Resolution of Ring-Substituted Phenylalanines by the Action of α-Chymotrypsin on their Ethyl Esters

The D- and L-isomers of the following ring-substituted phenylalanines have been prepared from the racemates in 60–80% yield: tyrosine, o-tyrosine, m-tyrosine, p-chlorophenylalanine, p-fluorophenylalanine, and 3,4-dihydroxyphenylalanine. The resolutions were effected by digestion of the racemic ethyl esters with α-chymotrypsin at pH 5.0 (pH-stat). The L-isomers were crystallized from the concentrated digests; the D-isomers were obtained by saponification of the remaining D-esters after extraction into ethyl acetate. Analysis of the isomers by column chromatography indicated optical purities greater than 99.5%. The racemic esters were prepared with ethanol/hydrogen chloride and were used without isolation.