Ionic liquid supported synthesis of tricyclic pyrimido[1,2-a]benzimidazoles by a telescoped Michael/hetero annulation strategy†
A telescoped sequence involves the reaction of cationic imidazolium attached 2-aminobenzimidazoles with in situ generated 1,1-dicyano-2-aryl ethylenes was explored for the regioselective synthesis of pyrimido[1,2-a]benzimidazoles. The perceived regioselectivity was presumed in terms of preferential Michael addition of 2-aminobenzimidazole followed by intramolecular annulation to the exclusive formation of 4-iminopyrimidines on an ionic liquid support. A plausible mechanistic pathway for their selective formation is discussed and fully supported by X-ray analysis. The present strategy reveals both the amine function and the ring nitrogen in substituted 2-aminobenzimidazoles are active sites for nucleophilic attack on α,β-unsaturated nitriles.