2-Azidobenzyl (4-nitrophenyl) carbonate (414 mg, 1.317 mmol) was added to a suspension of Boc-Lys-OH (Compound tk39) (295 mg, 1.198 mmol) and sodium bicarbonate (252 mg, 2.99 mmol) in 1,4-dioxane (3 mL)-water (2 mL). The reaction mixture was stirred at room temperature for 22 hours and then cooled in an ice bath and diluted with ethyl acetate and water, and a saturated aqueous potassium bisulfate solution (0.7 mL) was added to the resulting mixture. The mixture was extracted with ethyl acetate (×2), and the organic phase was washed with water (10 mL×2) and brine (10 mL) and dried over sodium sulfate. Following concentration under reduced pressure, the resulting crude product was purified by normal-phase silica gel column chromatography (hexane/ethyl acetate) to afford (S)-6-((((2-azidobenzyl)oxy)carbonyl)amino)-2-((tert-butoxycarbonyl)amino)hexanoic acid (Compound tk40) (431.5 mg, 85%).
