3-Nitro-1,2,4-triazole [Coupling Agent]

Experimental procedure: to a solution of N-Boc-Val-OH (160 mg, 0.74 mmol) and Et₃N (140 µ L, 1.0 mmol) in THF is added 1 (395 mg, 0.44 mmol). The mixture is concentrated, coevaporated with pyridine and then with toluene, and redissolved in THF (4 mL). DIPEA (0.38 mL, 2.2 mmol), BOP-Cl (0.28 g, 1.1 mmol), and 3-nitro-1,2,4-triazole (0.13 g, 1.1 mmol) were added successively. The reaction mixture is stirred at rt for 1 h. Chromatography on silica gel with 0.5% Et₃N and 20 － 100% EtOAc in hexanes gives 2 (0.39 g, 81% yield).