Chiral Protonated Amino Acid Ester Discrimination by Acyclic Chiral Hosts Including D-Mannofuranose Moieties in Fast Atom Bombardment Mass Spectrometry Coupled with the Enantiomer Labeled Guest Method

Abstract

Chiral discrimination by novel chiral hosts 3 and 4, which consist of an oxyethylene chain and 2,3:5,6-di-O-cyclohexylidene-β-D-mannofuranose moieties at both ends, was examined considering several protonated amino acid isopropyl ester guests using fast atom bombardment mass spectrometry (FABMS) with the enantiomer deuterium-labeled guest method. Host 3 showed chiral discrimination toward several guests but with slightly smaller chiral discrimination abilities than host 2a, which consists of a bis-(oxyethylene) chain and 1,2:3,4-di-O-isopropylidene-α-D-galactopyranoside moieties at both ends. Host 4 showed no enantioselectivity, possibly because chiral hosts do not show a pseudo-ring conformation having large open spaces in complexations with a chiral guest in regard to the steric effects of cyclohexylidene groups. In addition, to understand the chiral recognition mechanisms, the binding sites of hosts 3 and 4 were elucidated by ultraviolet-visible (UV-VIS) and proton nuclear magnetic resonance (¹H-NMR) spectroscopy.