Formal Syntheses of (±)-Platensimycin and (±)-Platencin via a Dual-Mode Lewis Acid Induced Cascade Cyclization Approach

A mild
and efficient dual-mode Lewis acid induced Diels–Alder
(DA)/carbocyclization cascade cyclization reaction has been developed
for construction of the tricyclic core of ent-kaurenoids
in one pot with the aid of a theoretical study on the π,σ-Lewis
acidities of a variety of Lewis acids. With ZnBr2 as the
dual-mode Lewis acid, a series of substituted enones and dienes underwent
DA/carbocyclization cascade cyclization reaction smoothly at room
temperature and provided the tricyclic cyclized products in one pot
with good yields and high diastereoselectivity. The tricyclic cyclized
product has been successfully utilized as a common intermediate for
formal syntheses of (±)-platensimycin and (±)-platencin.