Chiral recognizable host-guest interactions detected by fast-atom bombardment mass spectrometry: application to the enantiomeric excess determination of primary amines

The enantiomeric excess (ee) of organic amine compounds has been determined using fast-atom bombardment (FAB) mass spectrometry based on the chiral host-guest complexation systems. The method uses a 1 : 1 mixture of the given chiral crown ether hosts (H-RRR, and HSSSS-dn), one of whose enantiomers is isotopically labeled, for the ee-determination of a given amine salt guest (F+). The peak intensity ratio {I[(H-RRRR + G)(+)]/I([HSSSS-dn + G)(+)] = IRIS} of the two diastereomeric host-guest complex ions clearly changes with the change in the ee of the guest. The intensity excess (I-e) of the two complex ion peaks is newly defined as I-e = (IRIS - 1) / (IRIS + 1). The two sets of mass spectrometrically obtained I-e values vs a set of ee values of phenylalanine methyl ester and leucine methyl ester hydrochlorides show excellent linear relationships through the zero point (R-2 = 0.9989 and R-2 = 0.9970, respectively). This indicates the potential utility of the present ee-determination method to within +/- 3% ee. Furthermore, based on the solution equilibrium distributions of the complex ions, the linearity is mathematically justified. Therefore, the ee-determination can be simplified as ee (%) = (\I-e\ / \I-e(100)\) x 100: here, I-e(100) is the corresponding I-e value obtained using an enantiomerically pure (100% ee) guest as a reference. The present chiral dimethoxyphenyl crown ether host pairs employed are effective (IRIS greater than or equal to 1.5) for the ee-determination of most a-amino acid esters, but not effective (IRIS approximate to 1.0) for most simple alkylamines.