Step 1: Iodomethane (1.01 mL, 16.3 mmol) was slowly added dropwise to a mixed solution of THF (14.82 mmol) and DMF (25 mL) of (2S)-5-(benzyloxy)-2-{[(tert-butoxy)carbonyl]amino}-5-oxopentanoic acid (5.00 g) along with K2CO3 (2.25 g, 16.3 mmol). The reaction mixture was stirred at room temperature for about 3 hours before iodomethane (1.01 mL, 16.3 mmol) was added again. Subsequently, EtOAc was added to the reaction mixture and washed three times with 10% Na2S2O3 solution and then dried with MgSO4. After concentration under reduced pressure to remove the solvent, the crude product was purified by silica gel column chromatography (100:1, followed by 50:1 CHCl3/MeOH) to afford methyl (S)-5-benzyl-2-((tert-butoxycarbonyl)amino)pentanedioate (4.20 g, 12.23 mmol, 82% yield). m/z of C18H25NO6 was detected by ESIMS as 352 (M + H ).
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