[CH:1](NC(C)C)(C)C.[Li]CCCC.[Li+].CC([N-]C(C)C)C.[C:21]([O:25][C:26]([N:28]1[CH2:32][C:31]([F:34])([F:33])[CH2:30][C@H:29]1[C:35]([OH:37])=[O:36])=[O:27])([CH3:24])([CH3:23])[CH3:22].CI>C1COCC1>[C:21]([O:25][C:26]([N:28]1[CH2:32][C:31]([F:33])([F:34])[CH2:30][C:29]1([CH3:1])[C:35]([OH:37])=[O:36])=[O:27])([CH3:24])([CH3:22])[CH3:23]To a cooled (−78° C.) solution of diisopropylamine (4.9 mL, 0.032 mol) in dry THF (15 mL), n-BuLi (0.030 mol) was added dropwise under argon atmosphere, stirred the reaction mixture for 30 min at −78° C., then for one hour at −20° C. The resulting LDA solution was transferred to a solution of (S)-4,4-difluoro-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester (2.7 g, 0.010 mol) in THF (15 mL), via cannula and then stirred for 30 minutes at −78° C., then at 0° C. for one hour. The reaction mixture was once again cooled to −78° C., and methyl iodide (0.54 mL, 0.010 mol) was added over 10 min and then warmed to RT over 2 h. After completion of the reaction, the reaction mixture was quenched with 1N HCl (20 mL) and extracted with DCM. The organic layer was concentrated under reduced pressure using a rotary evaporator to give a crude gum, which was purified through column chromatography (10-20% ethyl acetate:hexane, basic alumina, diameter of column—2.5 cm, Height of alumina—approx. 7 inch) to give the desired (rac)-4,4-difluoro-2-methyl-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester (1.1 g) as a white solid.
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