Cl[C:2]([O:4][CH2:5][C:6]1[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=1)=[O:3].[Br:12][C:13]1[CH:18]=[CH:17][C:16]([CH2:19][CH2:20][NH:21][CH3:22])=[CH:15][CH:14]=1.C(N(CC)CC)C>C(Cl)Cl>[Br:12][C:13]1[CH:14]=[CH:15][C:16]([CH2:19][CH2:20][N:21]([CH3:22])[C:2](=[O:3])[O:4][CH2:5][C:6]2[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=2)=[CH:17][CH:18]=1Benzyl chloroformate (0.97 mL, 6.7 mmol) was added dropwise to a solution of 23B (1.1 g, 5.1 mmol)in CH2Cl2 (26 mL) and triethylamine (1 mL, 7.5 mmol). The reaction mixture washed with brine, dried over Na2SO4 and concentrated in vacuo. Purification by flash chromatography (0% to 50% EtOAc in hexanes) afforded 28C (1.15 g, 65%) as a clear oil. 1H NMR (400 MHz, CDCl3) δ ppm 2.70-2.94 (m, 5 H) 3.47 (m, 2 H) 5.08 (m, 2 H) 6.90-7.12 (m, 2 H) 7.26-7.44 (m, 7 H).
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