Periselective intramolecular [4+2] cycloadditions of ketenimines: synthesis of pyrido[1,2-a]benzimidazoles

A new synthesis of pyrido[1,2-a]benzimidazole derivatives is described. Easily available N-(2-propenylideneaminophenyl)ketenimines underwent an intramolecular [4+2] cycloaddition reaction, in which the cumulated CC bond of the ketenimine fragment acts as the 2π-electron component and the α,β-unsaturated imine function as the 4π-electron component, yielding the titled compounds in a periselective manner.