An Antibody-Catalyzed Allylic Sulfoxide-Sulfenate Rearrangement

Antibodies SZ-cis-39C11 and SZ-trans-28F8, which were elicited in response to N-aryl-3-methoxyphenyl proline derivatives, catalyze the [2,3]-sigmatropic rearrangement of allylic sulfoxides to sulfenates. Reduction of the sulfenates with dithiothreitol in situ yields allylic alcohols as the final product. The antibodies achieve rate accelerations in the range 10(2)-10(3) over background and exhibit distinctive hapten-dependent substrate specificity and enantio- and diastereoselectivity. Of particular note is the effective chirality transfer from the sulfoxide center to the product alcohol in the SZ-cis-39C11-catalyzed conversion of (Z)-2-(4-methoxyphenyl)-but-2-en-1-yl 4-nitrophenyl sulfoxide. These properties can be contrasted with those of bovine serum albumin (BSA) which accelerates the same reactions to a comparable extent but does not discriminate between substrate isomers. Partitioning of substrate from aqueous solution into the less polar environment of the protein pocket can account for much of the observed rate enhancement, whereas specific conformational constraints programmed by the haptens must orient the flexible substrate within the induced antibody-combining sites so as to favor certain reaction pathways over others. These studies thus expand the scope of antibody catalysis to an important new class of pericyclic reactions and illustrate how medium effects can be exploited together with conformational constraint to control reactivity and selectivity.