Iron-catalyzed [3+2+1] annulation of 2-aminobenzimidazoles/3-aminopyrazoles and aromatic alkynes using N,N-dimethylaminoethanol as a one carbon synthon for the synthesis of pyrimido[1,2-a]benzimidazoles and pyrimido[1,2-b]indazoles

A simple and efficient method for the synthesis of pyrimido[1,2-a]benzimidazoles and pyrimido[1,2-b]indazoles from 2-aminobenzimidazoles/3-aminoindazoles, alkynes and N,N-dimethylaminoethanol by a three-component [3 + 2 + 1] annulation catalyzed by FeCl3 has been established, where N,N-dimethylaminoethanol was applied as a methine source. Good tolerance of the reaction makes this method applicable to construct biologically active pyrimido[1,2-a]benzimidazoles and pyrimido[1,2-b]indazoles. In addition, regioselective aryl substituted pyrimido[1,2-a]benzimidazoles were synthesized in the presence of TfOH. Replacement of alkynes with acetaldehydes gave the same pyrimido[1,2-a]benzimidazoles and pyrimido[1,2-b]indazoles.