The method was carried out with reference to the literature (Roy et al., 2011). In a 250 mL three-necked flask equipped with a condenser tube, thermometer and dropping funnel, coumarin (10 g, 68.5 mmol) and 50 mL of concentrated sulfuric acid (0.935 mol) were added. After cooling the reaction system to -10 °C, a mixture consisting of concentrated sulfuric acid (15 mL, 0.28 mol) and fuming nitric acid (5 mL, 0.12 mol) was added slowly and dropwise over a period of 1 hour, during which the reaction temperature was kept below -5 °C. Upon completion of the reaction, the mixture was decanted into ice water, the precipitate was collected by filtration and recrystallized from acetic acid. A white solid 11.26 g of 6-nitro-2H-benzopyran-2-one was obtained in 86% yield with a melting point of 191-192.5 °C (literature value 188-190 °C). The structure of the product was confirmed by 1H NMR and 13C NMR: 1H NMR (400.13 MHz, CDCl3) δ 6.60 (d, J = 9.5 Hz, 1H), 7.48 (d, J = 9.0 Hz, 1H), 7.83 (d, J = 9.5 Hz, 1H), 8.42 (dd, J = 9.0 Hz, 2.5 Hz, 1H). 8.46 (d, J = 2.5 Hz, 1H); 13C NMR (100.62 MHz, CDCl3) δ 118.1, 118.8, 118.8, 123.7, 126.6, 142.2, 144.0, 157.5, 158.8.
