Xanthones from fungi, lichens, and bacteria: the natural products and their synthesis

All that glitters is not gold: Panning cytotoxic natural products and derivatives with a fused tricyclic backbone by the estimation of their leadlikeness for cancer treatment

Tricyclic compounds call the attention because of their pharmacological properties, and are considered a preferred platform for the development of drugs. Especially, in cancer treatment, these planar compounds are known for their ability to stack with DNA base pairs, acting as intercalators. In this sense, natural products (NPs) are a prodigal source of polycyclic compounds, comprising classes, such as carbolines, anthraquinones and xanthones. However, most of these compounds lack suitable physico-chemical properties, compatible to oral bioaviability. In this perspective, this paper aims to overview the role of tricyclic cores in the development of cytotoxic compounds, focusing on the leadlikeness estimation of the most prominent NP classes and their synthetic derivatives.

Characterisation of the biosynthetic pathway to agnestins A and B reveals the reductive route to chrysophanol in fungi

Two new dihydroxy-xanthone metabolites, agnestins A and B, were isolated from Paecilomyces variotii along with a number of related benzophenones and xanthones including monodictyphenone. The structures were elucidated by NMR analyses and X-ray crystallography. The agnestin (agn) biosynthetic gene cluster was identified and targeted gene disruptions of the PKS, Baeyer-Villiger monooxygenase, and other oxido-reductase genes revealed new details of fungal xanthone biosynthesis. In particular, identification of a reductase responsible for in vivo anthraquinone to anthrol conversion confirms a previously postulated essential step in aromatic deoxygenation of anthraquinones, e.g. emodin to chrysophanol.

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.

Ceric Ammonium Sulfate (CAS) Mediated Oxidations of Benzophenones Possessing a Phenolic Substituent for the Synthesis of Xanthones and Related Products

Work previously published by our group described novel methodology for the synthesis of xanthones and related products from phenolic benzophenones in a reaction mediated by ceric ammonium sulfate (CAS). In this paper we further explore this novel reaction by subjecting an additional set of phenolic benzophenones to CAS to afford a range of compounds, including xanthones, 9 H-xanthen-2,9(4a H)-diones, 3 H-spiro[benzofuran-2,1'-cyclohexa[2,5]diene]-3,4'-diones, and biaryl compounds. A comparison of these reactions with the more commonly used oxidant ceric ammonium nitrate (CAN) was also conducted. Based on these results, greater insight into the reaction mechanism has been gained. In addition, the conversion of the synthesized xanthen-2,9(4a H)-diones to xanthones by treatment with sodium dithionite is described.

A bifunctional pyrazolone–chromone synthon directed organocatalytic double Michael cascade reaction: forging five stereocenters in structurally diverse hexahydroxanthones

The merging of two or more known natural product-based scaffolds is a powerful and routine strategy to develop bioactive small molecules.

Silver-catalyzed cascade radical cyclization of 2-(allyloxy)arylaldehydes with cyclopropanols: access to chroman-4-one derivatives

A convenient silver-catalyzed cascade radical cyclization of 2-(allyloxy)arylaldehydes with cyclopropanols was developed to synthesize carbonyl-containing alkyl-substituted chroman-4-one derivatives.

Tandem synthesis of 4-aminoxanthones is controlled by a water-assisted tautomerization: a general straightforward reaction

Iminolactone-aminofuran tautomerization is the rate-determining step in the multicomponent synthesis of xanthones from 3-carbonylchromones, isocyanides and dienophiles.

Stereocontrolled construction of six vicinal stereogenic centers on a hexahydroxanthone framework through a formal [2+1+3] annulation

The first example of a bifunctional donor–donor 3C synthon formed in situ from an activated methine with nitromethane through a [2+1] Michael addition, further directing a one-pot organocascade Michael/Henry cycloaddition was developed.

Bifunctional oxindole-chromone 4C building block directed asymmetric synthesis of bispirocyclic hexahydroxanthones featuring five contiguous stereocenters and two side-by-side oxindoles

Optically active small molecules based on privileged natural product frameworks and rich in three-dimensional complexity are in high demand.

An asymmetric iminium ion catalysis-enabled cascade cycloaddition reaction of chromone-oxindole synthons with enals: construction of a spirooxindole–hexahydroxanthone framework

We report the first asymmetric iminium ion catalysis-enabled cascade cycloaddition reaction of bifunctional chromone-oxindole synthons and α,β-unsaturated aldehydes.

Base-Promoted Denitrogenative/Deoxygenative/Deformylative Benzannulation of N-Tosylhydrazones with 3-Formylchromones for Diverse and Polyfunctionalized Xanthones

A simple and efficient base-promoted denitrogenative/deoxygenative/deformylative benzannulation is developed for the construction of biologically interesting polyfunctionalized xanthones starting from N-tosylhydrazones and two molecules of 3-formylchromones. This unprecedented protocol proceeds via a cascade diazo formation/Michael addition/denitrogenation/[4 + 2] cycloaddition/elimination/ring opening. The synthesized xanthones possess potent UV-filter, fluorescent sensor, and antioxidant properties.

Influence of Hydroxyl Functional Group on the Structure and Stability of Xanthone: A Computational Approach

The present work addresses computational research focused on the energetic and structural properties of four isomers monohydroxyxanthone, using the G3(MP2)//B3LYP method, in order to evaluate the influence of the hydroxyl (-OH moiety) functional group on the xanthone molecule. The combination of these computational results with previous experimental data of these compounds enabled the determination of their enthalpies, entropies and Gibbs energies of formation, in the gaseous phase, and consequently to infer about the relative thermodynamic stability of the four isomers. Other issues were also addressed for the hydroxyxanthone isomers, namely the conformational and the tautomeric equilibrium analysis of the optimized molecular structures, the frontier orbitals, and the electrostatic potential energy maps. Complementarily, an energetic study of the intramolecular O - H ⋯ O hydrogen bond for 1-hydroxanthone was also performed.

Xanthone-derived polyketides from the Thai mangrove endophytic fungus Phomopsis sp. xy21

Six new xanthone-derived polyketides, named phomoxanthones F-K (1-6), along with three known ones, were isolated from Phomopsis sp. xy21, which was isolated as an endophytic fungus from the Thai mangrove Xylocarpus granatum. Phomoxanthone F (1) represents the first xanthone-derived polyketide containing a 10a-decarboxylated benzopyranone nucleus that was substituted by a 4-methyldihydrofuran-2(3H)-one moiety at C10a. Phomoxanthones G (2) and H (3) are highly oxidized xanthone-derived polyketides containing a novel 5-methyl-6-oxabicyclo[3.2.1]octane motif. This is the first report of a C6-O-C12 bridge in xanthone-derived polyketides. Additionally, a plausible biogenetic pathway for these xanthone-derived polyketides is proposed.

Chitosan oligosaccharides affect xanthone and VOC biosynthesis in Hypericum perforatum root cultures and enhance the antifungal activity of root extracts

Water-soluble chitosan oligosaccharides (COS) affect xanthone and volatile organic compound content, as well as antifungal activity against human pathogenic fungi of extracts obtained from Hypericum perforatum root cultures. Several studies have demonstrated the elicitor power of chitosan on xanthone biosynthesis in root cultures of H. perforatum. One of the major limitations to the use of chitosan, both for basic and applied research, is the need to use acidified water for solubilization. To overcome this problem, the elicitor effect of water-soluble COS on the biosynthesis of both xanthones and volatile organic compounds (VOCs) was evaluated in the present study. The analysis of xanthones and VOCs was performed by HPLC and GC-MS headspace analysis. The obtained results showed that COS are very effective in enhancing xanthone biosynthesis. With 400 mg L^-1 COS, a xanthone content of about 30 mg g^-1 DW was obtained. The antifungal activity of extracts obtained with 400 mg L^-1 COS was the highest, with MIC₅₀ of 32 µg mL^-1 against Candida albicans and 32-64 µg mL^-1 against dermatophytes, depending on the microorganism. Histochemical investigations suggested the accumulation of isoprenoids in the secretory ducts of H. perforatum roots. The presence of monoterpenes and sesquiterpenes was confirmed by the headspace analysis. Other volatile hydrocarbons have been identified. The biosynthesis of most VOCs showed significant changes in response to COS, suggesting their involvement in plant-fungus interactions.

Lichen Xanthones as Models for New Antifungal Agents

Due to the emergence of multidrug-resistant pathogenic microorganisms, the search for new antimicrobial compounds plays an important role in current medicinal chemistry research. Inspired by lichen antimicrobial xanthones, a series of novel chlorinated xanthones was prepared using five chlorination methods (Methods A⁻E) to obtain different patterns of substitution in the xanthone scaffold. All the synthesized compounds were evaluated for their antimicrobial activity. Among them, 3-chloro-4,6-dimethoxy-1-methyl-9H-xanthen-9-one 15 showed promising antibacterial activity against E. faecalis (ATCC 29212 and 29213) and S. aureus ATCC 29213. 2,7-Dichloro-3,4,6-trimethoxy-1-methyl-9H-xanthen-9-one 18 revealed a potent fungistatic and fungicidal activity against dermatophytes clinical strains (T. rubrum, M. canis, and E. floccosum (MIC = 4⁻8 µg/mL)). Moreover, when evaluated for its synergistic effect for T. rubrum, compound 18 exhibited synergy with fluconazole (ΣFIC = 0.289). These results disclosed new hit xanthones for both antibacterial and antifungal activity.

Cytotoxic Tetrahydroxanthone Dimers from the Mangrove-Associated Fungus Aspergillus versicolor HDN1009

Three new tetrahydroxanthone dimers, 5-epi-asperdichrome (1), versixanthones N (2), and O (3), were isolated from the mangrove-derived fungus Aspergillus versicolor HDN1009. Their structures, including the absolute configurations, were elucidated by NMR, HRMS, and circular dichroism (CD) experiments. Among them, compound 1 was the second example of tetrahydroxanthone dimers, which dimerized by a rare diaryl ether linkage and showed promising antibacterial activities against Vibrio parahemolyticus, Bacillus subtilis, Mycobacterium phlei, and Pseudomonas aeruginosa, with MIC values ranging from 100 μM to 200 μM; whilst compounds 2 and 3 exhibited extensive cytotoxicities against five cancer cell lines (HL-60, K562, H1975, MGC803, and HO-8910), with IC₅₀ values ranging from 1.7 μM to 16.1 μM.

Aspergixanthones I⁻K, New Anti-Vibrio Prenylxanthones from the Marine-Derived Fungus Aspergillus sp. ZA-01

Marine-derived fungi are a rich source of structurally diverse metabolites. Fungi produce an array of compounds when grown under different cultivation conditions. In the present work, different media were used to cultivate the fungus Aspergillus sp. ZA-01, which was previously studied for the production of bioactive compounds, and three new prenylxanthone derivatives, aspergixanthones I–K (1–3), and four known analogues (4–7) were obtained. The absolute configuration of 1 was assigned by ECD experiment and the Mo₂(AcO)₄ ICD spectrum of its methanolysis derivative (1a). All the compounds (1–7) were evaluated for their anti-Vibrio activities. Aspergixanthone I (1) showed the strongest anti-Vibrio activity against Vibrio parahemolyticus (MIC = 1.56 μM), Vibrio anguillarum (MIC = 1.56 μM), and Vibrio alginolyticus (MIC = 3.12 μM).

A comprehensive review on xanthone derivatives as α-glucosidase inhibitors

α-Glucosidase plays an important role in carbohydrate metabolism and is therefore an attractive therapeutic target for the treatment of diabetes, obesity and other related complications. In the last two decades, considerable interest has been given to natural and synthetic xanthone derivatives in this field of research. Herein, a comprehensive review of the literature on xanthones as inhibitors of α-glucosidase activity, their mechanism of action, experimental procedures and structure-activity relationships have been reviewed for more than 280 analogs. With this overview we intend to motivate and challenge researchers (e.g. chemistry, biology, pharmaceutical and medicinal areas) for the design of novel xanthones as multipotent drugs and exploit the properties of this class of compounds in the management of diabetic complications.

Absolute Configurations of 14,15-Hydroxylated Prenylxanthones from a Marine-Derived Aspergillus sp. Fungus by Chiroptical Methods

Determination of the absolute configrations for natural products is one of the most important and challenging tasks, especially when the molecules display high conformational flexibility. In this paper, eight new prenylxanthones, aspergixanthones A-H (1-8), and one known analogue (9), were isolated from the marine-derived fungus Aspergillus sp. ZA-01. The absolute configurations of C-14 and C-15 in 1-8 were difficult to be assigned due to the high conformational flexibility of the chains. To solve this problem, the experimental ECD, ORD, and VCD spectra of 1 were combined for analysis with the corresponding theoretical predictions for its different diastereomers. This study suggested that a concerted application of more than one chiroptical methods could be used as a preferable approach for the stereochemical characterizations of flexible molecules. Compounds 1-9 were evaluated for their cytotoxic and antibacterial activities. Among them, 6 showed cytotoxicity against the A-549 cell line with the IC₅₀ value of 1.1 μM, and 7 exhibited antibacterial activity against Micrococcus lysodeikticus with the MIC value of 0.78 μg/mL.

Enantioselective Total Synthesis of Chromanone Lactone Homo- and Heterodimers

A one pot borylation/Suzuki-Miyaura reaction of the 4-bromochromanone lactones 21 and 23, respectively, followed by cleavage of the methyl ether moieties gave the homodimeric chromanone lactones 10 and 11. Reaction of a 1:1 mixture of 21 and 23 under otherwise identical conditions gave a 1:1:2-mixture of the two homodimers 10 and 11 and the heterodimer 12. This is the first example of the preparation of a heterodimeric chromanone lactone. For the enantioselective synthesis of the starting material, phenol 17 was transformed into the chromane 18 using a Wacker-type cyclisation with 99 % ee and 80 % yield.

Enantioselective Total Synthesis of Blennolide H and Phomopsis-H76 A and Determination of Their Structure

This work reports on the enantioselective total synthesis of the two dimeric natural chromanone lactones phomopsis-H76 A (5) and blennolide H (6). Both syntheses could be achieved from chromane 11, which was obtained by an enantioselective Wacker-type cyclization with >99 % ee. The dimerization of the corresponding monomers was performed using a palladium-catalyzed Suzuki reaction. Moreover, within this work it was possible to revise the absolute configuration of phomopsis-H76 A and determine the relative as well as absolute configuration of blennolide H.

Convergent Synthesis of Kibdelone C

The synthesis of kibdelone C, a polycyclic natural xanthone isolated from a soil actinomycete, was achieved through a convergent approach. A 6π-electrocyclization was applied to construct the highly substituted dihydrophenanthrenol fragment (B-C-D ring). InBr₃-promoted lactonization was employed to build the isocoumarin ring, which served as a common precursor for the formation of isoquinolinone ring (A-B ring). A key DMAP-mediated oxa-Michael/aldol cascade reaction was developed to install the tetrahydroxanthone fragment (E-F ring). This approach provides a new solution to prepare its derivatives and structurally related natural products.

Secondary metabolism in the lichen symbiosis

Lichens, which are defined by a core symbiosis between a mycobiont (fungal partner) and a photobiont (photoautotrophic partner), are in fact complex assemblages of microorganisms that constitute a largely untapped source of bioactive secondary metabolites. Historically, compounds isolated from lichens have predominantly been those produced by the dominant fungal partner, and these continue to be of great interest for their unique chemistry and biotechnological potential. In recent years it has become apparent that many photobionts and lichen-associated bacteria also produce a range of potentially valuable molecules. There is evidence to suggest that the unique nature of the symbiosis has played a substantial role in shaping many aspects of lichen chemistry, for example driving bacteria to produce metabolites that do not bring them direct benefit but are useful to the lichen as a whole. This is most evident in studies of cyanobacterial photobionts, which produce compounds that differ from free living cyanobacteria and are unique to symbiotic organisms. The roles that these and other lichen-derived molecules may play in communication and maintaining the symbiosis are poorly understood at present. Nonetheless, advances in genomics, mass spectrometry and other analytical technologies are continuing to illuminate the wealth of biological and chemical diversity present within the lichen holobiome. Implementation of novel biodiscovery strategies such as metagenomic screening, coupled with synthetic biology approaches to reconstitute, re-engineer and heterologously express lichen-derived biosynthetic gene clusters in a cultivable host, offer a promising means for tapping into this hitherto inaccessible wealth of natural products.

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.

Secondary Metabolites of Mangrove-Associated Strains of Talaromyces

Boosted by the general aim of exploiting the biotechnological potential of the microbial component of biodiversity, research on the secondary metabolite production of endophytic fungi has remarkably increased in the past few decades. Novel compounds and bioactivities have resulted from this work, which has stimulated a more thorough consideration of various natural ecosystems as conducive contexts for the discovery of new drugs. Thriving at the frontier between land and sea, mangrove forests represent one of the most valuable areas in this respect. The present paper offers a review of the research on the characterization and biological activities of secondary metabolites from manglicolous strains of species belonging to the genus Talaromyces. Aspects concerning the opportunity for a more reliable identification of this biological material in the light of recent taxonomic revisions are also discussed.

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.

A Multifunctional Monooxygenase XanO4 Catalyzes Xanthone Formation in Xantholipin Biosynthesis via a Cryptic Demethoxylation

Xantholipin and several related polycyclic xanthone antibiotics feature a unique xanthone ring nucleus within a highly oxygenated, angular, fused hexacyclic system. In this study, we demonstrated that a flavin-dependent monooxygenase (FMO) XanO4 catalyzes the oxidative transformation of an anthraquinone to a xanthone system during the biosynthesis of xantholipin. In vitro isotopic labeling experiments showed that the reaction involves sequential insertion of two oxygen atoms, accompanied by an unexpected cryptic demethoxylation reaction. Moreover, characterizations of homologous FMOs of XanO4 suggested the generality of the XanO4-like-mediated reaction for the assembly of a xanthone ring in the biosynthesis of polycyclic xanthone antibiotics. These findings not only expand the repertoire of FMO activities but also reveal a novel mechanism for xanthone ring formation.

Biomimetic Total Synthesis of (±)-Griffipavixanthone via a Cationic Cycloaddition-Cyclization Cascade

We report the concise, biomimetic total synthesis of the dimeric, Diels-Alder natural product griffipavixanthone from a readily accessible prenylated xanthone monomer. The key step utilizes a novel intermolecular [4+2] cycloaddition-cyclization cascade between a vinyl p-quinone methide and an in situ generated isomeric diene promoted by either Lewis or Brønsted acids. Experimental and computational studies of the reaction pathway suggest that a stepwise, cationic Diels-Alder cycloaddition is operative.

Formation of Phenalenone Skeleton by an Unusual Rearrangement Reaction

The frame rearrangement reaction of dinaphthyl ketones, possessing hydroxy groups at appropriate positions, into phenalenone derivatives under acidic conditions was discovered serendipitously. Although this rearrangement had limited scope, its mechanism was unusual, involving the division of naphthalene rings into one phenalenone ring and one benzene ring. The reaction mechanism was elucidated by direct determination of intermediate structures using ¹H NMR measurements. The generated phenalenones are expected to be key intermediates toward natural products and functional materials.

A fruitful decade for fungal polyketides from 2007 to 2016: antimicrobial activity, chemotaxonomy and chemodiversity

The last three decades have shown that the fungi can be ‘biofactories’ of novel, bioactive secondary metabolites that produce numerous natural products with novel skeletons and biological activities. Particularly in the last 10 years, large numbers of antimicrobial fungal secondary metabolites have been discovered. This review provides an overview of key, defining developments of the last 10 years regarding the discovery of antimicrobial activity, chemotaxonomy and chemodiversity of fungal polyketides.

α-Pyrone derivatives with cytotoxic activities, from the endophytic fungus Phoma sp. YN02-P-3

Four new α-pyrone derivatives phomones C-F (1-4) together with four known compounds (5-8) were isolated from the endophytic fungus Phoma sp. YN02-P-3. Compound 1 is the first example of 6-α,β-unsaturated ester-2-pyrone dimers via intermolecular symmetrical [2+2] cycloaddition. The chemical structures of these compounds were determined from spectroscopic data (1D/2D NMR, MS and IR). The acetylated product (9) of 1 along with compounds 1-8 were then tested for their cytotoxicity against HL-60, PC-3 and HCT-116 cell lines. Compounds 2, 3, 5 and 9 with acetyl groups showed significant inhibitory activities against the three cell lines with IC₅₀ values in the range 0.52-9.85μM. while compounds 1, 4 and 6-8 that possess no acetyl group showed no inhibitory activity (IC₅₀>50μM), indicating that the acetyl group at 10- or 12- are essential for their cytotoxic activities. The structure-activity relationships of these phomones were also reported.

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.