N#N.[CH2:3]([O:5][C:6]([C:8]1[N:9]=[C:10]([C:13](=[O:15])[CH3:14])[O:11][CH:12]=1)=[O:7])[CH3:4].COC([O:21][CH3:22])OC.[C:23]([O-])([O-])=O.[Na+].[Na+]>C(O)CO>[CH2:3]([O:5][C:6]([C:8]1[N:9]=[C:10]([C:13]2([CH3:14])[O:21][CH2:22][CH2:23][O:15]2)[O:11][CH:12]=1)=[O:7])[CH3:4]In a flame dried round-bottomed flask equipped with a magnetic stir bar and under inert atmosphere (N2), a solution of 2-acetyl-oxazole-4-carboxylic acid ethyl ester (7.0 g, 38.0 mmol) in ethylene glycol (42.7 mL) was treated with trimethylorthoformate (10.5 mL, 96.0 mmol) followed by LiBF4 (0.73 g, 8.0 mmol). The reaction mixture was heated at 95° C. until completion of the reaction. Aq. 0.5 M Na2CO3 was added and the mixture was extracted with Et2O. The org. layer was washed with aq. 0.5 M Na2CO3 (2×), dried over Na2SO4, filtered, and the solvent was removed under reduced pressure to give the title compound as a yellow oil. LC-MS-conditions 01: tR=0.76 min; [M+H]+=227.99.
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