Separation of atropisomeric 1,4,5,6-tetrahydropyrimidinium salts by chiral HPLC and determination of their enantiomerization barriers

Chiral separation, determination of absolute configuration, and high-performance liquid chromatography detection of enantiomeric 3-hydroxyhexadecanoyl-CoA

The enoyl-coenzyme A (CoA) hydratase catalyzes the hydration of 2-enoyl-CoA to yield 3-hydroxyacyl-CoA in mitochondrial and peroxisomal β-oxidation. However, the stereospecificities of these hydratases differ from each other. To provide clear evidence of the stereospecificities of hydratases, the absolute configuration of 3-hydroxyhexadecanoyl-CoAs was determined, and they were subjected to a high-performance liquid chromatography (HPLC) using a chiral separation column. The retention time of 3(R)-hydroxyhexadecanoyl-CoA was shorter than that of 3(S)-hydroxyhexadecanoyl-CoA. The HPLC analysis carried out using a chiral separation column is considered to be useful for the study of enoyl-CoA hydratase.

Effective HPLC resolution of [4]heterohelicenium dyes on chiral stationary phases using reversed-phase eluents

[4]Heterohelicenium cations 1a-c adopt a twisted helical structure that renders them chiral. They are configurationally stable and their enantiomers have been resolved, for the first time, by HPLC on Chiralcel OD-RH and Chirobiotic TAG chiral stationary phases (CSPs). Chiral cations 1a-c have been resolved by HPLC using water-based eluents containing KPF(6) as additive. The elution order of the analyte enantiomers was determined by on-line CD detection, and was found to be opposite on the two CSPs. The effect of mobile phase composition and analyte structure on retention and enantioselectivity was investigated.

Studies of structure and dynamics in a nominally symmetric twisted amide by NMR and electronic structure calculations

Amides that are twisted around the C—N bond show unusual spectroscopy and reactivity when compared with planar amides. The diacyl derivatives of 3,4,7,8-tetramethyl-2,5-dithioglycoluril are intriguing examples of this class, since the crystal structures show that the two acyl groups are twisted by different amounts on either side of the molecule owing to a combination of steric and electronic effects. However, the 1H NMR spectra in solution at room temperature exhibit only one acyl resonance, so there must be fast interconversion among pairs of equivalent structures of each compound. We have prepared a number of derivatives with different acyl groups, both on the glycoluril framework as well as on its dithio analogue. The chemical exchange in solution was slowed down sufficiently by cooling to see individual sites for only two compounds: the dithiodipivaloyl and the dithiodiadamantyl derivatives. The barriers were estimated at 41 kJ mol–1 for the dipivaloyl derivative and 45 kJ mol–1 for diadamantyl derivative. The results show that rotation around the twisted amide bond is slowed by both the steric size of the acyl group and the presence of the thioureido group vs. the ureido group in the glycoluril core. In the solid-state 13C NMR spectra, there is no evidence for any dynamics, even for the diacetyl derivative at ambient temperature. Electronic structure calculations predict a geometry for the dipivaloyl derivative very close to that observed in the crystal structure. These results indicate that the crystal confines, but does not distort the molecule. A mechanism for the exchange is proposed. The relevance of these results to the mechanism of Claisen-like condensations in diacylglycolurils is also discussed.Key words: 1H and 13C NMR, exchange, dynamics, CP/MAS, solids, line shape analysis, amides, twisted amides, atropisomers, glycoluril.