Desulfurization of epidithio-2, 5-piperazinediones (6 and 7) by triphenylphosphine in THF gave dimeric compounds (8 and 15). In the presence of water or phenol the desulfurization gave 3-hydroxy (9) or 3-phenoxy-2, 5-piperazinedione (10). Reaction of 6c with triphenylphosphine in ethyl vinyl ether yielded 2, 5-diazabicyclo [2.2. 2] octane-3, 6-dione (11), indicating the presence of 1, 4-dipole (B) as an intermediate of the desulfurization. ...