Highly Enantioselective Extraction of Underivatized Amino Acids by the Uryl-Pendant Hydroxyphenyl-Binol Ketone

The hydroxyphenyl chiral ketone, (S)-3, reacts with D-amino acids bearing hydrophobic side chains exclusively over the L-amino acids in a two-phase liquid-liquid extraction, and thus acts as a highly stereoselective extractant. Calculations for the energy-minimized structures for the imine diastereomers and the comparison of the selectivities with other phenyl ketones, (S)-4 and (S)-5, demonstrate that the hydrogen bond between the carboxylate group and the phenolic hydroxyl group contributes to the remarkable enantioselectivities. The multiple hydrogen bonds present in the imine of (S)-3 reinforce the rigidity, and results in the difference between the stabilities of the imine diastereomers. The imine could be hydrolyzed in methanolic HCl solution, and the extraction of the evaporated residues revived the organic layer of (S)-3, which could enter into a new extractive cycle and leaves the D-amino acid with enantiomeric excess (ee) values of over 97% in the aqueous layer.