[C:1]([O:5][C:6]([N:8]1[C:12]([CH3:16])([C:13]([OH:15])=O)[CH2:11][O:10][C:9]1([CH3:18])[CH3:17])=[O:7])([CH3:4])([CH3:3])[CH3:2].C(N1C=CN=C1)(N1C=CN=C1)=O.O[NH:32][C:33](=[NH:35])[CH3:34]>CN(C=O)C>[CH3:17][C:9]1([CH3:18])[N:8]([C:6]([O:5][C:1]([CH3:2])([CH3:3])[CH3:4])=[O:7])[C:12]([CH3:16])([C:13]2[O:15][N:35]=[C:33]([CH3:34])[N:32]=2)[CH2:11][O:10]1To a solution of 430 mg of 3-(tert-butoxycarbonyl)-2,2,4-trimethyl-1,3-oxazolidine-4-carboxylic acid in 4 ml of DMF was added 296 mg of 1,1′-carbonyldiimidazole, followed by stirring at room temperature for 2 hours. To the reaction mixture was added 146 mg of N-hydroxyacetamidine, followed by stirring at room temperature for 1 hour, at 110° C. for 2 hours, and then at 130° C. overnight. The reaction mixture was left to be cooled to room temperature and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel column chromatography to obtain 390 mg of tert-butyl 2,2,4-trimethyl-4-(3-methyl-1,2,4-oxadiazol-5-yl)-1,3-oxazolidine-3-carboxylate.
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