Unified approach to indolosesquiterpene alkaloids: total syntheses of anti-viral agents, xiamycin A and oridamycin A

A unified catalytic asymmetric total synthesis of naturally occurring anti-viral indolosesquiterpene alkaloids, xiamycin A (1a) and oridamycin A (2a), has been accomplished. The In(III)-catalyzed regio- and diastereoselective ene-yne cyclization has been utilized to construct the pentacyclic skeleton 6 (>20 : 1 dr) having an exocyclic olefin. Diastereoselective hydroboration oxidation afforded the advanced intermediate β-hydroxy aldehyde 5a. Finally, a late-stage highly diastereoselective methylation followed by stereocenter adjustment completed the total synthesis of xiamycin A (1a) and oridamycin A (2a).