Intramolecular alkylation of α,β-unsaturated ketones: a total synthesis of (±)-isolongifolene and an approach to the synthesis of zizaane-type sesquiterpenoids

A total synthesis of (±)-isolongifolene 6, a C15 hydrocarbon derived from the acid-catalyzed rearrangement of the sesquiterpenoid longifolene, is described. The known carbomethoxyoctalone 10 was transformed via a 10-step sequence into the unsaturated keto tosylate 9. The key step of the overall synthesis involved the efficient intramolecular γ alkylation of 9 to produce the tricyclic α,β-unsaturated ketone 5. The latter compound was converted into the enone 33, which had previously served as a precursor for the synthesis of (±)-isolongifolene 6.