Two Distinct Gold-Catalyzed Oxidative Annulations of 1,5-Allenynes with Nitrones to Yield 1-Naphthol Derivatives Bearing 2,3-versus 3,4-Fused Nitroxy Rings

Gold-catalyzed nitrone oxidations of 1,5-allenynes are postulated to involve the formation of gold-enolate containing all carbon 1,3-dipole-like species Int-4, further yielding 1-naphthols fused with a nitroxy ring. 1,5-Allenynes bearing internal alkynes (R = aryl and alkyl) afford 1-naphthols bearing 3,4-fused nitroxy rings, whereas their terminal alkyne analogous (R = H) deliver 1-naphthols with 2,3-fused nitroxy rings. Our DFT study confirms the intermediacy of alpha-oxo-gold carbenes that trap the tethered allenes to generate gold-containing 1,3-dipole-like species before [3 + 3]-annulations with a second nitrone. Our preliminary results indicate that quinoline oxides also generate similar gold-enolate containing 1,3-dipole-like intermediates Int-4 for further [3 + 3]-annulations.