Enantioselective Liquid–Liquid Extraction of Underivatized Amino Acids with Simple Chiral Aminophenyl-Aldehyde

The derivative of aminophenyl-aldehyde with an asymmetric carbon and an uryl group, (S)-2, was synthesized. The combination of (S)-2 and aliquat-336 in CDCl₃ extracted underivatized amino acids in water layer by imine formation with enantioselectivities of 12/1 for Phe, 13/1 for Val, and 12/1 for Leu, which are comparable with those of previously reported binaphthol-based extractor (S)-1. The enantioselectivities of (S)-2 is remarkable considering the low molecular weight compared to (S)-1. Density functional theory computations and experimental data demonstrate that imine bond is strengthened by resonance-assisted hydrogen bond with the nearby NH group.