Pyrolysis of the title compound in benzene is described. Although tetraphenyldiazocyclopentadiene and other related diazo compounds undergo various photochemical changes, 1 it is reported to be recovered unchanged when refluxed in benzene. 2,3 On heating with high boiling solvents like mesitylene, tetraphenyldiazocyclopentadiene gives tetraphenylcyclopentene and nuclear CH insertion products. 2.3 In contrast, we find 8-diazo-7,9diphenylcyclopent(a)acenaphthylene 2 undergoes ready thermal decomposition in benzene to give products involving the intermediacy of the corresponding carbene. The increased reactivity of the diazo compound 2 can probably be attributed to the better delocalization characteristics in the resulting carbene through annelation by the acenaphthylene system. 4 Acecyclone was converted to the diene.1 in 80% yield by a modified procedure.5 The title compound was prepared by the reaction of the diene with p-toluenesulphonylazide in 94% yield. 6 When the diazo compound.2 was refluxed in benzene for 2.5 h, the starting material disappeared and the crude reaction mixture was chromatographed over silica gel. Elution with petrol furnished 5% of 7, 12_diphenylbenzo(k)fluoranthene L. m.p. 279' (Reported 268') . h _(JZtOH): 245nm(6 58,870), 299nm(G 40,480), 312nm(6 68,060), 331nm (0 12,870), 346nm(E 10,120), 368nm(E 8,277), 386nm(E 17,480), 41Onm(6 18,400); -t Vcm KEr) : 3030,