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_RRRR and H_SSSS-dn), one of whose enantiomers is isotopically labeled, for the ee-determination of a given amine salt guest (G⁺). The peak intensity ratio {I[(H_RRRR + G)⁺] / I[(H_SSSS-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 (R² = 0.9989 and R² = 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)|) × 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 ≥ 1.5) for the ee-determination of most α-amino acid esters, but not effective (IRIS ≈ 1.0) for most simple alkylamines.