Asymmetric Transformation of N-Acyl-Dl-amino Acids

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Abstract
The asymmetric transformation which converted N-acyl-Dl-amino acids to the desired optically active isomers was studied. N-Acylamino acids such as N-butyrylproline, N-acetylleucine, and N-benzoylphenylglycine were easily racemized in the presence of a catalytic amount of acetic anhydride in melted states or in acetic acid or chloroform solutions. The racemic modification of these N-acylamino acids crystallized as a true racemic mixture suitable for optical resolution by a preferential crystallization procedure under the conditions of racemization. By combining preferential crystallization of the desired enantiomer by seeding from a supercooled melt or a supersaturated solution of a racemic modification and simultaneous racemization of the opposite isomer, N-acyl-Dl-amino acid was partially converted to an optically active isomer and the whole reaction mixture became optically active (10–40% enantiomeric excess).