A Synthetic Route to 3-(Heteroaryl)-7-hydroxycoumarins Designed for Biosensing Applications

A straightforward method to synthesize 3-(2-benzimidazolyl)-7-hydroxycoumarins, based on the condensation reaction of 7-acetoxy-3-formylcoumarin with various C- and/or N-substituted ortho-phenylenediamine derivatives is presented. This unusual approach proved particularly effective for introducing different hydrophilic groups (carboxylic or sulfonic acids or trimethylalkylammonium moieties) onto the heteroaryl scaffold, leading to cyan-green emitting coumarins that were both water-soluble and strongly fluorescent under physiological conditions. The further extension of this condensation reaction to bis(2-aminophenyl)diselenide enabled the first synthesis of 3-(2-benzoselenazolyl)-7-hydroxycoumarin. The potential utility of these new 7-hydroxycoumarins was demonstrated through the synthesis and spectroscopic and analyte-responsive behavior of fluorogenic probes suitable for sensing biologically relevant thiols and urokinase, a protease that plays a key role in cancer invasion and metastasis.