Six grams of DL-phenylalanine, 5.4 g of N-benzenesulfonyl-L-aspartic acid and 1.8 g of sulfuric acid were dissolved in 110 ml of water. While this solution was continuously stirred at 50° C., a 10% aqueous solution of caustic soda (7.3 ml) was added over five hours. The resulting crystal was separated, washed with 10 ml of water and dried to give 3.1 g of an L-phenylalanine/N-benzenesulfonyl-L-aspartic acid salt. Analysis of this salt revealed that the optical purity of the L-phenylalanine was 94%.
