Gold-Catalyzed Cascade Reactions of Furan-ynes with External Nucleophiles Consisting of a 1,2-Rearrangement: Straightforward Synthesis of Multi-Substituted Benzo[b]furans

A gold-catalyzed cycloisomerization of silyl-protected 2-(1-alkynyl)-2-alken-1-(2-furanyl)-1-ols with various nucleophiles including water, alcohol, aniline, sulfonamide, and electron-rich arene has been developed. The method provides a highly efficient access to 5,7-disubstituted or 2,5,7-trisubstituted benzo[b]furans with a wide diversity of substituents under mild reaction conditions, which are not easily available by other methods. Remarkably, an interesting rearrangement of the alkyl group from C2 to the C3 position of the furan ring takes place during the cyclization process. The following gold-assisted allylic substitution enables an elaboration of benzo[b]furans on its side chain of the C5 position with a wide range of functional groups.