[CH:1]([C:3]1[C:12]2[C:11](=[O:13])[O:10][C:9](C)(C)[O:8][C:7]=2[CH:6]=[CH:5][CH:4]=1)=[CH2:2].C[O-].[Na+].CO>Cl>[CH:1]([C:3]1[CH:4]=[CH:5][CH:6]=[C:7]([OH:8])[C:12]=1[C:11]([O:10][CH3:9])=[O:13])=[CH2:2]To 5-ethenyl-2,2-dimethyl-4H-1,3-benzodioxin-4-one (4.01 g, 19.6 mmol) was added 0.5 M sodium methoxide in methanol (85 mL, 42.5 mmol) at rt. Aqueous 1 N HCl (100 mL) was added to the solution after 15 min. The cloudy solution was extracted with ether (2×100 mL). The combined organic extracts were washed with H2O (50 mL) and brine (50 mL), dried over Na2SO4, filtered, and concentrated to afford 2-ethenyl-6-hydroxybenzoic acid, methyl ester (2.0 g, 57%) as a yellow oil. This material was used without further purification in the next step: 1H NMR (CDCl3) δ3.96 (s, 3H), 5.26 (dd, J=10.8, 1.5 Hz, 1H), 5.49 (dd, J=17.3, 1.5 Hz, 1H), 6.95 (m, 2H), 7.23-7.39 (m, 2H), 11.12 (s, 1H).
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