Synthesis of the alkenyl-substituted tetracyclic core of the bisabosquals

HCl-catalyzed deprotection and cyclization of benzylic alcohol 15 cleanly provided tricycle 16 by a cis-selective intramolecular Diels–Alder reaction. Acetylation of the phenol, bis epoxidation, and base-catalyzed hydrolysis and cyclization afforded tetracycle 19 with the bisabosqual skeleton, but the opposite stereochemistry at the tertiary alcohol stereocenter. Selective dehydration of the tertiary alcohol to form the exocyclic alkene, ozonolysis, reductive deoxygenation of the side chain epoxide, and addition of MeMgBr to the ketone from the less hindered face gave tertiary alcohol 24 with the tetracyclic core of bisabosqual A (1).