Diastereoselective Synthesis oftrans‐anti‐Hydrophenanthrenes via Ti‐mediated Radical Cyclization and Total Synthesis of Kamebanin

Abstract Ent ‐kaurenes consist of an ABC‐ring based on a trans ‐ anti ‐hydrophenanthrene skeleton and a D ring with an exomethylene. Highly oxygen‐functionalized ent ‐kauren‐15‐ones have promising antiinflammatory pharmacological activity. In this study, we developed a novel diastereoselective synthesis of trans ‐ anti ‐hydrophenanthrenes via a Ti‐mediated reductive radical cyclization. We also demonstrated the applicability of this method by developing the first total synthesis of (±)‐kamebanin (longest linear sequence; 17 steps, overall yield; 6.5 %). Furthermore, this synthesis provided a formal semi‐pinacol rearrangement for the construction of the quaternary carbon at C8 and a novel Thorpe‐Ziegler‐type reaction for the construction of the D‐ring.