A Bifunctional Lewis Acid Induced Cascade Cyclization to the Tricyclic Core of ent-Kaurenoids and Its Application to the Formal Synthesis of (±)-Platensimycin

A mild and efficient bifunctional Lewis acid induced cascade cyclization reaction has been developed for construction of the tricyclic core of ent-kaurenoids. With ZnBr(2) as the bifunctional Lewis acid, a series of substituted enones and dienes underwent cascade cyclization smoothly at room temperature and provided the tricyclic products in one pot with good yields (75-91%) and high diastereoselectivity. The cyclized product has been successfully employed for the formal synthesis of (±)-platensimycin.