Developments in the Construction of Benzo[b]thiophenes from 2‐Alkynylthioanisoles by Electrophilic and Radical Cyclization Reactions

Benzo[b]thiophene moieties are important skeletons which have been widely found in many natural products, pharmaceuticals, as well as functionalized materials. 2‐Alkynylthioanisoles, a class of bifunctional compounds bearing methylthio and alkynyl groups, are easily accessible from Pd‐catalyzed Sonogashira coupling between 2‐halothioanisoles and terminal alkynes, which have been widely employed as synthons in the construction of benzo[b]thiophenes through an electrophilic or a radical pathway. This review summarizes the related developments with a particular focus on the reaction mechanism. We hope it will help in future research in this area and believe that with the discovery of new catalytic systems, the industrial synthesis of benzo[b]thiophenes from 2‐alkynylthioanisoles will be achieved soon.