An expeditious Nazarov cyclization strategy toward the hydroazulene core of guanacastepene A.

The hydroazulene core of guanacastepene A has been synthesized in five steps from commercially available starting materials using a classical Nazarov cyclization to install the stereochemistry in the cyclopentanone diastereoselectively. In the presence or absence of Lewis bases, a hydroazulenone or a spirocyclic ketone generated via a novel Wagner-Meerwein rearrangement is obtained with excellent selectivity and yield.