[O:1]=[C:2]1[CH2:6][CH2:5][N:4]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH2:3]1.CO[CH:16](OC)[N:17]([CH3:19])[CH3:18]>>[CH3:16][N:17](/[CH:19]=[C:6]1/[CH2:5][N:4]([C:7]([O:9][C:10]([CH3:13])([CH3:12])[CH3:11])=[O:8])[CH2:3][C:2]/1=[O:1])[CH3:18]tert-Butyl 3-oxopyrrolidine-1-carboxylate (40 g, 216 mmol) was treated with N,N-dimethylformamide dimethyl acetal (267 g, 2241 mmol) and heated at 105° C. for 40 min. The solution was cooled and evaporated under reduced pressure and the resulting orange solid was treated with hexane (200 mL) and cooled in the refrigerator for 64 h. The resulting brownish-yellow solid obtained as such was collected by filtration, dried and used in the next step without further purification. LC/MS: 241.1 (M+1).
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