Synthesis of bis–indolylmethanes, tris–indolylmethanes and 3, 3ʹʹ- diindolyloxindole derivatives using cellulose sulfuric acid as the biodegradable solid acid catalyst under heterogeneous condition

Reaction of indoles with structurally diverse aldehydes and ketones in the presence of catalytic amount of cellulose sulfuric acid as the heterogeneous, reusable and efficient catalyst gave the corresponding bis-indolyl methanes, tris-indolyl methenes and 3, 3ʹ-diindolyloxindole derivatives in good to excellent yields at room temperature.