For the synthesis of di-tert-butyl (S)-4-oxopyrrolidine-1,2-dicarboxylate (16c), (S)-1-(tert-butoxycarbonyl)-4-oxopyrrolidine-2-carboxylic acid (16b, 1.35 g, 5.9 mmol) was dissolved in anhydrous dichloromethane (27 mL) and the solution was cooled to 0 °C. Subsequently, tert-butanol (1.7 mL, 17.7 mmol) and 4-dimethylaminopyridine (DMAP, 72 mg, 0.59 mmol) were sequentially added to the solution. After stirring for 5 min, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC-HCl, 1.19 g, 6.2 mmol) was added. The reaction mixture was gradually warmed to room temperature and continued to stir overnight. Upon completion of the reaction, the reaction was quenched with saturated sodium bicarbonate solution and extracted three times with dichloromethane. The organic layers were combined, washed with saturated brine, dried over anhydrous magnesium sulfate and filtered. The solvent was removed by concentration under reduced pressure and the resulting residue was purified by fast column chromatography (eluent: n-hexane/ethyl acetate, 8:1) to afford the target product 16c (0.96 g, 57% yield) as a light yellow oil.
