(R)-2-amino-N-benzyl-3-(tert-butyldisulfanyl)propanamide (Compound tk93) (60 mg, 0.201 mmol) and triethylamine (0.034 ml, 0.241 mmol) were dissolved in dichloromethane (1 ml), and the solution was cooled to 0° C. 3-Nitro-2-pyridinesulfenyl chloride (46.0 mg, 0.241 mmol) was added and the mixture was stirred at 0° C. for 2 hours. The reaction solution was concentrated under reduced pressure, and the resulting residue was purified by normal-phase silica gel column chromatography (hexane/ethyl acetate) to afford (R)—N-benzyl-3-(tert-butyldisulfanyl)-2-(((3-nitropyridin-2-yl)thio)amino)propanamide (Compound tk94) (87 mg, 96%).
