Bromoacetonitrile → (S)-cyanomethyl 6-((((2-azidobenzyl)oxy)carbonyl)amino)-2-((tert-butoxycarbonyl)amino)hexanoate

N,N-Diisopropylethylamine (52 μL, 0.298 mmol) and subsequently bromoacetonitrile (94 μL, 1.352 mmol) were added to a solution of (S)-6-((((2-azidobenzyl)oxy)carbonyl)amino)-2-((tert-butoxycarbonyl)amino)hexanoic acid (Compound tk40) (114 mg, 0.270 mmol) in acetonitrile (0.4 mL) at room temperature under a nitrogen atmosphere. The reaction mixture was stirred at the same temperature for 1.5 hours and then concentrated under reduced pressure, and the resulting crude product was purified by normal-phase silica gel column chromatography (hexane/ethyl acetate) to afford (S)-cyanomethyl 6-((((2-azidobenzyl)oxy)carbonyl)amino)-2-((tert-butoxycarbonyl)amino)hexanoate (Compound tk41) (140.2 mg, quant.).