[CH2:1]([O:3][C:4]([N:6]1[CH2:11][CH2:10][NH:9][CH2:8][CH2:7]1)=[O:5])[CH3:2].[Cl:12][CH2:13][C:14](Cl)=[O:15].CCN(C(C)C)C(C)C>C(Cl)Cl>[Cl:12][CH2:13][C:14]([N:9]1[CH2:8][CH2:7][N:6]([C:4]([O:3][CH2:1][CH3:2])=[O:5])[CH2:11][CH2:10]1)=[O:15]piperazine-1-carboxylic acid ethyl ester (200 mg, 1.26 mmol) was treated with chloroacetyl chloride (186 mg, 1.65 mmol) in anhydrous CH2Cl2 (10 mL) containing DIEA (490 mg, 3.80 mmol) at 0° C. The mixture was stirred and warmed to 27° C. for 4 h. The mixture was washed with 1 M HCl(aq). The organic phase was dried over MgSO4, and concentrated under reduced pressure to give ethyl 4-(2-chloroacetyl)piperazine-1-carboxylate (275 mg, 93% yield). C9H15ClN2O3; transparent oil; 1H NMR (400 MHz, CDCl3) δ 4.15 (2H, q, J=7.2 Hz) 4.06 (2H, s), 3.60 (2H, m), 3.54 (2H, m), 3.49 (4H, m), 1.27 (3H, t, J=7.2 Hz); 13C NMR (100 MHz, CDCl3) δ 164.3, 154.3, 61.4, 45.8, 43.5, 43.1, 41.8, 40.8, 14.7; ESI-HRMS calcd for C9H16ClN2O3: 235.0849. found: m/z 235.0852 [M+H]+.
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