Mechanistic studies and improvement of coinage metal-catalyzed transformation of alkynyloxiranes to furans: an alcohol addition-cyclization-elimination cascade.

In the presence of alcohol Ag or Au salts or complexes catalyze the conversion of alkynyloxiranes to substituted furans. Both catalysts are effective, and a large furan diversity can be obtained in high yield with one or the other catalyst. Mechanistic studies revealed that a cascade pathway and not the sometimes reported direct intramolecular nucleophilic addition of oxirane oxygen atom to intermediate acetylene-metal pi-complex occurs. Under the defined conditions, the intermediate formation of epoxide opening products has been identified. Depending on the catalyst, one or both of the latter cyclized to dihydrofurans, and further elimination of the alcohol led to the corresponding furans. These results highlight the duality between oxophilicity and alkynophilicity of Ag or Au salts.