HPLC quantitation of the four stereoisomers of benzoxathiepin derivatives with cellulose phenyl type chiral stationary phase and circular dichroism detection

A New Benzyl Phenethanolamine-β-Cyclodextrin Bonded Phase: Chiral Chromatography Performance and Application for LC-MS/MS Analysis of Pesticide Enantiomers in Fruits and Vegetables

Background

At present, the research on achiral drug and pesticide residue detection methods is still the mainstay at home and abroad, and there is still a lack of systematic research on the enantiomeric analysis of chiral drugs and pesticides.

Objective

In order to prepare a novel chiral stationary phase, whose “multi-mode” chiral separation chromatographic performance and its utility was verified.

Method

An S-(-)-2-benzylamino-1-phenylethanol mono-derivative β-cyclodextrin bonded stationary phase (BzCSP) was prepared based on the “thiol-ene” addition reaction. The chiral compounds including four types of chiral compounds were used as “probes,” and their chiral chromatographic properties were evaluated. Furthermore, a new LC-MS/MS method for the determination of the enantiomeric residues of three chiral pesticides in five kinds of fruits and vegetables was established.

Results

The study found that the novel stationary phase was suitable for a variety of chromatographic modes (normal phase mode, reversed-phase mode, polar organic mode). The resolutions of hexaconazole (Hex), tebuconazole (Teb), and triticonazole (Trit) enantiomers could be up to 2.31, 1.68, and 1.48, respectively, within 30 min under reversed-phase chromatography. Based on the optimal chromatographic and mass spectrum conditions, a new LC-MS/MS quantitative method for the Hex, Teb, and Trit enantiomers was established by multi-reaction positive ion monitoring (MRM). The detection limits (LODs) of enantiomers were less than 0.89 µg/kg for Hex, 0.93 µg/kg for Teb, and 0.93 µg/kg for Trit, and the averaged recoveries of enantiomers were in the range of 75.8–106.3% for Hex, 77.4–116.3% for Teb, and 78.7–113.4% for Trit. The method had good reproducibility with the RSDs (<5%) for intraday and (<7%) for interday.

Conclusions

The established method had the characteristics of good selectivity, high sensitivity, strong resistance to matrix interference, and good reproducibility. It is indicated that the stationary phase prepared by the “thiol-ene” addition reaction is a new type of multi-mode stationary phase, which has a good development value.

Highlights

The study reported a new method for the rapid preparation of a rare “multi-mode” chiral stationary phase BzCSP based on the “thiol-ene” addition reaction and verified the practicability of BzCSP including good selectivity, high sensitivity, strong resistance to matrix interference, and good reproducibility.

Enzyme- and Chemo-enzyme-Catalyzed Stereodivergent Synthesis

Multiple stereoisomers can be found when a substance contains chiral carbons in its chemical structure. To obtain the desired stereoisomers, asymmetric synthesis was proposed in the 1970s and developed rapidly at the beginning of this century. Stereodivergent synthesis, an extension of asymmetric synthesis in organic synthesis with the hope to produce all stereoisomers of chiral substances in high conversion and selectivity, enriches the variety of available products and serves as a reference suggestion for the synthesis of their derivatives and other compounds. Since biocatalysis has outstanding advantages of economy, environmental friendliness, high efficiency, and reaction at mild conditions, the biocatalytic reaction is regarded as an efficient strategy to perform stereodivergent synthesis. Thus, in this review, we summarize the stereodivergent synthesis catalyzed by enzymes or chemo-enzymes in cases where a compound contains two or three chiral carbons, i.e., at most four or eight stereoisomers are present. The types of reactions, including reduction of substituent ketones, cyclization reactions, olefin addition, and nonredox transesterification reactions, are also discussed for the understanding of the progress and application of biocatalysis in stereodivergent synthesis.

[Separation of chiral pharmaceutical drugs by chromatographic and electrophoretic techniques]

SUMMARY

A large number of pharmaceutical drugs possess one or more centers of asymmetry giving rise to enantiomers whose pharmacological properties and toxicity are often different. At successive stages of drug discovery, the enantiomers of any chiral molecule must be isolated and analyzed and their enantiomeric purity determined. The electrophoretic and chromatographic techniques have become the most important tools to routinely determine the enantiomeric purity of pharmaceutical molecules. Liquid chromatography (LC) is the most widely used because of the large number of columns marketed, the variety of selectivities available and the ease at which analytical results can be scaled up to the preparative level. In particular, more than 80% of enantioseparations of pharmaceutical molecules are successful with polysaccharide-derivative stationary phases (cellulose, amylose) for multiple system solvents (normal phase, polar organic phase or reverse phase). Complementary selectivities can be achieved more rapidly with other types of stationary phase (glycopeptides, Pirkle, cyclodextrins) but their application is hindered by problems of stability (proteins) or transfer to the preparative scale (cyclodextrins). At the present time, glycopeptide phases offer very promising prospects for the separation of amino acids (and derivatives) and peptide enantiomers. In addition, because of its faster analysis and environmental benefits, supercritical chromatography (SFC) has given rise to renewed interest. Capillary electrophoresis (CE) is an orthogonal technique complementary to chromatographic methods. Its principle involves the formation of diastereoisomer complexes after addition of anionic (HS-beta-CD, HS-gamma-CD CM-beta-CD) or neutral (TM-beta -CD, HP-beta-CD, DM-beta-CD, HP-gamma-CD) cyclodextrins to the running buffer. Compared to LC, CE analyses are cheaper (no chiral column, no solvent, low consumption of chiral selector) and peak efficiencies are higher by one order of magnitude. Furthermore, the mechanism of separation in CE is much simpler to understand and predict. However, the low capacity of CD column prevents its use at the preparative scale and consequently hampers its development as an analytical technique. Today, the increasing number of new drug candidate molecules produced daily, and for which the determination of enantiomeric purity is required before further development, encourages the pharmaceutical industry to seek fast chiral analysis methods based on simple protocols. The speed of analysis is more important than resolution. Thus, screening strategies are implemented with HPLC, SFC and CE including the selection of a limited number of chiral selectors with strong powers of chiral recognition.

Validation of a method using an achiral liquid chromatography sorbent and a circular dichroism detector

The known HPLC method using an achiral C8 silica sorbent and a circular dichroism (CD) detector for the determination of efaroxan enantiomeric excess has been validated. After optimization of the mobile phase, the enantiomers were detected at 278 nm offering maximum ellipticity between two optically active forms. The calibration curve of the anisotropy factor (g) versus the enantiomeric excess was linear with a correlation coefficient (r2) of 0.9985. The accuracy of the method was assessed by comparing the enantiomeric excess obtained by measuring the g factor (C8 column, CD and UV detections) with those determined by enantioselective HPLC (Chiralpak AD-H column, UV detection). Statistical tests (level of confidence of 95%) were assessed to compare the two orthogonal methods. The straight line gave a correlation coefficient of 0.9995, an intercept not significantly different from zero (0.0549) and a slope of 1.026. The precision evaluated on retention time (RSD<0.6%), g factor (RSD<8.3%) and CD peak area (RSD<7.5%) was suitable both in term of intra- and inter-day precisions. The proposed method has the advantages of being fast and precise without using expensive chiral column. Non-enantioselective HPLC-CD was suitable for the simultaneous determination of the optical and chemical purity of efaroxan.

Sensitivity improvement of circular dichroism detection in HPLC by using a low-pass electronic noise filter: Application to the enantiomeric determination purity of a basic drug

The quality control of chiral drugs requires the determination of their enantiomeric purity. Nowadays, circular dichroism (CD) spectroscopy is gaining increasing importance in pharmaceutical analysis because of the commercially available CD detector in liquid chromatography. The separation of the two enantiomers of a basic drug (efaroxan) was achieved by high performance liquid chromatography using an amylose-derivated column with both UV and CD detections. A baseline-resolved separation (resolution: 5) was obtained after optimization of the mobile phase composition with hexane-ethanol-diethylamine (90:10:0.05; v/v/v). The use of a commercial low-pass electronic noise filter of the CD signal has improved the signal-to-noise ratio by a factor twelve and allowed the quantitation of each enantiomer in the 1.25-300 microg ml(-1) concentration range. The CD linear calibration curve, expressed in terms of stereoisomer height ratio versus concentration ratio, was plotted over the 0.4-6% range. A correlation coefficient greater than 0.999 was obtained by least-squares regression and the limit of detection for the distomer/eutomer ratio was estimated at 0.14%. Although the method validation showed good repeatability on the retention times (RSD < 0.9%), on the peak height ratios (RSD < 8.7%) of each enantiomer only up to 99.2% enantiomeric purity was achieved.