Benzo[b]thiophenes are widely applied scaffolds in pharmaceutical and materials sciences. Thus, the synthesis of benzo[b]thiophenes has attracted great attention over the past few decades; a number of synthetic routes for 2-substituted or 2,3-disubstituted benzo[b]thiophenes through intra- and intermolecular reactions have been reported. However, in case of synthesis of 2-amido benzo[b]thiophenes, few cases using halogen reagents (I2, NBS, and NCS) and transition-metal catalysts (Au and Rh) have been disclosed so far.
Recently, Toyota group reported the silica-gel assisted synthesis of 2-bromo benzo[b]thiophenes from the corresponding 2-(bromoethynyl)adamantylsulfide. Based on the preliminary results, we have tried the synthesis of 2-amido benzo[b]thiophenes from (o-thioanisole) substituted ynamides and desired products were obtained in high yields. Under careful optimization, one-pot reaction of ynamide coupling and cyclization reaction could be achieved by use of silica-gel as the promoter. This practical one-pot reaction affords an eco-friendly and economical synthetic route to 2-amido benzo[b]thiophenes with high yields in mild condition. This method is not relying on expensive transition-metal catalysts, no poisonous halogenated reagent, no additional purification of ynamide intermediates, and no extraction procedure of reaction mixture, which shows a power of this chemistry.