C=O.[C:3]([O:7][C:8]([NH:10][CH2:11][CH2:12][C:13]1[N:14]=[C:15](/[CH:18]=[CH:19]/[C:20]2[CH:25]=[CH:24][CH:23]=[CH:22][CH:21]=2)[O:16][CH:17]=1)=[O:9])([CH3:6])([CH3:5])[CH3:4].[C:26](OCC)(=O)C.C(=O)([O-])O.[Na+]>C1(C)C=CC=CC=1.C1(C)C=CC(S(O)(=O)=O)=CC=1>[C:3]([O:7][C:8]([N:10]1[CH2:11][CH2:12][C:13]2[N:14]=[C:15](/[CH:18]=[CH:19]/[C:20]3[CH:25]=[CH:24][CH:23]=[CH:22][CH:21]=3)[O:16][C:17]=2[CH2:26]1)=[O:9])([CH3:6])([CH3:4])[CH3:5]Paraformaldehyde (54.5 mg) and p-toluenesulfonic acid (7.2 mg) were added to a solution of 4-[2-(tert-butoxycarbonylamino)ethyl]-2-(trans-styryl)oxazole (190 mg) in toluene (15 ml) at room temperature. After heating under reflux for 1 hour, the reaction mixture was allowed to cool, and ethyl acetate (15 ml) and a saturated aqueous solution (15 ml) of sodium hydrogen-carbonate were added to the reaction mixture to separate an organic layer. After the water layer was extracted with ethyl acetate (10 ml), the resultant organic layers were combined and dried over anhydrous sodium sulfate, and the solvent was distilled off under reduced pressure. The residue was purified by column chromatography on silica gel (hexane:ethyl acetate=3:1→2:1) to obtain the title compound (153 mg) as a colorless oil.
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