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 parts per thousand (DHQ)(2)PHAL and gives access to a-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 C-2 unit ready to be converted in useful building blocks. A further one-pot protection/deprotection sequence catalyzed by Zn(ClO4)(2)center dot 6H(2)O preserved the a-hydroxycarboxylates from racemization by means of a parasitic alcohol-catalyzed retroaldol reaction.