Synthesis of new chiral xanthone derivatives acting as nerve conduction blockers in the rat sciatic nerve

Rubraxanthone: A Promising Xanthone From Guttiferae Family with Diverse Biological Activities

The transformation of primary metabolites into secondary metabolites is a natural process that results in the formation of secondary metabolites, which are also referred to as natural products. Among the secondary metabolites that are highly isolated from the Guttiferae family, xanthone is one of the most significant ones. Rubraxanthone (1) is a xanthone that was successfully extracted from Guttiferae. The biological synthesis of this prenylated xanthone occurs through the shikimate pathway with L-phenylalanine as the precursor. It has been reported that rubraxanthone possesses a wide range of valuable biological activities, including antibacterial, anticancer and antiplatelet characteristics. This review emphasised the significance of rubraxanthone as a secondary metabolite in Guttiferae, identifying it as a potential compound in pharmaceutical, botany and chemistry with account its biological properties and future research pathway.

Synthesis and Anti-Inflammatory Evaluation of a Library of Chiral Derivatives of Xanthones Conjugated with Proteinogenic Amino Acids

In recent decades, the relationship between drug chirality and biological activity has been assuming enormous importance in medicinal chemistry. Particularly, chiral derivatives of xanthones (CDXs) have interesting biological activities, including enantioselective anti-inflammatory activity. Herein, the synthesis of a library of CDXs is described, by coupling a carboxyxanthone (1) with both enantiomers of proteinogenic amino esters as chiral building blocks (2-31), following the chiral pool strategy. The coupling reactions were performed at room temperature with good yields (from 44 to 99.9%) and very high enantiomeric purity, with most of them presenting an enantiomeric ratio close to 100%. To afford the respective amino acid derivatives (32-61), the ester group of the CDXs was hydrolyzed in mild alkaline conditions. Consequently, in this work, sixty new derivatives of CDXs were synthetized. The cytocompatibility and anti-inflammatory activity in the presence of M1 macrophages were studied for forty-four of the new synthesized CDXs. A significant decrease in the levels of a proinflammatory cytokine targeted in the treatment of several inflammatory diseases, namely interleukin 6 (IL-6), was achieved in the presence of many CDXs. The amino ester of L-tyrosine (X1AELT) was the most effective in reducing IL-6 production (52.2 ± 13.2%) by LPS-stimulated macrophages. Moreover, it was ≈1.2 times better than the D-enantiomer. Indeed, enantioselectivity was observed for the majority of the tested compounds. Thus, their evaluation as promising anti-inflammatory drugs should be considered.

Development and evaluation of Pirkle-type chiral stationary phase for flash chromatography

Liquid chromatography is the most applied methodology for enantioseparation in preparative and semi-preparative scale; however, flash chromatography is seldom the first choice. This work proposes a new sustainable method to achieve pure enantiomers in mg scale. Herein, the functionalization of silica for flash chromatography columns with a suitable chiral selector, for subsequent quantitative enantioseparation of chiral compounds, is described. Accordingly, the Whelk-O®1 chiral selector was bonded to flash silica and packed into a reused solid phase extraction cartridge. For the evaluation of the enantioselective performance of the flash column, the enantiomers of a chiral derivative of xanthone were quantitatively enantioseparated with an average recovery of 70% and an enantiomer ratio (e.r.) of 99% and 97% for each enantiomer. Evaluation with the anti-inflammatory drug naproxen was also performed, resulting in an average recovery of 95% and 89% and 95% e.r. for each enantiomer. The flash column showed high stability and load ability, versatility, and good reproducibility.

Xanthones: A Class of Heterocyclic Compounds with Anticancer Potential

Xanthones (9H xanthen-9-one) are an important class of heterocyclic compounds containing oxygen and a moiety of gamma-pirone, dense with a two-benzene ring structure, distributed widely in nature. Naturally occurring xanthones are found in micro-organisms and higher plants as secondary metabolites in fungi and lichens. Compounds of the family Caryophyllaceae, Guttiferae and Gentianaceae, are the most common natural source of xanthones. The structure of the xanthones nucleus, coupled with its biogenetic source, imposes that the carbons are numbered according to the biosynthetic pact. The characteristics oxygenation pattern of xanthones earlier is mixed shikimateacetate biogenesis. The major class of xanthones includes simple oxygenated, non-oxygenated, xanthonolignoids, bisxanthones, prenylated and related xanthones, miscellaneous xanthones. Their great pharmacological importance and interesting scaffolds were highly encouraged by scientists to investigate either the synthesis design or natural products for cancer treatment. Because currently used antitumor drugs possess high toxicity and low selectivity, efficacious treatment may be compromised. This review is limited to the antitumor activity of xanthones and the chemistry of xanthone core, which may help provide fundamental knowledge to the medicinal chemist for new and advanced research in drug development.

Enantioselectivity of Chiral Derivatives of Xanthones in Virulence Effects of Resistant Bacteria

Antimicrobial peptides are one of the lines of defense produced by several hosts in response to bacterial infections. Inspired by them and recent discoveries of xanthones as bacterial efflux pump inhibitors, chiral amides with a xanthone scaffold were planned to be potential antimicrobial adjuvants. The chiral derivatives of xanthones were obtained by peptide coupling reactions between suitable xanthones with enantiomerically pure building blocks, yielding derivatives with high enantiomeric purity. Among 18 compounds investigated for their antimicrobial activity against reference strains of bacteria and fungi, antibacterial activity for the tested strains was not found. Selected compounds were also evaluated for their potential to inhibit bacterial efflux pumps. Compound (R,R)-8 inhibited efflux pumps in the Gram-positive model tested and three compounds, (S,S)-8, (R)-17 and (R,S)-18, displayed the same activity in the Gram-negative strain used. Studies were performed on the inhibition of biofilm formation and quorum-sensing, to which the enantiomeric pair 8 displayed activity for the latter. To gain a better understanding of how the active compounds bind to the efflux pumps, docking studies were performed. Hit compounds were proposed for each activity, and it was shown that enantioselectivity was noticeable and must be considered, as enantiomers displayed differences in activity.

Xanthones as P-glycoprotein modulators and their impact on drug bioavailability

Introduction: P-glycoprotein (P-gp) is an important efflux pump responsible for the extruding of many endogenous and exogenous substances out of the cells. P-gp can be modulated by different molecules - including xanthone derivatives - to surpass the multidrug resistance (MDR) phenomenon through P-gp inhibition, or to serve as an antidotal strategy in intoxication scenarios through P-gp induction/activation.Areas covered: This review provides a perspective on P-gp modulators, with particular focus on xanthonic derivatives, highlighting their ability to modulate P-gp expression and/or activity, and the potential impact of these effects on the pharmacokinetics, pharmacodynamics and toxicity of P-gp substrates.Expert opinion: Xanthones, of natural or synthetic origin, are able to modulate P-gp, interfering with its protein synthesis or with its mechanism of action, by decreasing or increasing its efflux capacity. These modulatory effects make the xanthonic scaffold a promising source of new derivatives with therapeutic potential. However, the mechanisms beyond the xanthones-mediated P-gp modulation and the chemical characteristics that make them more potent P-gp inhibitors or inducers/activators are still understudied. Furthermore, a new window of opportunity exists in the neuropathologies field, where xanthonic derivatives with potential to modulate P-gp should be further explored to optimize the prevention/treatment of brain pathologies.

Chiral derivatives of xanthones and benzophenones: Synthesis, enantioseparation, molecular docking, and tumor cell growth inhibition studies

Liquid chromatography enantioseparation and determination of enantiomeric purity of synthetized xanthone and benzophenone derivatives comprising one or more chiral moieties are reported. High enantioselectivity and resolution were observed in (S,S)‐Whelk‐O1 chiral stationary phase (CSP) for the enantiomeric mixtures of compounds comprising an aromatic ring linked to the stereogenic center(s), with α values ranging from 1.35 to 4.15 and Rs values ranging from 2.22 to 13.87. Among all the tested enantiomeric mixtures, those comprising three chiral moieties positioned in the xanthone scaffold gave the best chromatographic results. Enantiomers comprising an alkyl chain linked to the stereogenic centers were enantioseparated on a Lux® Celullose‐2 CSP. For both CSPs, the elution was performed in polar organic mode. The enantiomeric ratio (e.r.) values were always higher than 99%. Additionally, assessment of chiral recognition mechanisms on (S,S)‐Whelk‐O1 CSP was performed by molecular docking approach, which are in accordance with the chromatographic parameters. The nature and number of chiral moieties in the central aromatic scaffold of either xanthone or benzophenone derivatives are proved to be crucial for enantiorecognition. The evaluation of the growth inhibition of human tumor cell lines revealed that (S,S)‐(+)‐5 was the most potent compound, with values of GI50 of 12.83 ± 2.09 μM for A375‐C5 melanoma, 12.40 ± 1.16 μM for MCF‐7 breast adenocarcinoma, and 13.06 ± 1.29 μM for NCI‐H460 non‐small cell lung cancer. In some cases, the growth inhibitory effects demonstrated to be dependent on the stereochemistry of the compounds.

From Natural Products to New Synthetic Small Molecules: A Journey through the World of Xanthones

This work reviews the contributions of the corresponding author (M.M.M.P.) and her research group to Medicinal Chemistry concerning the isolation from plant and marine sources of xanthone derivatives as well as their synthesis, biological/pharmacological activities, formulation and analytical applications. Although her group activity has been spread over several chemical families with relevance in Medicinal Chemistry, the main focus of the investigation and research has been in the xanthone family. Xanthone derivatives have a variety of activities with great potential for therapeutic applications due to their versatile framework. The group has contributed with several libraries of xanthones derivatives, with a variety of activities such as antitumor, anticoagulant, antiplatelet, anti-inflammatory, antimalarial, antimicrobial, hepatoprotective, antioxidant, and multidrug resistance reversal effects. Besides therapeutic applications, our group has also developed xanthone derivatives with analytical applications as chiral selectors for liquid chromatography and for maritime application as antifouling agents for marine paints. Chemically, it has been challenging to afford green chemistry methods and achieve enantiomeric purity of chiral derivatives. In this review, the structures of the most significant compounds will be presented.

Chirality of Central Nervous System (CNS) Acting Drugs: A Formidable Therapeutic Hurdle Against CNS Diseases

BACKGROUND

Over fifty percent of drugs being used clinically are chiral and 90% of them are racemates. Unfortunately, they have both adverse and beneficial effects on body systems.

METHODS

Because of the erratic effects of chiral compounds on body functional systems, literature search was carried out with a view to identify CNS chiral drugs, their clinical advantages and disadvantages, unique physicochemical properties and structural modifications into safer drugs.

RESULTS

Findings have shown that majority of CNS and non-CNS acting drugs have chiral functional groups that may occur as either dextrorotatory (clockwise) or levorotatory (anticlockwise) or racemates which are inert. Sometimes, the enantiomers (optical isomers) could undergo keto-enol tautomerism, appearing in either acidic or basic or inert form. Chiral CNS acting drugs have agonistic and antagonistic effects, clinical advantages, disadvantages, and special clinical applications, possible modifications for better therapeutic effects and possible synthesis of more potent drugs from racemates. Clockwise chirality may be more effective and safer than the drugs with anticlockwise chirality. When chiral drugs are in racemate state they become inert and may be safer than when they are single. Also, diastereoisomers may be more dangerous than stereoisomers.

CONCLUSION

Therefore, chiral compounds should be adequately studied in lab rodents and primates, and their mechanisms of actions should be comprehensively understood before being used in clinical setting. Since many of them are toxic, their use should be based on principle of individualized medicine. Their molecular weights, functional groups, metabolites, polymers and stereoisomers could be valuable tools for their modifications.

Recent advances in the synthesis of xanthones and azaxanthones

A useful chemical toolbox for (aza)xanthones from 2012 to 2020 that covers the optimization of known procedures and novel methodologies.

New chiral stationary phases for liquid chromatography based on small molecules: Development, enantioresolution evaluation and chiral recognition mechanisms

Recently, we reported the development of new chiral stationary phases (CSPs) for liquid chromatography (LC) based on chiral derivatives of xanthones (CDXs). Based on the most promising CDX selectors, 12 new CSPs were successfully prepared starting from suitable functionalized small molecules including xanthone and benzophenone derivatives. The chiral selectors comprising one, two, three, or four chiral moieties were covalently bonded to a chromatographic support and further packed into LC stainless-steel columns (150 × 2.1 mm I.D.). The enantioselective performance of the new CSPs was evaluated by LC using different classes of chiral compounds. Specificity for enantioseparation of some CDXs was observed in the evaluation of the new CSPs. Besides, assessment of chiral recognition mechanisms was performed by computational studies using molecular docking approach, which are in accordance with the chromatographic parameters. X-Ray analysis was used to establish a chiral selector 3D structure.

Enantioseparation, recognition mechanisms and binding of xanthones on human serum albumin by liquid chromatography

Aim: To develop a method for enantioseparation of several chiral derivatives of xanthones (CDXs) by LC using a human serum albumin-chiral stationary phase (HSA-CSP) and screening CDX-HSA affinity. Additionally, recognition mechanisms were investigated. Materials & methods: The influence of organic modifier, buffer type, pH and ionic strength of mobile phase, and temperature were explored. The affinity was determined by measuring the retention times and further calculation of bound percentage. Chiral recognition mechanisms were investigated by docking. Results: Enantioselectivity and resolution values ranged from 1.40 to 9.16 and 1.51 to 4.97. Bound percentages ranged from 79.02 to 99.99%. Conclusion: LC systematic study and binding affinity of CDXs on HSA-CSP are presented here for the first time, expanding the applications of HSA-CSP for this class of compounds.

Synthetic Chiral Derivatives of Xanthones: Biological Activities and Enantioselectivity Studies

Many naturally occurring xanthones are chiral and present a wide range of biological and pharmacological activities. Some of them have been exhaustively studied and subsequently, obtained by synthesis. In order to obtain libraries of compounds for structure activity relationship (SAR) studies as well as to improve the biological activity, new bioactive analogues and derivatives inspired in natural prototypes were synthetized. Bioactive natural xanthones compromise a large structural multiplicity of compounds, including a diversity of chiral derivatives. Thus, recently an exponential interest in synthetic chiral derivatives of xanthones (CDXs) has been witnessed. The synthetic methodologies can afford structures that otherwise could not be reached within the natural products for biological activity and SAR studies. Another reason that justifies this trend is that both enantiomers can be obtained by using appropriate synthetic pathways, allowing the possibility to perform enantioselectivity studies. In this work, a literature review of synthetic CDXs is presented. The structures, the approaches used for their synthesis and the biological activities are described, emphasizing the enantioselectivity studies.

Chiral Derivatives of Xanthones with Antimicrobial Activity

According to the World Health Organization, the exacerbated use of antibiotics worldwide is increasing multi-resistant infections, especially in the last decade. Xanthones are a class of compounds receiving great interest in drug discovery and development that can be found as natural products or obtained by synthesis. Many derivatives of xanthones are chiral and associated with relevant biological activities, including antimicrobial. The aim of this review is to compile information about chiral derivatives of xanthones from natural sources and their synthesized examples with antimicrobial activity.

Carboxyxanthones: Bioactive Agents and Molecular Scaffold for Synthesis of Analogues and Derivatives

Xanthones represent a structurally diverse group of compounds with a broad range of biological and pharmacological activities, depending on the nature and position of various substituents in the dibenzo-γ-pyrone scaffold. Among the large number of natural and synthetic xanthone derivatives, carboxyxanthones are very interesting bioactive compounds as well as important chemical substrates for molecular modifications to obtain new derivatives. A remarkable example is 5,6-dimethylxanthone-4-acetic acid (DMXAA), a simple carboxyxanthone derivative, originally developed as an anti-tumor agent and the first of its class to enter phase III clinical trials. From DMXAA new bioactive analogues and derivatives were also described. In this review, a literature survey covering the report on carboxyxanthone derivatives is presented, emphasizing their biological activities as well as their application as suitable building blocks to obtain new bioactive derivatives. The data assembled in this review intends to highlight the therapeutic potential of carboxyxanthone derivatives and guide the design for new bioactive xanthone derivatives.

Enantiomeric Resolution and Docking Studies of Chiral Xanthonic Derivatives on Chirobiotic Columns

A systematic study of enantioresolution of a library of xanthonic derivatives, prepared “in-house”, was successfully carried out with four commercially available macrocyclic glycopeptide-based columns, namely Chirobiotic^TM T, Chirobiotic^TM R, Chirobiotic^TM V and Chirobiotic^TM TAG. Evaluation was conducted in multimodal elution conditions: normal-phase, polar organic, polar ionic and reversed-phase. The effects of the mobile phase composition, the percentage of organic modifier, the pH of the mobile phase, the nature and concentration of different mobile phase additives on the chromatographic parameters are discussed. Chirobiotic^TM T and Chirobiotic^TM V, under normal-phase and reversed-phase modes, respectively, presented the best chromatographic parameters. Considering the importance of understanding the chiral recognition mechanisms associated with the chromatographic enantioresolution, and the scarce data available for macrocyclic glycopeptide-based columns, computational studies by molecular docking were also carried out.

Lipophilicity assessement in drug discovery: Experimental and theoretical methods applied to xanthone derivatives

For the last several years, searching of new xanthone derivatives (XDs) with potential pharmacological activities has remained one of the main areas of interest of our group. The optimization of biological activity and drug-like properties of hits and leads is crucial at early stage of the drug discovery pipeline. Lipophilicity is one of the most important drug-like properties having a great impact in both pharmacokinetics and pharmacodynamics processes. In this work, we describe the lipophilicity of a small library of bioactive XDs, previously synthesized by our group, using different methods: computational, vortex-assisted liquid-liquid microextraction coupled with high-performance liquid chromatography (VALLME-HPLC), reversed-phase high-performance thin layer chromatography (RP-HPTLC), reversed-phase high-performance liquid chromatography (RP-HPLC), and biomembrane model by the partition between micelles and aqueous phase. The different results obtained by the used methods were compared and discussed. The methodologies and data gathered in this study will expand the investigation of lipophilicity of XDs, an important class of compounds in medicinal chemistry.

New chiral stationary phases based on xanthone derivatives for liquid chromatography

Six chiral derivatives of xanthones (CDXs) were covalently bonded to silica, yielding the corresponding xanthonic chiral stationary phases (XCSPs). The new XCSPs were packed into stainless-steel columns with 150 x 4.6 mm i.d. Moreover, the greening of the chromatographic analysis by reducing the internal diameter (150 x 2.1 mm i.d.) of the liquid chromatography (LC) columns was also investigated. The enantioselective capability of these phases was evaluated by LC using different chemical classes of chiral compounds, including several types of drugs. A library of CDXs was evaluated in order to explore the principle of reciprocity as well as the chiral self-recognition phenomenon. The separation of enantiomeric mixtures of CDXs was investigated under multimodal elution conditions. The XCSPs provided high specificity for the enantiomeric mixtures of CDXs evaluated mainly under normal-phase elution conditions. Furthermore, two XCSPs were prepared with both enantiomers of the same xanthonic selector in order to confirm the inversion order elution.

Chiral Derivatives of Xanthones: Investigation of the Effect of Enantioselectivity on Inhibition of Cyclooxygenases (COX-1 and COX-2) and Binding Interaction with Human Serum Albumin

Searching of new enantiomerically pure chiral derivatives of xanthones (CDXs) with potential pharmacological properties, particularly those with anti-inflammatory activity, has remained an area of interest of our group. Herein, we describe in silico studies and in vitro inhibitory assays of cyclooxygenases (COX-1 and COX-2) for different enantiomeric pairs of CDXs. The evaluation of the inhibitory activities was performed by using the COX Inhibitor Screening Assay Kit. Docking simulations between the small molecules (CDXs; known ligands and decoys) and the enzyme targets were undertaken with AutoDock Vina embedded in PyRx—Virtual Screening Tool software. All the CDXs evaluated exhibited COX-1 and COX-2 inhibition potential as predicted. Considering that the (S)-(−)-enantiomer of the nonsteroidal anti-inflammatory drug ketoprofen preferentially binds to albumin, resulting in lower free plasma concentration than (R)-(+)-enantiomer, protein binding affinity for CDXs was also evaluated by spectrofluorimetry as well as in in silico. For some CDXs enantioselectivity was observed.

Resolution, determination of enantiomeric purity and chiral recognition mechanism of new xanthone derivatives on (S,S)-whelk-O1 stationary phase

The enantioresolution and determination of the enantiomeric purity of 32 new xanthone derivatives, synthesized in enantiomerically pure form, were investigated on (S,S)-Whelk-O1 chiral stationary phase (CSP). Enantioselectivity and resolution (α and R_S ) with values ranging from 1.41-6.25 and from 1.29-17.20, respectively, were achieved. The elution was in polar organic mode with acetonitrile/methanol (50:50 v/v) as mobile phase and, generally, the (R)-enantiomer was the first to elute. The enantiomeric excess (ee) for all synthesized xanthone derivatives was higher than 99%. All the enantiomeric pairs were enantioseparated, even those without an aromatic moiety linked to the stereogenic center. Computational studies for molecular docking were carried out to perform a qualitative analysis of the enantioresolution and to explore the chiral recognition mechanisms. The in silico results were consistent with the chromatographic parameters and elution orders. The interactions between the CSP and the xanthone derivatives involved in the chromatographic enantioseparation were elucidated.

New chiral derivatives of xanthones: synthesis and investigation of enantioselectivity as inhibitors of growth of human tumor cell lines

A highly efficient and practical methodology for synthesis of new chiral derivatives of xanthones (CDXs) in enantiomerically pure form has been developed. According to this approach, thirty CDXs (3-32) were synthesized by coupling a carboxyxanthone (1) and a carboxymethoxyxanthone (2) with both enantiomers of commercially available chiral building blocks, namely six amino alcohols, one amine and one amino ester. The activation of the carboxylic acid group of the xanthonic scaffold was carried out with the coupling reagent O-(benzotriazol-1-yl)-N-N-N'-N'-tetramethyluronium tetrafluoroborate (TBTU), in the presence of a catalytic amount of TEA in anhydrous THF. The coupling reactions with the chiral blocks were performed at room temperature with short reactions times, excellent yields (ranging from 94% to 99%), and very high enantiomeric excess. The synthesized CDXs were evaluated for their effect on the in vitro growth of three human tumor cell lines, namely A375-C5 (melanoma), MCF-7 (breast adenocarcinoma), and NCI-H460 (non-small cell lung cancer). The most active compound was CDX 15 being active in all human tumor cell lines with values of GI50 of 32.15±2.03μM for A375-C5, 22.55±1.99μM for MCF-7, and 14.05±1.82μM for NCI-H460. Nevertheless, some CDXs showed cell-type selectivity. Furthermore, the growth inhibitory effects, in some cases, demonstrated to be depending on the stereochemistry of the CDXs. An interesting example was observed with the enantiomers 3 and 4, which demonstrated high enantioselectivity for MCF-7 and NCI-H460 cell lines. It can be inferred that the effects on the growth of the human tumor cell lines can be ascribed not only to the nature and positions of substituents on the xanthonic scaffold but also to the stereochemistry of the CDXs. Some considerations regarding structure-activity relationship within this class of compounds will be highlighted.