1: hydroxybenzotriazole (HOBt), NH4Cl, dicyclohexylcarbodiimide (DCCI), NEt3 / CH2Cl2 / 1.5 h / 25 °C 2: HCl (gas) / ethyl acetate / 0.08 h / -25 °C 3: 68 percent / dicyclohexylcarbodiimide (DCCI), hydroxybenzotriazole (HOBt), NEt3 / CH2Cl2 / 1 h / 25 °C 4: mercaptoethanol / dimethylformamide / 0.5 h / 25 °C View Scheme