[N+:1]([C:4]1[S:8][C:7]([CH:9]=[O:10])=[CH:6][CH:5]=1)([O-:3])=[O:2].S(=O)(=O)([OH:13])N.Cl([O-])=O.[Na+].C(OCC)(=O)C>O1CCOCC1.O>[N+:1]([C:4]1[S:8][C:7]([C:9]([OH:13])=[O:10])=[CH:6][CH:5]=1)([O-:3])=[O:2]A mixture of 5-nitrothiophene-2-carbaldehyde (1.685 g, 10.72 mmol) and sulfamic acid (1.249 g, 12.87 mmol) in dioxane (30 ml) was cooled to 0° C., and a solution of sodium chlorite (1.940 g, 21.44 mmol) in water (14 ml) was added drop-wise. The mixture was allowed to warm to RT and stirred for 2 hours. A second batch of 5-nitrothiophene-2-carbaldehyde (1.966 g, 12.51 mmol) was reacted under the same conditions using the molar ratios described above. The two reaction mixtures were combined and portioned between ethyl acetate and water. The organic layer was extracted twice with 5% NaHCO3 and the organic phase was discarded. The basic aqueous phase was acidified with 2N HCl (pH=2) and extracted twice with ethyl acetate. The combined organic layers were washed with brine and dried over sodium sulfate; the solvent was removed affording 5-nitrothiophene-2-carboxylic acid (Int. 53) (3.6 g, 20.79 mmol, MS/ESI+173.9 [MH]+).
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