A new approach to the bicyclo[4.3.0] ring system of natural products from the liverwort: total synthesis of (±)-chiloscyphone and (±)-isochiloscyphone

Construction of the tricyclic derivative 5, a common intermediate for the synthesis of sesquiterpenoids 1–4, was accomplished by using the intramolecular Diels–Alder reaction. Availability of 5 was demonstrated by effective total synthesis of chiloscyphone 1 and isochiloscyphone 2.