Dl-Phenylalanine → L-phenylalanine N-carbobenzoxy-L-aspartic acid

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Eight grams of DL-phenylalanine, 10 g of N-carbobenzoxy-L-aspartic acid and 2.4 g of sulfuric acid were dissolved in 90 ml of water. A 10% aqueous solution (9.7 ml) of caustic soda was added to the resulting solution over six hours while it was continuously stirred at 50° C. As a result, half of the sulfuric acid was neutralized, and the molar amount of the remaining sulfuric acid was a quarter of the amount of DL-phenylalanine. (It will be noted that in the other examples also, the amount of sulfuric acid was adjusted to be one mol per 2 mols of the finally present D-phenylalanine.) The resulting crystal was separated, washed with 10 ml of water and dried to give 8.2 g of L-phenylalanine/N-carbobenzoxy-L-aspartic acid salt. An analysis revealed that the optical purity of the L-phenylalanine contained in this salt was 98%. The molar ratio of L-phenylalanine to N-carbobenzoxy-L-aspartic acid contained in this salt was 1:1.5. An X-ray diffraction pattern of this salt in powder form had peaks at 2θ=4, 16.2, 17.4, 17.6, 18.6, 19.1, 24.6 and 28.8 (degrees).

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