SYNTHESIS OF ETHISOLIDE, ISOAVENACIOLIDE, AND AVENACIOLIDE

Syntheses of the trio of related mold metabolites ethisolide (1), isoavenaciolide (2), and avenaciolide (3) in racemic and optically active forms are described. Glycolate Claisen rearrangements governed by chelation control of enolate geometry and diastereofacial bias in the [3,3] shifts establish the functional and stereochemical details in butyrolactone precursors (15b, 22, and 19) of the natural products. Lactone rearrangements via complementary bis(transesterification) (22 --> 23) and carboxylate displacement/lactonization (26 --> 27 --> 3) reactions provided the diastereomeric isoavenaciolide and avenaciolide structures. Differential reactivities among potential transacylation substrates 15a, 15b, 21, 22, 24, and 26 are attributed to steric interactions or strain in bridged bicyclic intermediates 32, 34, and 36.