Studies on C20-Diterpenoid Alkaloids: Synthesis of the Hetidine Framework and Its Application to the Synthesis of Dihydronavirine and the Atisine Skeleton

The full details of a synthesis of the hetidine framework of the C-20-diterpenoid alkaloids and its conversion to the atisine core structure are reported. The application of the hetidine framework to the synthesis of dihydronavirine, which is the formal reduction product of the natural product navirine, is also described. Key to the success of these studies is the use of a Ga(III)-catalyzed cycloisomerization reaction of alkynylin denes to prepare a [6-7-6] framework that was advanced to the hetidine skeleton. Furthermore, a Michael/aldol sequence was developed for the construction of the bicyclo[2.2.2] framework that is characteristic of the hetidines and atisines.