An alkaline solution containing l-phenylalanine (l-Phe) and one of Dl-valine, Dl-leucine, and Dl-isoleucine is allowed to selectively form a precipitate composed of l-Phe and the aliphatic d-amino acid by adjusting the pH of initial solution to around 5.5 with hydrochloric acid. 1H NMR spectra in deuterium oxide, elemental analyses, and infrared spectra of the precipitates indicate that they are adducts of the aliphatic d-amino acid and l-Phe in the molar ratio of 1:1. The free aliphatic Dl-amino acids with high optical purity (84–100%) may be recovered from the adducts in 49–63% yield. An attempt was made to obtain both the aliphatic d- and Dl-amino acids by using an aqueous solution containig the aliphatic Dl-amino acid and l-Phe as an initial solution. The aliphatic l-amino acids obtained had optical purity of 47–85%.

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An aqueous solution containing l-phenylalanine (l-Phe) and Dl-valine, Dl-leucine or Dl-isoleucine in the molar ratio of 1:2 gave adduct composed of equimolar amounts of l-Phe and the amino acids abounding in d-isomer as crystals. From the adducts, the recovered free aliphatic amino acids had about 100% optical purity.

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