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1.
A solvent mixture containing dioxane, acetonitrile, and hexane was found to be suitable as a mobile phase to resolve oxazepam enantiomers by chiral stationary phase high performance liquid chromatography using covalent Pirkle columns. The resolved oxazepam enantiomers in this solvent mixture had a racemization half-life greater than 3 days at 23°C. When desiccated at 0°C as dried residue, OX enantiomers were stable for at least 50 days with less than 2% racemization. The conditions which stabilized OX enantiomers significantly facilitated the determination of racemization half-lives of OX enantiomers in a variety of aqueous and nonaqueous solvents and at different temperatures. © 1992 Wiley-Liss, Inc. 相似文献
2.
By computer simulation of experimental dynamic gas chromatographic elution profiles, the rotational energy barrier ΔG= of racemic 2,2′-diisopropylbiphenyl has been determined as 114.6–115.0 kJ/mol (75–100°C). These data are in good agreement with a value that was determined previously by measuring the racemization kinetics of an enriched sample. This indicates that there is no measurable catalytic or inhibitory effect of the stationary phase. © 1994 Wiley-Liss, Inc. 相似文献
3.
A new method has been developed for the rapid determination of D-cysteine contents in synthetic peptides. It is based on the reduction of cystine residues, when present, with tris- alkylphosphines, selective derivatization of the cysteine residues with 4-vinylpyridine, followed by acid hydrolysis of the (4-pyridylethyl)cysteine –peptides. Baseline enantiomeric resolution of theD ,L -S-β-(4-pyridylethyl)cysteine, and thus quantification ofD - enantiomer contents at levels ≤1%, is easily achieved by capillary zone electrophoresis exploiting the host–guest complexation principle with crown ethers or by gas chromatography on chiral glass capillary columns upon conventional derivatization of the hydrolysate. The acid-stability of the (4-pyridylethyl)cysteine derivative prevents racemization via thiazoline intermediates and allows for standardization of the acid hydrolysis-dependent racemization. 相似文献
4.
Racemic ethyl 2,3-dibromopropionate, commercially available at low price, is a key intermediate used in the synthesis of several heterocycle fragments, which are present in many biologically active compounds. Surprisingly, the enantiomers are not commercially available and have never been described in the literature. In this work, we undertook two different strategies to obtain these enantiomers, which are enantioselective synthesis and preparative HPLC enantioseparation of commercially available racemate on multigram scale. The first strategy has proved inadequate because racemization occurred during the synthesis (ee ≈ 9-50%). Conversely, the second strategy produced a very good enantioseparation of commercially available racemate (ee > 99.5% for both enantiomers) on multigram scale. 相似文献
5.
High‐Performance Liquid Chromatographic Resolution of Neutral and Cationic Hetero[6]Helicenes 下载免费PDF全文
Geraldine M. Labrador Johann Bosson Zachary S. Breitbach Yeeun Lim Eric R. Francotte Rocchina Sabia Claudio Villani Daniel W. Armstrong Jérôme Lacour 《Chirality》2016,28(4):282-289
Cationic hetero[6]helicenes 1+ , 2+ and 3+ have been recently disclosed. Herein we report on their enantiomeric separation using high‐performance liquid chromatography. Separation of the antipodes can be achieved in preparative scale on neutral adducts with Chiralcel OD‐I or Chiralpak ID CSP. Selectivity factors of 1.90, 1.67, and 1.96 were obtained for 1-H , 2-H , and 3-H , respectively. Separation can also be performed on the carbenium ions on regular Chiralpak IA CSP using water‐containing eluents, thus allowing for enantiomeric purity determinations in aqueous environments. Resolution of neutral and cationic helicenes is also achieved on more recently developed LARIHC columns. The versatility of the cyclofructan phases allows for baseline separations for both cases and their loading capabilities are demonstrated. Finally, the configurational stability of 1+ , 2+ , and 3+ was measured. For each replacement of an oxygen atom by an amino group, the racemization barrier increases significantly (ΔG? = 29.8, 36.3 and >37 kcal mol‐1 for 1+ , 2+ , and 3+ respectively). Chirality 28:282–289, 2016. © 2016 Wiley Periodicals, Inc. 相似文献
6.
Jianlin Han Marion Jean Christian Roussel Hiroki Moriwaki Vadim A. Soloshonok 《Chirality》2019,31(4):328-335
Herein, we disclose the design of a model Ni(II) complex of glycine Schiff base possessing single‐nitrogen stereogenic center, which was successfully used for high‐performance liquid chromatography (HPLC)‐assisted assessment of its configurational stability. The major finding is that the configurational stability of the Ni(II)‐coordinated nitrogen is profoundly dependent on the reaction conditions used, in particular the solvent, and can range from inconsequential (t½ less than 5 min) to virtually completely stable (t½ 90 y). The discovery reported in this study most likely to be of certain theoretical and synthetic value. 相似文献
7.
Even though amino acids are important trace components in the brewing of beers, they have not been extensively evaluated in these beverages. Studies involving the enantiomeric composition of these amino acids are even less prevalent. A brief summary of the brewing process for malt beverages is given. The total concentration and enantiomeric composition of three amino acids (leucine, phenylalanine, and proline) were determined in 25 different beers. Proline tended to have the highest average absolute concentration and the lowest percentage of the D -enantiomer in most samples. In some cases the relative amounts of D -phenylalanine and D -leucine exceeded 10% of the individual amino acids. The enantiomeric composition of the amino acids in different beer samples did not vary as extensively as the absolute concentrations. The reason for the concentration differences between proline and the other amino acids is discussed. A knowledge of amino acid concentrations and enantiomeric compositions appears to be useful in characterizing specific beers and brewing processes. © 1996 Wiley-Liss, Inc. 相似文献
8.
We present two methods for the enantioselective analysis of thioridazine (THD) and thioridazine 2-sulfone (THD 2-SO(2)) in human plasma based on liquid-liquid extraction with diethyl ether and chiral resolution of the enantiomers on Chiralpak AD and Chiralcel OD-H columns, respectively. After validation, the methods were used to study the degradation and racemization of both drug and metabolite. Our results showed that both enantiomers of THD and THD 2-SO(2) were stable at varying temperatures, pH, and ionic strengths; however, solubility problems for THD and THD 2-SO(2) enantiomers were observed, mainly at pH 8.5. The influence of light on the stability of the THD and THD 2-SO(2) enantiomers was also studied. Degradation of the THD enantiomers was observed under UV light (254 and 366 nm) while THD 2-SO(2) enantiomers were stable at these wavelengths and also when exposed to visible light. 相似文献
9.
Tautomerism of 4-hydroxy-2,5-dimethyl-3(2H)-furanone: evidence for its enantioselective biosynthesis
Chiral natural flavor compounds exhibit characteristic enantiomeric excesses due to stereoselective, enzymatically catalyzed reactions during biogenesis. Although the enzymatic formation of the strawberry key flavor compound 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF; Furaneol(R)) is anticipated, the naturally occurring compound is racemic. As racemization due to keto-enol-tautomerism of HDMF could account for this observation, HDMF was investigated by (1)H-NMR spectroscopy tracing the exchange of the proton bound to the furanone-ring at C2 with deuteron from the medium (D(2)O). In addition, the racemization rate of HDMF was directly determined by cyclodextrin-modified capillary electrophoresis of enantiomerically enriched HDMF stored at different pH values. Tautomerism and the racemization rate of HDMF was lowest at pH values between 4 and 5. However, tautomerism and thus racemization was catalyzed under stronger acidic conditions (pH 2) and especially at pH values greater than 7, the value published for plant cell cytosol. Approximately 50% of the protons at C2 were exchanged with deuteron within 1 h at pH 7.2. Therefore, in order to demonstrate the enzymatic formation of HDMF, incubation experiments with Zygosaccharomyces rouxii as well as strawberry protein extract were carried out under slightly acidic conditions (pH 5), the most suitable pH value for studies on the enantiomeric ratio of HDMF. In both experiments the formation of enantiomerically enriched HDMF could be demonstrated for the first time, whereas incubation experiments under neutral conditions resulted in the detection of racemic HDMF. 相似文献
10.
Native chemical ligation in dimethylformamide can be performed chemoselectively without racemization
Marc Dittmann Muheeb Sadek Ralf Seidel Martin Engelhard 《Journal of peptide science》2012,18(5):312-316
Native chemical ligation of unprotected peptides in organic solvents has been previously reported as a fast, efficient, and suitable method for coupling of hydrophobic peptides. However, it has not been determined whether the reaction can be carried out without possible side reactions or racemization. Here, we present a study on the chemoselectivity of this method by model reactions designed to test the reactivity of Arg and Lys side chains as well as that of α‐amino groups. A possible racemization of the C‐terminal amino acid of the N‐terminal peptide was also investigated. The results show that ligation in organic solvents can be conducted chemoselectively without side reactions with other nucleophilic groups. Furthermore, no racemization of the C‐terminal amino acid was observed if both educts were added simultaneously. Thus, native chemical ligation can be performed either in aqueous buffer systems or in organic solvents paving the way for the synthesis of larger hydrophobic peptides and/or membrane proteins. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd. 相似文献