An NMR Chiral Solvating Agent for the Chiral Recognition of the Two Enantiomers of N-(3,5-Dinitrobenzoyl)-α-amino Acids

Abstract

Two enantiomers of chiral drugs often show different pharmaceutical activities. Consequently, the study on the stereochemical dependence of the pharmacokinetics and the pharmaceutical activities of chiral drugs is important. Determination of the enantiomeric composition of chiral drugs is, therefore, essential. Enantiomeric composition of chiral compounds has been determined most widely by highperformance liquid chromatographic chiral separations on chiral stationary phases (CSPs). Another important method of determining the enantiomeric composition of chiral compounds is the NMR techniques utilizing chiral solvating agent (CSA). The two transient diastereomeric adducts formed between a CSA and each of the two enantiomers of chiral analytes might induce anisochronous NMR resonances and consequently the enantiomeric composition of chiral analytes can be easily assessed from the NMR spectrum. Most CSAs, which have been developed and successfully utilized in the determination of the enantiomeric composition of chiral compounds by NMR spectroscopy, contain aromatic functional group(s) to induce magnetic anisotropic influence and to invoke π-π interaction with analytes. Derivatives of optically active trans-1,2-diaminocyclohexane has also been utilized as NMR CSAs and these CSAs have also been designed to contain aromatic functional group(s) in order to induce magnetic anisotropic influence and to invoke π-π interaction with analytes. Consequently, CSAs derived from optically active trans-1,2diaminocyclohexane have been utilized for the determination of enantiomeric composition of chiral compounds containing aromatic functional group(s). For example, CSA 1 and CSA 2 derived from (1R,2R)-1,2-diaminocyclohexane shown in Figure 1 have been successfully applied to the determination of enantiomeric composition of chiral carboxylic acids by NMR spectroscopy. Recently, we found that a liquid chromatographic chiral stationary phase (CSP 3, Figure 1) derived from optically active (1S,2S)-1,2-diaminocyclohexane is quite successful for the resolution of N-(3,5-dinitrobenzoyl)-α-amino acids even though the CSP does not contain any aromatic functional group, but contains only a simple primary amino group and a ureide tethering group. From these results, we inferred that CSA 4, which has the same structure as the chiral selector of CSP 3, might show nonequivalences for the two enantiomers of N-(3,5-dinitrobenzoyl)-α-amino acids by NMR spectroscopy. CSA 4 was prepared simply by treating (1S,2S)-1,2diaminocyclohexane with ethyl isocyanate. CSA 4 thus prepared was used to see the H NMR chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine. As shown in Figure 2, the chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine was observed at the NMR peaks corresponding to the protons on the 3,5-dinitrophenyl ring (denoted by a in Figure 1). In addition the chemical shift nonequivalence of the two enantiomers of N-(3,5-dinitrobenzoyl)phenylglycine was observed to be greatest when the ratio of CSA 4 and N-(3,5-dinitrobenzoyl)phenylglycine is 2:1 at ambient temperature (22 ± 0.5 C). The H NMR experimental results for the chemical shift nonequivalences of the two enantiomers of five N-(3,5dinitrobenzoyl)-α-amino acids in the presence of two equivalents of CSA 4 are summarized in Table 1. As shown