Thermolysis of a 5, 5-dialkyl-Δ3-1, 3, 4-oxadiazolin-2-one in nitrobenzene containing an aryl isocyanate at 150° C affords a 1, 3-diaryl-5, 5-dialkylimidazolidine-2, 4-dione, an N- arylcarbamoyl-3, 3-dialkyloxindole, and a 3-aryl-2-arylimino-5, 5-dialkyl-1, 3-oxazolidin-4- one. Those products arise from attack of a diazoalkane, generated in situ from the oxadiazolinone by thermal cycloreversion, on the isocyanate function. Two imidazolidine ...