Synthesis of Diterpenoid Steenkrotin A

Key words

steenkrotin A - terpenoids - O–H bond insertion - carbonyl-ene reaction - Ueno–Stork reaction - ketyl-olefin cyclization - aldol condensation

Significance

Ding and co-workers report the first total synthesis of (±)-steenkrotin A, a diterpenoid isolated from an African shrub. Structurally, this natural product is characterized by a pentacyclic skeleton, embedded with a sterically congested tetrahydrofuran moiety. The authors focused first on the construction of tricycle I, which set the stage for further elegant transformations to access the target.

Comment

The tetrahydrofuran subunit was installed by a Rh-catalyzed O–H bond insertion and a carbonyl-ene reaction. A SmI₂-mediated Ueno–Stork reaction and a ketyl-olefin cyclization were used to construct spirocycle L. Interestingly, (±)-steenkrotin A could be accomplished by a cascade aldol condensation–vinylogous retro-aldol–aldol reaction, followed by elimination.