Er(OTf)3-catalyzed synthesis of fluorescent 7-aminocoumarins

7-Aminocoumarins are commonly used as fluorophores for application in molecular monitoring, cell imaging and many other fields. Herein, a series of fluorescent 7-aminocoumarins were prepared via Er(OTf)3-catalyzed Pechmann condensation of 3-aminophenols with β-ketoesters in 19–96% yields. Among them, 7-(dimethylamino)-4-(pyridin-4-yl)-2H-chromen-2-one (3j) displayed a fluorescence maximum with a wavelength of 557 nm and a large Stokes shift of 166 nm in DMSO, and showed strong fluorescence in sub acid to alkaline environment. Besides, 3j exhibited excellent photo stability and stable fluorescence intensity in the presence of some metal cations and biomolecules in neutral solution.