General procedure for the synthesis of N-Cbz-3-pyrrolidinone from N-CBZ-3-hydroxypyrrolidine: intermediate 6.iii (1.10 g) was dissolved in dichloromethane (DCM, 8 mL) and cooled to 0 °C. Under stirring, N,N-diisopropylethylamine (DIPEA, 2.5 mL) was added slowly and dropwise, followed by a solution of sulfur trioxide pyridine complex (1.79 g) in dimethyl sulfoxide (DMSO, 6.5 mL). The reaction mixture was stirred continuously at 0 °C for 1 h, after which the reaction was quenched by the addition of water (6 mL). The aqueous phase was extracted with a solvent mixture of ether/hexane (1:1, 3 × 5 mL) and the organic phases were combined. After the organic phase was concentrated under vacuum, the residue obtained was solubilized with a solvent mixture of ether/hexane (1:1) and purified by silica gel column chromatography (n-hexane/ethyl acetate, 5:5) to give 1.05 g (96% yield) of the light yellow oily product N-Cbz-3-pyrrolidinone. The product was characterized by 1H NMR (DMSO-d6; δ ppm): 2.48-2.61 (2H, m); 3.61-3.80 (4H, m); 5.09 (2H, s); 7.27-7.41 (5H, m).
