Franchetine, a unique 7,17-seco type of norditerpenoid alkaloid, possesses a highly congested polycyclic architecture coupled with nine stereogenic centers. Here we present an efficient synthetic approach for the intact hexacyclic framework of franchetine from the known tricyle 16 in 20 steps. The synthesis features a diastereoselective 6-exo-tet radical cyclization for construction of ring A and a unique oxidative Wagner-Meerwein-type rearrangement to realize the functionalized [3.2.1] bridging ring CD.

An efficient successive Michael/double aldol/dehydration reaction approach for the spiro-linked DEFF′ tetracyclic core ring systems of logeracemin A was developed.

No abstract