The Acid Chloride Procedure

Abstract

p-Toluenesulfonyl-l-isoleucine [2] (2.9 g, 10 mmol) is dissolved in dry ether (15 ml), phosphorus pentachloride [3, 4] (2.2 g, 10.6 mmol) is added and the suspension is shaken until almost all the PCl₅ dissolves. The solution is decanted from the small amount of undissolved material and the ether evaporated in vacuo with a water aspirator. The phosphorus oxychloride remaining in the residue is removed by continuing the evaporation on an oil pump for about one hour. The acid chloride (about 3 g) is a colorless oil [5]. It is dissolved in dry ether and added, in several portions, to a suspension of glycine ethyl ester hydrochloride (2.5 g, 18 mmol) in dry ether (50 ml) containing triethylamine (3.5 ml = 2.52 g, 25 mmol). The mixture is allowed to stand at room temperature with occasional shaking for 12 hours. The precipitate is collected on a filter, washed with ether and then triturated and washed with water to extract triethylammonium chloride. The dry product (3.34 g, 90%) [6] melts at 159–160 °C. Recrystallization of a sample from ethanol raises the m.p. by 1 °C.