[NH2:1][C:2]1[S:3][CH:4]=[CH:5][C:6]=1[C:7]([O:9][CH3:10])=[O:8].C(Cl)(Cl)Cl.[C:15](Cl)(Cl)=[S:16].C(=O)(O)[O-].[Na+]>O>[N:1]([C:2]1[S:3][CH:4]=[CH:5][C:6]=1[C:7]([O:9][CH3:10])=[O:8])=[C:15]=[S:16]In a sulfonation flask, 50.2 g (0.32 mol) of methyl 2-aminothiophene-3-carboxylate are added to 480 ml of chloroform and 320 ml of water. Then 40.5 g (0.35 mol) of thiophosgene and 1000 ml of saturated aqueous sodium bicarbonate solution are added simultaneously in 40 minutes under stirring. The stirring continued for 1 h at room temperature and then the organic phase is separated. The water phase is extracted twice with chloroform and the organic phase dried over sodium sulfate. After removal of the chloroform in the water-jet vacuum 61.3 g of a dark oil is obtained, which is further purified by column chromatography over silica gel (eluant: ethyl acetate/hexane=1:5). 41.5 g of methyl 2-isothiocyanatothiophene-3-carboxylate are obtained in the form of a brown powder having a melting point of 63-65° C.
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