Collective Total Synthesis of Casbane Diterpenes: One Strategy, Multiple Targets.

Of the more than 100 casbane diterpenes known to date, only the eponymous parent hydrocarbon casbene itself has ever been targeted by chemical synthesis. Outlined herein is a conceptually new approach that brings not a single but a variety of casbane derivatives into reach, especially the more highly oxygenated and arguably more relevant members of this family. The key design elements are a catalyst controlled intramolecular cyclopropanation with or without subsequent equilibration, chain extension of the resulting stereoisomeric cyclopropane building blocks via chemoselective hydroboration/cross coupling, and the efficient closure of the strained macrobicyclic framework by ring closing alkyne metathesis. Of arguably highest relevance is the fact that a hydroxy-directed catalytic trans -hydrostannation allows for late-stage diversity. These virtues are manifested in concise total syntheses of depressin, yuexiandajisu A, and ent -pekinenin C; the latter turned out to be identical with euphorhylonal A, which had obviously been misassigned in the literature.