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The novel C 20 H 12 cyclopenta-fused polycyclic aromatic hydrocarbon benz[l]acephenanthrylene (2) and its isomer benz[ j]acephenanthrylene (3) have been obtained by flash vacuum thermolysis of 2-(1-chloroethenyl)benzo[c]phenanthrene (6) and 6-(1-chloroethenyl)chrysene (7), respectively. At T у 900 ЊC 2 and T у 1000 ЊC 3 rearrange selectively to the abundant combustion effluent benzo[j]fluoranthene (1). No evidence for the presence of the related rearrangement products benz[l]aceanthrylene (12) and benz[j]aceanthrylene ( 13), respectively, is found. Semi-empirical AM1 calculations provide a rationalization for these observations; the conversion of 2 and 3 into 1, instead of 12 and 13, respectively, via consecutive ring-contraction-ring-expansion processes and vice versa is favoured.

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Pure isobenzofuran was obtained in quantitative decomposing the adduct of ( 11) and 3,6-di-(2-pyridyl)-syield from flash-vacuum thermolysis of the readily accessible 1,4-epoxy-1,2,3,4-tetrahydronaphthalene ; Diels-tetrazine a t 120 "C in vacuo.: reactivity Alder reactions were studied. of isobenzofuran with olefins of various