N[C:2]1[O:3][CH:4]=[C:5]([C:7]([O:9][CH2:10][CH3:11])=[O:8])[N:6]=1.O.O.C1(C)C=CC(S(O)(=O)=O)=CC=1.N([O-])=O.[Na+].[I-:29].[K+].C(=O)(O)[O-].[Na+].S([O-])([O-])(=O)=S.[Na+].[Na+]>C(#N)C.O>[I:29][C:2]1[O:3][CH:4]=[C:5]([C:7]([O:9][CH2:10][CH3:11])=[O:8])[N:6]=12.00 g (12.81 mmol) of ethyl 2-amino-1,3-oxazole-4-carboxylate are added to a solution of 8.00 g (38.43 mmol) of p-toluenesulfonic acid dihydrate in 48 ml of acetonitrile. The suspension is cooled to 0° C., and a solution of 1.77 g (25.62 mmol) sodium nitrite and 5.32 g (32.02 mmol) potassium iodide in 7.2 ml of water is then added. The mixture is stirred at 0° C. for 10 min and, after warming to RT, further overnight. The mixture is then diluted with 200 ml of water. By addition of 1 M aqueous sodium bicarbonate solution, the pH is adjusted to 9. 24 ml of 2 M sodium thiosulfate solution are then added. The aqueous phase is extracted three times with in each case 30 ml of ethyl acetate. The combined organic phases are dried over magnesium sulfate. After filtration, the solvent is removed on a rotary evaporator. The residue is purified by column chromatography on silica gel 60 (mobile phase gradient: cyclohexane/ethyl acetate 300:1→2:1).
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