Chiral discrimination of secondary alcohols by both1H-NMR and HPLC after labeling with a chiral derivatization reagent, 2-(2,3-anthracenedicarboximide)cyclohexane carboxylic acid

New xanthine oxidase inhibitors from the fruiting bodies of Tyromyces fissilis

Excessive uric acid production, which causes gout and hyperuricemia, can be blocked by inhibiting xanthine oxidase (XO). However, some agents to block on XO often cause side effects, thereby necessitating the identification of new inhibitors. During the screening of XO inhibitors from various mushroom extracts, we found that a methanolic extract of the fruiting bodies of Tyromyces fissilis, an inedible and non-toxic fungus, showed inhibitory activity. Both n-hexane and ethyl acetate layers, obtained by partitioning this extract exhibited XO inhibitory activity. Subsequently, using an activity-guided separation method, eight active compounds (1-8) were isolated. The structures of five of the new compounds, 2-4, 6, and 7, were elucidated by spectral analysis and chemical derivatization. All compounds had a salicylic acid moiety with an aliphatic group at the C-6 position. Notably, 2-hydroxy-6-pentadecylbenzoic acid (1) showed the highest level of XO noncompetitive inhibition (58.9 ± 2.2% at 25 µM).

The molecular structure of plant sporopollenin

Sporopollenin is a ubiquitous and extremely chemically inert biopolymer that constitutes the outer wall of all land-plant spores and pollen grains¹. Sporopollenin protects the vulnerable plant gametes against a wide range of environmental assaults, and is considered a prerequisite for the migration of early plants onto land². Despite its importance, the chemical structure of plant sporopollenin has remained elusive¹. Using a newly developed thioacidolysis degradative method together with state-of-the-art solid-state NMR techniques, we determined the detailed molecular structure of pine sporopollenin. We show that pine sporopollenin is primarily composed of aliphatic-polyketide-derived polyvinyl alcohol units and 7-O-p-coumaroylated C16 aliphatic units, crosslinked through a distinctive dioxane moiety featuring an acetal. Naringenin was also identified as a minor component of pine sporopollenin. This discovery answers the long-standing question about the chemical make-up of plant sporopollenin, laying the foundation for future investigations of sporopollenin biosynthesis and for the design of new biomimetic polymers with desirable inert properties.

The molecular structure of plant sporopollenin

Sporopollenin is a ubiquitous and extremely chemically inert biopolymer that constitutes the outer wall of all land-plant spores and pollen grains¹. Sporopollenin protects the vulnerable plant gametes against a wide range of environmental assaults, and is considered a prerequisite for the migration of early plants onto land². Despite its importance, the chemical structure of plant sporopollenin has remained elusive¹. Using a newly developed thioacidolysis degradative method together with state-of-the-art solid-state NMR techniques, we determined the detailed molecular structure of pine sporopollenin. We show that pine sporopollenin is primarily composed of aliphatic-polyketide-derived polyvinyl alcohol units and 7-O-p-coumaroylated C16 aliphatic units, crosslinked through a distinctive dioxane moiety featuring an acetal. Naringenin was also identified as a minor component of pine sporopollenin. This discovery answers the long-standing question about the chemical make-up of plant sporopollenin, laying the foundation for future investigations of sporopollenin biosynthesis and for the design of new biomimetic polymers with desirable inert properties.

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 Male Produced Aggregation Pheromone of a Strawberry Sap Beetle, Lobiopa insularis (Coleoptera: Nitidulidae)

The nitidulid beetle Lobiopa insularis is an important pest of strawberry crops in the United States and Brazil. Both larvae and adults feed on ripe strawberries, causing 20-70% loss in production during serious infestations. Aiming at the development of efficient, clean, and highly specific pest management systems, semiochemicals, especially pheromones, are particularly useful. Analyses of the extracts of both males and females obtained from aeration of live beetles showed the presence of three male specific compounds, 2-nonanone, 2-undecanone, and 2-undecanol (in an enantiomeric ratio of S:R = 3.5:1). This is the first record of ketones and an alcohol as pheromone components in Nitidulidae. These compounds were emitted by males in amounts of 0.3:6:1.5 ng per insect within 24 h (1:30:3), respectively, during the scotophase, indicating nocturnal sexual activity. Field tests with pitfall traps containing different mixtures of compounds and ripe strawberries as a co-attractant summed up to five treatments with 25 replications. As a result, 59% males and 41% females (1:0.7) were caught, indicating the L. insularis pheromone to cause aggregation of both sexes. Results of the field tests showed that the attractivity of the binary mixture of ketones (T3) differed from the control (T5), from traps with 2-undecanone alone (T4), and from the mixture of 2-undecanone and racemic 2-undecanol (T2). Moreover, the activity of the ternary mixture of compounds (T1) was not different from that of T3, indicating that the racemic alcohol did not positively influence trap catches. In future applications, a mixture of synthetic strawberry-derived compounds that are attractive to L. insularis may substitute rapidly decaying fruit in the field, maintaining catches for longer periods. Because of its efficiency and low cost, a mixture of 2-undecanone and 2-nonanone is recommended to catch adult L. insularis in the field.

Real time band selective F1 -decoupled proton NMR for the demixing of overlay spectra of chiral molecules

The small chemical shift dispersion and complex multiplicity pattern in proton NMR limit quantifications, for instance the determination of enantiomeric excess (ee) for an enantiomeric mixture. Herein, we present a simple proton-proton correlation experiment with band selective homonuclear (BASH) decoupling in both F₁ and F₂ dimensions, for the removal of scalar and residual dipolar couplings to provide collapsed singlet for each chemical site. The method has been demonstrated to separate the severely overlapped spectra of enantiomers using both chiral isotropic and anisotropic phases as well as a small biomolecule, particularly for the diastereotopic protons and also for the determination of ee. Copyright © 2016 John Wiley & Sons, Ltd.

HPLC Separation of Diastereomers: Chiral Molecular Tools Useful for the Preparation of Enantiopure Compounds and Simultaneous Determination of Their Absolute Configurations

To obtain enantiopure compounds, the so-called chiral high performance liquid chromatography (HPLC) method, i.e., HPLC using a chiral stationary phase, is very useful, as reviewed in the present Special Issue. On the other hand, normal HPLC (on silica gel) separation of diastereomers is also useful for the preparation of enantiopure compounds and also for the simultaneous determination of their absolute configurations (ACs). The author and coworkers have developed some chiral molecular tools, e.g., camphorsultam dichlorophthalic acid (CSDP acid), 2-methoxy-2-(1-naphthyl)propionic acid (MαNP acid), and others suitable for this purpose. For example, a racemic alcohol is esterified with (S)-(+)-MαNP acid, yielding diastereomeric esters, which are easily separable by HPLC on silica gel. The ACs of the obtained enantiopure MαNP esters can be determined by the ¹H-NMR diamagnetic anisotropy method. In addition, MαNP or CSDP esters have a high probability of giving single crystals suitable for X-ray crystallography. From the X-ray Oak Ridge thermal ellipsoid plot (ORTEP) drawing, the AC of the alcohol part can be unambiguously determined because the AC of the acid part is already known. The hydrolysis of MαNP or CSDP esters yields enantiopure alcohols with the established ACs. The mechanism and application examples of these methods are explained.

Crystalline Structure ofN-(S)-2-Heptyl (1R,2R)-2-(2,3-Anthracenedicarboximido)cyclohexanecarboxamide That Differs from Its Preferred Conformation in the Solvent Used for Crystallization

The crystalline structure of N-(S)-2-heptyl (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexamide (1), which was crystallized from methanol, was determined by an X-ray analysis and had a different conformation from its preferred one in CD3OD by a 1H-NMR analysis. Inter- and intra-molecular CH-pi interaction in a crystal plays a very important role in crystal packing. The preferred conformation of the amide derivative in a solution allows us to exploit (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexanecarbonyl chloride as a conversion reagent to determine the absolute configuration of chiral amines by 1H-NMR.

Aminoindanol-based chiral derivatizing agents for the determination of the absolute configuration of carboxylic acids

New chiral derivatizing agents have been prepared through a simple, short-step synthesis. The absolute configuration of α-chiral carboxylic acids can be assigned on the basis of the NMR chemical shift difference between diastereomeric esters. Because of the modular structures of the agents, the anisotropic effect could be easily manipulated to afford large chemical shift differences even in polar solvents.

Cyclic glycolipids from glandular trichome exudates of Cerastium glomeratum

Fourteen cyclic glycolipids, named glomerasides A-N, have been isolated from the glandular trichome exudate of Cerastium glomeratum (Caryophyllaceae). Their structures were determined by spectroscopic analysis of the glycolipids, as well as by application of the Ohrui-Akasaka method to the fatty acid methyl esters derived from the glycolipids and GCMS studies of trimethylsilyl ether derivatives of the methyl esters. The various glomerasides have a glycosidic linkage between the anomeric hydroxy group of the glucose and the C-11, C-10 or C-9 positions of the docosanoyl moiety. They also contained an ester linkage between the C-6 hydroxy group of the glucose ring and the carboxyl group of the oxygenated fatty acid to form their macrocyclic structures. The glucose moiety was optionally acetylated and/or malonylated at the C-2 or C-3 hydroxy groups. Among these compounds, the 1,6'-cyclic ester of 11(R)-(2-O-acetyl-β-d-glucopyranosyloxy)docosanoic acid (glomeraside D) was the most abundant (25%).

Demixing of severely overlapped ¹H NMR resonances and interpretation of anomalous intensity pattern of dipolar coupled A₃ spins in a weakly aligning medium

We report a single (13)C spin edited selective proton-proton correlation experiment to decipher overcrowded (13)C coupled proton NMR spectra of weakly dipolar coupled spin systems. The experiment unravels the masked (13)C satellites in proton spectrum and permits the measurement of one bond carbon-proton residual dipolar couplings in I(3)S and for each diastereotopic proton in I(2)S groups. It also provides all the possible homonuclear proton-proton residual couplings which are otherwise difficult to extract from the broad and featureless one dimensional (1)H spectrum, in addition to enantiodifferentiation in a chiral molecule. Employment of heteronuclear ((13)C) decoupling in the evolution period results in complete demixing of overlapped signals from enantiomers. The observed anomalous intensity pattern in strongly dipolar coupled methyl protons in methyl selective correlation experiment has been interpreted using polarization operator formalism.

Development of highly potent chiral discrimination methods that solve the problems of the diastereomer method

The development of highly potent chiral discrimination methods that solve the problems of the diastereomer method, in which it is impossible to discriminate the diastereomers having chiral centers separated by more than four bonds, is described. On the basis of the results obtained, a new hypothesis, Induced Chiral Fields that the achiral reversed phase can provide chiral fields depending on the structures of the eluents, is proposed to explain the significant results of separation of the diastereomers derived from newly developed chiral and fluorescent labeling reagents and optical isomers by reversed-phase HPLC, which was hitherto impossible.

Selectivity Enhancement for trans-2-(2,3-Anthracenedicarboximido)-cyclohexane-derived Diastereomers in HPLC by Using an Ordered Organic Stationary Phase

2-(2,3-anthracenedicarboximido)cyclohexane derivatives (AC) have been known as the evolutionary diastereomerizing reagents for enantiomer discrimination in HPLC with ODS. However, a substantial separation of diastereomers can be observed only at lower temperatures, such as -40 degrees C. Therefore, in this work, poly(octadecyl acrylate)-grafted silica, ODAn was applied as an alternative stationary phase to ODS for the separation of AC-derived diastereomers. As a result, complete separation was achieved even under the conventional condition: for example, methanol as the mobile phase and 0 degrees C as the column temperature. An investigation on the temperature dependency of the selectivity demonstrated that ODAn shows a remarkable increase in selectivity at temperatures below 30 degrees C, which almost agreed with the peak-top temperature of the endothermic peak in a DSC thermogram for ODA35 immersed in a mobile phase. The better separation would be derived from a highly ordered structure of ODAn and a carbonyl-pi interaction with AC-derived diastereomers.

Chiral discrimination of primary amines by HPLC after labeling with a chiral derivatization reagent,trans-2-(2,3-anthracenedicarboximido)-cyclohexanecarbonyl chloride

Enantiomeric discrimination of chiral primary amines was performed by both reversed-phase HPLC and normal-phase HPLC after labeling with a chiral fluorescent derivatization reagent, (1R,2R)- and (1S,2S)-trans-2-(2,3-anthracenedicarboximido)cyclohexanecarbonyl chloride. Use of HPLC permits separation of diastereomeric derivatives of amines up to C30 which have a primary amino group at the middle of the alkyl chain. The derivatives of primary amines having an anteiso alkyl chain, which has a chiral branched-methyl at the n-3 position of the alkyl chain, were also separated by HPLC, and it was also possible to separate niphatesine D by reversed-phase HPLC after derivatization.