A solution of diethyl phosphoramidate (30.6 g, 0.2 mol) in tetrachloromethane (50 mL) was slowly added dropwise with stirring and efficient external cooling to a solution of oxalyl chloride (27.9 g, 0.22 mol) in the same solvent (150 mL). The reaction temperature was kept at -5 to 0 °C. After completion of the addition, the temperature of the mixture was slowly increased to 50 °C and this was maintained for 1 h. The mixture was then cooled to room temperature, filtered, and concentrated in vacuo. The residual liquid product was distilled in vacuo (bp 85–87 °C/10 Torr); yield 22.9 g (64%).
