Synthesis of the tetracyclic core of the bisabosquals

HCl-catalyzed deprotection and cyclization of 8b provided tricycle 9b cleanly. Epoxidation of 9b afforded tetracycle 13 with the wrong stereochemistry at the tertiary alcohol. Selective elimination of the tertiary alcohol to give the exocyclic methylene compound, alkene cleavage to form the ketone with OsO4 and NaIO4, and addition of MeMgBr to the ketone from the least hindered face gave tertiary alcohol 16 with the tetracyclic core of bisabosqual A (1).