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-(pyrrolylmethyl)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]carbazole benzyl ester could be hydrogenolysed to the corresponding carboxylic acid.