O=[C:2]1[O:7][C:6](=[O:8])[C:5]2[CH:9]=[C:10]([C:13]([OH:15])=[O:14])[CH:11]=[CH:12][C:4]=2[NH:3]1>CN(C)C1C=CN=CC=1.CN(C)C=O.CO.C(OCC)(=O)C>[NH2:3][C:4]1[CH:12]=[CH:11][C:10]([C:13]([OH:15])=[O:14])=[CH:9][C:5]=1[C:6]([O:7][CH3:2])=[O:8]150 mg (1.21 mmol) of 4-dimethylaminopyridine are added to 2.5 g (12.1 mmol) of 2,4-dioxo-1,4-dihydro-2H-3,1-benzoxazine-6-carboxylic acid [described in J. Med. Chem., (1981), 24(6), 735-742] in solution in 10 ml of dimethylformamide and 10 ml of methanol, and the mixture is heated at 60° C. for 3 hours. The reaction medium is concentrated under reduced pressure. The residue is taken up in water and extracted with ethyl acetate. The organic phase is washed with a saturated sodium chloride solution, dried over sodium sulphate and concentrated under reduced pressure. The solid obtained is taken up in ethyl acetate, filtered and dried. 1.98 g of a white powder are obtained.
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