Synthetic Studies on Dicyclopenta[a,d]cyclooctane Terpenoids: Construction of the Core Structure of Fusicoccins and Ophiobolins on the Route Involving a Wagner-Meerwein Rearrangement

The total diastereoselective synthesis of dicyclopenta[a,d]cyclooctane core skeleton of tricyclic terpenoids, fusicoccins, and ophiobolins is reported. The synthesis commences from 2-methylcyclopent-2-en-1-one and leads first to the easily accessible intermediary cyclopenta[8]annulene 18. The subsequent steps include two key transformations: shifting the angular methyl group from the angular to the neighboring position employing a carbocationic rearrangement (26 → 28) and construction of a quaternary stereogenic center via alkylation of α-methylcyclooctanone intermediate (38 → 48). In the context of the latter transformation, a series of model experiments on alkylation of 2-methylcyclooctan-1-one were conducted. The stereochemical assignments were verified by X-ray analyses of the key structures 39 and 50.