Total Synthesis and Structural Confirmation of ent-Galbanic Acid and Marneral

By capitalizing on a highly selective Claisen rearrangement, ent-galbanic acid 1 and (+)-marneral 2 have been synthesized. The relative configurations of (+)-1 and (+)-2 were unambiguously established by X-ray crystallographic analysis of the precursors 11a and 20, with the absolute configuration ensuing from their derivation from R-pulegone. In this way, the controversial issue of the configuration of galbanic acid was unequivocally settled.