OO.[Cl:3][C:4]1[CH:5]=[CH:6][C:7]([C:11]([OH:13])=[O:12])=[N:8][C:9]=1[Cl:10].[OH:14]S([O-])=O.[Na+]>FC(F)(F)C(O)=O>[Cl:3][C:4]1[C:9]([Cl:10])=[N+:8]([O-:14])[C:7]([C:11]([OH:13])=[O:12])=[CH:6][CH:5]=150% Hydrogen peroxide (38 g, 0.35 mol) was carefully added to a mechanically stirred mixture of trifluoroacetic acid (350 mL) and 5,6-dichloropyridine-2-carboxylic acid (56.4 g, 0.29 mol) at 79° C. After one hour, the reaction mixture was poured into 1 L of saturated aqueous NaHSO3, stirring vigorously and cooling in an ice bath. The precipitate was collected and dried to provide 5,6-dichloropyridine-2-carboxylic acid-N-oxide (62.9 g, 0.30 mol), mp 160° C.
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