Gold(I)-Catalyzed Ring Expansion of Alkynylcyclopropyl Allyl Ethers to Construct Tetrasubstituted Methylenecyclobutanones: A Mechanistic Investigation about the Character of Catalytic Amount of Water

A gold(I)-catalyzed ring expansion of alkynylcyclopropyl allyl ethers to produce tetrasubstituted methylenecyclobutanones in moderate to good yields has been disclosed in this paper. This reaction proceeded through an intramolecular [3,3]-sigmatropic rearrangement followed by [1,2]-allyl shift pathway in the presence of catalytic amount of water upon gold(I) catalysis. The intriguing reaction mechanism has been proposed on the basis of deuterium and O-18-labeling experiments, Mass spectroscopic analysis, H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopic tracking and density functional theory (DFT) calculations. The further transformation of these methylenecyclobutanones into polycyclic skeleton has been realized by a practical three-step synthetic procedure. Several other transformations have been also indicated.