Synthesis of the novel fused pi-electronic systems by the intramolecular electrophilic attack of tropylium ion

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Fused full pi-conjugated systemss containing seven-membered ring are molecules of both theoretical and experimental interest due to their unique electronic structures, and various kinds of such compounds have been prepared. However the reported synthetic methods generally suffered from tedious multistep procedures and low yields at the stage of construction of the unsaturated seven-membered ring. We pay our attention to electrophilic nature of tropylium ion, and developed two facile synthetic methods of the compounds containing conjugated seven-membered ring. Our synthetic stratery was to use commercially available 1,3,5-cycloheptatriene as the seven-membered unit.1) The facile and novel synthesis of fused full conjugated cyclic compounds containing tropylium ion by an intramolecular Friedel-Crafts type reaction of o-aryl-, and o-heteroaryl-tropyliobenzenes. Cyclohepta [a] thienonaphthalenylium ions and cyclohepta [a] furonaphthalenylium ion which are the isoelectronic cations of triphenylene, and cyclohepta [a] benzofuro [c] naphthalenylium ions, cyclohepta [alpha] benzothieno [c] naphthalenylium ions and cyclohepta [a] indoro [c] naphthalenylium ion which are the isoelectronic carbocations of the potent carcinogenic benzo [g] chrysene, were obtained in reasonable yields.2) The facile and novel synthesis of heteroaromatic fused azurene derivatives.beta- (10-Benz [a] -azulenyl) -a, b-unsaturated ketone and beta- (9-azuleno [1,2-b] thienyl) -alpha, beta-unsaturated ketones were obtained from omicron- [2-furyl] tropyliobenzenes and 1-tropylio-2- [2-furyl] thiophenes, respectively, in reasonable yields. The mechanism involving a novel ring-cyclization by intramolecular attack of the tropylium ion to the 2-position of the furan ring is postulated. X-ray crystal structure of two of beta- (9-azuleno [1,2-b] thienyl) -alpha, beta-unsaturated ketones have been determined.