Metal-Free Oxidative Cydization of Alkynyl Aryl Ethers to Benzofuranones

摘要

Recently, reactions of N-oxides with alkynes have attracted significant attention; several groups were involved and many synthetically useful transformations have been reported. In nearly all of the reactions gold catalysts are used as reactivators for the triple bond. After oxygen transfer and release of pyridine, α-oxo gold carbenes are obtained as intermediates (Scheme 1, upper part). These highly reactive species can be used to generate a diverse set of valuable target molecules. Apart from these transition-metal-catalyzed reactions only one report was published by the Gong group in which a metal-free process for this kind of chemistry was presented (Scheme 1, lower part). For this particular transformation stoichiometric amounts of MsOH turned out to be crucial to promote the reaction, and the authors speculate that a nitrogen-assisted protonation of the alkyne initiates the reaction cascade.