D-(−)-Quinic acid: a chiron store for natural product synthesis

Synthesis of alcohols: streamlined C1 to Cn hydroxyalkylation through photoredox catalysis

Naturally occurring and readily available α-hydroxy carboxylic acids (AHAs) are utilized as platforms for visible light-mediated oxidative CO2-extrusion furnishing α-hydroxy radicals proved to be versatile C1 to Cn hydroxyalkylating agents. The direct decarboxylative Giese reaction (DDGR) is operationally simple, not requiring activator or sacrificial oxidants, and enables the synthesis of a diverse range of hydroxylated products, introducing connectivity typically precluded from conventional polar domains. Notably, the methodology has been extended to widely used glycolic acid resulting in a highly efficient and unprecedented C1 hydroxyhomologation tactic. The use of flow technology further facilitates scalability and adds green credentials to this synthetic methodology.

Liquid chromatography-tandem mass spectrometry profile and antioxidant, antimicrobial, antiproliferative, and enzyme activities of Thymus pectinatus and Thymus convolutus: in vitro and in silico approach

BACKGROUND

The objective of this study is to investigate the antiproliferative, antioxidant, antimicrobial, and enzyme activity capacities and phytochemical compositions of Thymus pectinatus (TP), Thymus convolutus (TC), which are endemic to Türkiye. Quantitative analysis of phenolic compounds in the extracts was conducted using liquid chromatography-tandem mass spectrometry, targeting 53 phenolic compounds.

RESULTS

Rosmarinic acid, quinic acid, and cynaroside were identified as the major compounds, exhibiting quantitative variation in both extracts. The extracts had a high total phenolic content, with 113.57 ± 0.58 mg gallic acid equivalents (GAE)/g extract for TP and 130.52 ± 1.05 mg GAE/g extract for TC. Furthermore, although both extracts exhibited high total flavonoid content; the TP extract (75.12 ± 1.65 mg quercitin equivalents (QE)/g extract) displayed a higher flavonoid content than the TC extract (30.24 ± 0.74 mg QE/g extract) did. The extracts had a promising antiproliferative effect on C6, HeLa, and HT29 cancer cell lines with a less cytotoxic effect (10.5-14.2%) against normal cells. Both extracts exhibited very potent inhibitory activity against the xanthine oxidase enzyme, with half-maximal inhibitory concentration values of respectively 2.07 ± 0.03 μg mL-1 and 2.76 ± 0.06 μg mL-1 and moderate activity against tyrosinase and α-glucosidase. Docking simulations proved that rosmarinic acid and cynaroside, the major components of the extracts, were the most potent inhibitors of xanthine oxidase. According to antimicrobial activity results, the TC extract exhibited moderate activity against Staphylococcus aureus, and the TP extract had strong activity against both Enterococcus faecium and S. aureus.

CONCLUSION

These findings emphasize the beneficial effects of the two endemic Thymus species on human health and suggest their potential use as plant-derived bioactive agents. © 2024 Society of Chemical Industry.

Phytochemical profiles and evaluation of the biological potential of ethyl acetate and n-butanol fractions of the aerial parts of Cistus albidus L

The phytochemical profiles of the ethyl acetate (EAFCA) and n-butanol (BFCA) fractions of the aerial parts of Cistus albidus L., were characterised by a developed LC-ESI-MS/MS method, using 53 fingerprint phytochemicals, revealing the presence of 26 and 22 compounds, respectively with quinic acid, gallic acid, protocatechuic acid, catechin, gentisic acid, tannic acid, ellagic acid, quercitrin, astragalin as the major compounds of the EAFCA, in very higher amounts than in the BFCA. The antioxidant activity was evaluated using six methods (DPPH•, ABTS•+, GOR•, CUPRAC, FRAP and Phenanthroline). In agreement with its highest polyphenolic content (430.12 ± 1.02 µg GAE/mg of extract) and largest amounts of identified polyphenolics, the EAFCA exhibited a higher antioxidant activity than the BFCA. Additionally, the EAFCA showed the highest acetylcholinesterase (AChE) inhibition (IC50 of 25.1 ± 2.58 µg/mL). Moreover, both EAFCA and BFCA demonstrated high photoprotective activity, with sun protection factor (SPF) values of 36.49 and 36.52, respectively.

Study on the Distribution of Low Molecular Weight Metabolites in Mango Fruit by Air Flow-Assisted Ionization Mass Spectrometry Imaging

Mass spectrometry imaging is a novel molecular imaging technique that has been developing rapidly in recent years. Air flow-assisted ionization mass spectrometry imaging (AFAI-MSI) has received wide attention in the biomedical field because of its features such as not needing a pretreatment sample, having high sensitivity, and wide coverage of metabolite detection. In this study, we set up a mass spectrometry imaging method for analyzing low molecular metabolites in mango fruits by the AFAI-MSI method. Compounds such as organic acids, vitamin C, and phenols were detected from mango tissue by mass spectrometry under the negative ion scanning mode, and their spatial distribution was analyzed. As a result, all the target compounds showed different distributions. Citric acid was mainly distributed in the pulp. Malic acid, quinic acid, and vitamin C universally existed in the pulp and peel. However, galloylglucose isomer and 5-galloylquinic acid were predominantly found in the peel. These results show that AFAI-MSI can be used for the analysis of mango fruit endogenous metabolites conveniently and directly, which will facilitate the rapid identification and in situ characterization of plant endogenous substances.

Virtual Screening of Natural Chemical Databases to Search for Potential ACE2 Inhibitors

The angiotensin-converting enzyme II (ACE2) is a multifunctional protein in both health and disease conditions, which serves as a counterregulatory component of RAS function in a cardioprotective role. ACE2 modulation may also have relevance to ovarian cancer, diabetes, acute lung injury, fibrotic diseases, etc. Furthermore, since the outbreak of the coronavirus disease in 2019 (COVID-19), ACE2 has been recognized as the host receptor of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The receptor binding domain of the SARS-CoV-2 S-protein has a strong interaction with ACE2, so ACE2 may be a potent drug target to prevent the virus from invading host cells for anti-COVID-19 drug discovery. In this study, structure- and property-based virtual screening methods were combined to filter natural product databases from ChemDiv, TargetMol, and InterBioScreen to find potential ACE2 inhibitors. The binding affinity between protein and ligands was predicted using both Glide SP and XP scoring functions and the MM-GBSA method. ADME properties were also calculated to evaluate chemical drug-likeness. Then, molecular dynamics (MD) simulations were performed to further explore the binding modes between the highest-potential compounds and ACE2. Results showed that the compounds 154-23-4 and STOCK1N-07141 possess potential ACE2 inhibition activities and deserve further study.

Catalytic Enantioselective Alkylation of Prochiral Enolates

The asymmetric alkylation of enolates is a particularly versatile method for the construction of α-stereogenic carbonyl motifs, which are ubiquitous in synthetic chemistry. Over the past several decades, the focus has shifted to the development of new catalytic methods that depart from classical stoichiometric stereoinduction strategies (e.g., chiral auxiliaries, chiral alkali metal amide bases, chiral electrophiles, etc.). In this way, the enantioselective alkylation of prochiral enolates greatly improves the step- and redox-economy of this process, in addition to enhancing the scope and selectivity of these reactions. In this review, we summarize the origin and advancement of catalytic enantioselective enolate alkylation methods, with a directed emphasis on the union of prochiral nucleophiles with carbon-centered electrophiles for the construction of α-stereogenic carbonyl derivatives. Hence, the transformative developments for each distinct class of nucleophile (e.g., ketone enolates, ester enolates, amide enolates, etc.) are presented in a modular format to highlight the state-of-the-art methods and current limitations in each area.

Deoxygenative Divergent Synthesis: En Route to Quinic Acid Chirons

The installation of vicinal mesylate and silyl ether groups in a quinic acid derivative generates a system prone for stereoselective borane-catalyzed hydrosilylation through a siloxonium intermediate. The diversification of the reaction conditions allowed the construction of different defunctionalized fragments foreseen as useful synthetic fragments. The selectivity of the hydrosilylation was rationalized on the basis of deuteration experiments and computational studies.

13C-1H coupling constants as a conformational tool for structural assignment of quinic and octulosonic acid

A complete set of NMR coupling constants (¹J_C-H, ²J_C-H, ³J_C-H, and ³J_H-H) were calculated for the eight stereoisomers of quinic acid, at the B3LYP/6-311G(d,p)/PCM(methanol) level of theory. The Fermi contact term of the coupling constants was computed with a modified, uncontracted, version of the 6-311G(d,p) basis set, with additional tight polarization functions. ¹H and ¹³C NMR chemical shifts were determined at the same level using the gauge-invariant atomic orbital (GIAO) method. The magnitude of the spin-spin coupling constants was found to be affected by the orientation (axial or equatorial) of the coupling proton and the orientation of the hydroxy group on the coupling carbon, whereas the chemical shifts depend on the presence or absence of electron-withdrawing hydroxy groups attached to the carbon atoms involved. Graphical Abstract Nuclear magnetic resonance coupling constants, computed with density functional theory, can be used to differentiate and identify the different stereoisomers of quinic acid.

Regulation of phytochemicals in fruits and berries by environmental variation-Sugars and organic acids

Sugars and organic acids are important phytochemicals contributing to the nutrition and sensory properties of fruits and berries. Their contents are closely correlated to the genetic background of plants as well as to the environmental conditions during growth. This review focuses on the recent researches on the metabolism of these compounds in fruits and berries in response to the variation of environmental conditions, including temperature, radiation, and water supply. A great deal of investigations indicates that the influence of environmental factors on the composition of fruits/berries depended largely on the genetic background. Moreover, the metabolic regulation in response to environmental changes also varies between different plant developmental stages. Nevertheless, some general trends, like the positive correlation between light intensity and sugar content, were observed in most investigations. In grapes (Vitis vinifera L.), the content of malic acid always decreases as light intensity increases, and as the water supply decreases. PRACTICAL APPLICATIONS: The contents of sugars and organic acids, and especially their relative ratio, are important indicators determining the taste and quality of fruits and fruit products. In this review, we summarized the investigations carried out on the regulation of these sensory contributing primary metabolites in fruits and berries in relation to the variation of environmental conditions. It was indicated that various factors, such as plant genotype, growing period, and interaction between environmental factors, might contribute to the impact of environmental changes on the composition of fruits/berries. The article not only provides comprehensive knowledges in food chemistry and plant physiology but also provide important background knowledge for berry cultivation and breeding, as well as useful guidelines for utilization of fruits and berries in food industry.

Novel comprehensive multidimensional liquid chromatography approach for elucidation of the microbosphere of shikimate-producing Escherichia coli SP1.1/pKD15.071 strain

Shikimic acid is a intermediate of aromatic amino acid biosynthesis and the preferred starting material for production of the most commonly prescribed anti-influenza drug, Tamiflu. Its six-membered carbocyclic ring is adorned with several chiral centers and various functionalities, making shikimic acid a valuable chiral synthon. When microbially-produced, in addition to shikimic acid, numerous other metabolites are exported out of the cytoplasm and accumulate in the culture medium. This extracellular matrix of metabolites is referred to as the microbosphere. Due to the high sample complexity, in this study, the microbosphere of shikimate-producing Escherichia coli SP1.1/pKD15.071 was analyzed by liquid chromatography and comprehensive two-dimensional liquid chromatography coupled to photodiode array and mass spectrometry detection. GC analysis of the trimethylsilyl derivatives was also carried out in order to support the elucidation of the selected metabolites in the microbosphere. The elucidation of the metabolic fraction of this bacterial strain might be of valid aid for improving, through genetic changes, the concentration and yield of shikimic acid synthesized from glucose. Graphical abstract.

Phytochemical Analysis and Antifungal Activity of Extracts from Leaves and Fruit Residues of Brazilian Savanna Plants Aiming Its Use as Safe Fungicides

The increasing demand for safe food without preservatives or pesticides residues has encouraged several studies on natural products with antifungal activity and low toxicity. In this study, ethanolic extracts from leaves and fruit residues (peel and seeds) of three Brazilian savanna species (Acrocomia aculeata, Campomanesia adamantium and Caryocar brasiliense) were evaluated against phytopathogenic fungi. Additionally, the most active extract was chemically characterized by ESI-MS and its oral acute toxicity was evaluated. Extracts from C. brasiliense (pequi) peel and leaves were active against Alternaria alternata, Alternaria solani and Venturia pirina with minimal inhibitory concentrations between 350 and 1000 µg/mL. When incorporated in solid media, these extracts extended the lag phase of A. alternata and A. solani and reduced the growth rate of A. solani. Pequi peel extract showed better antifungal activity and their ESI-MS analysis revealed the presence of substances widely reported as antifungal such as gallic acid, quinic acid, ellagic acid, glucogalin and corilagin. The oral acute toxicity was relatively low, being considered safe for use as a potential natural fungicide.

Catalytic enantioselective total synthesis of (+)-eucomic acid

A catalytic enantioselective synthesis of (+)-eucomic acid is reported. A palladium-catalyzed asymmetric allylic alkylation is employed to access the chiral tetrasubstituted α-hydroxyacid moiety found in the natural product. The protecting group strategy was investigated, and a protecting group manipulation was made without any appreciable deleterious effects in the allylic alkylation reaction. Non-natural (+)-eucomic acid is synthesized in a longest linear sequence of 13 steps.

Quinic acid derivatives inhibit dengue virus replication in vitro

BACKGROUND

Dengue is the most prevalent arboviral disease in tropical and sub-tropical areas of the world. The incidence of infection is estimated to be 390 million cases and 25,000 deaths per year. Despite these numbers, neither a specific treatment nor a preventive vaccine is available to protect people living in areas of high risk.

RESULTS

With the aim of seeking a treatment that can mitigate dengue infection, we demonstrated that the quinic acid derivatives known as compound 2 and compound 10 were effective against all four dengue virus serotypes and safe for use in a human hepatoma cell line (Huh7.5). Both compounds were non-virucidal to dengue virus particles and did not interfere with early steps of the dengue virus life cycle, including binding and internalization. Experiments using a replicon system demonstrated that compounds 2 and 10 impaired dengue virus replication in Huh7.5 cells. Additionally, the anti-dengue virus effects of the quinic acid derivatives were preserved in human peripheral blood mononuclear cells.

CONCLUSIONS

Taken together, these data suggest that quinic acid derivatives represent a novel chemical class of active compounds that could be used to combat dengue virus infection.

Synthesis and Biological Activity of 2-Methylene Analogues of Calcitriol and Related Compounds

In an attempt to prepare vitamin D analogues that are superagonists, (20R)- and (20S)-isomers of 1α-hydroxy-2-methylenevitamin D3 and 1α,25-dihydroxy-2-methylenevitamin D3 have been synthesized. To prepare the desired A-ring dienyne fragment, two different approaches were used, both starting from the (-)-quinic acid. The obtained derivative was subsequently coupled with the C,D-ring enol triflates derived from the corresponding Grundmann ketones, using the Sonogashira reaction. Moreover, (20R)- and (20S)-1α,25-dihydroxy-2-methylenevitamin D3 compounds with an (5E)-configuration were prepared by iodine catalyzed isomerization. All four 2-methylene analogues of the native hormone were characterized by high in vitro activity. As expected, the 25-desoxy analogues were much less potent. Among the synthesized compounds, two of them, 1α,25-dihydroxy-2-methylenevitamin D3 and its C-20 epimer, were found to be almost as active as 2-methylene-19-nor-(20S)-1α,25-dihydroxyvitamin D3 (2MD) on bone but more active in intestine.

Synthesis and Antibacterial Activity of Alkylated Diamines and Amphiphilic Amides of Quinic Acid Derivatives

Different series of N-alkylated diamines and their derivatives condensed to quinic acid were synthesized and tested for antibacterial properties against Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Mycobacterium tuberculosis. The lipophilic chain and carbohydrate moiety modulate the antibacterial activity and the compounds showed a structure-activity relationship. Overall, 11 compounds displayed better activity than chloramphenicol against Gram-positive and Gram-negative bacteria. Monoalkylated amines 2a-h displayed an activity similar to that of ethambutol against Mycobacterium tuberculosis.

The shikimate dehydrogenase family: functional diversity within a conserved structural and mechanistic framework

Shikimate dehydrogenase (SDH) catalyzes the NADPH-dependent reduction of 3-deydroshikimate to shikimate, an essential reaction in the biosynthesis of the aromatic amino acids and a large number of other secondary metabolites in plants and microbes. The indispensible nature of this enzyme makes it a potential target for herbicides and antimicrobials. SDH is the archetypal member of a large protein family, which contains at least four additional functional classes with diverse metabolic roles. The different members of the SDH family share a highly similar three-dimensional structure and utilize a conserved catalytic mechanism, but exhibit distinct substrate preferences, making the family a particularly interesting system for studying modes of substrate recognition used by enzymes. Here, we review our current understanding of the biochemical and structural properties of each of the five previously identified SDH family functional classes.

Formal total syntheses of classic natural product target molecules via palladium-catalyzed enantioselective alkylation

Pd-catalyzed enantioselective alkylation in conjunction with further synthetic elaboration enables the formal total syntheses of a number of "classic" natural product target molecules. This publication highlights recent methods for setting quaternary and tetrasubstituted tertiary carbon stereocenters to address the synthetic hurdles encountered over many decades across multiple compound classes spanning carbohydrate derivatives, terpenes, and alkaloids. These enantioselective methods will impact both academic and industrial settings, where the synthesis of stereogenic quaternary carbons is a continuing challenge.

Facile synthesis of the cyclohexane fragment of enacloxins, a series of antibiotics isolated from Frateuria sp. W-315

An efficient and good yield synthesis of the cyclohexane moiety of enacyloxins, a series of antibiotics isolated from Frateuria sp. W-315, was achieved from d-quinic acid using a successive Barton-McCombie deoxygenation.

Enantioselective decarboxylative alkylation reactions: catalyst development, substrate scope, and mechanistic studies

α-Quaternary ketones are accessed through novel enantioselective alkylations of allyl and propargyl electrophiles by unstabilized prochiral enolate nucleophiles in the presence of palladium complexes with various phosphinooxazoline (PHOX) ligands. Excellent yields and high enantiomeric excesses are obtained from three classes of enolate precursor: enol carbonates, enol silanes, and racemic β-ketoesters. Each of these substrate classes functions with nearly identical efficiency in terms of yield and enantioselectivity. Catalyst discovery and development, the optimization of reaction conditions, the exploration of reaction scope, and applications in target-directed synthesis are reported. Experimental observations suggest that these alkylation reactions occur through an unusual inner-sphere mechanism involving binding of the prochiral enolate nucleophile directly to the palladium center.

Synthetic and structural studies on macrocyclic amino cyclitols--conformational chameleons

Starting from quinic acid the synthesis of 1,4-butanediol-linked macrocyclic aminocyclitols 30, 32, 34, 36 and 38 is described. Assembly was achieved by olefin cross-metathesis of appropriate cyclohexyl allyl ethers followed by ring-closing metathesis of bis-O-allyl homodimers. In all five cases studied, the only products that were formed were those resulting from direct ring-closing metathesis; the formation of larger rings was not detected. These macrocycles exhibited diverse conformational behaviour which included formation of stable separable conformers 31a and 31b as well as conformationally dynamic macrocycles 35 in which a ring flip in one cyclohexane chair conformer induces a ring flip of the other cyclohexane ring through the linking chains of the macrocycles. The activation energy for the inversion of the chair conformation in this process was determined to be about 38 kJ mol(-1), which is about 7 kJ mol(-1) lower than the activation energy for the ring flip of the unsubstituted cyclohexane ring. In all cases, the conformational studies strongly suggest that intramolecular H-bonding between 1,3-diaxially oriented amido and alcohol or ether groups exerts a decisive contribution to the overall stabilisation of the preferred cyclohexane chair conformation.

First Total Synthesis of Antitumor Natural Product (+)- and (−)-Pericosine A: Determination of Absolute Stereo Structure†

The first total synthesis of (+)- and (-)-pericosine A has been achieved, enabling the revision and determination of the absolute configuration of this antitumor natural product as methyl (3S,4S,5S,6S)-6-chloro-3,4,5-trihydroxy-1-cyclohexene-1-carboxylate. Every step of this total synthesis proceeded well with excellent stereoselectivity. Structures of the intermediates in crucial steps were confirmed by detailed 2D NMR analysis.

A unified synthetic approach to the pyrazinone dragmacidins

This review describes recent developments from our laboratory involving the synthesis of the structurally complex, pyrazinone-containing dragmacidin alkaloids.

Studies for the transformation of carbocycles into carbohydrates: approach toward the synthesis of higher sugar derivatives

A highly stereocontrolled synthesis of a beta-D-ribo-hept-6-ulopyranosuronamide derivative, a useful intermediate for the synthesis of other higher sugars, has been developed using naturally occurring (-)-quinic acid as a chiral starting material. The transformation of carbocycle to carbohydrate, a key step in this sequence, occurred in a one-pot reaction: an ozonolysis carried out under mild conditions.