Lewis acid-mediated intramolecular addition of silyl enol ethers to internal unactivated alkynes

The EtAlCl2-mediated intramolecular addition of silyl enol ethers to both terminal and internal unactivated alkynes, bearing alkyl and phenyl susbstituents at the alkyne moiety, gave mono- and bicyclic beta,gamma -unsaturated ketones in good to excellent yields. On the other hand, the silyloxy-substituted cyclic vinylsilanes were obtained in moderate to high yields when the reactions were catalyzed by B(C6F5)(3) in the presence of triethylsilane. All the reactions proceeded via endo-fashion exclusively. The mechanisms of these regiospecific Lewis acid-assisted carbocyclizations are proposed.