Rapid isolation and identification of minor natural products by LC–MS,LC–SPE–NMR and ECD: Isoflavanones,biflavanones and bisdihydrocoumarins from Ormocarpum kirkii

The combination of the hyphenated techniques LC–MS and LC–SPE–NMR constitutes a powerful platform for the rapid isolation and identification of minor components from natural sources. Electronic circular dichroism (ECD) is a useful tool to determine the absolute configuration of small quantities of chiral molecules. In order to search for minor constituents present in an Ormocarpum kirkii extract, these techniques were applied for the separation and structure elucidation of a series of isoflavanones, biflavanones and biscoumarins. After optimization of chromatographic conditions and subsequent isolation, MS and 1D and 2D NMR data were collected. Experimental and calculated ECD spectra were used in conjunction with NMR data to confirm the absolute configuration of these compounds. Eight compounds were identified for the first time and six have been previously reported. The present approach offers a strategy for accelerating research on natural products.     

Molecular tweezers for enantiodiscrimination in NMR: Di‐(R,R)‐1‐[10‐(1‐hydroxy‐2,2,2‐trifluoroethyl)‐9‐anthryl]‐2,2,2‐trifluoroethyl benzenedicarboxylates

A series of new chiral molecular tweezers, di‐(R,R)‐1‐[10‐(1‐hydroxy‐2,2,2‐trifluoroethyl)‐9‐anthryl]‐2,2,2‐trifluoroethyl phthalate (2), isophthalate (3) and terephthalate (4), were synthesized and their structure studied by NMR and molecular mechanics. Their effectiveness as chiral solvating agents for the determination of the enantiomeric purity of chiral compounds using NMR was demonstrated. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Strategies for dereplication of natural compounds using high-resolution tandem mass spectrometry

Complete structural elucidation of natural products is commonly performed by nuclear magnetic resonance spectroscopy (NMR), but annotating compounds to most likely structures using high-resolution tandem mass spectrometry is a faster and feasible first step. The CASMI contest 2016 (Critical Assessment of Small Molecule Identification) provided spectra of eighteen compounds for the best manual structure identification in the natural products category. High resolution precursor and tandem mass spectra (MS/MS) were available to characterize the compounds. We used the Seven Golden Rules, Sirius2 and MS-FINDER software for determination of molecular formulas, and then we queried the formulas in different natural product databases including DNP, UNPD, ChemSpider and REAXYS to obtain molecular structures. We used different in-silico fragmentation tools including CFM-ID, CSI:FingerID and MS-FINDER to rank these compounds. Additional neutral losses and product ion peaks were manually investigated. This manual and time consuming approach allowed for the correct dereplication of thirteen of the eighteen natural products.     

Microscale methodology for structure elucidation of natural products

Advances in microscale spectroscopic techniques, particularly microcryoprobe NMR, allow discovery and structure elucidation of new molecules down to only a few nanomole. Newer methods for utilizing circular dichroism (CD) have pushed the limits of detection to picomole levels. NMR and CD methods are complementary to the task of elucidation of complete stereostructures of complex natural products. Together, integrated microprobe NMR spectroscopy, microscale degradation and synthesis, are synergistic tools for the discovery of bioactive natural products and have opened new realms for discovery among extreme sources including compounds from uncultured microbes, rare invertebrates and environmental samples.     

An improved method for determining enantiomeric excess by 13C‐NMR in chiral liquid crystal media

By using a combination of inverse gated ¹H decoupled ¹³C‐NMR experiments 1 with short acquisition times and NMR Cryo‐probe technology, the sample requirements and experimental times necessary to accurately measure enantiomeric excess of small chiral molecules has been reduced 16‐fold. Quality ¹³C‐NMR spectra can now be obtained from a 1 to 5 mg sample in 12 minutes. The enantiomeric excess determination achieved from the average integration of all the ¹³C‐resonances in the spectrum is comparable to enantiomeric excess measured by chiral SFC. The advantage of the NMR method is that enantiomeric excess can rapidly be measured in situ on practical amounts of enantioselective reaction products without the need for chromatographic separation or chemical modification and with substantially less solvent waste. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Application of L-proline derivatives as chiral shift reagents for enantiomeric recognition of carboxylic acids

Four proline-derived chiral receptors 5-8 were readily synthesized starting from L-proline. The enantiomeric recognition ability of chiral receptors was examined with a series of carboxylic acids by (1) H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds with each of the enantiomers of guest molecules were determined by using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate that the hosts form 1:1 instantaneous complexes with all guests. The receptors exhibited different chiral recognition abilities toward the enantiomers of racemic guests. Among the chiral receptors used in this study, prolinamide 6 was found to be the best chiral shift reagent and is effective for the determination of the enantiomeric excess of chiral carboxylic acids.     

Synthesis and enantiomeric analysis of cyclotriphosphazene derivatives with one centre of chirality

A series of chiral cyclotriphosphazene compounds 2-9 in which the spiro 3-amino-1-propanoxy moiety provides the one centre of chirality have been synthesised and characterised by elemental analysis, MS, ¹H and ³¹P NMR spectroscopies. The enantiomers of newly synthesised compounds have been analysed by the changes in the ³¹P NMR spectra on addition of a Chiral Solvating Agent (CSA), (S)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. HPLC methods have been developed for the enantiomeric separations of chiral cyclotriphosphazenes containing one centre of chirality. It is found that chiral HPLC gave a good resolution of enantiomers of the racemic compounds 2-9 with resolution factors between 2.49 and 7.50 making them good candidates for enantiomeric separations and determination of absolute configuration.     

Polarimetric detection in high-performance liquid chromatography

Chiroptical detection for HPLC is particularly useful as a selective detection method for chiral molecules, and in enantiomeric purity determination with partial chiral separation or without chiral separation. The recent development of laser-based polarimeters with microdegree sensitivity has increased the applicability of optical rotation detection in HPLC. The detection limit of these instruments is submicrogram on-column for many chiral compounds in analytical HPLC. A variety of applications of the selective detection of optically active molecules are reviewed. The use of polarimetric detection with partial chiral separation is considered, both as an aid to method development and for enantiomeric purity determination. Finally applications to enantiomeric purity determination without chiral separation are reviewed, with the dual use of nonchirally selective and chiroptical detectors to determine the total amount and optical purity of the analyte. Determinations of chiral purity for samples of high enantiomeric excess are described, which with laser-based instrumentation may give accuracies of better than +/- 1% with sample loadings of 50 micrograms on an achiral column. Applications to the study of enantioselective reactions are also considered, with determination of enantiomeric excess in near-racemates to better than +/- 0.1%.     

Chiral purity determination of tobacco alkaloids and nicotine-like compounds by 1H NMR spectroscopy in the presence of 1,1′-binaphthyl-2,2′-diylphosphoric acid

The enantiomeric purity of several tobacco alkaloids and nicotine-like compounds was determined using ¹H NMR (300 MHz) spectroscopy in the presence of (-)-(R)-1,1′binaphthyl-2,2′-diylphosphoric acid (BNPPA) as a chiral complexing agent. The most significant signal splitting resulting from diastereoisomeric complexation are seen for chemical shifts in the proximity of the pyridinyl nitrogen. Chemical shift data exclude any contribution of the pyrrolidinyl protons to chiral recognition, but when the pyrrolidine ring is replaced by a piperidine ring, i.e., for compounds such as rac-anabasine and rac-anatabine, non-equivalence between enantiomers was observed for protons close to the piperidine ring. A new approach for the preparation of the pure (-)-(S)-and (+)-(R)-enantiomers of nornicotine, a tobacco alkaloid and metabolite of nicotine, was developed. The optically pure enantiomers thus obtained were used to establish the minimum sensitivity of the NMR spectroscopic method of chiral analysis. These findings provide a new, general, and facile method for the determination of enantiomeric purity of tobacco alkaloids and nicotine-like compounds. © 1996 Wiley-Liss, Inc.     

Enantiomeric resolution of galanthamine and related drugs used in anti-Alzheimer therapy by means of capillary zone electrophoresis employing derivatized cyclodextrin selectors

An analytical assay is presented for the determination of the enantiomeric composition of galanthamine and related synthetic and natural compounds. (−)-Galanthamine is isolated from Galanthus nivalis and is used in this optical pure form in the therapy of Alzheimer’s disease. Recent efforts for a total synthesis of unichiral (−)-galanthamine is connected with the need for a fast and reliable assay for the determination of the optical purity of the end product, as well as for optimizing and controlling the final steps in total synthesis particularly the asymmetric transformation of narwedine. In this paper the enantiomeric resolution of these compounds is reported employing a capillary electrophoretic system with β-cyclodextrin derived chiral selectors. With the proposed system a number of galanthamine and narwedine derived analogous compounds could be separated, including 1-bromo- and N-alkyl-substituted compounds.     

A NMR shift method for determination of the enantiomeric composition of hydroperoxides formed by lipoxygenase.

A method is presented for determination of the enantiomeric composition of hydroxyperoxides formed by enzymic oxygenation of unsaturated fatty acids. After reduction of the hydroperoxy group with NaBH4, and esterification, the positional isomers of the resulting hydroxy compounds are separated by high performance liquid chromatography. The latter are subsequently subjected to a chiral derivatization to form diastereomeric alpha-methoxy-alpha-trifluoromethylphenylacetate esters. Determination of the diastereomeric composition by a NMR shift experiment furnishes the enantiomeric composition of the parent hydroperoxides. The method has been applied to the hydroperoxides formed by incubation of linoleic acid by corn germ or soybean lipoxygenase. Our results indicate that under the conditions used the hydroperoxides are mainly enantiospecifically formed.     

Stereoisomeric flavour compounds XLV: Structure analysis of 2-alkoxy-5-alkyl-tetrahydrofurans

The synthesis of several 2-alkoxy-5-alkyl-tetrahydrofurans is of interest in our investigations of structure–function relationships of chiral flavour compounds. For the preparation of the enantiomeric acetals the unambiguous configurational assignment of the cis and trans series of these compounds is indispensable. By means of crystalline acetal derivatives the absolute structure of a model compound in the cis and the trans configuration is revealed by X-ray measurement and correlated with the corresponding cis and trans configurated aroma compounds. The first complete structure elucidation of the class of 2-alkoxy-5-alkyltetrahydrofurans has been carried out.     

Enantiomeric recognition of chiral 3,3-bridged-1,1'-binaphthol dimer toward alpha-phenylethylamine and alpha-amino acid ester

The 1,1'-binaphthol-based dimers with p-phenylenebis(2-ethynyl) spacer, (+)-6 and (+)-2, were synthesized as chiral host compounds. (1)H NMR, UV-vis, and fluorescent titration were used to evaluate the enantiomeric recognition abilities of the chiral host dimers toward the guest amine 7 and alpha-amino acid ester 8. The chiral BINOL-based dimers were found to have good enantiomeric recognition ability. The computer simulation of the host-guest complex molecules was carried out to describe the conformational changes of both naphthyl ring in the molecule of chiral host dimer after complexation with the guest molecule.     

The Enantiomeric Separation of Tetrahydrobenzimidazoles Cyclodextrins‐ and Cyclofructans

High performance liquid chromatography (HPLC) and capillary electrophoresis (CE) were used to examine the enantiomeric separation of a series of 17 racemic tetrahydrobenzimidazole analytes. These compounds were prepared as part of a synthetic program directed towards a select group of pyrrole‐imidazole alkaloids. This group of natural products has a unique framework of pyrrole‐ and guanidine‐containing fused rings which can be constructed through the intermediacy of a tetrahydrobenzimidazole scaffold. Several bonded cyclodextrin‐ (both native and derivatized) and derivatized cyclofructan‐based chiral stationary phases were evaluated for their ability to separate these racemates via HPLC. Similarly, several cyclodextrin derivatives and derivatized cyclofructan were evaluated for their ability to separate this set of chiral compounds via CE. Enantiomeric selectivity was observed for the entire set of racemic compounds using HPLC with resolution values up to 3.0. Among the 12 different CSPs, enantiomeric recognition was most frequently observed with the Cyclobond RN and LARIHC CF6‐P, while the Cyclobond DMP yielded the greatest number of baseline separations. Fifteen of the analytes showed enantiomeric recognition in CE with resolution values as high as 5.0 and hydroxypropyl‐γ‐cyclodextrin was the most effective chiral additive. Chirality 25:133–140, 2013. © 2012 Wiley Periodicals, Inc.     

Fluorescence determination of enantiomeric composition of pharmaceuticals via use of ionic liquid that serves as both solvent and chiral selector

A new method has been developed for the sensitive and accurate determination of enantiomeric compositions of a variety of drugs, including propranolol, naproxen, and warfarin. The method is based on the use of the fluorescence technique to measure diastereomeric interactions between both enantiomeric forms of a drug with an optically active room temperature ionic liquid (RTIL) followed by partial least squares analysis of the data. The chiral RTIL used in this study, S-[(3-chloro-2-hydroxypropyl) trimethylammonium] [bis((trifluoromethyl)sulfonyl)amide] (S-[CHTA](+) [Tf(2)N](-)), is a novel chiral RTIL that has been synthesized successfully recently in our laboratory in optically pure form using a simple one-step reaction with commercially available reagents. The high solubility power and strong enantiomeric recognition ability make it possible to use this chiral RTIL to solubilize a drug and to induce diastereomeric interactions for the determination of enantiomeric purity, that is, to use it as both solvent and chiral selector. Enantiomeric compositions of a variety of pharmaceutical products with different shapes, sizes, and functional groups can be determined sensitively (microgram concentration) and accurately (enantiomeric excess as low as 0.30% and enantiomeric impurity as low as 0.08%) by use of this method.     

Recent advances in methods of direct optical resolution

Very great advances have been made in the field of direct optical resolution of organic compounds by chromatographic techniques. Chiral capillary gas chromatography now permits a determination of the enantiomeric composition of a few nanograms of a compound present in a mixture of many others. Coupled with high resolution mass spectrometry the technique will additionally permit structural elucidation; of great interest in pheromone research and related areas. Analytical separations of enantiomers are now also carried out by high-performance liquid chromatography (HPLC) methods based on a variety of principles. Basically, two main types are used, differing as to whether the mobile phase has to be a chiral medium or not. Two-dimensional HPLC, whereby compounds separated on a non-chiral column are progressively and automatically transferred to a chiral column for optical resolution, has been used successsfully for chiral amino acid separations. Many different chiral sorbents for preparative LC and HPLC resolutions have been prepared; some of these are now used in columns capable of producing pure enantiomers from a given racemate at a rate of the order of one gram/hour in continuous, automatic HPLC procedures. Apart from all important applications of these results of optical resolution technology, an increased knowledge of the underlying chiral recognition phenomena responsible for enantioselection has also been achieved.     

Stereoselectivity in acid-catalyzed heteronucleophilic substitution of enantiomeric 3-O-methyloxazepam and 3-O-ethyloxazepam

Kinetics of acid-catalyzed heteronucleophilic substitution and racemization of enantiomeric MeOX in ethanol and enantiomeric EtOX in methanol were studied by quenching reaction products at various times by neutralization. Enantiomeric contents of remaining substrate and reaction product were determined by chiral stationary phase high-performance liquid chromatography. The experimental procedure allowed the determination of the stereoselectivity (i.e., the enantiomeric ratio of a substitution product formed from an enantiomerically pure substrate) involved in the heteronucleophilic substitution reactions. The stereoselectivity was found to vary between 58:42 and 87:13, depending on the acid concentration, substrate, solvent, and temperature. The enantiomeric purity of remaining substrates was identical to that of the starting substrate, indicating that the enantiomeric substrates did not undergo a ring-opening reaction. The results provided additional evidence supporting the mechanism proposed earlier in acid-catalyzed stereoselective heteronucleophilic and homonucleophilic substitutions and the resulting racemization of enantiomeric 3-alkoxy-1,4-benzodiazepines in alcoholic solvents. © 1995 Wiley-Liss, Inc.     

Water‐soluble calix[4]resorcinarenes as chiral NMR solvating agents for bicyclic aromatic compounds

Water‐soluble calix[4]resorcinarenes with proline, 3‐hydroxyproline, and 4‐hydroxyproline substituent groups are evaluated as chiral NMR solvating agents on a series of bicyclic aromatic compounds with naphthyl, indole, dihydroindole, and indane rings. The substrates interact with the calixresorcinarene through insertion of the aromatic ring into the cavity. Most of the substrates are analyzed as cationic species, although one anionic species is analyzed. All of the substrates exhibit enantiomeric discrimination in the ¹H‐NMR spectrum with one or more of the calixresorcinarenes. In most cases, the hydroxyproline derivatives are more effective at causing enantiodifferentiation than the corresponding proline derivative. Presumably, the hydroxyl group on the proline moieties is involved in interactions with the substituent groups of the substrate that are important in creating chiral recognition. The enantiomeric discrimination in the ¹H‐NMR spectrum is large enough for many resonances to permit the analysis of enantiomeric purity. Chirality 2009. © 2009 Wiley‐Liss, Inc.     

Generation of molecular chiral asymmetry through stirred crystallization

In our earlier work we established that stirred crystallization of achiral compounds that crystallize in enantiomeric forms result in spontaneous chiral symmetry breaking. The asymmetry thus spontaneously generated is confined to the solid state. In this article, we present a case in which the crystal enantiomeric excess (CEE) can be converted to molecular enantiomeric excess (EE) through a solid state reaction which relates the enantiomeric form of the crystal to the enantiomeric form of the product. Such a process not only provides a means of detecting the CEE generated in stirred crystallization but it is also a means through which chiral asymmetry generated spontaneously is "propagated" to generate chiral compounds with enantiomeric excess.     

Diamagnetic lanthanide tris beta-diketonates as organic-soluble chiral NMR shift reagents

Diamagnetic lanthanium(III) and lutetium(III) tris beta-diketonate complexes of 3-(trifluoroacetyl)-d-camphor, 3-(heptafluorobutyryl)-d-camphor, and d,d-dicampholylmethane are shown to be effective chiral NMR shift reagents for determining the enantiomeric purity of compounds with hard Lewis base functional groups. These include substrates with amine, alcohol, epoxide, sulfoxide, and oxaxolidine moieties. Enantiomeric discrimination is observed in the (1)H NMR spectrum. Diamagnetic lanthanide complexes represent an alternative to paramagnetic varieties that often cause too much line broadening in the NMR spectra. The choice of which metal to use varies with substrate. Similarly, there is no consistent trend with ligand as not one of the complexes is consistently better than the others for all substrates. The enantiomeric discrimination also varies with solvent. Comparisons show that the chiral recognition was usually larger in benzene-d(6) than in chloroform-d or cyclohexane-d(12).