[C:1]([C:4]1[C:5]([O:9][CH3:10])=[N:6][S:7][CH:8]=1)(=[O:3])N.S(=O)(=O)(O)[OH:12].N([O-])=O.[Na+]>O>[C:1]([C:4]1[C:5]([O:9][CH3:10])=[N:6][S:7][CH:8]=1)([OH:12])=[O:3]To a solution of 7.9 g. (0.05 mole) of 4-carbamoyl-3-methoxyisothiazole prepared as in Example 19 in 90 ml. of 80% sulfuric acid at 10°-15° C. was added slowly beneath the surface of the liquid a solution of 9.5 g. (0.137 mole) of sodium nitrite in 13 ml. of H2O. When addition was complete the solution was allowed to come to 25° C., and then was heated to 60° C., for several minutes. After cooling to 25° C. the reaction solution was poured onto ice to precipitate 3.64 g. (46%) of 4-carboxy-3-methoxyisothiazole as a white solid, m.p. 182°-185° C.
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