Enantioselective complexation of carbamoylated quinine and quinidine with N-blocked amino acids: vibrational and electronic circular dichroism study

Abstract

Infrared absorption (IR) and vibrational and electronic circular dichroism (VCD and ECD, respectively) spectra of tert-butylcarbamoylquinine (t-BuCQN) and pseudoenantiomeric tert-butylcarbamoylquinidine (t-BuCQD), denoted as selectors (SO), complexed with chiral (S) and (R)-3,5-dinitrobenzoylleucine (DNB-Leu) and achiral 3,5-dinitrobenzoylglycine (DNB-Gly), denoted as selectands (SA), in methanol and acetonitrile, with the spectra of pure SA and SO are reported. H-D exchange of exchangeable hydrogen atoms of SA and SO in deuterated methanol which occurs in IR and VCD experiments is exploited to identify Amide II and Amide III vibrational modes. The formation of preferentially bound complexes composed of sterically compatible combinations of DNB-Leu and SO are manifested by increased intensity of VCD bands assigned to vibrations of amide, carbamate, quinoline, and dissociated carboxylate group and also by increased ECD signals. The VCD technique revealed similarities between the strongly bound diastereomeric complex of chiral DNB-Leu and SO and the complex of achiral DNB-Gly and SO, highlighting the leading role of SO in the formation of SA-SO complex.