Enantioselective Annulation Using Nazarov Reagent: Synthesis of (+)Preoleanatetraene

The enantioselective synthesis of preoleanatetraene (1) has been accomplished via a convergent approach of two C-15 synthons. The key step of this synthesis has been an interesting enantioselective variant of the Robinson annulation using the Nazarov reagent and a chiral enamine to obtain the bicyclic moiety A. This asymmetric methodology opens the access to other irregular triterpene skeletons whose biogenetic implication should not be underestimated.