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 focuses 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 a-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 C -> O silyl shift, thus generating the new carbanion. This then reacts with the second electrophile, either interior intramolecularly. The versatility of these two formyl anion equivalents is demonstrated by selected examples from natural product synthesis. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007). .