[CH:1]([C:3]1[C:8]([C:9]([O:11][CH2:12][CH3:13])=[O:10])=[C:7]([O:14][CH3:15])[C:6]([O:16][CH3:17])=[CH:5][CH:4]=1)=[O:2].P([O-])(O)(O)=[O:19].[Na+].CC(=CC)C.Cl([O-])=O.[Na+]>C(O)(C)(C)C.O1CCCC1.O>[CH2:12]([O:11][C:9]([C:8]1[C:7]([O:14][CH3:15])=[C:6]([O:16][CH3:17])[CH:5]=[CH:4][C:3]=1[C:1]([OH:19])=[O:2])=[O:10])[CH3:13]
A solution of ethyl 6-formyl-2,3-dimethoxybenzoate (5.24 g, 22 mmol), sodium dihydrogen phosphate (3.60 g, 30 mmol) and 2-methyl-2-butene (10.3 ml, 96.8 mmol) in t-butanol (20 ml), tetrahydrofuran (20 ml) and water (20 ml) was stirred at room temperature for 10 min. To the obtained mixture was added sodium chlorite (6.76 g, 74.8 mmol) and the mixture was stirred at room temperature for 1 h. The reaction mixture was poured into water and extracted with ethyl acetate. The extract was washed with brine, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was recrystallized from ethyl acetate-n-hexane to give 2-(ethoxycarbonyl)-3,4-dimethoxybenzoic acid (4.51 g, 80.7%) as crystals.
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