Total Synthesis and Structural Confirmation of Curcusone I.

The first asymmetric total synthesis of curcusone I, an unusual Euphorbiaceae diterpenoid, has been accomplished, unambiguously confirming its true structure, as previously revised by computational studies. The common [5-7-6] tricyclic core, which is found in rhamnofolane-type curcusones as well as in daphnane and tigliane diterpenes, was efficiently and diastereoselectively assembled via a Rh-catalyzed selenium-involved [2,3]-sigmatropic rearrangement cascade and an additive-controlled SmI2-mediated cyclization. The oxabicyclo[3.2.1]octane framework of curcusone I was successfully constructed by a unique transannular 5-endo-trig oxa-Michael addition. All eight desired stereocenters were installed diastereoselectively.