M?NP acid, a powerful chiral molecular tool for preparation of enantiopure alcohols by resolution and determination of their absolute configurations by the1H NMR anisotropy method

Constituents of Chimaphila japonica and Their Diuretic Activity

Three new phenols (1-3), one new cyclohexanol (4), two known phenols (5-6), and six known flavonoids (7-12) were isolated from the n-butanol of the 75% ethanol extract of all plants of Chimaphila japonica Miq. Among them, compound 5 was named and described in its entirety for the first time, and compounds 9 and 10 were reported in C. japonica for the first time. The structures of all compounds were confirmed using a comprehensive analysis of 1D and 2D NMR and HRESIMS data. Biological results show that compounds 4, 7, and 11 exhibited potent diuretic activity. The modes of interaction between the selected compounds and the target diuretic-related WNK1 kinase were investigated in a preliminary molecular docking study. These results provided insight into the chemodiversity and potential diuretic activities of metabolites in C. japonica.

Chiral organic stereochemistry: Chiral HPLC, chiral auxiliaries, CD spectroscopy, X-ray crystallography, and light-powered chiral molecular motors

In the studies of chiral organic stereochemistry, it is important to use enantiopure compounds. For this purpose, the chiral HPLC (High-Pressure Liquid Chromatography) columns containing chiral stationary phases were invented by Y. Okamoto and coworkers for enantio-separating various racemic compounds. In addition, the use of chiral auxiliaries is also useful for preparing enantiopure compounds and also for determining their absolute configurations, where covalent-bonded diastereomers are separated by HPLC on silica gel. In this review article, these HPLC methods will be discussed together with the applications to some interesting organic compounds including light-powered chiral molecular motors.

Uncovering potential interspecies signaling factors in plant-derived mixed microbial culture

Microbes use signaling factors for intraspecies and interspecies communications. While many intraspecies signaling factors have been found and characterized, discovery of factors for interspecies communication is lagging behind. To facilitate the discovery of such factors, we explored the potential of a mixed microbial culture (MMC) derived from wheatgrass, in which heterogeneity of this microbial community might elicit signaling factors for interspecies communication. The stability of Wheatgrass MMC in terms of community structure and metabolic output was first characterized by 16S ribosomal RNA amplicon sequencing and liquid chromatography/mass spectrometry (LC/MS), respectively. In addition, detailed MS analyses led to the identification of 12-hydroxystearic acid (12-HSA) as one of the major metabolites produced by Wheatgrass MMC. Stereochemical analysis revealed that Wheatgrass MMC produces mostly the (R)-isomer, although a small amount of the (S)-isomer was also observed. Furthermore, 12-HSA was found to modulate planktonic growth and biofilm formation of various marine bacterial strains. The current study suggests that naturally derived MMCs could serve as a simple and reproducible platform to discover potential signaling factors for interspecies communication. In addition, the study indicates that hydroxylated long-chain fatty acids, such as 12-HSA, may constitute a new class of interspecies signaling factors.

Chirality: An Important Phenomenon Regarding Biosynthesis, Perception, and Authenticity of Flavor Compounds

Chirality plays an important role in flavor research. This will be outlined using selected examples from the key areas analysis, authenticity assessment, biogenesis, and odor perception. Developments of analytical techniques, in particular the use of chiral stationary phases in capillary gas chromatography, enabled the determination of naturally occurring configurations of chiral volatiles at trace levels. Thus, knowledge of pathways and enzymes involved in the biogenesis of chiral substances was acquired, and enantioselective analysis has become a prominent tool in the authenticity assessment of flavorings. Increasing information is also available on structure-odor relationships of chiral flavor compounds and the influence of their configurations on odor thresholds and odor properties. A substantial extension of these data sets and a further understanding of the role of chirality in the perception of aroma compounds is expected from studies on the enantioselectivity of odorant receptor systems. Developments in these areas will be put into a historical perspective, recent progress will be emphasized, and data gaps will be described.

Hydrodealkenylative C(sp3)-C(sp2) bond fragmentation

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity-particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)-C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Reinvestigation of the Absolute Configurations of Chiral β-Mercaptoalkanones Using Vibrational Circular Dichroism and 1H NMR Analysis

The absolute configurations of chiral β-mercaptoalkanones were previously assigned on the basis of the ¹H NMR anisotropy method using (S)-2-methoxy-2-(1-naphthyl)propionic acid ((S)-MαNP) as the chiral auxiliary. This study presents a reinvestigation of the configurations of 4-mercapto-2-pentanone 1, 4-mercapto-2-heptanone 2, and 2-mercapto-4-heptanone 3. Enantiomers of 1, 2, and 3 were obtained by lipase-catalyzed hydrolyses of the respective acetylthioalkanones. Upon derivatization with (S)-MαNP, the configurations of the reaction products were deduced based on the order of the HPLC elution of the diastereoisomeric thioesters, assuming that the sector rule previously developed for secondary alcohols is also valid for thiols. In addition, the configurations were experimentally determined by vibrational circular dichroism (VCD) and ¹H NMR analyses after esterification with (R)-hydratropic acid (HTA) and 2-methoxy-2-phenylacetic acid (MPA). The assignments of the configurations using VCD and NMR analyses of HTA- and MPA-thioesters were in agreement. However, they were opposite to those deduced for (S)-MαNP thioesters via the sector rule. Consequently, the formerly assigned configurations of β-mercaptoalkanones deduced via investigation of (S)-MαNP-derivatives have to be revised.

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.

Chemoenzymatic synthesis and cytotoxicity of oenanthotoxin and analogues

We developed a synthetic scheme for the synthesis of naturally occurring (14R)-oenanthotoxin and several analogs. Key-steps of this synthesis were an efficient homo-coupling of alkynes and a chemoenzymatic resolution of racemic oenanthotoxin using novozyme 435 and vinyl acetate. The compounds were screened for their cytotoxic activity using a photometric sulforhodamine B assays and several human tumor cell lines. Oenanthotoxin and many derivatives thereof were cytotoxic to tumor cell lines as well as to non-malignant mouse fibroblasts. The highest activity was determined for human ovarian cancer cells A2780 with EC50 = 3.8 μM.

Cyclic lipodepsipeptides verlamelin A and B, isolated from entomopathogenic fungus Lecanicillium sp

Verlamelin and its new derivative (verlamelin B) were isolated from fermentation broth of entomopathogenic fungus Lecanicillium sp. HF627. As the structural elucidation of verlamelin so far was only preliminary, we studied and determined the absolute structure of these two compounds to be cyclo(5S-hydroxytetradecanoic acid-D-alloThr/Ser-D-Ala-L-Pro-L-Gln-D-Tyr-L-Val). This is the first study that precisely analyzed the structure of verlamelin.

New chiral derivatizing agents: convenient determination of absolute configurations of free amino acids by 1H NMR

The chiral carbonate reagents 5 allow for the direct and unambiguous determination of the absolute configurations of a wide range of free amino acids using (1)H NMR. By using a approximately 3:1 mixture of (S)-5 and (R)-5, absolute configurations of the corresponding carbamates are determined by only analyzing the nitrogen protons.

Determination of absolute configuration--an overview related to this special issue

Rapid progress in asymmetric synthesis stimulated a further development of methods and techniques for the determination of absolute configuration of chiral molecules. In recent years the direct methods, i.e. X-ray diffraction analysis, circular dichroism (vibrational and electronic), Raman optical activity, optical rotation measurements, as well as indirect methods for relative configuration assignment with the use of NMR spectroscopy or enzymatic transformations, are receiving increasing attention not only by specialists in the field but also by synthetic and structural chemists alike. This paper provides a short overview of the methods currently used, as well as references to contributions collected in this Thematic Issue of Chirality.

Synthesis of enantiopure 2-aryl-2-methoxypropionic acids and determination of their absolute configurations by X-ray crystallography

Racemic 2-aryl-2-methoxypropionic acids were enantioresolved by the use of (S)-(-)-phenylalaninol 4. For instance, racemic 2-methoxy-2-phenylpropionic acid (+/-)-7 was condensed with phenylalaninol (S)-(-)-4 yielding a diastereomeric mixture of amides, which was easily separated by HPLC on silica gel affording the first-eluted amide (-)-13a and the second-eluted amide (+)-13b: alpha = 3.19, Rs = 3.49. The absolute configuration of amide (-)-13a was determined to be (R;S) by X-ray crystallography by reference to the S configuration of the phenylalaninol moiety. Amide (R;S)-(-)-13a was converted to oxazoline (R;S)-(-)-14a, from which enantiopure 2-methoxy-2-phenylpropionic acid (R)-(-)-7 was recovered. Other 2-aryl-2-methoxypropionic acids, (R)-(-)-8, (R)-(-)-9, (R)-(+)-10, (R)-(-)-11, and (R)-(-)-12, were similarly prepared in enantiopure forms with the use of phenylalaninol (S)-(-)-4, and their absolute configurations were clearly determined by X-ray crystallography or by chemical correlation.

New route to enantiopure MαNP acid, a powerful resolution and chiral1H NMR anisotropy reagent

MalphaNP acid (+/-)-1, 2-methoxy-2-(1-naphthyl)propionic acid, was enantioresolved by the use of phenylalaninol (S)-(-)-4; a diastereomeric mixture of amides formed from acid (+/-)-1 and amine (S)-(-)-4 was easily separated by fractional recrystallization and/or HPLC on silica gel, yielding amides (R;S)-(-)-5a and (S;S)-(+)-5b. Their absolute configurations were determined by X-ray crystallography by reference to the S configuration of the phenylalaninol moiety. Amide (R;S)-(-)-5a was converted to oxazoline (R;S)-(+)-8a, from which enantiopure MalphaNP acid (R)-(-)-1 was recovered. In a similar way, enantiopure MalphaNP acid (S)-(+)-1 was obtained from amide (S;S)-(+)-5b. These reactions provide a new route for the large-scale preparation of enantiopure MalphaNP acid, a powerful chiral reagent for the enantioresolution of alcohols and simultaneous determination of their absolute configurations by (1)H NMR anisotropy.

Enantiomeric separation and determination of absolute stereochemistry of asymmetric molecules in drug discovery—Building chiral technology toolboxes

The application of Chiral Technology, or the (extensive) use of techniques or tools for the determination of absolute stereochemistry and the enantiomeric or chiral separation of racemic small molecule potential lead compounds, has been critical to successfully discovering and developing chiral drugs in the pharmaceutical industry. This has been due to the rapid increase over the past 10-15 years in potential drug candidates containing one or more asymmetric centers. Based on the experiences of one pharmaceutical company, a summary of the establishment of a Chiral Technology toolbox, including the implementation of known tools as well as the design, development, and implementation of new Chiral Technology tools, is provided.

Preparation of single-enantiomer 2-methyl-4-heptanol, a pheromone of Metamasius hemipterus, using (S)-2-methoxy-2-(1-naphthyl)propionic acid

To investigate the resolution of secondary alcohols using 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid), 2-methyl-4-heptanol, one of the aggregation pheromones of Metamasius hemipterus, was resolved using (S)-MalphaNP acid. As a chiral-resolving agent, MalphaNP acid is superior to 3,3,3-trifluoro-2-methoxy-2-phenylpropionic acid (MTPA) in terms of HPLC separation and NMR shielding. A better separation of diastereomeric MalphaNP esters was observed when n-hexane-THF was used as the eluent for silica gel HPLC. The solvolysis of the diastereomeric MalphaNP esters gave (R)-2-methyl-4-heptanol and its enantiomer; enantiopure (S)-MalphaNP acid was also recovered. In addition, the preferred conformation of the MalphaNP ester was confirmed using methyl (R)-3-hydroxyvalerate as an authentic compound.

1,1‘-Binaphthyl-2,2‘-diyl Phosphoroselenoyl Chloride as a Chiral Molecular Tool for the Preparation of Enantiomerically Pure Alcohols and Amines

Enantiomerically pure phosphoroselenoyl chloride bearing a binaphthyl group was synthesized. This phosphoroselenoyl chloride was used to discriminate and resolve simple secondary alcohols. Stereospecific conversions of diastereomerically pure phosphoroselenoic acid esters, obtained by reaction of the chloride with simple secondary alcohols, to enantiomerically pure alcohols and amines were also achieved.

Preparation of enantiopure Wieland–Miescher ketone and derivatives by the MαNP acid method. Substituent effect on the HPLC separation

Enantiopure Wieland-Miescher ketone (4, W-M ketone) and derivatives were prepared by the enantioresolution with 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid 1). Various racemic derivatives of 4 were esterified with acid (S)-(+)-1 yielding diastereomeric MalphaNP esters, which were separated by HPLC on silica gel. It was clarified that the HPLC separation of diastereomers depended on the substituent of the derivatives, leading to the working hypothesis that MalphaNP acid esters of alcohols with less polar and more bulky aliphatic substituents are more effectively separated. The best separation was obtained in the case of tert-butyldimethylsilyl (TBDMS) ether derivative (12a/12b): separation factor alpha=1.80, and resolution factor, Rs=1.30. The (1)H NMR spectra of separated MalphaNP esters showed anomalously large magnetic anisotropy effects, from which their absolute configurations were determined. Solvolysis or reduction of the separated MalphaNP esters yielded alcohols, which were converted to enantiopure W-M ketones 4. The results thus provided another route for preparation of enantiopure ketones (8aR)-(-)-4 and (8aS)-(+)-4.

Synthesis of enantiopure phthalides including 3-butylphthalide, a fragrance component of celery oil, and determination of their absolute configurations

Enantiopure phthalides 2 and 5-8 were synthesized via enantioresolution of the corresponding alcohols with a chiral auxiliary of camphorsultam dichlorophthalic acid, (1S,2R,4R)-(-)-CSDP acid 3, followed by solvolysis with KOH in MeOH and the catalytic oxidation of chiral glycols with iridium complex 28. The absolute configurations of phthalides 2 and 5-8 were determined by applying the (1)H-NMR anisotropy method of MalphaNP acid (4), 2-methoxy-2-(1-naphthyl)propionic acid, to the chiral synthetic precursory alcohols. In the case of 3-phenylphthalide (R)-(-)-7, the absolute configuration determined by the (1)H-NMR anisotropy method using MalphaNP acid 4 agreed with that by the X-ray crystallographic method. By applying these methods, 3-butylphthalide (S)-(-)-2, a fragrance component of essential oil of celery, has been synthesized in an enantiopure form, and its absolute configuration was unambiguously determined.

Investigations of molecular recognition aspects related to the enantiomer separation of 2-methoxy-2-(1-naphthyl)propionic acid using quinine carbamate as chiral selector: An NMR and FT-IR spectroscopic as well as X-ray crystallographic study

The chiral recognition mechanism of a cinchona alkaloid-based chiral stationary phase (CSP) showing high enantiomer discrimination potential for 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid) was investigated. Conformational and structural analyses of the 1:1 complexes of 9-O-(tert-butylcarbamoyl) quinine selector (SO) and MalphaNP acid (selectand, SA) were carried out employing NMR spectroscopy in solution, Fourier-transform infrared (FT-IR) spectroscopy, and solid-state X-ray diffraction analysis. Intramolecular NOEs of a soluble analogue of the CSP afforded the conformational states of the free and complexed form of the selector. The (1)H-NMR spectra revealed that the free form of the SO constitutes anti-open as well as anti-closed and/or syn-closed conformers. Upon complexation with the (S)-MalphaNP acid enantiomer to form the more stable diastereomeric associate, a conformational transition of the selector takes place, resulting in the synthesis of the anti-open conformer nearly exclusively. FT-IR spectra reveal that, besides the primary ion-pairing interaction, stereoselective hydrogen bonding stabilizes the more stable complex via the amide hydrogen of the SO. X-ray diffraction analysis of 9-O-(tert-butylcarbamoyl)quinine and (S)-MalphaNP acid complex further revealed the occurrence of a bidentate H-bond-mediated ionic interaction between SO and SA as well as the lack of pi-pi interaction in the 1:1 complex, and corroborated the conclusions derived from spectroscopic and chromatographic studies.