1,1'-Oxalyldiimidazole

Experimental procedure²⁾: A stirred solution of N-protected α-amino acid (10 mmol) in acetonitrile (30 mL) is cooled to 10 °C and treated with 1,1'-oxalyldiimidazole (2.09 g, 11 mmol) while maintaining the reaction temperature at below 15 °C. Stirring is continued for 1 h at 15-20 °C, then a mixture of α-amino acid ester hydrochloride (10 mmol) and triethylamine (10 mmol) in acetonitrile (5 mL) is added. The resultant mixture is stirred for 2 h at 10-15 °C. After removal of the solvent in vacuo, the residue is dissolved in ethyl acetate and the organic solution is washed with 5% NaHCO₃, 2% HCl, and brine, then dried over anhydrous Na₂SO₄. The organic solvent is evaporated off in vacuo to give the crude dipeptide, which is purified by recrystallization.