Broad-Spectrum Enantioselective Diels−Alder Catalysis by Chiral, Cationic Oxazaborolidines

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 pattern which differs from that observed with α,β-enals. The different reaction channels can be understood in terms of pathways via complexes 3 (for α,β-enals) and 4 (for α,β-enones and esters).