Crystallise the amide repeatedly from EtOH/diethyl ether. The (±)-isomer crystallises from H2O and has m 157-158o. [Greenstein & Winitz The Chemistry of the Amino Acids J. Wiley, Vol 3 p 1838 1961. de Jong Recl Trav Chim Pays-Bas 19 288 1900, Fischer & Otto Chem Ber 36 2106 1903, Beilstein 4 H 295.]