[O:1]=[C:2]1[CH2:7][O:6][C:5]2[CH:8]=[CH:9][C:10]([CH:12]=[O:13])=[N:11][C:4]=2[NH:3]1.[OH:14]OS([O-])=O.[K+]>CN(C=O)C>[O:1]=[C:2]1[CH2:7][O:6][C:5]2[CH:8]=[CH:9][C:10]([C:12]([OH:14])=[O:13])=[N:11][C:4]=2[NH:3]1This acid was prepared from 3-Oxo-3,4-dihydro-2H-pyrido[3,2-b][1,4]oxazine-6-carboxaldehyde (900 mg) by oxidation with Oxone (potassium peroxymonosulphate) (3.7 g) in a DMF solution (50 mL). After 1.5 hours at room temperature, dilution with water (50 mL) filtration and drying in vacuo afforded the acid as an off-white solid (687 mg, 70%): LCMS (ES) m/e 195 (M+H)+; 1H NMR δ 7.81 (d, J=8.1 Hz, 1H), 7.41 (d, J=8.1 Hz, 1H), 4.77 (s, 2H).
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