Base-promoted cyclization of 3-organoselenyl-methylene-2-alkynyl
aryl propargyl ethers has been developed for the synthesis of 3-butylselanyl-methylene
benzofurans, 3-methyl-2-alkynyl-benzofurans, and 4-iodo-benzo[b]furan-fused selenopyrans. Under potassium tert-butoxide as the base and tetrahydrofuran as the solvent, at room
temperature, 3-organoselenyl-methylene-2-alkynyl aryl propargyl ethers
were converted into 3-butylselanyl-methylene benzofurans via a 5-exo-dig mode. Using the same substrate, changing the solvent
to dimethylsulfoxide, 3-methyl-2-alkynyl-benzofurans were selectively
obtained in good yields. From 3-butylselanyl-methylene benzofurans,
4-iodo-benzo[b]furan-fused selenopyrans were prepared
through a nucleophilic cyclization promoted by molecular iodine. The
optimization of the reaction conditions showed that the solvents governed
the regioselectivity of this cyclization and the initial formation
of the dimsyl anion by the reaction of dimethylsulfoxide with potassium tert-butoxide was crucial for the 3-methyl-2-alkynyl-benzofuran
preparation. We also proposed the mechanism for the formation of the
products, demonstrated that the methodology can be scaled up, and
showed the application of the prepared compounds as substrate in further
transformations.