Dimethyl Sulfoxide → 1-Benzyl 2-methyl (2S)-4-oxopyrrolidine-1,2-dicarboxylate

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To solution of methyl sulfoxide (23.90 ml, 337 mmol) in DCM (100 ml) at −78° C. was added oxalyl chloride (2 M in DCM, 84 ml, 168 mmol) dropwise. The formed solution was stirred at this temperature for 30 min. A solution of (2S,4R)-1-benzyl 2-methyl 4-hydroxypyrrolidine-1,2-dicarboxylate (21.38 g, 77 mmol) in DCM (100 ml) was added dropwise at −78° C. The formed slurry was stirred at −78° C. for 2 hr before addition of N,N-Diisopropylethylamine (66.7 ml, 383 mmol) dropwise. The final solution was stirred at room temperature 3 h. The mixture was washed with iced 1M HCl, 5% citric acid, and then brine, dried over MgSO4, filtered, and evaporated. The residual light brown oil was purified by silica gel column chromatography, eluted with 4:1, 3:1, then 2:1 hexane-EtOAc to afford (S)-1-benzyl 2-methyl 4-oxopyrrolidine-1,2-dicarboxylate (14.8 g, 70% yield) as light brown viscous oil. 1H NMR (CDCl3) δ 2.58-2.63 (m, 1H), 2.90-2.99 (m, 1H), 3.62, 3.77 (s, 3H, rotamers), 3.95-4.02 (m, 2H), 4.82-4.89 (m, 1H), 5.11-5.24 (m, 2H), 7.32-7.39 (m, 5H).

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