Expedient Construction of the ABEF Azatetracyclic Ring Systems of Lycoctonine-Type and 7,17-seco-Type C19-Diterpenoid Alkaloids

A synthetic strategy for the modeling construction of the highly bridged azatetracyclic ABEF ring system of numerous lycoctonine-type C-19-diterpenoid alkaloids bearing a characteristic oxygenated quaternary center at C-7 has been successfully developed. The tetracyclic core was constructed rapidly from a readily prepared 6,7-bicyclic AB ring precursor through a 13-step sequence via an advanced tetracyclic N,O-acetal intermediate, which belong to another core structure of natural 7,17-seco-type alkaloids. The key step involves an SmI2-promoted intramolecular radical coupling reaction of an N,O-acetal with a carbonyl group, mimicking a plausible biogenetic transformation.