The Assignment of Absolute Configuration by NMR

An Efficient Chemoenzymatic Approach towards the Synthesis of Rugulactone

Rugulactone is a natural product isolated from the plant Cryptocarya rugulosa. It has shown very important biological activity as an inhibitor of the nuclear factor κB (NF-κB) activation pathway. A new chemoenzymatic approach towards the synthesis of rugulactone is presented here. The chirality, induced to the key intermediate by a stereoselective enzymatic reduction utilizing NADPH-dependent ketoreductase, is described in detail.

New Naphthoquinone Terpenoids from Marine Actinobacterium, Streptomyces sp. CNQ-509

A member of the marine streptomycete clade MAR4, Streptomyces sp. CNQ-509, has genetic potential for the biosynthesis of hybrid isoprenoids and produces several meroterpenoids such as naphterpin, nitropyrrolin and marinophenazine. Our research on the strain CNQ-509 led to the isolation of two new naphterpin derivatives (1 and 2) comprised of naphthoquinone and geranyl moieties along with the known terpenoid, debromomarinone. The two-dimensional structure of these compounds was determined through spectral data analysis using data from NMR, MS and UV spectroscopy. Furthermore, the full structures of 1 and 2 including absolute configurations were unequivocally established by a combination of NMR experiments and chemical modifications.

Toward the Synthesis of Phormidolides

A convergent and stereoselective approach for the synthesis of marine natural product (MNP) phormidolide D (PM D) is proposed. Two main disconnections divided PM D in three molecular fragments: the macrocyclic core 4, the stapling iodoalkene 9 corresponding to the central part of PMs, and the east fragment 5 that includes the unusual bromo-methoxy-diene moiety and a tetradecanoic acid ended with a (E)-dichloro-ene functionality. Procedures for the preparation of compounds 5, 9, and the never-reported fatty acids 7 and 8, present in PMs C and D, respectively, have been afforded with good yields and high degree of stereoselectivity. The absolute configuration of all of the generated stereocenters has been established. The reaction to link iodoalkene 9 and formylmacrolactone 4, using the Nozaki-Hiyama-Takai-Kishi coupling, gave an advanced synthetic intermediate with total stereocontrol. Finally, a deeper study of protecting groups and reaction conditions for the last step of the synthesis is needed. All the information gathered in this publication will be of great value to continue performing synthetic studies for the preparation of these NPs. The versatility and the presence of a common polyol chain in oscillariolide and PMs A-C would allow applying the same retrosynthesis for the synthesis of the mentioned MNP.

Can Stereoclusters Separated by Two Methylene Groups Be Related by DFT Studies? The Case of the Cytotoxic Meroditerpenes Halioxepines

QM/NMR-DFT (quantum mechanics combined with nuclear magnetic resonance parameters calculated by density functional theory approximations) studies allowed us to link two stereoclusters separated by two methylene groups present in the new meroditerpenes halioxepine B (2) and halioxepine C (3) and the known halioxepine (1), isolated from two Indonesian sponges of the genus Haliclona (Reniera). DP4 and DP4+ probabilities were used to discriminate the two diastereotopic arrangements of the two stereoclusters, whose unconnected relative configurations were determined by ROESY and J-based configurational analysis. To confirm the DFT studies, the full relative configuration of 1 was deduced using a mixture of benzene-d₆ and pyridine-d₅ as the NMR solvent. ROESY measurements connected the two stereoclusters and demonstrated that DFT calculations accurately predict the configuration when two methylenes separate the two stereoclusters. The different arrangements of the distant stereoclusters C-1/C-2/C-7 and C-10/C-15 for compounds 2 and 3 were deduced by DFT calculations and explained the opposite optical rotations observed for the two compounds. Halioxepines B (2) and C (3) display moderate cytotoxicity against different human cancer cell lines.

Heteroatom-Directed Acylation of Secondary Alcohols To Assign Absolute Configuration

Birman's HBTM catalyst is effective for the enantioselective acylation and kinetic resolution of benzylic secondary alcohols. The enantioselective acylation has now been extended to secondary alcohols bearing electron-withdrawing groups such as halides and other heteroatoms. The level of selectivity is modest to good and is sufficient for determining configuration using the competing enantioselective conversion method. A mathematical analysis identifies conditions for achieving maximum differences in conversion and, consequently, assigning configuration with greater confidence. The new method is effective for halohydrins and secondary-tertiary 1,2-diols and was used to confirm the configuration of two inoterpene natural products.

Marine Natural Peptides: Determination of Absolute Configuration Using Liquid Chromatography Methods and Evaluation of Bioactivities

Over the last decades, many naturally occurring peptides have attracted the attention of medicinal chemists due to their promising applicability as pharmaceuticals or as models for drugs used in therapeutics. Marine peptides are chiral molecules comprising different amino acid residues. Therefore, it is essential to establish the configuration of the stereogenic carbon of their amino acid constituents for a total characterization and further synthesis to obtain higher amount of the bioactive marine peptides or as a basis for structural modifications for more potent derivatives. Moreover, it is also a crucial issue taking into account the mechanisms of molecular recognition and the influence of molecular three-dimensionality in this process. In this review, a literature survey covering the report on the determination of absolute configuration of the amino acid residues of diverse marine peptides by chromatographic methodologies is presented. A brief summary of their biological activities was also included emphasizing to the most promising marine peptides. A case study describing an experience of our group was also included.

F-THENA: a chiral derivatizing agent for the determination of the absolute configuration of secondary aromatic alcohols with a self-validating system

F-THENA is designed as an alternative fluorine-containing chiral derivatizing agent (CDA). The fluorine atom functions exclusively as a reporter which can directly sense an anisotropic effect from an aromatic substituent of a chiral alcohol. In combination with chemical shift differences from both ¹⁹F NMR and ¹H NMR, the F-THENA method can successfully be used for determining the absolute configuration of chiral secondary aromatic alcohols with a self-validating system.

A chiral aluminum solvating agent (CASA) for 1H NMR chiral analysis of alcohols at low temperature

A chiral aluminum solvating agent (CASA) was demonstrated to be a general and efficient reagent for ¹H NMR chiral analysis of alcohols.

A photo-stable fluorescent chiral thiourea probe for enantioselective discrimination of chiral guests

Herein, a chiral thiourea Schiff base derived from (1R,2R)-1,2-cyclohexanediamine and tetraphenylethylene (TPE) was applied as a highly effective chiral sensor for the enantioselective discrimination of various acids and aminesviaion-pair and hydrogen-bond interaction.

α-Amylase and α-Glucosidase Inhibitory Activities of Chemical Constituents from Wedelia chinensis (Osbeck.) Merr. Leaves

As part of an ongoing search for new natural products from medicinal plants to treat type 2 diabetes, two new compounds, a megastigmane sesquiterpenoid sulfonic acid (1) and a new cyclohexylethanoid derivative (2), and seven related known compounds (3-9) were isolated from the leaves of Wedelia chinensis (Osbeck.) Merr. The structures of the compounds were conducted via interpretation of their spectroscopic data (1D and 2D NMR, IR, and MS), and the absolute configurations of compound 1 were determined by the modified Mosher's method. The MeOH extract of W. chinensis was found to inhibit α-amylase and α-glucosidase inhibitory activities as well as by the compounds isolated from this extract. Furthermore, compound 7 showed the strongest effect with IC₅₀ values of 112.8 ± 15.1 μg/mL (against α-amylase) and 785.9 ± 12.7 μg/mL (against α-glucosidase). Compounds 1, 8, and 9 showed moderate α-amylase and α-glucosidase inhibitory effects. Other compounds showed weak or did not show any effect on both enzymes. The results suggested that the antidiabetic properties from the leaves of W. chinensis are not simply a result of each isolated compound but are due to other components such as the accessibility of polyphenolic groups to α-amylase and α-glucosidase activities.

Chiral sensors for determining the absolute configurations of α-amino acid derivatives

Two simple ¹H NMR tests give the absolute configurations of α-amino acids.

A simple ternary ion-pair complexation protocol for testing the enantiopurity and the absolute configurational analysis of acid and ester derivatives

The ternary ion-pair complexation protocol for rapid testing of the enantiopurity and the assignment of absolute configurations of various acid and ester derivatives.

Chemical constituents from Taraxacum officinale and their α-glucosidase inhibitory activities

Three novel butyrolactones (1-3) and butanoates (4-6), namely taraxiroside A-F, were isolated from Taraxacum officinale along with twenty-two known compounds (7-28). Their chemical structures were elucidated by interpretation of spectroscopic data and comparison with those of literatures. All isolates were evaluated for their α-glucosidase inhibitory activities. Novel compounds 1-6 (IC₅₀ 145.3-181.3 μM) showed inhibitory activities similar to that of acarbose (IC₅₀ 179.9 μM). Compound 7 and 12 were the most potent inhibitor with IC₅₀ values of 61.2 and 39.8 μM respectively. Compounds 2 and 12 showed as mixed-type inhibition, whereas compound 7 and acarbose showed competitive inhibition.

Di(1-naphthyl) methanol ester of carboxylic acids for absolute stereochemical determination

The absolute stereochemistry of chiral carboxylic acids is determined as a di(1-naphthyl)methanol ester derivative. Computational scoring of conformations favoring either P or M helicity of the naphthyl groups, capable of exciton-coupled circular dichroic coupling, leads to a predicted stereochemistry for the derivatized carboxylic acids.

Chiral Molecular Science: How were the absolute configurations of chiral molecules determined? "Experimental results and theories"

Molecular chirality is a key concept in chemistry, bioscience, and molecular technology, like the invention of a light-powered chiral molecular motor explained in this review. Thus, the primary research subject is how to determine the absolute configuration (AC) of chiral compounds. This review article focuses on the principle, theory, and practice of the nonempirical methods for determining ACs of chiral compounds, i.e., the Bijvoet method in X-ray crystallography and the circular dichroism (CD) exciton chirality method, together with the historical aspects of AC determination. The theoretical equations of X-ray crystallography and exciton CD spectroscopy are explained in detail, and these equations are useful for readers to understand the principle and mechanism of these methods. This review also focuses on the relative methods, where the internal reference with known AC is used and the relative configuration is determined by X-ray crystallography and/or ¹ H nuclear magnetic resonance (NMR) diamagnetic anisotropy method. In these cases, CSDP acid and MαNP acid are useful for the chiral resolution of racemic alcohols, where their diastereomeric esters are easily separable by high-performance liquid chromatography (HPLC) on silica gel. Thus, these methods are useful for the preparation of enantiopure compounds and simultaneous determination of their ACs. In this review article, the above methods are explained mainly based on the author's own research results.

Heterocyclic pharmacochemistry of new rhinovirus antiviral agents: A combined computational and experimental study

Rhinovirus (RV), member of the Enterovirus genus, is known to be involved in more than half of the common colds. Through advances in molecular biology, rhinoviruses have also been associated with exacerbations of chronic pulmonary diseases (e.g. asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis). In the current investigation, we develop a novel series of 4,5-dimethoxybenzyl derivatives that potently inhibits rhinovirus replication. Compound (S)-7f blocks RV-B14 replication with an EC₅₀ value of 0.25 μM and shows a low toxicity in HeLa cells (CC₅₀ > 271 μM). Enantioseparation followed by an absolute configuration determination by a Mosher's method revealed the interest of enantiopure compounds. Molecular docking studies permitted the identification of key biological interactions within the drug-binding pocket and an in silico drug-like study revealed a good potential for the development of these derivatives.

Dibenzo-α-pyrones: a new class of larvicidal metabolites against Aedes aegypti from the endophytic fungus Hyalodendriella sp. Ponipodef12

BACKGROUND

In our search for new agrochemicals from endophytic fungi, the crude extract of the endophytic Hyalodendriella sp. Ponipodef12 associated with the hybrid 'Neva' of Populus deltoides Marsh × P. nigra L. was found to possess larvicidal activity against Aedes aegypti.

RESULTS

Fractionation of the extract has led to the isolation of 11 dibenzo-α-pyrones (1-11), including three new congeners: hyalodendriols A-C (1-3). The structures of the new compounds were elucidated by comprehensive spectroscopic analyses, including the modified Mosher's method for the assignment of the absolute configuration. Compounds 2-7 showed potent larvicidal activities against the fourth-instar larvae of A. aegypti with IC₅₀ values ranging from 7.21 to 120.81 µg mL^-1 . Among them, penicilliumolide D (6) displayed the strongest activity (IC₅₀ = 7.21 µg mL^-1 ). A structure-larvicidal activity relationship was discussed. The possible mode of action of these compounds was assessed for their acetylcholinesterase inhibitory activities. In addition, hyalodendriol C (3) displayed antibacterial activity against Bacillus subtilis and Xanthomonas vesicatoria, and exhibited strong inhibition against the spore germination of Magnaporthe oryzae.

CONCLUSION

Our study revealed dibenzo-α-pyrones to be a new class of larvicidal metabolites against A. aegypti. © 2016 Society of Chemical Industry.

5,6,7,8-Tetrahydro-2-(2-phenylethyl)chromones from artificial agarwood of Aquilaria sinensis and their inhibitory activity against acetylcholinesterase

Thirteen previously undescribed 5,6,7,8-tetrahydro-2-(2-phenylethyl)chromones named tetrahydrochromone A-M, together with nine known ones, were isolated from artificial agarwood (induced by holing) originating from Aquilaria sinensis (Lour.) Gilg. The structures of these compounds were unambiguously determined based on extensive NMR spectroscopic analyses, and the absolute configuration was resolved by CD analyses, X-ray crystallographic, chemical and Mosher's method. Tetrahydrochromone A, B, K-M, and Oxidoagarochromone An exhibited inhibitory activity against AChE with the percentage inhibition range from 17.5% to 47.9% (with Tacrine as the positive control; inhibition ratio: 66.7%) when tested at 50 μg/mL. Tetrahydrochromone A-E, F-J feature one methoxy and three hydroxys linked at the cyclohexene ring rather than usual four hydroxys, and tetrahydrochromone K-M represent the first examples of 7,8-epoxy tetrahydrochromones.

Determination of the Absolute Configuration of β-Chiral Primary Alcohols Using the Competing Enantioselective Conversion Method

A method for determining the absolute configuration of β-chiral primary alcohols has been developed. Enantioenriched alcohols were acylated in the presence of either enantiomer of the enantioselective acylation catalyst HBTM, and the faster reaction was determined by measuring product conversion using 1H NMR spectroscopic analysis. An empirical mnemonic was developed that correlates the absolute configuration of the alcohol to the faster reacting catalyst. Successful substrates for this method include primary alcohols that bear a "directing group" on the stereogenic center; directing groups include arenes, heteroarenes, enones, and halides.

Daryamide Analogues from a Marine-Derived Streptomyces species

Three new cyclohexene amine derivatives, daryamides D-F (1-3), a new arylamine derivative, carpatamide D (4), and a new ornithine lactamization derivative, ornilactam A (5), were isolated from the marine-derived Streptomyces strain SNE-011. Their structures, including absolute configurations, were elucidated on the basis of spectroscopic analysis and chemical methods. The carpatamide skeleton could be considered as the biosynthetic precursor of the daryamides.

The Berkeleylactones, Antibiotic Macrolides from Fungal Coculture

A carefully timed coculture fermentation of Penicillium fuscum and P. camembertii/clavigerum yielded eight new 16-membered-ring macrolides, berkeleylactones A-H (1, 4, 6-9, 12, 13), as well as the known antibiotic macrolide A26771B (5), patulin, and citrinin. There was no evidence of the production of the berkeleylactones or A26771B (5) by either fungus when grown as axenic cultures. The structures were deduced from analyses of spectral data, and the absolute configurations of compounds 1 and 9 were determined by single-crystal X-ray crystallography. Berkeleylactone A (1) exhibited the most potent antimicrobial activity of the macrolide series, with low micromolar activity (MIC = 1-2 μg/mL) against four MRSA strains, as well as Bacillus anthracis, Streptococcus pyogenes, Candida albicans, and Candida glabrata. Mode of action studies have shown that, unlike other macrolide antibiotics, berkeleylactone A (1) does not inhibit protein synthesis nor target the ribosome, which suggests a novel mode of action for its antibiotic activity.

Penicilones A-D, Anti-MRSA Azaphilones from the Marine-Derived Fungus Penicillium janthinellum HK1-6

Four new azaphilones, penicilones A-D (1-4), were isolated from the mangrove rhizosphere soil-derived fungus Penicillium janthinellum HK1-6. Their planar structures and absolute configurations were determined by extensive analysis of NMR spectroscopic data, ECD spectra, the modified Mosher's method, and chemical conversions. Interestingly, 1 and 2 had the opposite configuration at C-7 compared to the closely related chloro analogues 3 and 4. Ester hydrolysis of 2 and 4 afforded their parental azaphilones, named penicilones E (5) and F (6). Compounds 1-6 were evaluated for their antibacterial activities in vitro. Penicilones B-D (2-4) showed potent anti-MRSA (Staphylococcus aureus ATCC 43300, ATCC 33591) activities with MIC values ranging from 3.13 to 6.25 μg/mL.

Stereoselective preparation of quaternary 2-vinyl sphingosines and ceramides and their effect on basal sphingolipid metabolism

The dicyclohexylborane-mediated addition of allene 1 to (E)-2-tridecenal affords a quaternary protected 2-amino-2-vinyl-1,3-diol in good yield as a single diastereomer. This compound is readily transformed into the four stereoisomers of the quaternary (E)-2-vinyl analogs of sphingosine. The metabolic fate and the effect of these compounds on the basal sphingolipid metabolism in human A549 lung adenocarcinoma cells has been studied, together with the ceramide analog of the most relevant vinylsphingosine derivative.

Enantioselective Synthesis of (+)-Mitomycin K by a Palladium-Catalyzed Oxidative Tandem Cyclization

The mitomycins, a family of bioactive natural products, feature a compact 6/5/5-fused polycyclic ring structure densely decorated with highly reactive and/or fragile quinone, amino ketal, and aziridine as well as carbamate moieties. It is this striking feature that has defeated numerous synthetic attempts towards these apparently small molecules, rendering them one of the most formidable targets for total synthesis. We herein report the first enantioselective synthesis of (+)-mitomycin K, a representative of G series mitomycins. The key step of this synthesis is an enantioselective oxidative cyclization catalyzed by a palladium/(+)-sparteine system that had previously been developed by our group. The robustness of this method bodes well for further applications in the asymmetric total synthesis of natural products, particularly those with characteristic 6/5/5-fused pyrroloindole skeletons.

In-tube derivatization for determination of absolute configuration and enantiomeric purity of chiral compounds by NMR spectroscopy

We have developed an in-tube derivatization method using commercially available polymer-supported coupling agents to prepare derivatives of chiral compounds directly in NMR tube with high yield and purity. Because the method does not require any workup or purification, the configuration and enatiopurity can be quickly determined by NMR analysis for a small amount of chiral compounds, which is critical for today's fast-paced medicinal chemistry efforts in drug discovery. The application of the method was demonstrated for the derivatization of chiral amines, alcohols, diols, amino alcohols, thiols, and carboxylic acids using various chiral derivatizing agents and coupling agents. This article also serves as a practical guide for in-tube derivatization and selection of suitable chiral derivatizing agents and coupling agents for various types of chiral compounds. Copyright © 2016 John Wiley & Sons, Ltd.

Separation of Alkyne Enantiomers by Chiral Column HPLC Analysis of Their Cobalt-Complexes

Separation of the enantiomers of new chiral alkynes in strategic syntheses and bioorthogonal studies is always problematic. The chiral column high-performance liquid chromatography (HPLC) method in general could not be directly used to resolve such substrates, since the differentiation of the alkyne segment with the other alkane/alkene segment is not significant in the stationary phase, and the alkyne group is not a good UV chromophore. Usually, a pre-column derivatization reaction with a tedious workup procedure is needed. Making use of easily-prepared stable alkyne-cobalt-complexes, we developed a simple and general method by analyzing the in situ generated cobalt-complex of chiral alkynes using chiral column HPLC. This new method is especially suitable for the alkynes without chromophores and other derivable groups.

An epigenetic modifier induces production of (10'S)-verruculide B, an inhibitor of protein tyrosine phosphatases by Phoma sp. nov. LG0217, a fungal endophyte of Parkinsonia microphylla

Incorporation of the histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), to a culture broth of the endophytic fungus Phoma sp. nov. LG0217 isolated from Parkinsonia microphylla changed its metabolite profile and resulted in the production of (10'S)-verruculide B (1), vermistatin (2) and dihydrovermistatin (3). When cultured in the absence of the epigenetic modifier, it produced a new metabolite, (S,Z)-5-(3',4'-dihydroxybutyldiene)-3-propylfuran-2(5H)-one (4) together with nafuredin (5). The structure of 4 was elucidated by spectroscopic analyses and its absolute configuration was determined by application of the modified Mosher's ester method. The absolute structure of (10'S)-verruculide B was determined as 5-[(10'S,2'E,6'E)-10',11'-dihydroxy-3',7',11'-trimethyldodeca-2',6'-dien-1'-yl]-(3R)-6,8-dihydroxy-3-methylisochroman-1-one (1) with the help of CD and NOE data. Compound 1 inhibited the activity of protein tyrosine phosphatases (PTPs) 1B (PTP1B), Src homology 2-containing PTP 1 (SHP1) and T-cell PTP (TCPTP) with IC50 values of 13.7±3.4, 8.8±0.6, and 16.6±3.8μM, respectively. Significance of these activities and observed modest selectivity of 1 for SHP1 over PTP1B and TCPTP is discussed.

Improving the performance of J-modulated ADEQUATE experiments through homonuclear decoupling and non-uniform sampling

Homonuclear ¹³ C-¹³ C couplings at natural abundance can be measured using the J-modulated Adequate DoublE QUAntumTransfer Experiment (ADEQUATE) experiment. To somewhat ameliorate F₁ digitization requirements, a scaling factor was incorporated into the original pulse sequence. Non-uniform sampling provides an obvious avenue to further facilitate the acquisition of ¹ J_CC and ⁿ J_CC homonuclear coupling constant data. We introduce homonuclear decoupling (HD) analogous to that described for the 1,1-HD-ADEQUATE and 1,n-HD-ADEQUATE experiments and evaluate the combination of non-uniform sampling and HD on the acquisition of both ¹ J_CC and ⁿ J_CC homonuclear ¹³ C-¹³ C coupling constants using ibuprofen as a model compound. Copyright © 2015 John Wiley & Sons, Ltd.