Gold-Catalyzed Intramolecular [3+2] Cycloadditions of 1-Aryl-1-allene-6-enes

Treatment of 1-aryl-1-allen-6-enes with [PPh3AuCl]/AgSbF6 (5 mol %) in CH2Cl2 at 25 degrees C led to intramolecular [3+2] cycloadditions, giving cis-fused dihydrobenzo[a]fluorene products efficiently and selectively. The reactions proceeded with initial formation of trans/cis mixtures of 2-alkyl-1-isopropyl-2-phenyl-1,2-dihydronaphthalene cations B, which were convertible into the desired cis-fused cycloadducts through the combined action of a gold catalyst and a Bronsted acid. Theoretic calculation supports the participation of the trans-B cation as reaction intermediate. Although HOTf showed similar activity towards several 1-aryl-1-allen-6-enes, it lacks generality for this cycloaddition reaction.