Photochemical [2+2+1] annulation of 2-vinyloxy arylalkynes with bromomalonates via energy transfer

A photochemical [2 + 2 + 1] radical annulation of 2-vinyloxy arylalkynes with bromomalonates has been developed. This protocol affords a transition-metal-free, oxidant-free and base-free approach to cyclopenta[b]benzofurans under mild conditions. This tandem reaction involves sequential radical addition, 5-exo-dig cyclization, 1,5-H shift, 5-endo-trig annulation, radical coupling, and elimination processes. Significantly, the photoinduced C-Br bond homolysis of bromomalonates involves an energy transfer, and the solvent DMF acts as an acid-binding reagent.