Synthesis of Spirotricyclic Core of Bonnadiene

We herein describe a new approach for the efficient synthesis of the tricyclic core of diterpene bonnadiene. The synthetically challenging and unusual [6–7–5] spirotricyclic skeleton including the all-carbon quaternary stereocenter, was installed diastereoselectively via a type II [5 + 2] cycloaddition, followed by a unique vinylogous semipinacol rearrangement. The described chemistry demonstrates the feasibility of making the [6–7–5] spirotricyclic skeleton of the final product from the strained bridged [7–8–5] ring system.