[CH:1]([C:3]1[CH:15]=[CH:14][C:6]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])=[CH:5][C:4]=1[OH:16])=O.C(=O)([O-])[O-].[K+].[K+].Br[CH2:24][C:25]([O:27][CH2:28][CH3:29])=[O:26]>CN(C=O)C>[O:16]1[C:4]2[CH:5]=[C:6]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH:14]=[CH:15][C:3]=2[CH:1]=[C:24]1[C:25]([O:27][CH2:28][CH3:29])=[O:26]A solution of tert-butyl 4-formyl-3-hydroxybenzoate (1.2 g, 5.40 mmol) and potassium carbonate (1726 mg, 12.4 mmol) in DMF (17 ml) was admixed dropwise with ethyl bromoacetate (0.56 ml, 5.0 mmol) and stirred at 80° C., for 6 h. Then ethyl bromoacetate (0.28 ml, 2.5 mmol) was added again and the mixture was stirred at 80° C. overnight. After cooling to room temperature, the reaction mixture was added to ice-water and extracted with ethyl acetate. The organic phases were dried over sodium sulphate, filtered and concentrated to dryness under reduced pressure.
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