Cinchona alkaloid-catalyzed enantioselective direct aldol reaction of N-Boc-oxindoles with polymeric ethyl glyoxylate

The first enantioselective direct aldol addition of N-Boc-oxindoles to polymeric ethyl glyoxylate is presented. The reaction is performed by using as low as 0.1 mol% (DHQ)2PHAL and gives access to α- hydroxycarboxylate derivatives bearing adjacent secondary alcohol and quaternary stereocenters with high levels of diastereo- and enantiocontrol. The use of ethyl glyoxylate in its polymeric form represents an important advantage for synthetic applications and allows us to directly install a C2 unit ready to be converted in useful building blocks. A further one-pot protection/deprotection sequence catalyzed by Zn(ClO4) 2·6 H2O preserved the α-hydroxycarboxylates from racemization by means of a parasitic alcohol-catalyzed retroaldol reaction. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.