[CH3:1][C:2]1[NH:8][CH:7]=[C:6]([C:9]([OH:11])=[O:10])[C:4](=[O:5])[CH:3]=1.C(=O)([O-])[O-].[Ca+2].[I:17]Cl.S(=O)(O)[O-].[Na+]>CN(C)C=O>[I:17][C:3]1[C:4](=[O:5])[C:6]([C:9]([OH:11])=[O:10])=[CH:7][NH:8][C:2]=1[CH3:1]To 6-methyl-4-(1H)-pyridone-3-carboxylic acid (prepared according to the procedure described in J. Org. Chem., 1972, 37, 1145-1148) (10.0 g, 65.4 mmol) and calcium carbonate (6.55 g, 65.4 mmol) in N,N-dimethylformamide (100 ml) was added iodine monochloride (21.2 g, 130.8 mmol) in N,N-dimethylformamide (20 ml). After 6 hours sodium bisulfite solution (30 ml) was added and the solution poured into ice water (500 ml). The precipitate was collected by filtration washing with water and methanol to give a cream coloured solid (15.7 g, 86%). 1H NMR (250 MHz, DMSO-d6) δ15.42 (1H, br s), 13.33 (1H, br s), 8.50 (1H, s), 2.60 (3H, s); MS (ES+) m/z 280 [MH]+.
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