Effective Catalysts for Enantioselective Diels-Alder Reactions

摘要

The oxazaborolidine 1 is a valuable chiral catalyst for the borane-mediated enantioselective reduction of achiral unsymmetrical ke-tones. Protonation of 1 with triflic acid generates a chiral oxazaboro-lidinium cation 2, which is an extraordinarily useful broad-spectrum catalyst for numerous Diels-Alder reactions. It has been applied to the synthesis of many complex molecules. Because the oxazaborolidine 1 is quite a weak base, its full protonation can only be achieved using a very strong acid such as triflic acid or triflimide.