A new chiral BLA promoter for asymmetric aza Diels - Alder and aldol-type reactions of imines

Article abstract of DOI:10.1021/ja00102a019

A new type of chiral promoter for double asymmetric inductions of aza Diels - Alder and aldol-type reactions of imines is prepared from trialkyl borates (B(OMe)₃ or B(OPh)₃) and optically pure binaphthol; X-ray analysis of the boron complex demonstrates that it exists as a Br?nsted 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 β-amino acid esters with high enantioselectivity. The solution conformations of the BLA·imine complexes have been studied using ¹H 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 Br?nsted acid.