IntroductionFluorescence detection methods offer high sensitivity and experimental convenience.1–3Among many fluorophores, coumarin has been studied for a long time.4–7The fluorescence properties of coumarin derivatives depend on their molecular substitution, being influenced by the degree of intramolecular charge transfer (ICT) 8from the 6- and/or 7-substituents (electron donating) to the coumarin ring (electron accepting).9,10Some chemical conversions, including hydrolysis and oxidation, of the derivatives generate strongly fluorescent 7-hydroxycoumarin, and the resultant fluorescence increase has been used for several applications.11For example, 4-methyl-7-hydroxycoumarin phosphate monoester has been used for studies of the kinetic behavior of alkaline phosphatase.12We found that 7-phenoxycoumarin (2e:Scheme 1) does not fluoresce at all. We thought that this represented an opportunity to develop novel fluorescence probes by usingO-dearylation as a trigger. We have already reported thato- andp-phenoxyphenols areO-dearylated to yield phenol in theipso-substitution mode by highly reactive oxygen species.13,14Therefore, we expected that nonfluorescent 7-(hydroxyphenoxy)coumarins would beO-dearylated in theipso-substitution mode to yield 7-hydroxycoumarin upon reaction with highly reactive oxygen species, generating strong fluorescence.Synthesis of 7-hydroxycoumarin derivatives.Reagents and conditions: (1) cuprous chloride, 7-hydroxycoumarin potassium salt, pyridine, reflux, under argon; (2) trifluoromethanesulfonic acid, 2′,2′,2′-trifluoroethanol, 0 °C; (3) aqueous HCl, reflux; (4) Pd/C, H2, methanol, room temperature; (5) acetic anhydride, pyridine, room temperature.Here we describe the design and synthesis of several derivatives of 7-hydroxycoumarin,3a,3b,3cand3d(Scheme 1), as potential fluorescence probes for reactive oxygen species (ROS). Their chemical properties and reactivities towards ROS are also examined.