Tandem Michael Addition and Lactonization of Silyl Enolates and α,β-Unsaturated Ketones

Abstract

A highly diastereo- and enantioselective tandem reaction between silyl enolates and α,β-unsaturated ketones is presented. The initial Michael addition of the enolate to the ketone is followed by an intramolecular substitution of the aryl­oxy group by the created oxyanion, yielding the lactone. The reaction is mediated by a quaternary ammonium salt derived from cinchonidine. The authors have employed several enolates for this reaction and found both diastereo- and enantio­selectivities to be highly dependent on the steric bulk of the enolate compound. The biggest influence is exercised by the R1 group, but it was also revealed that the use of more sterically hindered aryl groups was beneficial for the selectivities. In contrast to that, changing the electronic properties of the aryl group was found to decrease the enantioselectivity.