Synthesis of 7‐Allylated Benzofuran Derivatives from o‐Allyloxyethynylbenzene via Claisen Rearrangement and TBAF‐Catalyzed Annulation

The 7‐allylated benzofuran derivatives were synthesized via a continuous reaction of Claisen rearrangement and annulation using o‐allyloxyetynylbenzene as starting material. In addition, it was observed that annulation of o‐alkynylphenol proceeded under mild conditions when carried out in the presence of a catalytic amount of TBAF. Furthermore, these continuous reactions could be achieved in a one‐pot reaction and afforded not only 7‐allylbenzofurans but also 7‐alkenylbenzofurans by controlling the reaction temperature and time of the annulation. Finally, we demonstrated that 7‐allylbenzofuran and 7‐alkenylbenzofuran could be converted into 7‐(2‐formylvinyl)benzofuran and 7‐formylbenzofuran derivatives, respectively. Therefore, various 7‐substituted benzofuran derivatives could be synthesized according to this synthetic strategy. The 7‐Allylated benzofuran derivatives were synthesized from o‐allyloxyethynylbenzene derivatives through two step reactions of Claisen rearrangement in the presence of Et2AlCl and annulation using TBAF. This continuous reaction proceeded under mild conditions by utilizing only inexpensive reagents in one‐pot operation.