Sequential five-membered ring formation via intramolecular alkene insertion and intramolecular allene insertion to a π-allylpalladium intermediate

Palladium(0)-catalyzed tandem cyclization of 6-allyl-2,7-octadienyl acetate 4 afforded the trans-fused bicyclo[3.3.0]octane 5 by intramolecular alkene insertion to a π-allylpalladium intermediate with 5-exo mode, followed by another intramolecular alkene insertion to the resulting σ-alkylpalladium intermediate. The reaction of 6, a methyl substituted derivative at the 7 position, also provided the novel trans-fused 7 and 8 whereas that of 11 which has dimethyl groups at the 2 and 7 position lead to 12 via 6-exo cyclization. Intramolecular reactions of the π-allylpalladium intermediates to allene moieties afforded two modes of cyclization (15 → 16 and 32 → 33). The former reaction was applied to a synthesis of isoiridomyrmecin (31). A domino cyclization of acyclic 41 was achieved via five and six consecutive carbon-carbon bond formations to successfully provide tricyclic 44 and tetracyclic 43.