Enantiospecific synthesis of the cubitane skeleton.

The fully substituted 12-membered macrocycle of the cubitane-type diterpenoids has been assembled in an enantioselective manner following a novel "bridge-and-cut" strategy. Hydroxyalkylation of (S)-carvone afforded a carvonylgeraniol, which underwent transannular cyclization on treatment with samarium diiodide in THF. Fragmentation of one of the shorter bridges of the resulting [8.2.2]bicycle liberated the 12-membered ring with the desired cis-arrangement of the isopropenyl side chains.