[N:1]([CH2:4][C:5]([C:7]1[CH:12]=[CH:11][C:10]([N+:13]([O-:15])=[O:14])=[CH:9][CH:8]=1)=[O:6])=[N+]=[N-].[CH3:16][O:17][C:18](=[O:24])[CH2:19][CH2:20][C:21](Cl)=O.C1(P(C2C=CC=CC=2)C2C=CC=CC=2)C=CC=CC=1>ClCCCl>[CH3:16][O:17][C:18](=[O:24])[CH2:19][CH2:20][C:21]1[O:6][C:5]([C:7]2[CH:12]=[CH:11][C:10]([N+:13]([O-:15])=[O:14])=[CH:9][CH:8]=2)=[CH:4][N:1]=1To a mixture of crude 2-azido-1-(4-nitrophenyl)-ethanone (8.2 mmol) and 1,2,-dichloroethane (80 ml) was added 3-chlorocarbonyl-propionic acid methyl ester (1.1 ml, 9.0 mmol) and triphenylphosphine (2.4 g, 9.0 mmol). The mixture was stirred at 85° C. for 12 hours. After cooling to room temperature, the mixture was purified by flash chromatography on silica gel eluting with ethyl acetate-hexane (1:4 followed by 1:1) to give 3-[5-(4-nitro-phenyl)-oxazol-2-yl]-propionic acid methyl ester (520 mg, 23% yield for 2 steps). 1H NMR (acetone-d6) δ 2.92 (t, 2H, J=7.1 Hz, CH2), 3.19 (t, 2H, J=7.1 Hz, CH2), 3.68 (s, 3H, CH3), 7.74 (s, 1H, Ar), 7.98 (d, 2H, J=9.0 Hz, Ar), 8.34 (d, 2H, J=9.0 Hz, Ar).
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