A new synthetic route to pyrrolo[3,2-b]carbazoles, 1H-benzofuro[3,2-f]indoles and 1H-[1]benzothieno[2,3-f]indoles

The heterocyclic systems pyrrolo[3,2-b]carbazole, 1H-benzofuro[3,2-f]indole and 1H-[1]benzothieno[2,3-f]indole were synthesised by Montmorillonite K10 clay-catalysed condensation of 5-acetoxymethyl-4-acetylpyrroles with various indoles, benzofuran and benzothiophene. Ring closure of the intermediate 3-(pyrrolylmethyl)indoles was achieved with clay, whereas 2-(pyrrolyl-methyl)benzofuran and 3-(pyrrolylmethyl)benzothiophene required toluene-p-sulfonic acid in ethanol or toluene for cyclisation to the tetracyclic compounds. When an indole and a benzyl pyrrole-2-carboxylate was used, the resultant pyrrolo[3,2-b]carbozole benzyl ester could be hydrogenolysed to the corresponding carboxylic acid.