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.