Chiral Recognition of Chiral (Hetero)Cyclic Derivatives Probed by Tetraaza Macrocyclic Chiral Solvating Agents via 1H NMR Spectroscopy

In the field of chiral recognition, chiral cyclic organic compounds, especially heterocyclic organic compounds, have attracted little attention and have been rarely studied as chiral substrates by means of 1H NMR spectroscopy. In this paper, enantiomers of thiohydantoin derivatives, representing typical five-membered N,N-heterocycles, have been synthesized and utilized for assignment of absolute configuration and analysis of enantiomeric excess. All enantiomers have been successfully differentiated with the assistance of novel tetraaza macrocyclic chiral solvating agents (TAMCSAs) by 1H NMR spectroscopy. Surprisingly, unprecedented nonequivalent chemical shift values (up to 2.052 ppm) of the NH proton of substrates have been observed, a new milestone in the evaluation of enantiomers. To better understand the intermolecular interactions between host and guest, Job plots and theoretical calculations of (S)-G1 and (R)-G1 with TAMCSA 1a were investigated and revealed significant geometric differentiation between the diastereomers. In order to evaluate practical applications of the present systems in analyzing optical purity of chiral substrates, enantiomeric excesses of a typical substrate (G1) with different optical compositions in the presence of a representative TAMCSA (1a) can be accurately calculated based on the integration of the NH proton's signal peaks. Importantly, this work provides a significant breakthrough in exploring and developing the chiral recognition of chiral heterocyclic organic compounds by 1H NMR spectroscopy.

Chiral Recognition and Resolution of Phosphoric Acids Using Octahedral Cobalt Complexes

Determining the chirality of phosphoric acids can be a challenging task. In this study, we present a novel approach for the chiral recognition of phosphates using cationic octahedral cobalt complexes. By utilizing ³¹P NMR spectroscopy, we are able to accurately measure the enantiopurities of chiral phosphoric acids after forming ion pairs with the cobalt complexes. We have successfully applied this method to a variety of chiral phosphoric acids derived from BINOL, H8-BINOL, SPINOL, VAPOL, and VANOL compounds, as well as ATP, and were able to efficiently resolve them in ³¹P{¹H} NMR spectra. Furthermore, we were able to achieve an optical resolution of a phosphoric acid with an enantiomeric excess of greater than 99%.

A One-Dimensional Phase-Modulated (PM) NMR Experiment for Differentiating Spin Systems in Small Molecules Mixtures: With Application to Chiral Discrimination and Bicomponent Organic Systems

A one-dimensional phase-modulated NMR experiment, which distinguishes the partially resolved peaks and accelerates the data acquisition due to reduced dimensionality, is reported for differentiating spin systems, with application to chiral discrimination. The multifarious utility of the technique is demonstrated in plenteous examples.

Selective Homodecoupled 1D-1H NMR Experiment for Unravelling Enantiomers

¹H NMR spectroscopy is a quick and easy tool to resolve enantiomers. However, the paradigm of resolution of enantiomers by ¹H NMR is a difficult task due to the minor chemical shift differences and overlap of the resonances of the enantiomers. In the current study, we have performed the conventional selective homodecoupling 1D-¹H NMR experiment to achieve chiral resolutions. The predominant features of the proposed methods are (a) high sensitivity as opposed to routinely employed pure shift NMR experiments, (b) easy optimization like conventional 1D-¹H NMR, and (c) an artifact-free spectrum. The application of the method has been exhibited in the measurement of proton chemical shift differences between diastereomers and enantiomer excess (ee). The broad use of the proposed method was demonstrated by employing the six samples utilizing the chiral solvating and derivatizing agents.

Enantioselective Recognition of Racemic Amino Alcohols in Aqueous Solution by Chiral Metal-Oxide Keplerate {Mo132 } Cluster Capsules

Determining the relative configuration or enantiomeric excess of a substance may be achieved using NMR spectroscopy by employing chiral shift reagents (CSRs). Such reagents interact noncovalently with the chiral solute, resulting in each chiral form experiencing different magnetic anisotropy; this is then reflected in their NMR spectra. The Keplerate polyoxometalate (POM) is a molybdenum-based, water-soluble, discrete inorganic structure with a pore-accessible inner cavity, decorated by differentiable ligands. Through ligand exchange from the self-assembled nanostructure, a set of chiral Keplerate host molecules has been synthesised. By exploiting the interactions of analyte molecules at the surface pores, the relative configuration of chiral amino alcohol guests (phenylalaninol and 2-amino-1-phenylethanol) in aqueous solvent was establish and their enantiomeric excess was determined by 1 H NMR using shifts of ΔΔδ=0.06 ppm. The use of POMs as chiral shift reagents represents an application of a class that is yet to be well established and opens avenues into aqueous host-guest chemistry with self-assembled recognition agents.

Discrimination of enantiomers of amides with two stereogenic centers enabled by chiral bisthiourea derivatives using 1H NMR spectroscopy

Enantiomers of a few new amides containing two stereogenic centers have been derived from d- and l-α-amino acids as guests for chiral recognition by 1H NMR spectroscopy. A variety of chiral amides with two or more stereogenic centers often exist in the products of catalytic asymmetric synthesis, natural products or their total synthetic products, and chiral drugs. It would be a challenging and meaningful work to explore their chiral recognition. For this purpose, a class of novel chiral bisthiourea derivatives 1-9 has been synthesized from (1S,2S)-(+)-1,2-diaminocyclohexane, d-α-amino acids, and isothiocyanates as chiral solvating agents (CSAs). CSAs 1-9 proved to afford better chiral discriminating results towards most amides with two stereogenic centers, which have been rarely studied as chiral substrates by 1H NMR spectroscopy. In particular, CSAs 7, 8 and 9, featuring 3,5-bis(trifluoromethyl)benzene residues, exhibit outstanding chiral discriminating capabilities towards all amides, providing well-separated 1H NMR signals and sufficiently large nonequivalent chemical shifts. To test their practical application in the determination of enantiomeric excess, 1H NMR spectra of chiral amides (G16) with different optical purities were measured in the presence of CSAs 7 and 8, respectively. Their ee values (up to 90%) were accurately calculated by the integration of the NH proton of the CONHPh group of G16. To better understand the chiral discriminating behavior, Job plots of (±)-G16 with CSA 7 and (±)-G17 with CSA 8 and the association constants (Ka) of (S,R)-G16 and (R,S)-G16 with CSA 7 were evaluated, respectively. In order to further reveal any underlying intermolecular hydrogen bonding interactions, theoretical calculations of the enantiomers of (S,R)-G16 and (R,S)-G16 with CSA 7 were performed by means of the hybrid density functional theory (B3LYP) with the standard basis sets of 3-21G of the Gaussian 03 program, respectively.

Facile NMR approach for profiling curcuminoids present in turmeric

Curcumin, together with demethoxycurcumin and bisdemethoxycurcumin as a whole called curcuminoids, is an active phytochemical constituent present in the turmeric. When it comes to their analysis, most will rely on UV-Visible spectroscopy, HPLC and LC-MS methods. Looking to improve productivity, time and simplicity, we are proposing a ¹H NMR based approach for curcuminoids analysis and its applications to different geographical regions. In the present work, sample preparation protocol is reported for the simultaneous determination of curcuminoids using ¹H NMR. For the quantification of curcuminoids, 6-7 ppm vinylic proton region in the ¹H NMR spectrum was used, where acetone was observed as the suitable solvent in terms of curcuminoids solubility and proper resolution of peak. The result shows that curcumin (46.8-59.50%) was major among all varieties, followed by DMC (22.15-27.70%) and BDMC (17.52-30.29%) except in Andhrapradesh variety, where BDMC (30.29%) was more than DMC (22.89%). These studies were further supported by HPLC analysis.

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.

Insight into old and new pure shift nuclear magnetic resonance methods for enantiodiscrimination

Enantiodiscrimination and their quantification using nuclear magnetic resonance (NMR) spectroscopy has always been a subject of great interest. Proton is the nucleus of choice for enantiodiscrimination due to its high sensitivity and ubiquitous presence in nature. Despite its advantages, enantiodiscrimination suffers from extensive signal splitting by the proton-proton scalar couplings, which give complex multiplets that spread over a frequency range of some tens of hertz. These multiplets often overlap, further complicating interpretation of the spectra and quantifications. In the present review, we discuss some of the recent developments in the pure shift ¹ H NMR based methods for enantiomer resolution and enantiodiscrimination. We also compare various pure shift methods used for enantiodiscrimination and measurement of enantiomeric excess, considering the fact that conventional ¹ H NMR fails to provide any detailed insight.

The robust, readily available cobalt(iii) trication [Co(NH2CHPhCHPhNH2)3]3+ is a progenitor of broadly applicable chirality and prochirality sensing agents

When NMR spectra of chiral racemic organic molecules containing a Lewis basic functional group are recorded in the presence of air and water stable salts of the cobalt(iii) trication [Co((S,S)-NH2CHPhCHPhNH2)3]3+ (23+), separate signals are usually observed for the enantiomers (28 diverse examples, >12 functional groups). Several chiral molecules can be simultaneously analyzed, and enantiotopic groups in prochiral molecules differentiated (16 examples). Particularly effective are the mixed bis(halide)/tetraarylborate salts Λ-23+ 2X-BArf- (X = Cl, I; BArf = B(3,5-C6H3(CF3)2)4), which are applied in CD2Cl2 or CDCl3 at 1-100 mol% (avg 34 and 14 mol%). Job plots establish 1 : 1 binding for Λ-23+ 2Cl-BArf- and 1-phenylethyl acetate (4) or 1-phenylethanol (10), and ca. 1 : 2 binding with DMSO (CD2Cl2). Selected binding constants are determined, which range from 7.60-2.73 M-1 for the enantiomers of 10 to 28.1-22.6 M-1 for the enantiomers of 4. The NH moieties of the C2 faces of the trication are believed to hydrogen bond to the Lewis basic functional groups, as seen in the crystal structure of a hexakis(DMSO) solvate of Λ-23+ 3I-. These salts rank with the most broadly applicable chirality sensing agents discovered to date.

Differentiation of enantiomeric anions by NMR spectroscopy with chiral bisurea receptors

Enantiomeric ¹H NMR signals of chiral anions are separated by forming 1 : 1 and 1 : 2 host–guest complexes with chiral bisurea.

Catalytic asymmetric synthesis of CF3-substituted tertiary propargylic alcohols via direct aldol reaction of α-N3 amide

Organofluorine compounds are found in several important classes of chemicals, such as pharmaceuticals, agrochemicals, and functional materials. Chemists have been immensely interested in the development of methodologies for expeditious access to fluorine containing building blocks. In this study, we report a new method for the catalytic asymmetric synthesis of CF₃-substituted tertiary propargylic alcohols with two contiguous stereogenic centers via the direct aldol reaction of an α-N₃ amide to trifluoromethyl ketones. The key to the success of this method is the identification of a catalyst comprising Cu(ii)/chiral hydroxamic acid to promote the desired aldol reaction, constructing a tetrasubstituted carbon in a highly stereoselective fashion. Despite substantial prior advances in asymmetric catalysis, this class of catalysts has not been utilized for the formation of carbon-carbon bond-forming reactions. Our mechanistic study sheds light on the unique profile of this catalytic system, where the Cu(ii) complex plays a bifunctional role of serving as a Lewis acid and a Brønsted base. Furthermore, the densely functionalized aldol adducts undergo chemoselective transformations, affording a series of fluorine containing chiral building blocks with widespread application.

Recent Advances in Multinuclear NMR Spectroscopy for Chiral Recognition of Organic Compounds

Nuclear magnetic resonance (NMR) is a powerful tool for the elucidation of chemical structure and chiral recognition. In the last decade, the number of probes, media, and experiments to analyze chiral environments has rapidly increased. The evaluation of chiral molecules and systems has become a routine task in almost all NMR laboratories, allowing for the determination of molecular connectivities and the construction of spatial relationships. Among the features that improve the chiral recognition abilities by NMR is the application of different nuclei. The simplicity of the multinuclear NMR spectra relative to ¹H, the minimal influence of the experimental conditions, and the larger shift dispersion make these nuclei especially suitable for NMR analysis. Herein, the recent advances in multinuclear (¹⁹F, ³¹P, ¹³C, and ⁷⁷Se) NMR spectroscopy for chiral recognition of organic compounds are presented. The review describes new chiral derivatizing agents and chiral solvating agents used for stereodiscrimination and the assignment of the absolute configuration of small organic compounds.

Pure shift 1 H NMR: what is next?

Currently, pure shift nuclear magnetic resonance (NMR) is an area of high interest. The aim of this contribution is to describe briefly how this technique has evolved, where it is now and what could be the next challenges in the amazing adventure of the development and application of pure shift NMR experiments. Copyright © 2016 John Wiley & Sons, Ltd.

Discrimination of Enantiomers of Dipeptide Derivatives with Two Chiral Centers by Tetraaza Macrocyclic Chiral Solvating Agents Using 1H NMR Spectroscopy

1H NMR spectroscopy is often used to discriminate enantiomers of chiral analytes and determine their enantiomeric excess (ee) by various chiral auxiliaries. In reported research, these studies were mainly focused on chiral discriminantion of chiral analytes with only one chiral center. However, many chiral compounds possessing two or more chiral centers are often found in natural products, chiral drugs, products of asymmetric synthesis and biological systems. Therefore, it is necessary to investigate their chiral discrimination by effective chiral auxiliaries using 1H NMR spectroscopy. In this paper, a new class of tetraaza macrocyclic chiral solvating agents (TAMCSAs) with two amide (CONH), two amino (NH) and two phenolic hydroxyl (PhOH) groups has been designed and synthsized for chiral discrimination towards dipeptide derivatives with two chiral centers. These dipeptide derivatives are important chiral species because some of them are used as clinical drugs and special dietary supplements for treatment of human diseases, such as L-alanyl-L-glutamine and aspartame. The results show that these TAMCSAs have excellent chiral discriminating properties and offer multiple detection possibilities pertaining to 1H NMR signals of diagnostic split protons. The nonequivalent chemical shifts (up to 0.486 ppm) of various types of protons of these dipeptide derivatives were evaluated with the assistance of well-resolved 1H NMR signals in most cases. In addition, enantiomeric excesses (ee) of the dipeptide derivatives with different optical compositions have been calculated based on integration of well-separeted proton signals. At the same time, the possible chiral discriminating behaviors have been discussed by means of Job plots, ESI mass spectra and a proposed theoretical model of (±)-G1 with TAMCSA 1c. Additionally, the association constants of enantiomers of (±)-G5 with TAMCSA 1a were calculated by employing the nonlinear curve-fitting method.

p-TsOH promotes 2′-amino-[1,1′-binaphthalene]-2-ol as a chiral solvating agent

Ternary complex of p-TsOH, NOBIN with hydroxy acids and their derivatives permit NMR spectroscopic chiral discrimination.

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.