F[C:2](F)(F)[C:3](O)=[O:4].[CH2:8]1[CH:12]2[CH2:13][C:14](=[O:16])[CH2:15][CH:11]2[CH2:10][NH:9]1.C(OC(OC(C)(C)C)=O)(OC(C)(C)C)=O.C(N(CC)CC)C>C(#N)C>[C:3]([N:9]1[CH2:10][CH:11]2[CH2:15][C:14](=[O:16])[CH2:13][CH:12]2[CH2:8]1)(=[O:4])[CH3:2]Hexahydro-cyclopenta[c]pyrrol-5-one trifluoroacetate 1f (717 mg, 3 mmol) was dissolved in 20 mL of acetonitrile followed by addition of di-tert-butyl dicarbonate (0.42 mL, 4.5 mmol) and triethylamine (0.98 mL, 9 mmol) upon cooling by an ice-water bath. The reaction mixture was stirred overnight at the same temperature. The mixture was concentrated and diluted with 50 mL of water. The mixture was extracted with ethyl acetate (50 mL×3). The combined organic extracts were washed with 50 mL of saturated brine, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography to obtain the title compound 2-acetyl-hexahydro-cyclopenta[c]pyrrol-5-one 5a (0.36 g, yield 72%) as a colorless oil
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