Synthesis of heterocyclic compounds by thermal cyclization reaction and microwave-assisted cadogan reaction

A number of nitrogencontaining heterocyclic compounds were synthesized by thermal cyclization of aromatic amines and microwave-assisted Cadogan reaction. Thermal cyclization reaction is achieved by vaporizing aromatic amines at 560°C or higher and passing the vapor through a column packed with calcium oxide catalyst which is heated at the reaction temperature of 400-650°C under nitrogen carrier gas. Thermal cyclization of 2-aminobiphenyl at 560°C gave carbazole in 80% yield. Thermal reaction of 2aminodiphenylmethane yielded acridine (75% at 600°C) while reaction of 2-aminobenzophenone gave acridone as a major product (21-32% at 450-600°C). When 2aminodiphenylamine was reacted, phenazine was obtained in good yield (77% at 600°C) and reaction of 4-aminophenanthrene produced 4H-benzo[def]carbazole in 61% yield at 600°C. The cyclization reactions are proposed to occur via aromatic nucleophilic substitution mechanism. The effect of substituents on thermal cyclization reaction was investigated by reacting Nalkylated 2-aminobiphenyls. Hence, (do not includethe reactions of 2methylaminobiphenyl, 2-dimethylaminobiphenyl and 2-benzylaminobiphenyl Winter at Niigata University