Imidazole Derivatives, III. Regiospecific Synthesis,Structure, and Fluorescence Properties of Highly substituted Imidazo[1,2‐a]pyridines and Pyrido[1,2‐a]benzimidazoles

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Abstract
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.

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