A NEW CHIRAL BLA PROMOTER FOR ASYMMETRIC AZA-DIELS-ALDER AND ALDOL-TYPE REACTIONS OF IMINES

A new type of chiral promoter for double asymmetric inductions of aza Diels-Alder and aldol-type reactions of imines is prepared from trialkyl berates (B(OMe)(3) or B(OPh)(3)) and optically pure binaphthol; X-ray analysis of the boron complex demonstrates that it exists as a Bronsted acid-assisted chiral Lewis acid(BLA). The aldol-type reactions of a number of N-benzhydrylimines derived from aromatic aldehydes with the ketene silyl acetal derived from tert-butyl acetate mediated by the chiral BLA afford beta-amino acid esters with high enantioselectivity. The solution conformations of the BLA.imine complexes have been studied using H-1 NMR analysis and difference NOE measurements. The absolute configurations of the adducts can be understood in terms of a rational model involving an intramolecular hydrogen binding interaction via a Bronsted acid.