A concise synthesis of the functionalized [5–7–6] tricyclic skeleton of guanacastepene A

The six membered ring of guanacastepene A was constructed by a Diels–Alder reaction of 1,1,4-trisubstituted diene to set up the correct relative stereochemistry at the C8 quaternary center and the remote C5 stereocenter. In 10 efficient steps from the Diels–Alder adduct 9, the desired highly functionalized [5–7–6] tricyclic skeleton 2 was synthesized. Key steps involve trimethylsilyl chloride (TMSCl) assisted Michael addition to form enol ether and the usage of the enol ether in the following intramolecular Mukaiyama aldol reaction to form the middle seven membered ring of guanacastepene A.