Imidazole derivatives. III: Regiospecific synthesis, structure, and fluorescence properties of highly substituted imidazo[1,2-a]pyridines and pyrido[1,2-a]benzimidazoles

1-(Arylacetyl)imidazoles 1 react with acetylenedicarboxylic esters to provide highly functionalized imidazo[1,2-a]pyridines 2 in up to 89% yield. The scope and limitations of this novel condensation reaction have been investigated, ad a mechanistic interpretation is presented. A strong effect on the yield of this reaction is observed for electron-donating and electronwithdrawing substituents in the para position of the aryl ring. Moreover, the transformation is shown to proceed in a regiospecific manner starting with 4,5-unsymmetrically substituted 1-(arylacetyl)imidazoles (1d–g) and is extended to the synthesis of the corresponding pyrido[1,2-a]benzimidazoles 12. The crystal structure of derivative 2a has been determined by X-ray analysis. Imidazo[1,2-a]pyridines 2 and pyrido[1,2-a]-benzimidazoles 12, obtained by this procedure, are highly fluorescent in the visible region with characteristically large Stokes shifts.