Divergent Asymmetric Synthesis of Bonnadiene and <i>ent</i>‐Polytrichastrene B

We report herein the first asymmetric total synthesis of bonnadiene and ent-polytrichastrene B, two diterpenoids that share a common spirotricyclic skeleton. Notably, ent-polytrichastrene B also features a highly strained, sterically congested tetrasubstituted cyclopropane moiety, presenting significant synthetic challenges in both its construction and stereochemical control. The synthesis features: (1) palladium-catalyzed enantioselective redox-relay Heck alkenylation between electron-withdrawing alkenyl triflate and primary alkenol to form the stereocenter at C7; (2) diastereoselective intramolecular [5+2] cycloaddition to rapidly assemble the unique [6,7,5] spirotricyclic skeleton; (3) sequential installation of the requisite alkyl groups and alkenes present in the targeted molecule by leveraging the functionalities positioned on the tricycle; (4) Fe-catalyzed hydrogen atom transfer (HAT)-initiated hydrogenation to stereospecifically reduce the tetrasubstituted Δ^10,11 olefin and install the contiguous stereocenters; (5) Fe-mediated HAT-initiated 3-exo-trig radical cyclization to rapidly forge the tetrasubstituted cyclopropane ring with excellent stereoselectivity.