Imidazole derivatives. III. Regioselective synthesis, structure, and fluorescence properties of highly substituted imidazo[1,2-a]pyridines and pyrido[1,2-a]benzimidazoles.
1-(Arylacetyl)imidazoles I (R = H, MeO, NO2, R1 = R2 = H; R = H, OMe, R1 = H, CO2Et, Me, R2 = Me; R = OMe, R1 = H, Ph, R2 = Ph), react with MeO2CC.tplbond.CCO2Me to provide highly functionalized imidazo[1,2-a]pyridines II in up to 89% yield. The scope and limitations of this novel condensation reaction have been investigated, and a mechanistic interpretation is presented. A strong effect on the yield of this reaction is obsd. for electron-donating and electron-withdrawing substituents in the para position of the aryl ring. Moreover, the transformation proceeded in a regiospecific manner starting with 4,5-unsym. substituted and is extended to the synthesis of pyrido[1,2-a]benzimidazoles III. The crystal structure of deriv. II (R = R1 = R2 = H) was detd. by X-ray anal. II and III are highly fluorescent in the visible region with characteristically large Stokes shifts.
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