[OH:1][C:2]1[CH:9]=[C:8]([OH:10])[CH:7]=[C:6]([CH2:11][O:12][CH3:13])[C:3]=1[CH:4]=[O:5].[CH2:14]1[CH2:19][O:18][CH:17]=[CH:16][CH2:15]1>ClCCl.CC1C=CC(S([O-])(=O)=O)=CC=1.C1C=C[NH+]=CC=1>[OH:1][C:2]1[CH:9]=[C:8]([O:10][CH:17]2[CH2:16][CH2:15][CH2:14][CH2:19][O:18]2)[CH:7]=[C:6]([CH2:11][O:12][CH3:13])[C:3]=1[CH:4]=[O:5]A solution of 2,4-dihydroxy-6-methoxymethylbenzaldehyde (4.08 g, 22.4 mmol) in dichloromethane (100 mL) was treated with PPTS (100 mg) and DHP (4.06 mL, 44.8 mmol) at room temperature overnight. The reaction was concentrated to dryness. The residue was purified by the flash column chromatography (silica, hexanes/ethyl acetate=3:1) to afford the title compound (5.18 g, 87%). 1H NMR (300 MHz, CD3OD) δ 12.34 (s, 1H), 10.12 (s, 1H), 6.58 (s, 1H), 6.56 (s, 1H), 5.58 (m, 1H), 4.63 (s, 2H), 3.84-3.81 (m, 1H), 3.66-3.62 (m, 1H), 3.40 (s, 3H), 1.95-1.86 (m, 2H), 1.75-1.62 (m, 4H).
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