Further observations on and novel products from acid-catalysed indole-pyrrole condensations: formation of pyrrolo[2,3-b]carbazoles

Abstract

The clay-catalysed reaction between indole and ethyl 5-acetoxymethyt-4-acetyl-3-methylpyrrole-2-carboxylate 1a has been shown to give 2-ethoxycarbonyl-3,4-dimethylpyrrolo[3,2-b]carbazole 3a as the major product, accompanied by 3.9% of the isomeric pyrrolo[2,3-b]carbazole 4a. The minor product was rationalised by the correspondingly small amount of indole-2-pyrrolylmethyl substituted product observed when an alternative pyrrole was used. The proportion of pyrrolo[2,3-b]carbazole 4 formed was shown to increase with the presence of strongly electron-donating groups in the indole. In the case of 5-methylindole, which gave 6.4% of the [2,3-b] isomer, when the reaction was run at a lower temperature, the intermediate 3-substituted pyrrolylmethylindole 7c could be isolated. Cyclisation of this, under the original reaction conditions gave only the pyrrolo[3,2-b]carbazole 3h. There was no equilibration between the isomeric pyrrolocarbazoles under the reaction conditions.

The electrophilic substitution of the pyrrolo[3,2-b]carbazole 3a was examined. The pyrrolylmethyl and dimethylallyl cations, pyridine hydrobromide perbromide and Vilsmeier formylation all gave predominant, prior attack at the 10-position of the pyrrolo[3,2-b]carbazole. For the 10-pyrrolylmethylpyrrolo[3,2-b]carbazole 6a bromination gave the 8-bromo and 6,8-dibromo derivatives.