[OH:1][C:2]1[CH:3]=[C:4]2[C:8](=[C:9]([CH3:12])[C:10]=1[CH3:11])[N:7]([CH:13]=[O:14])[CH2:6][CH2:5]2.Cl[CH2:16][C:17]([CH3:19])=[CH2:18].C(=O)([O-])[O-].[K+].[K+].O>CN(C=O)C>[CH3:11][C:10]1[C:9]([CH3:12])=[C:8]2[C:4]([CH2:5][CH2:6][N:7]2[CH:13]=[O:14])=[CH:3][C:2]=1[O:1][CH2:18][C:17]([CH3:19])=[CH2:16]A suspension of 2,3-dihydro-5-hydroxy-6,7-dimethyl-1H-indole-1-carbaldehyde (21.8 g, 0.11 mol), 3-chloro-2-methyl-1-propene (15 mL, 0.15 mol) and potassium carbonate (23.6 g, 0.17 mol) in DMF (200 mL) was stirred at 60° C. for 15 hours under the nitrogen atmosphere. Water was added to the reaction mixture, and the mixture was extracted with ethyl acetate four times. The combined organic layers were washed with water and saturated brine, dried over magnesium sulfate, treated with active carbon, filtered, and concentrated under reduced pressure. The residue was recrystallized from diisopropyl ether-hexane to obtain 20.6 g of title compound.
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