Oxidation of 2,6-Piperazinediones

1,4-Disubstituted 2,6-piperazinediones (1) are susceptible to oxidation at the methylene group in the piperazine ring, giving piperazinetetrone (4) on being treated with selenium dioxide in boiling dioxane. Treatment of 1,4-diphenyl-2,6-piperazinedione (1a) with either nitrobenzene or tosyl chloride and triethylamine in benzene gives cyclic dimer (9) of dehydrogenated 1a. Cycloadduct (10) is obtained in the presecne of N-phenylmaleimide. It seems that both 9 and 10 are formed via labile mesoionic intermediate (8). Piperazinedione (1a) reacted with tosyl chloride, pyridine, and benzoyl chloride to give mesoionic compound (11) having 3-benzoyl and 5-(p-tolylthio) groups.