[C:1]([O:5][C:6]([NH:8][C@H:9]([C:12]([OH:14])=[O:13])[CH2:10][OH:11])=[O:7])([CH3:4])([CH3:3])[CH3:2].[H-].[Na+].Cl[C:18]1[C:23]([N+:24]([O-:26])=[O:25])=[CH:22][CH:21]=[CH:20][N:19]=1.O>CN(C)C=O>[C:1]([O:5][C:6]([NH:8][C@H:9]([C:12]([OH:14])=[O:13])[CH2:10][O:11][C:18]1[C:23]([N+:24]([O-:26])=[O:25])=[CH:22][CH:21]=[CH:20][N:19]=1)=[O:7])([CH3:4])([CH3:2])[CH3:3]To a stirring solution of N-t-butyloxycarbonyl-L-serine (2.26 g) in N,N-dimethylformamide (15 mL) under argon at −20° C. was slowly added sodium hydride (60% oil dispersion, 0.88 g gross weight). The resulting mixture was warmed to 0° C. for 1 h, then recooled to −20° C. before a solution of 2-chloro-3-nitropyridine (1.6 g) in N,N-dimethylformamide (10 mL) was added over 10 min. The resulting red solution was stirred at −20° C. for 1 h. Water (100 mL) was then added, and the mixture was washed with diethyl ether (30 mL twice). The aqueous layer was adjusted to pH 7 with 1.0 M aqueous hydrochloric acid and extracted with ethyl acetate (30 mL three times). The combined extracts were washed with brine (50 mL), dried over anhydrous sodium sulfate, and evaporated under vacuum. The resulting residue was purified by silica gel chromatography eluting with ethyl acetate-hexane (1:2) to obtain N-t-butyloxycarbonyl-O-(3-nitro-2-pyridyl)-L-serine (2.46 g).
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