Total Synthesis of the Aglycone of IB-00208

Chemistry and biology of specialized metabolites produced by Actinomadura

Covering: up to the end of 2022In recent years rare Actinobacteria have become increasingly recognised as a rich source of novel bioactive metabolites. Actinomadura are Gram-positive bacteria that occupy a wide range of ecological niches. This review highlights about 230 secondary metabolites produced by Actinomadura spp., reported until the end of 2022, including their bioactivities and selected biosynthetic pathways. Notably, the bioactive compounds produced by Actinomadura spp. demonstrate a wide range of activities, including antimicrobial, antitumor and anticoccidial effects, highlighting their potential in various fields.

Chemistry and biosynthesis of bacterial polycyclic xanthone natural products

Covering: up to the end of 2021Bacterial polycyclic xanthone natural products (BPXNPs) are a growing family of natural xanthones featuring a pentangular architecture with various modifications to the tricyclic xanthone chromophore. Their structural diversities and various activities have fueled biosynthetic and chemical synthetic studies. Moreover, their more potent activities than the clinically used drugs make them potential candidates for the treatment of diseases. Future unraveling of structure activity relationships (SARs) will provide new options for the (bio)-synthesis of drug analogues with higher activities. This review summarizes the isolation, structural elucidation and biological activities and more importantly, the recent strategies for the microbial biosynthesis and chemical synthesis of BPXNPs. Regarding their biosynthesis, we discuss the recent progress in enzymes that synthesize tricyclic xanthone, the protein candidates for structural moieties (methylene dioxygen bridge and nitrogen heterocycle), tailoring enzymes for methylation and halogenation. The chemical synthesis part summarizes the recent methodology for the division synthesis and coupling construction of achiral molecular skeletons. Ultimately, perspectives on the biosynthetic study of BPXNPs are discussed.

A new mode of cyclobutenedione ring opening for the synthesis of 2-oxobut-3-enamides and tetrasubstituted furans

A dichotomy between the additions of organolithiums and lithium amides to cyclobutenediones is described wherein the former give carbonyl addition products while the latter induce ring opening by enone cleavage via O- to C-lithium transfer. This distinct mode of ring scission gives access to 2-oxobut-3-enamides and tetrasubstituted furans.

Rhodium-Catalyzed Aerobic Decomposition of 1,3-Diaryl-2-diazo-1,3-diketones: Mechanistic Investigation and Application to the Synthesis of Benzils

The conversion of 1,3-diaryl-2-diazo-1,3-diketones to 1,2-daryl-1,2-diketones (benzils) is reported based on a rhodium(II)-catalyzed aerobic decomposition process. The reaction occurs at ambient temperatures and can be catalyzed by a few dirhodium carboxylates (5 mol %) under a balloon pressure of oxygen. Moreover, an oxygen atom from the O₂ reagent is shown to be incorporated into the product, and this is accompanied by the extrusion of a carbonyl unit from the starting materials. Mechanistically, it is proposed that the decomposition may proceed via the interaction of a ketene intermediate resulting from a Wolff rearrangement of the carbenoid, with a rhodium peroxide or peroxy radical species generated upon the activation of molecular oxygen. The proposed mechanism has been supported by the results from a set of controlled experiments. By using this newly developed strategy, a large array of benzil derivatives as well as 9,10-phenanthrenequinone were synthesized from the corresponding diazo substrates in varying yields. On the other hand, the method did not allow the generation of benzocyclobutene-1,2-dione from 2-diazo-1,3-indandione because of the difficulty of inducing the initial rearrangement.

Synthetic Studies on the Kigamicins

The kigamicins are polycyclic aromatic natural products featuring a tetrahydroxanthone and up to four sugar residues. They are toxic to human cancer cells under nutrient-poor conditions. A synthesis of the natural product skeleton has been achieved from chiral pool materials. Key steps include a regioselective hydration of a diarylalkyne and two oxidative cyclizations.

A Thermally Induced Hydride Transfer from an Amine to an Allene Triggers an Annulation Reaction, Giving Dihydrofuropyridinones

A thermal rearrangement leading to dihydrofuropyridinones and related polycyclic ring systems from furanones and cyclobutenones is described. A key feature of the reaction is the thermally induced hydride transfer from a 3°-amine to a conjugated allene to trigger cyclization.

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.

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.

Natural Products and Their Mimics as Targets of Opportunity for Discovery

Diverse structural types of natural products and their mimics have served as targets of opportunity in our laboratory to inspire the discovery and development of new methods and strategies to assemble polyfunctional and polycyclic molecular architectures. Furthermore, our efforts toward identifying novel compounds having useful biological properties led to the creation of new targets, many of which posed synthetic challenges that required the invention of new methodology. In this Perspective, selected examples of how we have exploited a diverse range of natural products and their mimics to create, explore, and solve a variety of problems in chemistry and biology will be discussed. The journey was not without its twists and turns, but the unexpected often led to new revelations and insights. Indeed, in our recent excursion into applications of synthetic organic chemistry to neuroscience, avoiding the more-traveled paths was richly rewarding.

Synthesis of the Pentacyclic Core of Citreamicin η

The citreamicins comprise a novel class of polycyclic xanthone natural products that have not yet yielded to total synthesis. A concise 11-step synthesis of the pentacyclic core of citreamicin η is now reported that features the use of a general approach for the synthesis of 1,4-dioxygenated xanthones. The synthesis also showcases improved techniques for effecting regioselective bromination of certain substituted phenols and coupling of acetylides with hindered ketones.

Synthesis and Biological Evaluation of Kibdelone C and Its Simplified Derivatives

Poylcyclic tetrahydroxanthones comprise a large class of cytototoxic natural products. No mechanism of action has been described for any member of the family. We report the synthesis of kibdelone C and several simplified analogs. Both enantiomers of kibdeleone C show low nanomolar cytotoxicity toward multiple human cancer cell lines. Moreover, several simplified derivatives with improved chemical stability display higher activity than the natural product itself. In vitro studies rule out interaction with DNA or inhibition of topoisomerase, both of which are common modes of action for polycyclic aromatic compounds. However, celluar studies reveal that kibdelone C and its simplified derivatives disrupt the actin cytoseketon without directly binding actin or affecting its polymerization in vitro.