[F:1][C:2]1[C:11]([O:12][CH3:13])=[C:10](F)[CH:9]=[CH:8][C:3]=1[C:4]([O:6][CH3:7])=[O:5].C(=O)([O-])[O-].[K+].[K+].[CH2:21]([O:23][CH:24]([O:27][CH2:28][CH3:29])[CH2:25]Br)[CH3:22].O.C[S:32](C)=O>C(OCC)(=O)C>[CH2:21]([O:23][CH:24]([O:27][CH2:28][CH3:29])[CH2:25][S:32][C:10]1[CH:9]=[CH:8][C:3]([C:4]([O:6][CH3:7])=[O:5])=[C:2]([F:1])[C:11]=1[O:12][CH3:13])[CH3:22]
In 59 mL of dimethyl sulfoxide is dissolved 11.8 g of methyl 2,4-difluoro-3-methoxybenzoate, to which are added 23.0 g of potassium carbonate and 9.33 g of sodium hydrogen sulfide n-hydrate (purity 70%). The mixture is stirred at 60° C. for 2 hours. Then, 25 mL of bromoacetaldehyde diethyl acetal is added at the same temperature as above, and stirred at that temperature for 3 hours. Water and ethyl acetate are added to the reaction mixture, and the organic layer is separated. The organic layer is washed with saturated aqueous solution of sodium chloride and dried over anhydrous magnesium sulfate, and the solvent is distilled off under reduced pressure. The residue is purified by column chromatography (eluent: hexane-ethyl acetate=5:1) to obtain 19.0 g of methyl 4-[(2,2-diethoxyethyl)sulfanyl]-2-fluoro-3-methoxybenzoate as an oily product.
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