intermolecular domino reactions mediated by diphenyldiselenide (PhSeSePh). Diphenyldiselenide, which has just the right strength of carbon radical capturing ability, can be employed as a useful mediator for intermolecular domino reactions of unsaturated compounds. Upon irradiation through Pyrex with a tungsten lamp (hv > 300 nm), a sequential addition of diphenyldiselenide to alkynes bearing an electron-withdrawing