[OH:1][CH2:2][C@@H:3]([NH:8][C:9](=[O:15])[O:10][C:11]([CH3:14])([CH3:13])[CH3:12])[CH2:4][CH2:5][S:6][CH3:7].C(N(CC)CC)C.[CH3:23][S:24](Cl)(=[O:26])=[O:25]>C(Cl)Cl>[CH3:23][S:24]([O:1][CH2:2][C@@H:3]([NH:8][C:9]([O:10][C:11]([CH3:12])([CH3:14])[CH3:13])=[O:15])[CH2:4][CH2:5][S:6][CH3:7])(=[O:26])=[O:25]To a solution of (S)-tert-butyl 1-hydroxy-4-(methylthio)butan-2-ylcarbamate (1.0 g, 1.0 eq, Chem-Impex, Catalog #03206) in DCM at −25° C. was added triethylamine (2.5 eq), followed by methanesulfonyl chloride (2.0 eq). The mixture was stirred at −25° C. for 1 h and RT for 30 min, washed with 1 N HCl, sat. NaHCO3 and brine, dried and concentrated to give a residue, which was purified by flash chromatography column on silica gel using ethyl acetate and hexane (1:2) as eluent to give (S)-2-(tert-butoxycarbonylamino)-4-(methylthio)butyl methanesulfonate (1.1 g, 75%). 1H NMR (CDCl3, 300 MHz) d 4.25 (m, 2H, OCH2), 4.0 (br s, 1H, BocNHCH), 3.05 (s, 3H, SO2CH3), 2.55 (m, 2H, SCH2), 2.15 (s, 3H, SCH3), 1.85 (m, 2H, BocNHCHCH2CH2S), 1.45 (s, 9H, OC(CH3)3).
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