[OH:1][C:2]1[CH:11]=[CH:10][C:5]2[S:6][C:7](=[O:9])[O:8][C:4]=2[CH:3]=1.C(=O)([O-])[O-].[K+].[K+].Cl[CH2:19][C:20]1[CH:24]=[CH:23][S:22][CH:21]=1>CN(C)C=O>[S:22]1[CH:23]=[CH:24][C:20]([CH2:19][O:1][C:2]2[CH:11]=[CH:10][C:5]3[S:6][C:7](=[O:9])[O:8][C:4]=3[CH:3]=2)=[CH:21]1A mixture of 6-hydroxy-1,3-benzoxathiol-2-one (15 g) and potassium carbonate (13.8 g) in dimethylformamide (200 mL) is stirred for 20 minutes, and is then treated, dropwise, with 3-chloromethylthiophene (11.8 g). The mixture is stirred at 60° C. for 16 hours. The reaction mixture is then concentrated in vacuo, and partitioned between water (200 mL) and ethyl acetate (200 mL). The aqueous layer is extracted with ethyl acetate (100 mL) and with diethyl ether (100 mL), and the combined aqueous layers are washed with brine, dried over magnesium sulphate, and evaporated to give a yellow solid, which is recrystallised from ethanol (200 mL) to give 6-(3-thienylmethoxy)-1,3-benzoxathiol-2-one (8.9 g), in the form of pale yellow crystals.
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