[Li]CCCC.[C:6](Br)([CH3:8])=[CH2:7].CON(C)[C:13](=[O:30])[C@@H:14]([NH:22][C:23](=[O:29])[O:24][C:25]([CH3:28])([CH3:27])[CH3:26])[CH2:15][CH:16]1[CH2:20][CH2:19][NH:18][C:17]1=[O:21].[NH4+].[Cl-]>C1COCC1>[CH3:8][C:6](=[CH2:7])[C:13](=[O:30])[C@@H:14]([NH:22][C:23](=[O:29])[O:24][C:25]([CH3:26])([CH3:27])[CH3:28])[CH2:15][CH:16]1[CH2:20][CH2:19][NH:18][C:17]1=[O:21]
n-BuLi (2.5 M, 3.17 mL, 7.9 mmol) was added dropwise to a solution of isopropenyl bromide (0.9 g, 8.3 mmol) in THF (15.0 mL) at −78° C. and the mixture was stirred at −78° C. for 30 min. A solution of tert-butyl ((2S)-1-(methoxy(methyl)amino)-1-oxo-3-(2-oxopyrrolidin-3-yl) propan-2-yl)carbamate (500 mg, 1.58 mmol) in THF (5.0 mL) was added dropwise. The reaction mixture was stirred at −78° C. for 3 h and then allowed to warm to ambient temperature and stirred for 12 h. Saturated aqueous NH4Cl (50 mL) was added and the resulting mixture was extracted with EtOAc (50 mL×2). The combined extracts were washed with brine, dried over anhydrous sodium sulfate, and concentrated. The residue was purified by flash column chromatography on silica gel (petroleum ether/EtOAc=20:1) to afford tert-butyl ((2S)-4-methyl-3-oxo-1-(2-oxopyrrolidin-3-yl)pent-4-en-2-yl)carbamate (200 mg, 42% yield) as a yellow oil.
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