C(=O)([O-])[O-].[K+].[K+].[CH2:7]([O:9][C:10]1[CH:23]=[CH:22][C:13](/[CH:14]=[C:15]2/[C:16](=[O:21])[NH:17][C:18](=[O:20])[S:19]/2)=[CH:12][CH:11]=1)[CH3:8].Br[CH2:25][CH2:26][CH2:27][NH:28][C:29](=[O:35])[O:30][C:31]([CH3:34])([CH3:33])[CH3:32]>CN(C=O)C>[CH2:7]([O:9][C:10]1[CH:23]=[CH:22][C:13](/[CH:14]=[C:15]2/[C:16](=[O:21])[N:17]([CH2:25][CH2:26][CH2:27][NH:28][C:29](=[O:35])[O:30][C:31]([CH3:34])([CH3:33])[CH3:32])[C:18](=[O:20])[S:19]/2)=[CH:12][CH:11]=1)[CH3:8]Potassium carbonate (350 mg, 2.53 mmol) was added to a solution of compound 2 (210 mg, 0.84 mmol) in 4 mL of anhydrous DMF at 0° C. After stirring of reaction mixture for 20 min, a solution of tert-butyl (3-bromopropyl)carbamate (200 mg, 0.84 mmol) in anhydrous DMF (3 mL) was added and then the reaction mixture was stirred at room temperature for 3 h. The reaction was quenched by adding water and extracted with ethyl acetate. The ethyl acetate layer was washed with 10% NH4Cl aqueous solution followed by water and dried over anhydrous Na2SO4, filtered, concentrated and purified by silica column chromatography (20% ethyl acetate in hexanes) to afford compound 21a (78.8%, 270 mg) as a white solid.
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