Short Route to Cassane-Type Diterpenoids: Synthesis of the Supposed Structure of Benthaminin 1

A short route toward aromatic cassane diterpenes from labdane terpenoids has been developed. In the key step, the aromatic ring with the oxygenated function at C-12 and the characteristic carbon group at C-14 of the target compounds is elaborated via a Diels–Alder/aromatization sequence of a furanosesquiterpene and methyl propiolate. On this basis, the synthesis of the proposed structure of benthaminin 1 from trans-communic acid has been achieved. The physical properties of the synthetic compound are somewhat different from those reported for the natural product.