[C:1]([O:5][C:6]([N:8]1[CH2:13][C@H:12]([CH2:14][OH:15])[NH:11][CH2:10][C@H:9]1[CH3:16])=[O:7])([CH3:4])([CH3:3])[CH3:2].[CH:17](=O)[C:18]1[CH:23]=[CH:22][CH:21]=[CH:20][CH:19]=1.C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+]>ClCCCl>[C:1]([O:5][C:6]([N:8]1[CH2:13][C@H:12]([CH2:14][OH:15])[N:11]([CH2:17][C:18]2[CH:23]=[CH:22][CH:21]=[CH:20][CH:19]=2)[CH2:10][C@H:9]1[CH3:16])=[O:7])([CH3:4])([CH3:3])[CH3:2]A mixture of (2R,5R)-5-hydroxymethyl-2-methyl-piperazine-1-carboxylic acid tert-butyl ester (14.58 g, 63 mmol), benzaldehyde (7.39 g, 70 mmol), sodium triacetoxyborohydride (16.0 g, 76 mmol) and 1,2-dichloroethane (100 mL) was stirred at 20° C. then quenched with saturated aqueous sodium hydrogen carbonate (500 mL). The mixture was extracted with DCM (3×100 mL) and the combined organic extracts dried (Na2SO4) and evaporated in vacuo to give an oil. Chromatography (SiO2; gradient elution with 0-25% EtOAc in 40-60 petroleum ether) gave the title compound (15.4 g, 76%) as an oil. MS: [M+H]+=321. 1H NMR (CDCl3): 7.42-7.12 (5H, m), 4.12-3.88 (2H, m), 3.85-3.55 (4H, m), 3.27 (1H, dd), 2.91-2.79 (1H, m), 2.73 (1H, dd), 2.63 (1H, s), 2.33 (1H, dd), 1.49 (9H, s), 1.21 (3H, d).
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