Synthesis of oxygenated spongiane-type diterpenoids from carvone

A new diastereoselective approach to oxygenated spongiane diterpenes starting from (R)-(−)-carvone is described. The carvone is incorporated as the B ring in the final spongiane framework using a B→AB→ABC→ABCD approach, which involves an intramolecular Diels–Alder reaction and the regioselective ring-opening of a dihydrofuran ring as key synthetic steps. The structure of the key intermediate in this approach has been verified by X-ray crystallography.