2-Azidobenzyl (4-nitrophenyl) carbonate (168 mg, 0.534 mmol) was added to a suspension of (R)-2-amino-N-benzyl-3-(tert-butyldisulfanyl)propanamide (Compound tk93) (145 mg, 0.486 mmol) and sodium bicarbonate (102 mg, 1.215 mmol) in 1,4-dioxane (2 mL) at room temperature. The reaction mixture was stirred at the same temperature for 15.5 hours, followed by addition of ethyl acetate and water. The resulting mixture was extracted with ethyl acetate, and the organic phase was washed with water (5 mL×2) and brine (5 mL) and dried over sodium sulfate. Following concentration under reduced pressure, the resulting residue was purified by normal-phase silica gel column chromatography (hexane/ethyl acetate) and then purified by reverse-phase silica gel column chromatography (0.05% aqueous trifluoroacetic-acid solution/0.05% trifluoroacetic acid-acetonitrile solution) to afford (R)-4-azidobenzyl (1-(benzylamino)-3-(tert-butyldisulfanyl)-1-oxopropan-2-yl)carbamate (Compound tk95) (148.9 mg, 65%).
