A New Stereoselective Synthesis of (-)-Isoavenaciolide and (-)-Avenaciolide

摘要

The synthesis of isoavenaciolide and avenaciolide in their natural enantiomeric forms are described. In both syntheses, α-(phenylthio)-β-[(methoxycarbonyl)methyl]-γ-lactones obtained by the base-induced cyclization of enantiomerically enriched γ-[(phenylthio)acyl] α,β-unsaturated esters were used as starting materials. In the isoavenaciolide synthesis the key step is the Stereoselective hydroxylation of the enolate generated in the (methoxycarbonyl)methyl chain that permits the bis-lactonization by a double transesterification. The configuration in the quaternary center vicinal to the carbonyl group in the ring was critical in order to obtain successfully the α-methylene lactone. In avenaciolide, the Stereoselective synthesis of the bis-lactone unit was performed taking advantage of the presence of the phenyl sulfide group which by previous activation was used as leaving group to obtain the fused ring by an intramolecular substitution utilizing the carboxylate of the β-substituent as nucleophile. In this case, the α-methylene lactone was obtained by previously reported methodology.