CHIRAL RECOGNITION IN HOST-GUEST COMPLEXATION DETERMINED BY THE ENANTIOMER-LABELED GUEST METHOD USING FAST-ATOM-BOMBARDMENT MASS-SPECTROMETRY

Chiral recognition in host-guest complexations between crown ether hosts (H) and amino acid ester ammonium ion guests (G(+)) has been evaluated by fast atom bombardment (FAB) mass spectrometry (m-nitrobenzyl alcohol matrix). The method uses a 1/1 mixed (for example, G(R)(+) and G(s-dn)(+)) solution of the guest whose enantiomer is isotopically (deuterium) labeled. Chiral recognition of a given host is simply measured with a given guest from the peak intensity ratio of the two diastereomeric host-guest complex ions as I[(H + G(R))(+)]/I[(H + G(S-dn))(+)] drop I-R/I-S-dn. Both the degree and the direction of chiral recognition are characterized by the I-R/I-S-dn values in the range from 0.5 to 5.4 (I-R/I-S-d3) for the present host-guest combination systems studied. Among several synthetic chiral crown ethers and related natural host compounds, it has been found that host 5 possesses remarkably large guest dependence upon the chiral recognition properties: (1) toward primary amino acid ester guests 14-21, a high degree of (R)-enantiomer preference (I-R/I-S = 3.2-5.4), (2) toward phenylglycine ester guest 22, almost no enantiomer recognition (I-R/I-S = 1.1), and (3) toward secondary amino acid ester guest 24, a weak (S)-enantiomer preference (I-R/I-S = 0.7). It is also shown that the I-R/I-S values measured with the present concentrations are reasonably correlated with the relative thermodynamic stabilities in the corresponding host-guest equilibria in solution (I-R/I-S less than or equal to K-R/K-S) for three typical host-guest combination systems selected (1-22, 4-16, and 5-16). Accordingly, the present FABMS/EL (enantiomer-labeled guest) method can be proposed as a new and practically useful technique for determining chiral recognition properties in the highly structured chiral host-chiral guest complexations.