[N:1]1([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH2:6][CH2:5][NH:4][CH2:3][CH2:2]1.[C:14]([O:20][CH2:21][CH3:22])(=[O:19])[CH2:15][C:16]([CH3:18])=O.[BH-](OC(C)=O)(OC(C)=O)OC(C)=O.[Na+]>C(Cl)Cl>[CH2:21]([O:20][C:14](=[O:19])[CH2:15][CH:16]([N:4]1[CH2:5][CH2:6][N:1]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH2:2][CH2:3]1)[CH3:18])[CH3:22]A solution of tert-butyl piperazine-1-carboxylate (1 g, 5.37 mmol), ethyl acetoacetate (883 μl, 6.98 mmol) in DCM (30 ml) was stirred for 30 min at rt under nitrogen, then (2.4 g, 10.74 mmol) NaBH(OAc)3 was added. The reaction mixture was stirred at rt overnight, quenched by addition of water, stirred for 15 min and slowly diluted with a saturated aqueous solution of sodium bicarbonate (pH around 8-9). The reaction mixture was shaken for 1 h. After separation, the aqueous layer was extracted with DCM. The organic layer was dried over anhydrous magnesium sulfate, filtered and concentrated. The residue was purified by Biotage (Snap 50 g cartridge, MeOH/DCM: 0/100 to 5/95 over 20 CV, not UV active) to afford the desired product 513 (1.03 g, 3.43 mmol, 63% yield) as a colorless sticky oil. MS (m/z): 301.4 (M+H).
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