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

Abstract

Abstract 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.