Determining the Absolute Configuration of (+)-Mefloquine HCl, the Side-Effect-Reducing Enantiomer of the Antimalaria Drug Lariam

Chiral separation and spectroscopic characterization of mefloquine analogues

Mefloquine, a widely used antimalarial agent, has spurred ongoing research into the development of derivatives with enhanced efficacy and reduced side effects. In this investigation, we synthesized two compounds containing N-allyl or N-tert-butylacetamid groups. A chiral liquid chromatography with polysaccharide chiral stationary phase was utilized to separate the enantiomers of both derivatives. We employed spectroscopic chiroptical and non-polarizable methods such as electronic and vibrational circular dichroism, infrared absorption and ultraviolet spectroscopies. Combined with density functional theory calculations, the stable conformers were found in solution and their spectra were subsequently simulated. We elucidated the three-dimensional structure of the enantiomerically pure compounds and assigned the absolute configuration of all prepared derivatives using both experimental and simulated spectra.

Solid-State NMR Characterization of Mefloquine Resinate Complexes Designed for Taste-Masking Pediatric Formulations

Mefloquine (MQ) is an antimalarial medication prescribed to treat or malaria prevention.. When taken by children, vomiting usually occurs, and new doses of medication frequently need to be taken. So, developing pediatric medicines using taste-masked antimalarial drug complexes is mandatory for the success of mefloquine administration. The hypothesis that binding mefloquine to an ion-exchange resin (R) could circumvent the drug's bitter taste problem was proposed, and solid-state 13C cross-polarization magic angle spinning (CPMAS) NMR was able to follow MQ-R mixtures through chemical shift and relaxation measurements. The nature of MQ-R complex formation could then be determined. Impedimetric electronic tongue equipment also verified the resinate taste-masking efficiency in vitro. Variations in chemical shifts and structure dynamics measured by proton relaxation properties (e.g., T1ρH) were used as probes to follow the extension of mixing and specific interactions that would be present in MQ-R. A significant decrease in T1ρH values was observed for MQ carbons in MQ-R complexes, compared to the ones in MQ (from 100-200 ms in MQ to 20-50 ms in an MQ-R complex). The results evidenced that the cationic resin interacts strongly with mefloquine molecules in the formulation of a 1:1 ratio complex. Thus, 13C CPMAS NMR allowed the confirmation of the presence of a binding between mefloquine and polacrilin in the MQ-R formulation studied.

Synthesis and Antifungal Activity of Stereoisomers of Mefloquine Analogs

Cryptococcal neoformans and Candida albicans are among the most prevalent causes of life-threatening fungal infections globally. The high mortality associated with these infections despite current antifungal therapy highlights the need for new drugs. In our previous work, we demonstrated that an analogue of the clinically used antimalarial mefloquine, (8-chloro-2-(4-chlorophenyl)quinolin-4-yl)(piperidin-2-yl)methanol (4377), has both antifungal activity and the ability to penetrate the central nervous system. Herein we describe the synthesis and antifungal assay of all four stereoisomers of 4377. All four stereoisomers retain potent antifungal activity with the erythro enantiomers having MIC values of 1 and 4 μg/mL against C. neoformans and C. albicans, respectively, and threo enantiomers, MIC values of 2 and 8 μg/mL, respectively. These results indicate that the stereochemistry of the piperidine methanol group is not critical for the antifungal properties of 4377 and gives guidance to future medicinal chemistry optimization efforts.

Connection of Isolated Stereoclusters by Combining 13C-RCSA, RDC, and J-Based Configurational Analyses and Structural Revision of a Tetraprenyltoluquinol Chromane Meroterpenoid from Sargassum muticum

The seaweed Sargassum muticum, collected on the southern coast of Galicia, yielded a tetraprenyltoluquinol chromane meroditerpene compound known as 1b, whose structure is revised. The relative configuration of 1b was determined by J-based configurational methodology combined with an iJ/DP4 statistical analysis and further confirmed by measuring two anisotropic properties: carbon residual chemical shift anisotropies (13C-RCSAs) and one-bond 1H-13C residual dipolar couplings (1DCH-RDCs). The absolute configuration of 1b was deduced by ECD/OR/TD-DFT methods and established as 3R,7S,11R.

Chiral Vicinal Diamines Derived from Mefloquine

Novel 1,2-diamines based on the mefloquine scaffold prepared in enantiomerically pure forms resemble 9-amino-Cinchona alkaloids. Most effectively, 11-aminomefloquine with an erythro configuration was obtained by conversion of 11-alcohol into azide and hydrogenation. Alkylation of a secondary amine unit was needed to arrive at diastereomeric threo-11-aminomefloquine and to introduce diversity. Most of the substitution reactions of the hydroxyl group to azido group proceeded with net retention of the configuration and involved actual aziridine or plausible aziridinium ion intermediates. Enantiomerically pure products were obtained by the resolution of either the initial mefloquine or one of the final products. The evaluation of the efficacy of the obtained vicinal diamines in enantioselective transformations proved that erythro-11-aminomefloquine is an effective catalyst in the asymmetric Michael addition of nitromethane to cyclohexanone (up to 96.5:3.5 er) surpassing epi-aminoquinine in terms of selectivity.

Elucidating natural product structures using a robust measurement of carbon residual chemical shift anisotropy combined with DFT

Determination of configurations and conformations is an important step in the structural characterization of small molecules. Apart from utilizing isotropic J-couplings and nuclear overhauser effect (NOEs) measured in isotropic solution, anisotropic Nuclear Magnetic resonance (NMR) data such as residual dipolar couplings and residual chemical shift anisotropies (RCSAs) were also used to elucidate complex small molecule structures. Measuring RCSA has always been historically difficult due to the isotropic shift effect accompanied by molecular alignment and therefore only occasionally applied in a few examples. Here, we present a robust measurement of carbon RCSAs using a smaller gel-stretching device to determine the structures of a few small molecules. A systematic study on how different density functional theory computed anisotropies of the chemical shift anisotropy tensors impact RCSA data interpretation has also been discussed. We also discuss the effect of utilizing various carbons as reference nuclei for RCSA data extraction as well as the orientation behavior of estrone in orthogonal alignment media.

Relative configuration of micrograms of natural compounds using proton residual chemical shift anisotropy

3D molecular structure determination is a challenge for organic compounds or natural products available in minute amounts. Proton/proton and proton/carbon correlations yield the constitution. J couplings and NOEs oftentimes supported by one-bond ¹H,¹³C residual dipolar couplings (RDCs) or by ¹³C residual chemical shift anisotropies (RCSAs) provide the relative configuration. However, these RDCs or carbon RCSAs rely on 1% natural abundance of ¹³C preventing their use for compounds available only in quantities of a few 10’s of µgs. By contrast, ¹H RCSAs provide similar information on spatial orientation of structural moieties within a molecule, while using the abundant ¹H spin. Herein, ¹H RCSAs are accurately measured using constrained aligning gels or liquid crystals and applied to the 3D structural determination of molecules with varying complexities. Even more, deuterated alignment media allow the elucidation of the relative configuration of around 35 µg of a briarane compound isolated from Briareum asbestinum.

Determination of 3D molecular structures remains challenging for natural products or organic compounds available in minute amounts. Here, the authors determine the structure of complex molecules, including few micrograms of briarane B-3 isolated from Briareum asbestinums, through measurement of ¹H residual chemical shift anisotropy.

Design, Synthesis, and Antifungal Evaluation of Novel Quinoline Derivatives Inspired from Natural Quinine Alkaloids

Inspired by quinine and its analogues, we designed, synthesized, and evaluated two series of quinoline small molecular compounds (a and 2a) and six series of quinoline derivatives (3a-f) for their antifungal activities. The results showed that compounds 3e and 3f series exhibited significant fungicidal activities. Significantly, compounds 3f-4 (EC₅₀ = 0.41 μg/mL) and 3f-28 (EC₅₀ = 0.55 μg/mL) displayed the superior in vitro fungicidal activity and the potent in vivo curative effect against Sclerotinia sclerotiorum. Preliminary mechanism studies showed that compounds 3f-4 and 3f-28 could cause changes in the cell membrane permeability, accumulation of reactive oxygen species, loss of mitochondrial membrane potential, and effective inhibition of germination and formation of S. sclerotiorum sclerotia. These results indicate that compounds 3f-4 and 3f-28 are novel potential fungicidal candidates against S. sclerotiorum derived from natural products.

Accelerating the Throughput of Affinity Mass Spectrometry-Based Ligand Screening toward a G Protein-Coupled Receptor

Affinity mass spectrometry (MS) enables rapid screening of compound mixtures for ligands bound to a specific protein target, yet its current throughput is limited to individually assay pools of 400-2000 compounds. Typical affinity MS screens implemented in pharmaceutical industry laboratories identify putative ligands based on qualitative analysis of compound binding to the target whereas no quantitative information is acquired to discriminate high- and low-affinity ligands in the screening phase. Furthermore, these screens require purification of a stabilized form of the protein target, which poses a great challenge for membrane receptor targets. Here, we describe a new, potentially general affinity MS strategy that allows screening of 20,000 compounds in one pool for highly efficient ligand discovery toward a G protein-coupled receptor (GPCR) target. Quantitative measurement of compound binding to the receptor enables high-affinity ligand selection using both the purified receptor and receptor-embedded cell membranes. This high-throughput, label-free and quantitative affinity MS screen resulted in discovery of three new antagonists of the A2A adenosine receptor.

Attrition-Enhanced Deracemization of the Antimalaria Drug Mefloquine

Mefloquine is an important drug for prevention and treatment of malaria. It is commercially available as a racemic mixture, wherein only one enantiomer is active against malaria, while the other one causes severe psychotropic effects. By converting the drug into a compound that crystallizes as a racemizable racemic conglomerate, the deracemization of mefloquine into the desired enantiomer was achieved.

Current scenario of artemisinin and its analogues for antimalarial activity

Human malaria, one of the most striking, reemerging infectious diseases, is caused by several types of Plasmodium parasites. Whilst advances have been made in lowering the numbers of cases and deaths, it is clear that a strategy based solely on disease control year on year, without reducing transmission and ultimately eradicating the parasite, is unsustainable. Natural products have served as a template for the design and development of antimalarial drugs currently in the clinic or in the development phase. Artemisinin combine potent, rapid antimalarial activity with a wide therapeutic index and an absence of clinically important resistance. The alkylating ability of artemisinin and its semi-synthetic analogues toward heme related to their antimalarial efficacy are underlined. Although impressive results have already been achieved in malaria research, more systematization and concentration of efforts are required if real breakthroughs are to be made. This review will concisely cover the clinical, preclinical antimalarial and current updates in artemisinin based antimalarial drugs. Diverse classes of semi-synthetic analogs of artemisinin reported in the last decade have also been extensively studied. The experience gained in this respect is discussed.

Application of molecular docking and PSO-SVR intelligent approaches in antimalarial activity prediction of enantiomeric cycloguanil analogues

A series of antifolate compounds, i.e. 1-(4-chlorophenyl)-6,6-dimethyl-1,3,5-triazine-2,4-diamine, or cycloguanil analogues, have shown effective inhibiting properties against Plasmodium falciparum dihydrofolate reductase (PfDHFR). In this work, the stereoselectivity of PfDHFR to the R and S enantiomer of cycloguanil analogues was obtained from molecular docking calculations and integrated into QSAR study to obtain a more accurate prediction model. Results indicate that PfDHFR can bind to cycloguanil analogues in the R and S enantiomers. Cycloguanil analogues with alkyl chain substituent prefer the R enantiomer over S because they do not experience steric hindrance with the Phe58 side chain, while cycloguanil analogues with phenol chain substituent prefer the S enantiomer over R because they do not experience steric hindrance with Leu46 and Met55 side chains. Particle swarm optimization and support vector regression were used to select relevant descriptors and generate the effective prediction model, with a high statistical significance level (r²_training = 0.941; r²_test = 0.884).

One-pot synthesis of polyfunctionalized quinolines via a copper-catalyzed tandem cyclization

An efficient one-pot approach for the synthesis of polyfunctionalized quinolines was developed via a sequence of copper-catalyzed coupling reaction/propargyl-allenyl isomerization/aza-electrocyclization. Easily available starting materials, mild conditions, and a wide substrate scope make this approach potentially useful.

Di(ethylene glycol) methyl ether methacrylate (DEGMEMA)-derived gels align small organic molecules in methanol

Residual dipolar couplings (RDCs) constitute an important NMR parameter for structural elucidation in all areas of chemistry. In this study, di(ethylene glycol) methyl ether methacrylate (DEGMEMA)-based gels are introduced as alignment media for the measurement of RDCs of small organic molecules in polar solvents such as methanol. The low viscosity of methanol permits the execution of J-scaled BIRD HSQC experiments that yield very sharp lines in anisotropic conditions. The gels have excellent mechanical properties, and their compression and expansion in the swollen state can be reversed and performed multiple times. This process enables the easy loading and release of analytes. The excellent performance of these new aligning gels is demonstrated by analyzing the structure of the alkaloid retrorsine. Copyright © 2016 John Wiley & Sons, Ltd.

Asymmetric Synthesis of (+)-anti- and (-)-syn-Mefloquine Hydrochloride

The asymmetric (er > 99:1) total synthesis of (+)-anti- and (-)-syn-mefloquine hydrochloride from a common intermediate is described. The Sharpless asymmetric dihydroxylation is the key asymmetric transformation used in the synthesis of this intermediate. It is carried out on an olefin that is accessed in three steps from commercially available materials, making the overall synthetic sequence very concise. The common diol intermediate derived from the Sharpless asymmetric dihydroxylation is converted into either a trans- or cis-epoxide, and these are subsequently converted to (+)-anti- and (-)-syn-mefloquine, respectively. X-ray crystallographic analysis of derivatives of (+)-anti- and (-)-syn-mefloquine is used to lay to rest a 40 year argument regarding the absolute stereochemistry of the mefloquines. A formal asymmetric (er > 99:1) synthesis of (+)-anti-mefloquine hydrochloride is also presented that uses a Sharpless asymmetric epoxidation as a key step.

Elucidation of the Enantiodiscrimination Properties of a Nonracemic Chiral Alignment Medium through Gel-based Capillary Electrochromatography: Separation of the Mefloquine Stereoisomers

Enantiodiscrimination and enantioseparation are two highly important processes in chemistry, often performed by using NMR spectroscopy and chromatography. For a better understanding of the mechanistic details, the same system should be studied by both methods. In addition, isotropic and anisotropic NMR parameters should be obtained, the latter using alignment media so that residual dipolar couplings and chemical-shift anisotropies can be measured. Consequently, a chiral alignment medium was used for the first time in chiral gel-based capillary electrochromatography with the four stereoisomers of the antimalaria drug mefloquine as test compounds. Chromatographic data verify that enantiodiscrimination obtained with this alignment gel is caused by differences in the equilibrium constants related to associate formation. Hence, the chromatographic separation provides physicochemical data that form a basis for the understanding and optimization of alignment processes, and vice versa.

Q.E.COSY: determining sign and size of small deuterium residual quadrupolar couplings using an extended E.COSY principle

Residual quadrupolar couplings contain important structural information comparable with residual dipolar couplings. However, the measurement of sign and size of especially small residual quadrupolar couplings is difficult. Here, we present an extension of the E.COSY principle to spin systems consisting of a Spin 1 coupled to a spin ½ nucleus, which allows the determination of the sign of the quadrupolar coupling of the Spin 1 nucleus relative to the heteronuclear coupling between the spins. The so-called Q.E.COSY approach is demonstrated with its sign-sensitivity using variable angle NMR, stretched gels and liquid crystalline phases applied to various CD and CD3 groups. Especially the sign-sensitive measurement of residual quadrupolar couplings that remain unresolved in conventional deuterium 1D spectra is shown.

The Use of a Combination of RDC and Chiroptical Spectroscopy for Determination of the Absolute Configuration of Fusariumin A from the Fungus Fusarium sp

A new alkylpyrrole derivative, fusariumin A (1), was isolated from the culture broth of the fungus Fusarium sp. The absolute configuration of fuasiumin A has been established as (2'R,3'R) using a combination of RDC (residual dipolar coupling)-based NMR and DFT-supported chiroptical spectroscopy. It is worth to note that in this study without the aid of the RDC analysis, an unambiguous determination of configuration and conformation was not feasible due to the excessive conformational possibilities of this open-chain compound.

Absolute configuration of the antimalarial erythro-mefloquine – vibrational circular dichroism and X-ray diffraction studies of mefloquine and its thiourea derivative

The long-standing discussion of the absolute configuration of erythro-mefloquine is revisited, showcasing the strength of a combination of experimental and calculated vibrational circular dichroism spectroscopy.

Attenuation of Neuropathic Pain by Inhibiting Electrical Synapses in the Anterior Cingulate Cortex

BACKGROUND

Synaptic mechanisms and neuronal oscillations have been proposed to be responsible for neuropathic pain formation. Many studies have also highlighted the important role of electrical synapses in synaptic plasticity and in neuronal oscillations. Thus, electrical synapses may contribute to neuropathic pain generation. However, previous studies have primarily focused on the role of chemical synapses, while ignoring the role of electrical synapses, in neuropathic pain generation.

METHODS

The authors adopted microinjection, RNA interference techniques, and behavioral tests to verify the link between connexin 36 (Cx36) and neuropathic pain. They also studied the selective Cx36 blocker mefloquine in rat chronic constriction injury and spared nerve injury model of neuropathic pain. Electrophysiologic recordings were used to further confirm the behavioral data.

RESULTS

The authors found that Cx36, which constitutes the neuron-neuron electrical synapses, was up-regulated in the anterior cingulate cortex after nerve injury (n = 5). Meanwhile, Cx36-mediated neuronal oscillations in the gamma frequency range (30 to 80 Hz) (n = 7 to 8) and the neuronal synaptic transmission (n = 13 to 19) were also enhanced. Neuropathic pain was relieved by disrupting Cx36 function or expression in the anterior cingulate cortex. They also found that mefloquine, which are clinically used for treating malaria, affected gamma oscillations and synaptic plasticity, leading to a sustained pain relief in chronic constriction injury and spared nerve injury models (n = 7 to 12).

CONCLUSION

The electrical synapses blocker mefloquine could affect gamma oscillations and synaptic plasticity in the anterior cingulate cortex and relieve neuropathic pain. Cx36 may be a new therapeutic target for treating chronic pain.

Trioxolane-Mediated Delivery of Mefloquine Limits Brain Exposure in a Mouse Model of Malaria

Peroxidic antimalarial agents including the sequiterpene artemisinins and the synthetic 1,2,4-trioxolanes function via initial intraparasitic reduction of an endoperoxide bond. By chemically coupling this reduction to release of a tethered drug species it is possible to confer two distinct pharmacological effects in a parasite-selective fashion, both in vitro and in vivo. Here we demonstrate the trioxolane-mediated delivery of the antimalarial agent mefloquine in a mouse malaria model. Selective partitioning of the trioxolane-mefloquine conjugate in parasitized erythrocytes, combined with effective exclusion of the conjugate from brain significantly reduced brain exposure as compared to mice directly administered mefloquine. These studies suggest the potential of trioxolane-mediated drug delivery to mitigate off-target effects of existing drugs, including the adverse neuropsychiatric effects of mefloquine use in therapeutic and chemoprophylactic settings.

A Concise and Highly Enantioselective Total Synthesis of (+)-anti- and (-)-syn-Mefloquine Hydrochloride: Definitive Absolute Stereochemical Assignment of the Mefloquines

A concise asymmetric (>99:1 e.r.) total synthesis of (+)-anti- and (-)-syn-mefloquine hydrochloride from a common intermediate is described. The key asymmetric transformation is a Sharpless dihydroxylation of an olefin that is accessed in three steps from commercially available materials. The Sharpless-derived diol is converted into either a trans or cis epoxide, and these are subsequently converted into (+)-anti- and (-)-syn-mefloquine, respectively. The synthetic (+)-anti- and (-)-syn-mefloquine samples were derivatized with (S)-(+)-mandelic acid tert-butyldimethylsilyl ether, and a crystal structure of each derivative was obtained. These are the first X-ray structures for mefloquine derivatives that were obtained by coupling to a known chiral, nonracemic compound, and provide definitive confirmation of the absolute stereochemistry of (+)-anti- as well as (-)-syn-mefloquine.

Enantiotopic discrimination in the NMR spectrum of prochiral solutes in chiral liquid crystals

Theory and applications to stereo- and bio-chemistry of the discrimination of enantiotopic elements observed in the NMR spectra of prochiral solutes dissolved in chiral liquid crystals are comprehensively discussed.

Elucidating the crystal structure of the antimalarial drug (±)-mefloquine hydrochloride: a tetragonal hydrated species

Albeit widely studied, the structure of the antimalarial drug (±)-mefloquine hydrochloride is still a controversial issue. A combination of X-ray powder diffraction, theoretical calculations and Fourier transform–infrared spectroscopy is used to unambiguously determine the crystal structure of the molecule in its active pharmaceutical ingredient. It is demonstrated that water is incorporated into the structure, leading to a hydrated form and, most importantly, to significant differences in both structure and bonding. Such changes can lead to significant differences in biological activity.

Trapped in misbelief for almost 40 years: selective synthesis of the four stereoisomers of mefloquine

Here we report the synthesis of all four stereoisomers of mefloquine. Mefloquine (Lariam) is an important anti-malaria drug that is applied as a racemate of the erythro form. However, the (-)-isomer induces psychosis, while the (+)-enantiomer does not have this undesired side effect. There are six syntheses of which five lead to the wrong enantiomer without the authors of these syntheses noting that they had synthesized the wrong compound. At the same time physical chemistry investigations had assigned the absolute configuration correctly and the last enantioselective synthesis that took these results into account delivered the correct absolute configuration. Since various synthetic approaches failed to provide the correct stereoisomers in previous syntheses, we submit here a synthetic approach with a domino Sonogashira-6π-electrocyclisation as key step that confirmed synthetically the correct absolute configuration of all four isomers.

Absolute Configuration and Antimalarial Activity of erythro-Mefloquine Enantiomers

Mefloquine (MQ), an antimalarial drug, is used as a racemate of (-)- and (+)-enantiomers, which display biological differences. The question concerning their exact configuration remains a matter of debate. The absolute configuration of the two MQ enantiomers as well as their biological activity has been established, thus confirming the importance of the stereochemistry in the design of MQ analogues that have fewer adverse side effects.

RDC-based determination of the relative configuration of the fungicidal cyclopentenone 4,6-diacetylhygrophorone A12

The hygrophorones, a class of cyclopentenones isolated from fruiting bodies of the genus Hygrophorus (basidiomycetes), show promising antifungal activity. While the constitution of 4,6-diacetylhygrophorone A(12) (3) and the relative configuration of the stereogenic centers in the cyclopentenone ring were elucidated using standard NMR and MS techniques, the relative configuration of the exocyclic stereogenic center could not be assigned. By introducing a sample of 3 into an alignment medium and measuring anisotropic NMR parameters, namely, residual dipolar couplings, we were able to unambiguously determine the relative configuration of all three stereogenic centers in 4,6-diacetylhygrophorone A(12) simultaneously by fitting several structure proposals to the experimental data.

Recent advances in malaria drug discovery

This digest covers some of the most relevant progress in malaria drug discovery published between 2010 and 2012. There is an urgent need to develop new antimalarial drugs. Such drugs can target the blood stage of the disease to alleviate the symptoms, the liver stage to prevent relapses, and the transmission stage to protect other humans. The pipeline for the blood stage is becoming robust, but this should not be a source of complacency, as the current therapies set a high standard. Drug discovery efforts directed towards the liver and transmission stages are in their infancy but are receiving increasing attention as targeting these stages could be instrumental in eradicating malaria.

The absolute configuration of (+)- and (-)-erythro-mefloquine

The controversy over the absolute configuration of (+)-erythro-mefloquine, the less psychosis-causing enantiomer of the anti-malarial drug Lariam, has been resolved by Mosher ester crystallization. The configuration determined previously by physical methods is correct, whereas the configuration determined by three enantioselective syntheses is wrong.

MSpin-RDC. A program for the use of residual dipolar couplings for structure elucidation of small molecules

We developed a new program, MSpin-RDC for the analysis of residual dipolar coupling data. This software, specially designed for small molecule analysis, can directly read many molecular-modeling and popular chemistry file formats and accept RDC values as a simple free-format table. Alignment tensor can then be computed by singular value decomposition, as well as predicted using inertia and gyration tensor-based methodologies. Trial structures are then ranked according to their Cornilescu's quality factor (Q) values. Analysis of multiconformational problems and fitting of RDC data to relative populations can be accomplished using the single-tensor approximation.

Influence of solvent and salt concentration on the alignment properties of acrylamide copolymer gels for the measurement of RDC

The dependence of molecular alignment with solvent nature and salt concentration has been investigated for mechanically stretched polyacrylamide copolymer gels. Residual dipolar couplings (RDCs) were recorded for D(2)O, DMSO-d(6), and DMSO-d(6)/D(2)O solutions containing different proportions of the solvents and different sodium chloride concentrations. Alignment tensors were determined by fitting the experimental RDCs to the DFT-computed structure of N-methylcodeinium ion. Analysis of the tensors shows that the degree of alignment decreases with the proportion of DMSO-d(6) as well as with the concentration of sodium chloride, most likely due to enhanced ion-pair aggregation. Furthermore, rotation of the alignment tensor is observed when increasing the salt concentration.

Chiral discrimination of amines by anisotropic NMR parameters using chiral polyacrylamide-based gels

A new chiral alignment medium for dimethyl sulfoxide, methanol, and water as solvents was developed. Because both enantiomers of the gel are available, it is possible to enantiodiscriminate natural products such as strychnine HCl that naturally occurs as single enantiomer. With the two methods of achieving anisotropy, namely stretching and confinement, the degree of alignment can be adjusted, and the director changed from horizontal to vertical. This increases the applicability. Three compounds were enantiodiscriminated on the basis of residual dipolar coupling data: mefloquine HCl, strychnine HCl, and menthylamine HCl.