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0 °C

2.5N aqueous NaOH (1.2 mL) was added to a solution of 4-[4-(4-tert-butoxycarbonylaminobutyl)phenylamino]butyric acid ethyl ester 420 (0.45 g, 1.18 mmol) dissolved in a mixture of water (25 mL) and THF (30 mL). The reaction was stirred for 2 h. 12 N HCl was added dropwise until pH 3. The solvent was evaporated in vacuo. The residue was cooled to 0° C. and a solution of TFA/dichloromethane (10 mL) was added. The reaction was stirred for 30 minutes. The solvent was evaporated in vacuo. The residue was purified by flash chromatography (silica gel, 4:1:0.1, chloroform/methanol/concentrated ammonium hydroxide, v/v) to provide 460 (0.38 g, 100%). 1H NMR (300 MHz, CD3OD) δ 1.61 (m, 4H), 1.82 (m, 2H), 2.29 (t, 2H), 2.50 (m, 2H), 2.88 (m, 2H), 3.09 (t, 2H), 6.58 (d, 2H), 6.92 (d, 2H). m/z (ESI) 251.

1. customThe solvent was evaporated in vacuo
2. additiona solution of TFA/dichloromethane (10 mL) was added
3. stirringThe reaction was stirred for 30 minutes
4. customThe solvent was evaporated in vacuo
5. customThe residue was purified by flash chromatography (silica gel, 4:1:0.1, chloroform/methanol/concentrated ammonium hydroxide, v/v)