[OH-].[Na+].CO[C:5]1[CH:15]=[CH:14][C:13]([O:16][CH3:17])=[CH:12][C:6]=1[CH:7]=[CH:8][C:9]([OH:11])=[O:10].Cl[CH:19](Cl)[CH3:20].B(Br)(Br)Br>>[CH2:17]([O:16][C:13]1[CH:12]=[C:6]2[C:5](=[CH:15][CH:14]=1)[O:11][C:9](=[O:10])[CH:8]=[CH:7]2)[C:20]1[CH:19]=[CH:7][CH:6]=[CH:5][CH:15]=1Equip a 5-L, three-neck, round-bottom flask with a large blade mechanical stirrer, thermocouple, an addition funnel, Claisen adapter, reflux condenser, and a sodium hydroxide scrubber. Charge the flask with 2,5-dimethoxycinnamic acid (182.3 g, 865 mmol, 1.0 equiv) and dichloroethane (2.5 L). Add boron tribromide (163.5 mL, 433.2 g, 1.73 mol, 2.0 equiv.) dropwise over 1 h, keeping the temperature below 35° C. Gas evolution can be monitored as the temperature of the reaction is gradually increased to reflux (82° C.). Reflux for 12 h, cool to 5° C., and quench by the careful addition of water (1.0 L). Filter the resulting yellow-red suspension/emulsion through a glass frit and wash with dichloroethane (1.0 L) and heptane (1.0 L) to afford a brown solid. Dry the wet material in a vacuum oven (30 in., 35° C.) for 18 h, to afford the coumarin (180.3 g, 127% theory) as a brown solid: 1H NMR (300 MHz, DMSO-d6) δ 7.97 (d, J=9.6 Hz, 1H), 7.22 (d, J=9.9 Hz, 1H), 7.05 (m, 2H), 6.43 (d, J=9.6 Hz, 1H).
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