Facile Synthesis of Carbotetracycles via Gold(I)-Catalyzed Cyclization of Unactivated Cyclic 1-Aryl-6,8-dien-1-ynes

The gold(I)-catalyzed cyclization of the cyclohexa-1,3-diene bearing a terminal alkyne and a gem-diester in the tether proceeds via a cationic gold intermediate to give a tetrahydromethyleneindene derivative. Changing the gem-diester to a gem-diketone in the tether, the transient gold-stabilized carbocation is trapped intramolecularly by a methyl ketone to afford a tricyclo[5.4.0.0^4,8]undecane ring skeleton. Under the same reaction conditions, aryl-substituted cyclohexadienynes afford hexahydroacephenanthrylene rings in a diastereoselective fashion and in good yields. The reaction proceeded via attack of the aryl group onto the gold-stabilized carbocations followed by aromatization and protodemetalation to generate the tetracarbocycles. This cycloaddition can be applied to the synthesis of pentahydronapthoazulenes from aryl-substituted cycloheptadienynes and the gold(I) catalyst.