NMR Spectroscopic Determination of Enantiomeric Excess Using Small Prochiral Molecules

Importance of molecular symmetry for enantiomeric excess recognition by NMR

Recently prochiral solvating agents (pro-CSA) came under the spotlight for the detection of enantiopurity by NMR. Chemical shift non-equivalency in achiral hosts introduced by the presence of chiral guests yields observable resonance signal splitting (Δδ) correlating to the enantiomeric excess (e.e.). In this work, symmetry is our lens to explain porphyrin-based supramolecular receptor activity in a chiral environment. Based on extensive NMR analyses of the atropisomeric receptors, the host symmetry is shown to be affected by porphyrin nonplanarity and further desymmetrized in the presence of a chiral guest. As such, the exposed porphyrin inner core (N-H), with its strong hydrogen bond abilities, for the first time, has been exploited in enantiomeric composition analysis. Our approach in e.e. detection by N-H signals appearing in a previously underutilized region of the spectrum (below 0 ppm) shows chemical shift splitting (Δδ) three times more sensitive to enantiomeric compositions than previously reported systems.

Analyte Interactions with Oxoporphyrinogen Derivatives: Computational Aspects

Abstract:

The binding of anions by highly-coloured chromophore compounds is of interest from the point-of-view of the development of optical sensors for analyte species. In this review, we have summarised our work on the interactions between oxoporphyrinogen type host compounds and different analyte species using computational methods. The origin of our interest in sensing using oxoporphyrinogens stems from an initial finding involving anion-host interactions involving a conjugated oxoporphyrinogen molecule. This review starts from that point, introducing some additional exemplary anion binding data, which is then elaborated to include descriptions of our synthesis work towards multitopic and ion pair interactions. In all the projects, we have consulted computational data on host structure and host-guest complexes in order to obtain information about the interactions occurring during complexation. Density functional theory and molecular dynamics simulations have been extensively used for these purposes. Oxoporphyrinogens are highly colored synthetically flexible compounds whose interactions with anions, ion pairs, and other species have been modelled using computational methods.

Nuclear Magnetic Resonance Spectroscopy for Quantitative Analysis: A Review for Its Application in the Chemical, Pharmaceutical and Medicinal Domains

Nuclear magnetic resonance (NMR) is a rapid and accurate analytical tool for qualification and quantification. The capacity of NMR of being quantitative can also justify the calibration of other analytical methods. In pharmaceutical domain, quantitative NMR (qNMR) can be applied in the identification and quantification of drug simultaneously. The early drug development stage requires a minimum sample for analysis. Thus, priority should be given to utilize this technique to attain results with least investment, rapid analysis time and minimum sample consumption. This technique is a significant phenomenon to identify impurities, drug substance, residual solvents of in-process control (IPC) samples and characterizing the formulations. From an analyst's perspective, qNMR proved to be a routine practice in pharmaceutical industry to qualify any drug product. The absolute and relative methods offer great help in quantifying the component of interest in the process control samples and finished products. This review highlights the evolution of NMR application in the pharmaceutical industry, where determining the purity of drug substance, drug product and establishing the identity of impurities and its level are the challenging aspects. NMR in medicinal field emerging as a numero uno for Covid-19 severity detection and its dire consequences, accelerated vaccine development and the mapping of SAR-COV-2 RNA and proteins via chemical shift assignments.

Estimation of Enantiomeric Excess Based on Rapid Host–Guest Exchange

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.

Enantiomeric Excess Dependent Splitting of NMR Signal through Dynamic Chiral Inversion and Coligand Exchange in a Coordination Complex

Nuclear magnetic resonance (NMR) spectroscopy cannot be used to discriminate enantiomers, and NMR resonances of enantiomeric mixtures are generally not affected by enantiomeric excess (ee). Here, we report that a coordination complex (L·2Zn·3C), where L is a salen-like prochiral ligand and C is an exchangeable acetate coligand, exhibits symmetrical splitting of one of the ¹H NMR resonances of L with the degree of splitting linearly proportional to ee of the chiral guest coligand C, 2-phenoxypropionic acid. Despite the well-defined chirality in the crystal structure of L·2Zn·3C, concurrent fast chiral inversion and coligand exchange in solution renders L·2Zn·3C the primary example of prochiral solvating agent (pro-CSA) based on a coordination complex. Notably, the NMR resonances remain split even in dilute solution due to the lack of chiral guest dissociation in the coligand exchange system. This work provides new insights into chiral transfer events in metal-ligand complexes.

Optically Pure Aziridin-2-yl Methanols as Readily Available 1H NMR Sensors for Enantiodiscrimination of α-Racemic Carboxylic Acids Containing Tertiary or Quaternary Stereogenic Centers

Enantiopure aziridin-2-yl methanols 3–7 are used as highly effective sensors for enantiodiscrimination of α-racemic carboxylic acids containing tertiary or quaternary stereogenic centers. A linear correlation between theoretical and observed % ee values for CSA-3 and enantiomerically enriched samples of mandelic acid has been observed, indicating the possible application of these compounds in the ee determination. The free NH and OH groups in 3–7 ensure good recognition.

A new horizon for vibrational circular dichroism spectroscopy: a challenge for supramolecular chirality

The development of the solid state and time-step VCD methods opened a new horizon to reveal the mechanism of chirality amplification from microscopic to supramolecular scales.

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.