Synthesis of Polycyclic Aromatic Hydrocarbons via a Novel Annelation Method

Article abstract of DOI:10.1021/jo00132a026

A new general synthetic approach to polycyclic aromatic hydrocarbons is described.The method is based on the convenient availability of o-lithioarylamides from regiospecific metalation of N,N-diethylarylamides with alkyllithium-amine reagents.Addition of the o-lithioarylamide to an aryl ketone or aldehyde affords a lactone.Reduction of the latter with zinc and alkali or HI generates the free acid which undergoes cyclization with ZnCl2 and Ac2O and reduction with zinc and alkali or HI to furnish the fully aromatic polyarene.Compounds synthesized via this route include 3-methylcholanthrene, benzanthracene, dibenzanthracene, dibenzanthracene, benzopyrene, and their methyl derivatives.Overall yields are generally good.Competitive enolate anion formation depresses the yield in the initial step in the reactions of enolizable ketones.However, this pathway can be suppressed with substantial improvement in yield through deuterium exchange of the hydrogens α to the carbonyl.The last three steps of the general method can be condensed to only one step through reductive cyclization of the lactone intermediates with hydriodic acid in acetic acid.While tertiary lactones are resistant to HI under these conditions, the corresponding free acids undergo reductive cyclization under similar conditions.