11 research outputs found

    Estimation of Enantiomeric Excess Based on Rapid Host–Guest Exchange

    Get PDF
    Chiral molecules possess enantiomers that have non-superimposable chemical structures but exhibit identical nuclear magnetic resonance (NMR) spectra. This feature prevents the use of NMR spectroscopic methods for the determination of enantiomeric excesses (ee) of chiral molecules, using simple mixtures of their enantiomers. Recently, however, it was reported that the addition of a symmetrical prochiral molecule (a reporter or host) into a solution of chiral analyte can lead to estimation of ee through interactions involving rapid exchange of the chiral analyte (guest) in the formed host–guest complex. This is due to the ee-dependent splitting of NMR resonances of the prochiral host molecule based on averaging the chemical shift non-equivalency caused by the presence of a chiral guest. The mechanism is not dependent on diastereomer formation, and 1:1 host–guest complexes can also show ee-dependent NMR peak splitting. Prochiral molecules capable of ee sensing using the NMR technique are now referred to as so-called prochiral solvating agents (pro-CSAs). pro-CSAs represent a family of reagents distinct from the commonly used NMR chiral derivatizing reagents (where chiral auxiliaries are used to derivatize enantiomers to diastereomers) or chiral solvating agents (where chiral auxiliaries interact in an asymmetric manner with analyte enantiomers). pro-CSA methods are unique since neither pro-CSA nor NMR contains chiral factors, making the technique neutral with respect to chirality. Here, we review our recent work on this matter involving several different nominally achiral receptor molecules whose unique guest binding properties and solution characteristics (especially with regard to NMR spectroscopy) allow for the estimation of ee in the corresponding chiral guests

    NMR spectroscopic detection of chirality and enantiopurity in referenced systems without formation of diastereomers

    Get PDF
    Enantiomeric excess of chiral compounds is a key parameter that determines their activity or therapeutic action. The current paradigm for rapid measurement of enantiomeric excess using NMR is based on the formation of diastereomeric complexes between the chiral analyte and a chiral resolving agent, leading to (at least) two species with no symmetry relationship. Here we report an effective method of enantiomeric excess determination using a symmetrical achiral molecule as the resolving agent, which is based on the complexation with analyte (in the fast exchange regime) without the formation of diastereomers. The use of N,N′-disubstituted oxoporphyrinogen as the resolving agent makes this novel method extremely versatile, and appropriate for various chiral analytes including carboxylic acids, esters, alcohols and protected amino acids using the same achiral molecule. The model of sensing mechanism exhibits a fundamental linear response between enantiomeric excess and the observed magnitude of induced chemical shift non-equivalence in the 1H NMR spectra

    Chiral Solid Solutions for the NMR Analysis of Enantiomers: A Potential New Approach to Chiral Analysis

    Get PDF

    Supramolekulární komplexy oxoporfyrinogenů s organickými molekulami

    Get PDF
    Title: Supramolecular complexes of oxoporphyrinogens with organic molecules Author: Václav Březina Department: Department of Macromolecular Physics Supervisor: doc. RNDr. Lenka Hanyková, Dr., Department of Macromolecular Physics Abstract: Oxoporphyrinogens are flat macrocyclic molecules possessing binding and protonation sites, and capable of light absorption in the visible region. These properties are prerequisites for a colori- metric molecular sensor, i.e. a specific detector of other molecules in the sample. In this work, we studied chromic properties of three oxoporphyrinogens, OxP and its partially (Bz2OxP) and fully (Bz4OxP) N-benzylated derivatives. Their colorimetric response to organic acids is caused by protonation and subsequent formation of supramolecular host-guest complex. We have shown that colorimetric sensitivity is highest for OxP and gradually weakens for Bz2OxP and Bz4OxP since the N-benzylation blocks the central binding sites, decreasing binding affinity of the ox- oporphyrinogens. Furthermore, solvatochromic response of the oxoporphyrinogens to varying solvent polarity showed similar sensitivity decrease in Bz2OxP and Bz4OxP. The chromic and binding properties were studied by UV/vis and NMR spectroscopy, host-guest binding models were applied to describe the formation of...Oxoporfyrinogeny, ploché makrocyklické molekuly, na sebe dokáží navázat kyseliny či jiné látky a zároveň pohlcují světlo ve viditelném oboru. Tyto vlastnosti jsou předpokladem pro molekulární kolorimetrický senzor, detekující přítomnost konkrétních látek ve vzorku. V této práci jsme studovali chromismus tří oxoporfyrinogenů, OxP a jeho částečně (Bz2OxP) a úplně (Bz4OxP) N-benzylovaných derivátů. Jejich kolorimetrická odezva na organické kyseliny je způsobena protonací a následnou tvorbou supramolekulárního "host-guest" komplexu. Vysoká citlivost OxP na přítomnost kyseliny se postupně snižuje u Bz2OxP a Bz4OxP, neboť N-benzylace blokuje centrální vazebná místa a tím se snižuje vazebná afinita oxoporfyrinogenů. Kromě toho byla pozorována solvatochromická odezva oxoporfyrinogenů na měnící se polaritu rozpouštědla, kde se ukázala podobně snížená citlivost u Bz2OxP a Bz4OxP. Ke studiu chromismu a vazebných vlastností byly použity UV/vis a NMR spektroskopie, tvorba komplexů oxoporfyrinogen-kyselina byla popsána vazebnými modely typu "host-guest". Přítomnost chemické výměny v NMR spektrech protonovaného OxP a Bz2OxP ukázala na přítomnost několika dynamických procesů, mimo jiné prototropní tautomerizace (tj. změna místa protonace) nebo rotace objemných postranních skupin v Bz2OxP. Tyto procesy byly v NMR...Department of Macromolecular PhysicsKatedra makromolekulární fyzikyFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

    Chiral Auxiliaries and Chirogenesis

    Get PDF
    This Reprint Book highlights and overviews the most important and novel aspects of chiral auxiliary and chirogenesis in different natural/physical sciences and in modern technologies. In particular, some newly emerging classes of molecules used for these purposes are described. This book consists of four review articles and one research paper and is of interest for general chemistry readership, including graduate and postgraduate students, and for researchers specializing in the fields of chirality and stereochemistr

    Boron-BINOL catalysed asymmetric Mannich and aldol type reaction: Novel boronate esters

    Get PDF
    EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Praxis und Theorie der gaschromatographischen Enantiomerentrennung an chiralen Stationärphasen mit einfachen und binären Selektoren

    Get PDF
    1.Enantioselective properties of several important gas-chromatographic chiral stationary phases were improved using the binary-selector approach. Thus, the enantioselectivity of Chirasil-Val-C11 was combined with that of Chirasil-Dex and Lipodex E, respectively, in the corresponding binary-selector chiral stationary phases (CSPs). 2.An equation for the optimization of enantioseparation on binary-selector CSPs was derived and examined experimentally. Theoretical treatment of the enantioseparation on binary-selector CSPs was performed. 3.A new macromolecular chiral selector (Valdex) containing both a polar valine-diamide group and an apolar beta-cyclodextrin cavity was synthesized and its enantioselective properties in solution towards both polar and apolar analytes were studied by NMR spectroscopy. 4.Several additional examples of temperature-induced inversion of elution order of enantiomers on two important CSPs, Chirasil-Val-C11 and Chirasil-Dex, were discovered and thoroughly investigated. 5.Using the approximation function, enantiomerization parameters of 1,2-di-tert-butylpyrazolidine were identified. 6.The dramatic influence of a small enantiomeric impurity of highly enantioselective chiral selectors on the enantioseparation factor was confirmed experimentally. Enantioselective liquid chromatography on quinine and quinidine derivatives used as chiral selectors was employed for the investigation. 7.A method for the evaluation of nonenantioselective versus enantioselective interactions in liquid chromatography was derived. 8.Enantioselective gas-chromatography was proposed and successfully applied as a method for the differentiation between racemic compounds and conglomerates.1.Die enantioselektiven Eigenschaften von verschiedenen wichtigen gaschromatographischen chiralen Stationärphasen (CSP) wurden mit Hilfe des binären Selektor-Ansatzes verbessert. So wurde die Enantioselektivität von Chirasil-Val-C11 mit der von Chirasil-Dex bzw. Lipodex E in dem entsprechenden binären Selektor-CSP kombiniert. 2.Es wurde eine theoretische Beziehung für die Optimierung der Enantiomerentrennung an binären Selektor-CSPs abgeleitet und experimentell verifiziert. 3.Es wurde ein neuer makromolekularer chiraler Selektor (Valdex), der sowohl eine polare Valindiamid-Gruppe und eine apolare beta-Cyclodextrin-Kavität enthält, synthetisiert und es wurden dessen enantioselektiven Eigenschaften in Lösung gegenüber polaren und unpolaren Analyten NMR-spektroskopisch untersucht. 4.Es wurden verschiedene zusätzliche Beispiele der Temperatur-induzierten Inversion der Elutionsreihenfolge von Enantiomeren an zwei wichtigen CSPs, Chirasil-Val-C11 und Chirasil-Dex , entdeckt und ausführlich untersucht. 5.Mit Hilfe der Approximierungsfunktion wurden die Enantiomerisierungsparameter von 1,2-di-tert-butylpyrazolidin bestimmt. 6.Der dramatische Einfluß einer geringen Enantiomerenverunreinigung hoch-enantioselektiver chiraler Selektoren auf den Enantiomerentrennfaktor wurde experimentell verifiziert. Es wurde die enantioselektive Flüssigchromatographie an Chinin- und Chinidin-Derivaten als chirale Selektoren für die Untersuchung eingesetzt. 7.Es wurde eine Methode zur Evaluierung von nicht-enantioselektiven vs. enantioselektiven Wechselwirkungen in der Flüssigchromatographie entwickelt. 8.Die enantioselektive Gaschromatographie wurde als Methode zur Differenzierung zwischen Racematen und Konglomeraten vorgeschlagen und erfolgreich eingesetzt

    Dynamic Processes in Prochiral Solvating Agents (pro-CSAs) Studied by NMR Spectroscopy

    No full text
    Several dynamic processes, including tautomerism and macrocyclic inversion, in 1H-NMR prochiral solvating agents (pro-CSAs) are investigated. Various features of pro-CSA, including modes of interaction for complex formation, stoichiometry, binding strength and temperature effects were compared for three representative pro-CSA molecules. Structural effects of conjugated tetrapyrrole pro-CSA on the mechanism of enantiomeric excess determination are also discussed. Detailed analysis of species (complexes) and dynamic processes occurring in solution and their 1H-NMR spectral manifestations at various temperatures is presented
    corecore