Three-Component Chiral Derivatizing Protocols for NMR Spectroscopic Enantiodiscrimination of Hydroxy Acids and Primary Amines

BINOL as a Chiral Solvating Agent for Sulfiniminoboronic Acids

1H NMR spectroscopic studies using BINOL as a chiral solvating agent (CSA) for a scalemic sulfiniminoboronic acid (SIBA) have revealed concentration- and enantiopurity-dependent variations in the chemical shifts of diagnostic imine protons used to determine enantiopurity levels. 11B/15N NMR spectroscopic studies and X-ray structural investigations revealed that unlike other iminoboronate species, BINOL-SIBA assemblies do not contain N-B coordination bonds, with 1H NMR NOESY experiments indicating that intermolecular H-bonding networks between BINOL and the SIBA analyte are responsible for these variations. These effects can lead to diastereomeric signal overlap at certain er values that could potentially lead to enantiopurity/configuration misassignments. Consequently, it is recommended that hydrogen-bonding-CSA-based 1H NMR protocols should be repeated using both CSA enantiomers to ensure that any concentration- and/or er-dependent variations in diagnostic chemical shifts are accounted for when determining the enantiopurity of a scalemic analyte.

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.

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.

A chiral sensor for recognition of varied amines based on 19F NMR signals of newly designed rhodium complexes

A novel chiral octahedral rhodium complex containing fluorine has been developed to be an excellent chiral sensor for a variety of amines including diamines, monoamines, amino alcohols and amino acids, showing well distinguishable 19F NMR signals and an accurate measurement of enantiomeric determination.

Synthesis of Tripeptide Derivatives with Three Stereogenic Centers and Chiral Recognition Probed by Tetraaza Macrocyclic Chiral Solvating Agents Derived from d-Phenylalanine and (1 S,2 S)-(+)-1,2-Diaminocyclohexane via 1H NMR Spectroscopy

Enantiomers of a series of tripeptide derivatives with three stereogenic centers (±)-G1-G9 have been prepared from d- and l-α-amino acids as guests for chiral recognition by 1H NMR spectroscopy. In the meantime, a family of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a-1d has been synthesized from d-phenylalanine and (1 S,2 S)-(+)-1,2-diaminocyclohexane. Discrimination of enantiomers of (±)-G1-G9 was carried out in the presence of TAMCSAs 1a-1d by 1H NMR spectroscopy. The results indicate that enantiomers of (±)-G1-G9 can be effectively discriminated in the presence of TAMCSAs 1a-1d by 1H NMR signals of multiple protons exhibiting nonequivalent chemical shifts (ΔΔδ) up to 0.616 ppm. Furthermore, enantiomers of (±)-G1-G9 were easily assigned by comparing 1H NMR signals of the split corresponding protons with those attributed to a single enantiomer. Different optical purities (ee up to 90%) of G1 were clearly observed and calculated in the presence of TAMCSAs 1a-1d, respectively. Intermolecular hydrogen bonding interactions were demonstrated through theoretical calculations of enantiomers of (±)-G1 with TAMCSA 1a by means of the hybrid functional theory with the standard basis sets of 3-21G of the Gaussian 03 program.

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.

Chiral discrimination of α-hydroxy acids and N-Ts-α-amino acids induced by tetraaza macrocyclic chiral solvating agents by using 1H NMR spectroscopy

In the field of chiral recognition, reported chiral discrimination by 1H NMR spectroscopy has mainly focused on various chiral analytes with a single chiral center, regarded as standard chiral substrates to evaluate the chiral discriminating abilities of a chiral auxiliary. Among them, chiral α-hydroxy acids, α-amino acids and their derivatives are chiral organic molecules involved in a wide variety of biological processes, and also play an important role in the area of preparation of pharmaceuticals, as they are part of the synthetic process in the production of chiral drug intermediates and protein-based drugs. In this paper, several α-hydroxy acids and N-Ts-α-amino acids were used to evaluate the chiral discriminating abilities of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a-1d by 1H NMR spectroscopy. The results indicate that α-hydroxy acids and N-Ts-α-amino acids were successfully discriminated in the presence of TAMCSAs 1a-1d by 1H NMR spectroscopy in most cases. The enantiomers of the α-hydroxy acids and N-Ts-α-amino acids were assigned based on the change of integration of the 1H NMR signals of the corresponding protons. The enantiomeric excesses (ee) of N-Ts-α-amino acids 11 with different optical compositions were calculated based on the integration of the 1H NMR signals of the CH3 protons (Ts group) of the enantiomers of (R)- and (S)-11 in the presence of TAMCSA 1b. At the same time, the possible chiral discriminating behaviors have been discussed by means of the Job plots of (±)-2 with TAMCSAs 1b and proposed theoretical models of the enantiomers of 2 and 6 with TAMCSA 1a, respectively.

Highly efficient one-pot assembly of peptides by double chemoselective coupling

An efficient and faster methodology for the assembly of oligopeptides by a three-step one-pot protocol with high yields and virtually no epimerization.

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.

Chiral Recognition Studies of α-(Nonafluoro-tert-butoxy)carboxylic Acids by NMR Spectroscopy

Three chiral α-(nonafluoro-tert-butoxy)carboxylic acids (R)-1, (RS)-2, (R)-3 were synthesized to examine their application as chiral solvating agents with amines. As a model compound, first (S)- and/or (RS)-α-phenylethylamine was used, and their diastereomeric salts were investigated by (1)H and (19)F NMR and ECD spectroscopy. The NMR spectroscopic studies were carried out at room temperature using the slightly polar CDCl3 and apolar C6D6 as solvents in 5 mM and 54 mM concentrations. The difference of the chemical shifts (Δδ) in the diastereomeric complexes is comparable with other, well-known chiral derivatizing and solvating agents (e.g., Mosher's acid, Pirkle's alcohol). Diastereomeric salts of racemic acids (RS)-1 and (RS)-2 with biologically active amines (1R,2S)-ephedrine and (S)-dapoxetine were also investigated by (19)F NMR spectroscopy.

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.

A simple and rapid approach for testing enantiopurity of hydroxy acids and their derivatives using 1H NMR spectroscopy

A rapid and the simple chiral derivatizing protocol involving the coupling of 2-formylphenylboronic acid and an optically pure [1,1-binaphthalene]-2,2-diamine is introduced for the accurate determination of the enantiopurity of hydroxy acids and their derivatives.

Sugar-boronate ester scaffold tethered pyridyl-imine palladium(ii) complexes: synthesis and their in vitro anticancer evaluation

The anticancer activity of sugar-boronate ester containing palladium(ii) complexes is reported.

Chiral discrimination of β-telluride carboxylic acids by NMR spectroscopy

A simple and effective procedure for the chiral discrimination of β-telluride carboxylic acids by ¹H and ¹²⁵Te NMR spectroscopy is presented.

Enantio-differentiation of molecules with diverse functionalities using a single probe

Chiral discrimination of molecules with diverse functionalities using a single CSA.

A hydrogen bond rationale for the enantioselective β-alkenylboration of enones catalyzed by O-monoacyltartaric acids

DFT calculations suggest that O-monoacyl L-tartaric acids catalyze the asymmetric conjugate alkenylboration of enones through transition structures that are stabilized by hydrogen-bonding interactions. Formation of a five-membered acyloxyborane is proposed. The hydrogen of the free carboxy group derived from the catalyst interacts with the carbonyl group of the cyclic acyloxyborane, stabilizing the transition structure and reducing the flexibility of the system. Additional stabilizing nonclassical CH···O hydrogen-bond interactions seem to determine the observed enantioselectivity.

A versatile ternary ionic complex for chiral discrimination of molecules with diverse functionalities using 1H NMR

A novel chiral solvating agent for enantiodiscrimination by NMR spectroscopy is introduced.

Study of boron-nitrogen dative bonds using azetidine inversion dynamics

A method for probing the strength of B-N dative bonds is reported. The activation parameters for nitrogen inversion in a series of azetidines tethered to boronate esters have been quantified by VT-NMR and the measured barriers correlated with data obtained by (11)B NMR, X-ray crystallography and MP2 calculations.

Discrimination of α-amino acids using green tea flavonoid (-)-epigallocatechin gallate as a chiral solvating agent

We report a special, hitherto-unexplored property of (-)-epigallocatechin gallate (EGCG) as a chiral solvating agent for enantiodiscrimination of α-amino acids in the polar solvent DMSO. This phenomenon has been investigated by (1)H NMR spectroscopy. The mechanism of the interaction property of EGCG with α-amino acids has been understood as arising out of hydrogen-bonded noncovalent interactions, where the -OH groups of two phenyl rings of EGCG play dominant roles. The conversion of the enantiomeric mixture into diastereomers yielded well-resolved peaks for D and L amino acids permitting the precise measurement of enantiomeric composition. Often one encounters complex situations when the spectra are severely overlapped or partially resolved hampering the testing of enantiopurity and the precise measurement of enantiomeric excess (ee). Though higher concentration of EGCG yielded better discrimination, the use of lower concentration being economical, we have exploited an appropriate 2D NMR experiment in overcoming such problems. Thus, in the present study we have successfully demonstrated the utility of the bioflavonoid (-)-EGCG, a natural product as a chiral solvating agent for the discrimination of large number of α-amino acids in a polar solvent DMSO. Another significant advantage of this new chiral sensing agent is that it is a natural product and does not require tedious multistep synthesis unlike many other chiral auxiliaries.

Simple and efficient methods for discrimination of chiral diacids and chiral alpha-methyl amines

The three-component chiral derivatization protocols have been developed for (1)H, (13)C and (19)F NMR spectroscopic discrimination of chiral diacids by their coordination and self-assembly with optically active (R)-α-methylbenzylamine and 2-formylphenylboronic acid or 3-fluoro-2-formylmethylboronic acid. These protocols yield a mixture of diastereomeric imino-boronate esters which are identified by the well-resolved diastereotopic peaks with significant chemical shift differences ranging up to 0.6 and 2.1 ppm in their corresponding (1)H and (19)F NMR spectra, without any racemization or kinetic resolution, thereby enabling the determination of enantiopurity. A protocol has also been developed for discrimination of chiral alpha-methyl amines, using optically pure trans-1,2-cyclohexanedicarboxylic acid in combination with 2-formylphenylboronic acid or 3-fluoro-2-fluoromethylboronic acid. The proposed strategies have been demonstrated on large number of chiral diacids and chiral alpha-methyl amines.