[S:1]([CH2:7][C:8]([O:10][CH3:11])=[O:9])[CH2:2][C:3]([O:5][CH3:6])=[O:4].[O:12]1[CH:16]=[CH:15][C:14]([C:17](=O)[C:18](OCCCC)=[O:19])=[CH:13]1.C([O-])(C)(C)C.[K+].Cl>CO>[CH3:11][O:10][C:8]([C:7]1[S:1][C:2]([C:3]([O:5][CH3:6])=[O:4])=[C:18]([OH:19])[C:17]=1[C:14]1[CH:15]=[CH:16][O:12][CH:13]=1)=[O:9]A mixture of 26 g of dimethyl thiodiacetate and 32 g of butyl 2-(fur-3-yl)-2-oxo-acetate is added dropwise, over 30 minutes at 64°-68° C., to a mixture of 100 ml of methyl alcohol and 39.9 g of potassium tert.-butanolate. The reaction mixture is kept at this temperature for a further 3 hours, and then poured into a mixture of 179 g of ice and 58 g of concentrated hydrochloric acid. The resultant crystals are separated off, washed with ice water until free of chloride, and dried at 60 C./0.1 mbar. Yield: 27.9 g. The product is recrystallized from 250 ml of methyl alcohol, and 22 g of 2,5-bis-(methoxycarbonyl)-3-fur-3-yl-4-hydroxythiophene having a melting point of 102° C. are obtained.
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