A 1 L three-neck flask was charged with 340 mL of dry DMF. Two syringes were charged separately with solutions of diethylenetriaminepentaacetic acid dianhydride (3.00 g, 8.40 mmol) in 100 mL of DMF and 1,4-diaminobutane (0.740 g, 8.40 mmol) in DMF/DBN, (96 mL/4 mL). The solutions were added simultaneously at ambient temperature to the flask at a rate of 30 mL/hour. After the addition was complete, the reaction mixture was allowed to stir overnight, stripped to dryness, and 1N HCl added to adjust the pH to 10.4. The solution was applied to AG1-X8 (100-200 mesh, OAc-) resin and eluted with 0.5N acetic acid to yield 0.52 g (13.6%) as a white solid, after removal of acetic acid and lyophilization.
