Design of Brønsted Acid-Assisted Chiral Lewis Acid (BLA) Catalysts for Highly Enantioselective Diels−Alder Reactions

Brønsted acid-assisted chiral Lewis acid (BLA) was highly effective as a chiral catalyst for the enantioselective Diels−Alder reaction of both α-substituted and α-unsubstituted α,β-enals with various dienes. Hydroxy groups in optically active binaphthol derivatives and boron reagents with electron-withdrawing substituents were used as Brønsted acids and Lewis acids, respectively. Intramolecular Brønsted acids in a chiral BLA catalyst played an important role in accelerating the rate of Diels−Alder reactions and in producing a high level of enantioselectivity. In particular, excellent enantioselectivity was achieved due to intramolecular hydrogen bonding interaction and attractive π−π donor−acceptor interaction in the transition-state assembly by hydroxy aromatic groups in a chiral BLA catalyst.