The synthesis of Benzo[C]carbazoles

Abstract

The base/light-induced cyclisation (condensation) reaction between alkyl and carbonyl substituents on biaryl compounds discovered in the University of the Witwatersrand laboratories was used to synthesise benzo[c]carbazoles. Specifically, 5,7-dimethyl-7H-benzo[c]carbazole was synthesised from 2-methyl-1H-indole in 79 % yield over four steps. The reaction sequence involved bromination of 2-methyl-1H-indole at C-3 to give 3-bromo-2-methyl-1H-indole. The subsequent methylation of the amino group to give 3-bromo-1,2-dimethyl-1H-indole was followed by Suzuki coupling with acetophenone-2-boronic acid under non-aqueous reaction conditions to give 1-[2-(1,2-dimethyl-1H-indol-3-yl)phenyl]. Lastly, tBuOK/hν–induced cyclisation of the product yielded the desired benzo[c]carbazole. 10-Methoxy-5,7-dimethyl-7H-benzo[c]carbazole was synthesized analogously in 75% yield from 5-methoxy-2-methyl-1H-indole.