[CH3:1][O:2][C:3](=[O:15])[C@H:4]([NH2:14])[CH2:5][O:6][CH2:7][C:8]1[CH:13]=[CH:12][CH:11]=[CH:10][CH:9]=1.C(O)(=O)C.[F:20][C:21]1[CH:28]=[CH:27][C:24]([CH:25]=O)=[CH:23][CH:22]=1.C([BH3-])#N.[Na+]>CO.C(OCC)(=O)C>[CH3:1][O:2][C:3](=[O:15])[C@H:4]([NH:14][CH2:25][C:24]1[CH:27]=[CH:28][C:21]([F:20])=[CH:22][CH:23]=1)[CH2:5][O:6][CH2:7][C:8]1[CH:13]=[CH:12][CH:11]=[CH:10][CH:9]=1
A solution of (2R)-3-benzyloxy-2-aminopropanoic acid methyl ester (19 g, 78 mmol) in methanol (350 mL) under N2 was cooled to 0° C. and acetic acid (2 g, pH 2) was added, followed by the addition of 4-fluorobenzaldehyde, a compound of formula (N) (12.5 mL, 117 mmol) and sodium cyanoborohydride (7.3 g, 117 mmol) and 3 Å molecular sieves (15 g). The resulting mixture was stirred at ambient temperature for 3 hours, then filtered through Celite (MeOH). The filtrate was concentrated in vacuo to afford a clear liquid. This was taken up in ethyl acetate and washed sequentially with 10% aqueous Na2CO3 solution, water, then brine. The organic phase was then dried over MgSO4, filtered, and concentrated in vacuo. Purification by flash column chromatography on silica gel afforded 14.7 g (60% yield) of (2R)-3-benzyloxy-2-(N-(4-fluorobenzyl)amino)propanoic acid methyl ester, a compound of formula (O), as a clear, colorless oil; NMR (CDCl3) 7.4-7.2 (m, 7), 7.0 (dd, 2), 4.5 (d, 2), 3.9-3.5 (m, 8) ppm, MS (LSIMS) 317.
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