A concise synthesis of 7-desmethylasteriscanolide and the discovery of an unusual fragmentation reaction to the related asteriscunolide skeleton

A concise synthesis of 7-desmethylasteriscanolide is described. The key features include an efficient intramolecular [2+2] photocycloaddition of the alpha,beta-unsaturated acid-ether 29 followed by a Curtius rearrangement/ruthenium tetroxide/fragmentation sequence to yield the cyclooctane lactones 32 and 33. Attempted methylation of 32 uncovered an interesting retro-Michael type fragmentation to the related naturally occuring asteriscunolide skeleton. (C) 1997 Elsevier Science Ltd.