A Simple Synthesis of Dibenzopyracylene

Dibenzopyracylene (indeno[l ,2,3-cdjfluoranthene) (4) is of theoretical interest due to its electron system, which is regarded as a perturbed [12]annulene [4]. 4 wras only accessible by a multistep synthesis starting from fluoranthene [5]. It was also isolated in traces from the tar formed by pyrolysis of several low molecular compounds [6]. According to the results [3, 7, 8] of the CO eliminations from different aromatic carbonyl compounds by short time pyrolysis [9], it seemed to be possible to synthesize 4 from dibenzo[b,def]chrysene-7,14-dione (1) and benzo[rst]pentaphene-5,8dione (2). 1 is used as dyestuff (Indanthrengoldgelb GK) and is therefore available in large quantities. 1 was pyrolyzed therefore in a vacuum flow apparatus at different temperatures. Below 1000 °C the yield of pyrolysis products was small and nearly all of the starting material passed the pyrolysis tube unchanged. At 1200 °C the reaction conditions were optimal, higher temperatures were not available with the present apparatus. The main product was the desired hydrocarbon 4, which was isolated and identified by spectroscopy. The second product of pyrolysis was 13 H-naphtho[3,2,l-cd]fluoranthene-13-one (3), resulting from elimination of only the first CO. Other compounds, for instance re-