Protecting-Group-Free Synthesis of Cassane-Type Furan Diterpenes via a Decarboxylative Dienone-Phenol Rearrangement.

An expeditious route to obtaining cassane-type furan diterpenes starting from (+)-sclareolide, an inexpensive commercially available natural lactone, has been achieved by using a solvent-free Diels-Alder cycloaddition and an unprecedented decarboxylative dienone-phenol rearrangement as key steps. Its applicability is showcased by the first synthesis of (5α)-vouacapane-8(14),9(11)-diene. The synthesis, which requires no protecting group, is efficient and atom- and step-economical (10 steps, 20% global).