13C NMR Spectroscopy for the Differentiation of Enantiomers Using Chiral Solvating Agents

Direct Enantiomer Differentiation of Drugs and Drug-Like Compounds via Noncovalent Copper-Amino Acid Complexation and Ion Mobility-Mass Spectrometry

Drug enantiomers can possess vastly different pharmacological properties, yet they are identical in their chemical composition and structural connectivity. Thus, resolving enantiomers poses a great challenge in the field of separation science. Enantiomer separations necessitate interaction of the analyte with a chiral environment─in mass spectrometry-based analysis, a common approach is through a three-point interaction with a chiral selector commonly introduced during sample preparation. In select cases, the structural difference imparted through noncovalent complexation results in enantiomer-specific structural differences, facilitating measurement using a structurally selective analytical technique such as ion mobility-mass spectrometry (IM-MS). In this work, we investigate the direct IM-MS differentiation of chiral drug compounds using mononuclear copper complexes incorporating an amino acid chiral selector. A panel of 20 chiral drugs and drug-like compounds were investigated for separation, and four l-amino acids (l-histidine, l-tryptophan, l-proline, and l-tyrosine) were evaluated as chiral selectors (CS) to provide the chiral environment necessary for differentiation. Enantiomer differentiation was achieved for four chiral molecule pairs (R/S-thalidomide, R/S-baclofen, R/S-metoprolol, and d/l-panthenol) with two-peak resolution (Rp-p) values ranging from 0.7 (>10% valley) to 1.5 (baseline separation). Calibration curves relating IM peak areas to enantiomeric concentrations enabled enantiomeric excess quantitation of racemic thalidomide and metoprolol with residuals of 5.7 and 2.5%, respectively. Theoretical models suggest that CuII and l-histidine complexation around the analyte chiral center is important for gas-phase stereoselectivity. This study demonstrates the potential of combining enantioselective noncovalent copper complexation with structurally selective IM-MS for differentiating chiral drugs and drug-like compounds.

Enantiospecific Analysis of Carboxylic Acids Using Cinchona Alkaloid Dimers as Chiral Solvating Agents

Cinchona alkaloid derivatives as Brønsted base catalysts have attracted considerable attention in the field of asymmetric catalysis. However, their potential application as chiral solvating agents has not been described. In this research, we investigated the use of the Cinchona alkaloid dimer, namely, (DHQ)2PHAL, as a chiral solvating agent for discerning various mandelic acid derivatives through 1H NMR spectroscopy. The addition of catalytic amounts of DMAP facilitated this process. Our experimental results demonstrate that dimeric (DHQ)2PHAL exhibits remarkable chiral discrimination properties regarding the diagnostic split protons of 1H NMR signals (including 24 examples, up to 0.321 ppm). Furthermore, it serves as an excellent chiral discriminating agent and provides good resolution for racemic chiral phosphoric acid as determined by 31P NMR spectroscopy. The quality of enantiodifferentiation has also been evaluated by means of the parameter "resolution (Rs)". Significantly, this class of CSAs based on (alkaloid)2linker systems with an azaaromatic linker can be directly employed, which is commercially available in an enantiopure form at very low cost and exhibits promising potential in determining the enantiopurity of α-hydroxy acids by chemoselective and biocatalytic reactions.

NMR chiral recognition of lipoic acid by cinchonidine CSA: A stereocenter beyond the organic function

Alpha-lipoic acid is a natural product that possesses distinct pharmacological properties. Lipoic acid is a short-chain fatty acid containing an asymmetric carbon at five bonds of distance to the organic function. Herein, we developed a nuclear magnetic resonance protocol to access the chiral recognition of lipoic acid in a simple and rapid procedure employing cinchonidine as a cheap chiral solvation agent in deuterated chloroform. To optimize this method, a statistical design of the experimental model was performed to produce a clear understanding of the optimal concentration, temperature, and molar ratio parameters. Based on the obtained spectra, the cinchonidine H₈ -H₉ scalar coupling indicated a conformational preference in the chiral discrimination procedure. Density functional theory calculations established a proximity between the asymmetric center of lipoic acid and the aromatic moiety of cinchonidine, clarifying possible conformations in this ion-pair interaction. The described protocol demonstrates how far is far enough to chiral discrimination using a chiral solvation agent, expanding the method to compounds that contain a remote stereocenter.

Discrimination of stereoisomers by one-dimensional 13 C NMR: All 16 stereoisomers of 4-hydroxy-α-tocopherol resolved

All 16 resolved! A vitamin E-derived compound containing four chiral centers is the first example where all stereoisomers, that is, eight diastereomeric pairs of enantiomers, could be discriminated in a single NMR run. Measurement at 176 MHz in the presence of Pirkle's alcohol as a chiral solvating agent is a relatively robust, simple, easy-to-set-up, and fast method.

A Dimeric Thiourea CSA for the Enantiodiscrimination of Amino Acid Derivatives by NMR Spectroscopy

The reaction of benzoyl isothiocyanate with (1R,2R)-1,2-bis(2-hydroxyphenyl)ethylenediamine afforded a new thiourea chiral solvating agent (CSA) with a very high ability to differentiate 1H and 13C NMR signals of simple amino acid derivatives, even at low concentrations. The enantiodiscrimination efficiency was higher with respect to that of the parent monomer, a thiourea derivative of 2-((1R)-1-aminoethyl)phenol, thus putting into light the relevance of the cooperativity between the two molecular portions of the dimer in a cleft conformation stabilized by interchain hydrogen bond interactions. An achiral base additive (DABCO or DMAP) played an active role in the chiral discrimination processes, mediating the interaction between the CSA and the enantiomeric mixtures. The chiral discrimination mechanism was investigated by NMR spectroscopy through the determination of complexation stoichiometries, association constants, and the stereochemistry of the diastereomeric solvates.

Simultaneous Enantiospecific Detection of Multiple Compounds in Mixtures using NMR Spectroscopy

Chirality plays a fundamental role in nature, but its detection and quantification still face many limitations. To date, the enantiospecific analysis of mixtures necessarily requires prior separation of the individual components. The simultaneous enantiospecific detection of multiple chiral molecules in a mixture represents a major challenge, which would lead to a significantly better understanding of the underlying biological processes; for example, via enantiospecifically analysing metabolites in their native environment. Here, we report on the first in situ enantiospecific detection of a thirty-nine-component mixture. As a proof of concept, eighteen essential amino acids at physiological concentrations were simultaneously enantiospecifically detected using NMR spectroscopy and a chiral solvating agent. This work represents a first step towards the simultaneous multicomponent enantiospecific analysis of complex mixtures, a capability that will have substantial impact on metabolism studies, metabolic phenotyping, chemical reaction monitoring, and many other fields where complex mixtures containing chiral molecules require efficient characterisation.

Efficient Enantiodifferentiation of Carboxylic Acids Using BINOL-Based Amino Alcohol as a Chiral NMR Solvating Agent

A new optically active BINOL-amino alcohol has been designed and synthesized in a good yield and applied as chiral nuclear magnetic resonance (NMR) solvating agent for enantioselective recognition. Analysis by ¹H NMR spectroscopy demonstrated that it has excellent enantiodifferentiation properties toward carboxylic acids and non-steroidal anti-inflammatory drugs (14 examples). The non-equivalent chemical shifts (up to 0.641 ppm) of various mandelic acids were evaluated by the reliable peak of well-resolved ¹H NMR signals. In addition, enantiomeric excesses of the ortho-chloro-mandelic acid with different enantiomeric ratio were calculated based on integration of proton well-separated splitting signals.

Thiourea Derivative of 2-[(1R)-1-Aminoethyl]phenol: A Flexible Pocket-like Chiral Solvating Agent (CSA) for the Enantiodifferentiation of Amino Acid Derivatives by NMR Spectroscopy

Thiourea derivatives of 2-[(1R)-1-aminoethyl]phenol, (1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, (1R,2R)-(1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, and (R)-1-phenylethanamine have been compared as chiral solvating agents (CSAs) for the enantiodiscrimination of derivatized amino acids using nuclear magnetic resonance (NMR) spectroscopy. Thiourea derivative, prepared by reacting 2-[(1R)-1-aminoethyl]phenol with benzoyl isothiocyanate, constitutes an effective CSA for the enantiodiscrimination of N-3,5-dinitrobenzoyl (DNB) derivatives of amino acids with free or derivatized carboxyl functions. A base additive 1,4-diazabicyclo[2.2.2]octane(DABCO)/N,N-dimethylpyridin-4-amine (DMAP)/NBu4OH) is required both to solubilize amino acid derivatives with free carboxyl groups in CDCl3 and to mediate their interaction with the chiral auxiliary to attain efficient differentiation of the NMR signals of enantiomeric substrates. For ternary systems CSA/substrate/DABCO, the chiral discrimination mechanism has been ascertained through the NMR determination of complexation stoichiometry, association constants, and stereochemical features of the diastereomeric solvates.

R-VAPOL-phosphoric acid based 1H and 13C-NMR for sensing of chiral amines and acids

Enantiomers have significant importance in pharmaceuticals, biology and modern chemistry and therefore distinguishing and quantifying the enantiomeric forms is of utmost importance. Herein, we propose diphenyl-3,3'-biphenanthryl-4,4'-diyl phosphate (R-VAPOL-PA) as a promising chiral solvating agent to discriminate amines and acids of poly-functional groups such as chiral amines, amino alcohols and hydroxy acids. The methodological approach involves using the nature of hydrogen bonds and ion pairs as a mode of weak interactions to form diastereomers where the probe is associated with enantiomers. The resulting diastereomer difference in the NMR spectrum enables the chiral discrimination with a complete baseline peak separation and an accurate enantiomeric excess (ee) analysis. We also carried out density functional theory (DFT) calculations to understand the complex formation to explain enantiodiscrimination by analysing the formation and stability of different chiral complexes. The binding energy differences between enantiomeric forms revealed by DFT calculations are qualitatively in agreement with the diastereomer difference in the NMR spectrum and unequivocally establishes the suggested experimental protocol of R-VAPOL-PA-based enantiomeric discrimination.

Chiral Discrimination by a Binuclear Pd Complex Sensor Using 31P{1H} NMR

An axially chiral binuclear μ-hydroxo Pd complex (BPHP) first served as an excellent chiral sensor for discriminating a variety of analytes including amino alcohol, amino amide, amino acid, mandelic acid, diol, diamine, and monoamine by ³¹P{¹H} NMR. A detailed recognition mechanism was proposed based on the single crystal and mass spectrum of Pd-complexes. In general, BPHP sensor, through extracting the acidic hydrogen of an analyte by its Pd-OH group, forms stable diastereomeric complexes with two enantiomers of the analyte giving well distinguishable split ³¹P{¹H} NMR signals for chiral discrimination.

One-Dimensional 13C NMR Is a Simple and Highly Quantitative Method for Enantiodiscrimination

The discrimination of enantiomers of mandelonitrile by means of 1D ¹³C NMR and with the aid of the chiral solvating agent (S)-(+)-1-(9-anthryl)-2,2,2-trifluoroethanol (TFAE) is presented. ¹H NMR fails for this specific compound because proton signals either overlap with the signals of the chiral solvating agent or do not show separation between the (S)-enantiomer and the (R)-enantiomer. The ¹³C NMR method is validated by preparing artificial mixtures of the (R)-enantiomer and the racemate, and it is shown that with only 4 mg of mandelonitrile a detection limit of the minor enantiomer of 0.5% is obtained, corresponding to an enantiomeric excess value of 99%. Furthermore, the method shows high linearity, and has a small relative standard deviation of only 0.3% for the minor enantiomer when the relative abundance of this enantiomer is 20%. Therefore, the ¹³C NMR method is highly suitable for quantitative enantiodiscrimination. It is discussed that ¹³C NMR is preferred over ¹H NMR in many situations, not only in molecules with more than one chiral center, resulting in complex mixtures of many stereoisomers, but also in the case of molecules with overlapping multiplets in the ¹H NMR spectrum, and in the case of molecules with many quaternary carbon atoms, and therefore less abundant protons.

Real time band selective F1 -decoupled proton NMR for the demixing of overlay spectra of chiral molecules

The small chemical shift dispersion and complex multiplicity pattern in proton NMR limit quantifications, for instance the determination of enantiomeric excess (ee) for an enantiomeric mixture. Herein, we present a simple proton-proton correlation experiment with band selective homonuclear (BASH) decoupling in both F₁ and F₂ dimensions, for the removal of scalar and residual dipolar couplings to provide collapsed singlet for each chemical site. The method has been demonstrated to separate the severely overlapped spectra of enantiomers using both chiral isotropic and anisotropic phases as well as a small biomolecule, particularly for the diastereotopic protons and also for the determination of ee. Copyright © 2016 John Wiley & Sons, Ltd.

Chiral Recognition by Dissolution DNP NMR Spectroscopy of 13C-Labeled dl-Methionine

A method based on d-DNP NMR spectroscopy to study chiral recognition is described for the first time. The enantiodifferentiation of a racemic metabolite in a millimolar aqueous solution using a chiral solvating agent was performed. Hyperpolarized ¹³C-labeled dl-methionine enantiomers were differently observed with a single-scan ¹³C NMR experiment, while the chiral auxiliary at thermal equilibrium remained unobserved. The method developed entails a step forward in the chiral recognition of small molecules by NMR spectroscopy, opening new possibilities in situations where the sensitivity is limited, for example, when a low concentration of analyte is available or when the measurement of an insensitive nucleus, like ¹³C, is required. The advantages and current limitations of the method, as well as future perspectives, are discussed.

Chirality sensing of tertiary alcohols by a novel strong hydrogen-bonding donor - selenourea

In this paper, a novel type of chiral bisselenourea sensor was first synthesized and used as a strong hydrogen-bonding donor for highly efficient chiral recognition of a diverse range of tertiary alcohols.

Chemical sensors are powerful for the fast recognition of chiral compounds. However, the established sensing systems are less effective for chiral tertiary alcohols. The chiral tertiary alcohol group is an important structural unit in natural products and drug molecules, and its enantioselective recognition represents a significant and challenging task. In this paper, a novel type of chiral bisselenourea sensor was first synthesized and used as a strong hydrogen-bonding donor for highly efficient chiral recognition of a diverse range of tertiary alcohols. The obtained sharply split NMR signals are well-distinguishable with a large (up to 0.415 ppm) chemical shift nonequivalence. The NMR signal of the hydroxyl hydrogen atom was first employed for enantiomeric excess determination of tertiary alcohols, giving accurate results with <2% absolute errors. The 2D NOESY spectra and computational studies suggest that the geometrical differentiation of the formed diastereomeric complexes between the sensor and tertiary alcohols enables the chiral discrimination of the hydroxyl hydrogen signals of the tertiary alcohol in the ¹H NMR spectrum.

Precise Determination of Enantiomeric Excess by a Sensitivity Enhanced Two-Dimensional Band-Selective Pure-Shift NMR

Unambiguous identification and precise quantification of enantiomers in chiral mixtures is crucial for enantiomer specific synthesis as well as chemical analysis. The task is often challenging for mixtures with high enantiomeric excess and for complex molecules with strong (1)H-(1)H scalar (J) coupling network. The recent advancements in (1)H-(1)H decoupling strategies to suppress the J-interactions offered new possibilities for NMR based unambiguous discrimination and quantification enantiomers. Herein, we discuss a high resolution two-dimensional pure-shift zCOSY NMR method with homonuclear band-selective decoupling in both the F1 and F2 dimensions (F1F2-HOBS-zCOSY). This advanced method shows a sharp improvement in resolution over the other COSY methods and also eliminates the problems associated with the overlapping decoupling sidebands. The efficacy of this method has been exploited for precise quantification of enantiomeric excess (ee) ratio (R/S) up to 99:1 in the presence of very low concentrations of chiral lanthanide shift reagents (CLSR) or chiral solvating agents (CSA). The F1F2-HOBS-zCOSY is simple and can be easily implemented on any modern NMR spectrometers, as a routine analytical tool.

Broadband 1H homodecoupled NMR experiments: recent developments, methods and applications

In recent years, a great interest in the development of new broadband 1H homonuclear decoupled techniques providing simplified JHH multiplet patterns has emerged again in the field of small molecule NMR. The resulting highly resolved 1H NMR spectra display resonances as collapsed singlets, therefore minimizing signal overlap and expediting spectral analysis. This review aims at presenting the most recent advances in pure shift NMR spectroscopy, with a particular emphasis to the Zangger-Sterk experiment. A detailed discussion about the most relevant practical aspects in terms of pulse sequence design, selectivity, sensitivity, spectral resolution and performance is provided. Finally, the implementation of the different reported strategies into traditional 1D and 2D NMR experiments is described while several practical applications are also reviewed.

Simultaneous chirality sensing of multiple amines by (19)F NMR

The rapid detection and differentiation of chiral compounds is important to synthetic, medicinal, and biological chemistry. Palladium complexes with chiral pincer ligands are demonstrated to have utility in determining the chirality of various amines. The binding of enantiomeric amines induces distinct (19)F NMR shifts of the fluorine atoms appended on the ligand that defines a chiral environment around palladium. It is further demonstrated that this method has the ability to evaluate the enantiomeric composition and discriminate between enantiomers with chiral centers several carbons away from the binding site. The wide detection window provided by optimized chiral chemosensors allows the simultaneous identification of as many as 12 chiral amines. The extraordinary discriminating ability of this method is demonstrated by the resolution of chiral aliphatic amines that are difficult to separate using chiral chromatography.

Modified Kagan's amide: synthesis and application as a chiral solvating agent for hydrogen-bonding based chiral discrimination in NMR

A modified Kagan's amide, N-((S)-1-(3,5-bis(trifluoromethyl)phenyl)ethyl)-3,5-dinitrobenzamide has been designed, synthesized and screened as a Chiral Solvating Agent for discrimination of optically active substrates.

Simultaneous 1H and 13C NMR enantiodifferentiation from highly-resolved pure shift HSQC spectra

NMR enantiodifferentiation studies are greatly improved by the simultaneous determination of accurate ¹H and ¹³C chemical shift differences, even smaller than the NMR resonance line width, obtained from the analysis of highly resolved cross-peaks in spectral aliased pure shift (SAPS) HSQC spectra.