An organocatalytic strategy for stereoselective additions of a glyoxylate anion equivalent

Because of the possibility of manipulating their functional groups, alfa-ketoesters are regarded as interesting products, being precursors of highly functionalized compounds that find applications in different fields; therefore, several strategies have been developed for their stereoselective synthesis. Herein, we report an organocatalytic method for the preparation of chiral beta-substituted-alfa-ketoesters relying on the unpolung strategy; in particular, we employed 1,3-dithiane-2-carboxy derivatives as acyl anion mimics in a metal-free catalytic enantioselective addition to nitroalkenes. Different bifunctional catalysts, endowed with two functionalities through which coordinating both reaction partners, have been tested in the model reaction between nitrostyrene and 2-S-trifluoroethyl carboxy-thioester-1,3-dithiane. Among these, the thiourea-based ones revealed the best; in particular, catalyst derived from 9-amino-9-deoxy epiquinidine afforded the product in 71% yield and 87% e.e. at room temperature. Different reaction conditions were screened and the reaction scope was investigated; the best results were obtained in toluene at 0 °C (60% yield and 92% e.e.). The Michael addition products were transformed into alfa-ketoesters through subsequent conversion of the dithiane moiety into a carbonyl one and transesterification without loss of the stereochemical integrity, and into beta-lactam through dithiane removal and nitro-group reduction. Known compounds were obtained, allowing the determination of the absolute stereochemistry.