Synthesis and Fluorescent Properties of Halo-Substituted 3-Hydroxykynurenic Acid Derivatives

A series of 3-hydroxykynurenic acid derivatives bearing a halogen atom in different positions are synthesized by hydroamination of methyl acetylenedicarboxylate with haloanilines, in an aza-Michael type reaction. Subsequent heterocyclization with Eaton's reagent yields halo-substituted kynurenic acid (KA) methyl esters via an intramolecular Friedel-Crafts reaction. Finally, the hydroxylation is achieved by the Elbs oxidation. In order to investigate the fluorescence properties of this heterocyclic nucleus, the corresponding derivatives of KA and its methyl esters are also included. The fluorescent properties are studied in phosphate buffer at pH 7.4 and in DMSO. The presence of a hydroxyl group at the 3-position is essential for achieving optimal emission of KA derivatives. The location of the halogen atom is also decisive, and when it is in the para position with respect to the N atom, the inductive and resonance effects of the halogen substituent are favored, maximizing the fluorescence. At the same position, the best results are observed with Cl, where 6-chloro-3-hydroxykynurenic acid emerges as a highly fluorescent molecule. It exhibits an almost unitary fluorescence quantum yield, long lifetimes (>10 ns), and strong fluorescence, particularly in phosphate buffer pH 7.4, making it a potentially effective new probe for bioimaging.