[C:1]([O:5][C:6]([N:8]1[CH2:12][C@H:11]([OH:13])[CH2:10][C@H:9]1[CH3:14])=[O:7])([CH3:4])([CH3:3])[CH3:2].[CH3:15][S:16](Cl)(=[O:18])=[O:17].C(N(CC)CC)C>ClCCl>[C:1]([O:5][C:6]([N:8]1[CH2:12][C@H:11]([O:13][S:16]([CH3:15])(=[O:18])=[O:17])[CH2:10][C@H:9]1[CH3:14])=[O:7])([CH3:4])([CH3:2])[CH3:3]To a solution of (2R,4R)-1-(tert-butoxycarbonyl)-4-hydroxy-2-methylpyrrolidine (1.92 g) and methanesulfonyl chloride (955 μl) in dichloromethane (30 ml) was added dropwise triethylamine (1.99 ml) at 0° C. After one hour, the reaction mixture was washed in turn with 1N-hydrochloric acid and brine and the organic layer was dried over magnesium sulfate. Evaporation of the solvent gave an oil, which was chromatographed on silica gel (200 ml) eluting with a mixture of hexane and ethyl acetate (1:1, V/V) to give (2R,4R)-1-(tert-butoxycarbonyl)-4-methanesulfonyloxy-2-methylpyrrolidine (2.49 g).
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