Metal-free construction of tricyclic or tetracyclic compounds-acid-promoted synthesis of benzo[4,5]imidazo[2,1-a]isoindole and 1,2-dialkyl-2,3-dihydrobenzimidazoles

An environmental friendly, efficient, and easy to operate strategy for synthesizing of benzo[4,5]imidazo[2,1-a]isoindole and 1,2-dialkyl-2,3-dihydrobenzimidazoles via acid-promoted coupling of dialdehyde and o-diaminobenzene under metal-free conditions is described. A series of products were generated in good yields under mild conditions.