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 alpha,beta-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 alpha,beta-enals. The different reaction channels can be understood in terms of pathways via complexes 3 (for alpha,beta-enals) and 4 (for alpha,beta-enones and esters).
Ryu
Do Hyun
DH
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
Lee
Thomas W
TW
Corey
E J
EJ