Small and Versatile — Formyl Anion and Dianion Equivalents

This account provides a short overview of the synthetically important class of formyl anions and the less studied formyl dianion equivalents. A brief literature survey, which particularly focusses on asymmetric homologation strategies, is complemented by a detailed description of lithiated (dimethoxymethyl)diphenylphosphane oxide, which is an ideal formyl anion equivalent in reactions with aldehydes. Under acidic conditions the intermediate ketene O,O-acetals afford homologated carboxylic esters whereas asymmetric dihydroxylation of ketene acetals leads to collapse of the intermediate diols furnishing the corresponding α-hydroxy carboxylic esters with high stereocontrol. Additionally, it is demonstrated that the silyl-substituted carbanion may be looked upon as a latent dianion that can react either with two electrophiles or with bis-electrophiles, the latter transformation yielding ring-closure coproducts. It is advantageous if the electrophiles are epoxides or epoxides carrying a remote leaving group. The carbanion will ring-open the epoxide and allow a CO silyl shift, thus generating the new carbanion. This then reacts with the second electrophile, either inter- or intramolecularly. The versatility of these two formyl anion equivalents is demonstrated by selected examples from natural product synthesis.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)