Design of Bronsted acid-assisted chiral Lewis acid (BLA) catalysts for highly enantioselective Diels-Alder reactions

Bronsted acid-assisted chiral Lewis acid (BLA) was highly effective as a chiral catalyst for the enantioselective Diels-Alder reaction of both alpha-substituted and cr-unsubstituted alpha,beta-enals with various dienes. Hydroxy groups in optically active binaphthol derivatives and boron reagents with electron-withdrawing substituents were used as Bronsted acids and Lewis acids, respectively. Intramolecular Bronsted 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 pi-pi donor-acceptor interaction in the transition-state assembly by hydroxy aromatic groups in a chiral BLA catalyst.