Reduction with Sodium in Liquid Ammonia [1]

Abstract

A sample of p-toluenesulfonyl-l-glutaminyl-l-asparagine [2] is dried in a desiccator, over P₂O₅, overnight in.vacuo. The dried protected dipeptide acid (4.14 g, 10 mmol) is added with stirring to liquid ammonia (about 300 ml) [3]. Stirring is continued and clean sodium is added in small pieces [4] until the blue color, which develops on dissolution of the metal and disappears during the reaction, persists for about one minute. Approximately 1.45 g sodium is required to reach this point. Glacial acetic acid (5 ml) is added [5] and the ammonia is allowed to evaporate. To remove the residual ammonia [6] the flask is placed in a desiccator and evacuated with the help of a water aspirator for at least one hour. The residue is dissolved in distilled water (40 ml), the pH is adjusted to 6 with glacial acetic acid and the solution diluted with absolute ethanol (800 ml). Scratching of the walls with a glass rod induces crystallization of the free dipeptide. After overnight storage in the refrigerator, the crystals are collected on a filter, washed with absolute ethanol (50 ml) and dried in air. The product (about 2.1 g, 80%) melts with decomposition at 200–203 °C. Recrystallization from water-ethanol yields purified l-glutaminyl-l-asparagine melting at 210–211 °C dec; [α] ²¹_D + 20.8° (c 2.7, 0.5 N HC1). On elemental analysis correct values are obtained for C, H and N.