[OH:1][CH:2]1[CH2:7][CH2:6][NH:5][CH:4]([C:8]([O:10][CH3:11])=[O:9])[CH2:3]1.C(N(CC)CC)C.Cl[C:20]([O:22][CH2:23][C:24]1[CH:29]=[CH:28][CH:27]=[CH:26][CH:25]=1)=[O:21]>C(Cl)Cl.O>[OH:1][CH:2]1[CH2:7][CH2:6][N:5]([C:20]([O:22][CH2:23][C:24]2[CH:29]=[CH:28][CH:27]=[CH:26][CH:25]=2)=[O:21])[CH:4]([C:8]([O:10][CH3:11])=[O:9])[CH2:3]1To a cold (5° C.) solution of methyl 4-hydroxypiperidine-2-carboxylate, 13a, (5.17 g, 32.48 mmol) and triethylamine (6.00 mL, 43.05 mmol) in CH2Cl2 (135 mL) was added dropwise benzyl chloroformate (6.20 mL, 43.43 mmol) over 10 min. The resulting solution was stirred at 5° C. for 1 hour and then allowed to warm to room temperature. The reaction mixture was diluted with water and the layers were separated. The aqueous was re-extracted with CH2Cl2 and the combined organics were dried over MgSO4, filtered and evaporated to dryness. The crude was passed through a plug of silica gel, eluting with 30-80% EtOAc/Hexanes to afford the desired product, 13b.
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