C[O:2]/[CH:3]=[C:4]1\[CH2:5][CH2:6][O:7][C:8]2[C:13]\1=[CH:12][CH:11]=[C:10]([C:14]#[N:15])[CH:9]=2.BrB(Br)Br.C(=O)(O)[O-].[Na+]>ClCCl>[CH:3]([CH:4]1[C:13]2[C:8](=[CH:9][C:10]([C:14]#[N:15])=[CH:11][CH:12]=2)[O:7][CH2:6][CH2:5]1)=[O:2](4E)-4-(methoxymethylidene)-3,4-dihydro-2H-chromene-7-carbonitrile (230 mg, 1.1 mmol) was dissolved in dichloromethane (6 mL). The solution was cooled to −78° C. To above solution was added tribromoborane (1.7 mL, 1 M, 1.7 mmol) dropwise over a course of 5 min and subsequently stirred for 30 min at −78° C. To the reaction mixture was added saturated sodium bicarbonate, extracted with dichloromethane (2×20 mL). The organic phase was washed with brine (5 mL), dried over anhydrous Na2SO4, filtered, concentrated. The crude product was used without purification. 1H NMR (500 MHz, CDCl3, δ in ppm): 7.0-7.4 (3H, aromatic), 6.58 (1H, s), 4.55 (2H, m), 2.98 (2H, m).
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