An Efficient Synthesis of Dibenzo[a,l]tetracene and Dibenzo[a,j]tetracene and Their Identification in a Coal Tar Extract

An efficient synthesis of potentially carcinogenic dibenzo[a,l]tetracene 1 and dibenzo[a,j]tetracene 2 has been developed and the synthetic standards were used to establish their environmental presence. The synthesis involves Diels–Alder reaction of in situ generated ortho-quinodimethane with phenanthrene-1,4-dione, followed by reductive aromatization as key steps. This synthetic strategy resulted in good overall yield and the mixture of 1 and 2 is easily separable by reversed-phase liquid chromatography (RP-LC). Compounds 1 and 2 were identified in a complex mixture of polycyclic aromatic hydrocarbons from coal tar (Standard Reference Material 1597a) and thus could be potential environmental hazards.