The cationic chiral Lewis acids 1 and 2, generated by triflic acid protonation of the corresponding neutral oxazaborolidines, serve as excellent catalysts for Diels-Alder addition of cyclopentadiene to a wide variety of dienophiles. Adducts have been obtained in excellent yield and enantioselectivity from α, β-unsaturated esters, lactones, and cyclic ketones. The absolute facial selectivity for each of these substrates follows a common ...
