Eosin Y, a common organo-photocatalyst in visible-light photoredox processes, was found to show excellent catalytic activities for thermal redox reactions under a catalytic amount of Cu(OAc)2. With this catalytic system, vinyl azides and ketene silyl acetals combine to form formal [3 + 2] cycloadducts by α-ester radical addition without light irradiation. This method provides a mild and straightforward paradigm to prepare important synthons of five-membered ene-γ-lactams and bridge ring lactams. It is the first example of an eosin Y-catalyzed redox reaction in the dark.