Total Synthesis of Cyclotripeptidic Natural Products Anacine, Aurantiomide C, Polonimides A and C, and Verrucine F

The total synthesis of cyclotripeptidic natural products possessing a central piperazino[2,1-b]quinazolin-3,6-dione core is described through an original strategy involving the pivotal cyclocondensation of an electrophilic homoserine lactone intermediate. The alkylidene group was spontaneously installed by autoxidation during the cyclocondensation process, while the propionamide side chain was introduced through the nickel-catalyzed aminocarbonylation of a bromoethyl intermediate. This last reaction is unprecedented on such highly functionalized intermediates. Finally, we explored structural modifications and interconversions of the natural products. Overall, this work led to anacine, aurantiomide C, polonimides A and C, and verrucine F.

Tandem Hock and Friedel-Crafts reactions allowing an expedient synthesis of a cyclolignan-type scaffold

The Hock cleavage, which is compatible with tandem processes, was applied to the synthesis of 1-aryltetralines through a one-pot transformation from readily available benzyl(prenyl)malonate substrates. After the photooxygenation of the prenyl moiety, the resulting hydroperoxide was directly engaged in a Hock cleavage by adding a Lewis acid. The presence of an aromatic nucleophile in the reaction mixture and that of a benzyl moiety on the substrate resulted in tandem Friedel-Crafts reactions to form the 1-aryltetraline products. These compounds share a close analogy to the cyclolignan natural products. Experimental observations and a DFT study support the involvement of an aldehyde intermediate during the Friedel-Crafts reactions, rather than an oxocarbenium.

Mode-of-action of the natural herbicide radulanin A as an inhibitor of photosystem II

BACKGROUND

Radulanin A is a natural 2,5-dihydrobenzoxepin synthesized by several liverworts of the Radula genus. Breakthroughs in the total synthesis of radulanin A paved the way for the discovery of its phytotoxic activity. Nevertheless, its mode-of-action (MoA) has remained unknown so far and thus was investigated, in Arabidopsis thaliana.

RESULTS

Radulanin A phytotoxicity was associated with cell death and partially depended on light exposure. Photosynthesis measurements based on chlorophyll-a fluorescence evidenced that radulanin A and a Radula chromene inhibited photosynthetic electron transport with IC50 of 95 and 100 μm, respectively. We established a strong correlation between inhibition of photosynthesis and phytotoxicity for a range of radulanin A analogs. Based on these data, we also determined that radulanin A phytotoxicity was abolished when the hydroxyl group was modified, and was modulated by the presence of the heterocycle and its aliphatic chain. Thermoluminescence studies highlighted that radulanin A targeted the QB site of the Photosystem II (PSII) with a similar MoA as 3-(3,4-dichloropheny)-1,1-dimethylurea (DCMU).

CONCLUSION

We establish that radulanin A targets PSII, expanding QB sites inhibitors to bibenzyl compounds. The identification of an easy-to-synthesize analog of radulanin A with similar MoA and efficiency might be useful for future herbicide development. © 2023 Society of Chemical Industry.

Tandem InCl3-Promoted Hydroperoxide Rearrangements and Nucleophilic Additions: A Straightforward Entry to Benzoxacycles

The acid-catalyzed rearrangement of organic peroxides is generally associated with C-C-bond cleavages (Hock and Criegee rearrangements), with the concomitant formation of an oxocarbenium intermediate. This article describes the tandem process between a Hock or Criegee oxidative cleavage and a nucleophilic addition onto the oxocarbenium species (in particular a Hosomi-Sakurai-type allylation), under InCl₃ catalysis. It was applied to the synthesis of 2-substituted benzoxacycles (chromanes and benzoxepanes), including a synthesis of the 2-(aminomethyl)chromane part of sarizotan, and a total synthesis of erythrococcamide B.

Asymmetric synthesis of a stereopentade fragment toward latrunculins

Latrunculins are marine toxins used in cell biology to block actin polymerization. The development of new synthetic strategies and methods for their synthesis is thus important in order to improve, modulate or control this biological value. The total syntheses found in the literature all target similar disconnections, especially an aldol strategy involving a recurrent 4-acetyl-1,3-thiazolidin-2-one ketone partner. Herein, we describe an alternative disconnection and subsequent stereoselective transformations to construct a stereopentade amenable to latrunculin and analogue synthesis, starting from (+)-β-citronellene. Key stereoselective transformations involve an asymmetric Krische allylation, an aldol reaction under 1,5-anti stereocontrol, and a Tishchenko-Evans reduction accompanied by a peculiar ester transposition, allowing to install key stereogenic centers of the natural products.

Total Synthesis of Phytotoxic Radulanin A Facilitated by the Photochemical Ring Expansion of a 2,2-Dimethylchromene in Flow

The radulanins are biologically active bibenzyl natural products featuring a synthetically challenging 2,5-dihydro-1-benzoxepine core. In contrast with previous reports exhibiting lengthy strategies, we demonstrate the shortest synthesis of radulanin A to date, featuring a largely unexplored photochemical ring expansion reaction of a 2,2-dimethylchromene precursor. This work was adapted to a continuous-flow setup for larger-scale preparation, in view of biological investigations into the herbicidal properties of this natural product.

Sinunanolobatone A, an Anti-inflammatory Diterpenoid with Bicyclo[13.1.0]pentadecane Carbon Scaffold, and Related Casbanes from the Sanya Soft Coral Sinularia nanolobata

A novel diterpenoid, sinunanolobatone A (1), featuring an unprecedented bicyclo[13.1.0]pentadecane carbon framework, along with two new casbane diterpenoids (2 and 3), and five known related ones (4-8) were isolated from the Sanya soft coral Sinularia nanolobata. The structures of the new compounds were established by detailed spectroscopic analysis, X-ray diffraction analysis, chemical reactions, or a quantum chemical computation method. A plausible biosynthetic pathway of 1 was proposed. In bioassay, the novel compound 1 showed significant inhibitory activity against lipopolysaccharide (LPS) induced inflammation in BV-2 microglial cells.

Synthesis of a Biomimetic Tetracyclic Precursor of Aspochalasins and Formal Synthesis of Trichoderone A

Aspochalasins are leucine-derived cytochalasins. Their complexity is associated with a high degree of biosynthetic oxidation, herein inspiring a two-phase strategy in total synthesis. We thus describe the synthesis of a putative biomimetic tetracyclic intermediate. The constructive steps are an intramolecular Diels-Alder reaction to install the isoindolone core of cytochalasins, whose branched precursor was obtained from a stereoselective Ireland-Claisen rearrangement performed from a highly unsaturated substrate. This also constitutes a formal synthesis of trichoderone A.

Assessing the Role of Developmental and Environmental Factors in Chemical Defence Variation in Heliconiini Butterflies

Chemical defences in animals are both incredibly widespread and highly diverse. Yet despite the important role they play in mediating interactions between predators and prey, extensive differences in the amounts and types of chemical compounds can exist between individuals, even within species and populations. Here we investigate the potential role of environment and development on the chemical defences of warningly coloured butterfly species from the tribe Heliconiini, which can both synthesize and sequester cyanogenic glycosides (CGs). We reared 5 Heliconiini species in captivity, each on a single species-specific host plant as larvae, and compared them to individuals collected in the wild to ascertain whether the variation in CG content observed in the field might be the result of differences in host plant availability. Three of these species were reared as larvae on the same host plant, Passiflora riparia, to further test how species, sex, and age affected the type and amount of different defensive CGs, and how they affected the ratio of synthesized to sequestered compounds. Then, focusing on the generalist species Heliconius numata, we specifically explored variation in chemical profiles as a result of the host plant consumed by caterpillars and their brood line, using rearing experiments carried out on two naturally co-occurring host plants with differing CG profiles. Our results show significant differences in both the amount of synthesized and sequestered compounds between butterflies reared in captivity and those collected in the field. We also found a significant effect of species and an effect of sex in some, but not all, species. We show that chemical defences in H. numata continue to increase throughout their life, likely because of continued biosynthesis, and we suggest that variation in the amount of synthesized CGs in this species does not appear to stem from larval host plants, although this warrants further study. Interestingly, we detected a significant effect of brood lines, consistent with heritability influencing CG concentrations in H. numata. Altogether, our results point to multiple factors resulting in chemical defence variation in Heliconiini butterflies and highlight the overlooked effect of synthesis capabilities, which may be genetically determined to some extent.

Programmed Multiple C-H Bond Functionalization of the Privileged 4-hydroxyquinoline Template

The introduction of substituents on bare heterocyclic scaffolds can selectively be achieved by directed C-H functionalization. However, such methods have only occasionally been used, in an iterative manner, to decorate various positions of a medicinal scaffold to build chemical libraries. We herein report the multiple, site selective, metal-catalyzed C-H functionalization of a "programmed" 4-hydroxyquinoline. This medicinally privileged template indeed possesses multiple reactive sites for diversity-oriented functionalization, of which four were targeted. The C-2 and C-8 decorations were directed by an N-oxide, before taking benefit of an O-carbamoyl protection at C-4 to perform a Fries rearrangement and install a carboxamide at C-3. This also released the carbonyl group of 4-quinolones, the ultimate directing group to functionalize position 5. Our study highlights the power of multiple C-H functionalization to generate diversity in a biologically relevant library, after showing its strong antimalarial potential.

Complex Polypropionates from a South China Sea Photosynthetic Mollusk: Isolation and Biomimetic Synthesis Highlighting Novel Rearrangements

Placobranchus ocellatus is well known to produce diverse and complex γ-pyrone polypropionates. In this study, the chemical investigation of P. ocellatus from the South China Sea led to the discovery and identification of ocellatusones A-D, a series of racemic non-γ-pyrone polyketides with novel skeletons, characterized by a bicyclo[3.2.1]octane (1, 2), a bicyclo[3.3.1]nonane (3) or a mesitylene-substituted dimethylfuran-3(2H)-one core (4). Extensive spectroscopic analysis, quantum chemical computation, chemical synthesis, and/or X-ray diffraction analysis were used to determine the structure and absolute configuration of the new compounds, including each enantiomer of racemic compounds 1-4 after chiral HPLC resolution. An array of new and diversity-generating rearrangements is proposed to explain the biosynthesis of these unusual compounds based on careful structural analysis and comparison with six known co-occurring γ-pyrones (5-10). Furthermore, the successful biomimetic semisynthesis of ocellatusone A (1) confirmed the proposed rearrangement through an unprecedented acid induced cascade reaction.

Diversity-oriented synthesis of 17-spirosteroids

A diversity-oriented synthesis (DOS) approach has been used to functionalize 17-ethynyl-17-hydroxysteroids through a one-pot procedure involving a ring-closing enyne metathesis (RCEYM) and a Diels–Alder reaction on the resulting diene, under microwave irradiations. Taking advantage of the propargyl alcohol moiety present on commercially available steroids, this classical strategy was applied to mestranol and lynestrenol, giving a collection of new complex 17-spirosteroids.

Why has transparency evolved in aposematic butterflies? Insights from the largest radiation of aposematic butterflies, the Ithomiini

Defended species are often conspicuous and this is thought to be an honest signal of defences, i.e. more toxic prey are more conspicuous. Neotropical butterflies of the large Ithomiini tribe numerically dominate communities of chemically defended butterflies and may thus drive the evolution of mimetic warning patterns. Although many species are brightly coloured, most are transparent to some degree. The evolution of transparency from a warning-coloured ancestor is puzzling as it is generally assumed to be involved in concealment. Here, we show that transparent Ithomiini species are indeed less detectable by avian predators (i.e. concealment). Surprisingly, transparent species are not any less unpalatable, and may in fact be more unpalatable than opaque species, the latter spanning a larger range of unpalatability. We put forth various hypotheses to explain the evolution of weak aposematic signals in these butterflies and other cryptic defended prey. Our study is an important step in determining the selective pressures and constraints that regulate the interaction between conspicuousness and unpalatability.

Variation of chemical compounds in wild Heliconiini reveals ecological factors involved in the evolution of chemical defenses in mimetic butterflies

Evolutionary convergence of color pattern in mimetic species is tightly linked with the evolution of chemical defenses. Yet, the evolutionary forces involved in natural variations of chemical defenses in aposematic species are still understudied. Herein, we focus on the evolution of chemical defenses in the butterfly tribe Heliconiini. These neotropical butterflies contain large concentrations of cyanogenic glucosides, cyanide-releasing compounds acting as predator deterrent. These compounds are either de novo synthesized or sequestered from their Passiflora host plant, so that their concentrations may depend on host plant specialization and host plant availability. We sampled 375 wild Heliconiini butterflies across Central and South America, covering 43% species of this clade, and quantify individual variations in the different CGs using liquid chromatography coupled with tandem mass spectrometry. We detected new compounds and important variations in chemical defenses both within and among species. Based on the most recent and well-studied phylogeny of Heliconiini, we show that ecological factors such as mimetic interactions and host plant specialization have a significant association with chemical profiles, but these effects are largely explained by phylogenetic relationships. Our results therefore suggest that shared ancestries largely contribute to chemical defense variation, pointing out at the interaction between historical and ecological factors in the evolution of Müllerian mimicry.

Time resolved transient circular dichroism spectroscopy using synchrotron natural polarization

Ultraviolet (UV) synchrotron radiation circular dichroism (SRCD) spectroscopy has made an important contribution to the determination and understanding of the structure of bio-molecules. In this paper, we report an innovative approach that we term time-resolved SRCD (tr-SRCD), which overcomes the limitations of current broadband UV SRCD setups. This technique allows accessing ultrafast time scales (down to nanoseconds), previously measurable only by other methods, such as infrared (IR), nuclear magnetic resonance (NMR), fluorescence and absorbance spectroscopies, and small angle X-ray scattering (SAXS). The tr-SRCD setup takes advantage of the natural polarization of the synchrotron radiation emitted by a bending magnet to record broadband UV CD faster than any current SRCD setup, improving the acquisition speed from 10 mHz to 130 Hz and the accessible temporal resolution by several orders of magnitude. We illustrate the new approach by following the isomer concentration changes of an azopeptide after a photoisomerization. This breakthrough in SRCD spectroscopy opens up a wide range of potential applications to the detailed characterization of biological processes, such as protein folding and protein-ligand binding.

One-Pot Synthesis of Metastable 2,5-Dihydrooxepines through Retro-Claisen Rearrangements: Method and Applications

A one-pot methodology to synthesize metastable bicyclic 2,5-dihydrooxepines from cyclic 1,3-diketones and 1,4-dibromo-2-butenes through the retro-Claisen rearrangement of syn-2-vinylcyclopropyl diketone intermediates is reported. DFT calculations were performed to understand the reaction selectivity and mechanisms towards [1,3]- or [3,3]-sigmatropic rearrangements, highlighting the crucial influence of the temperature. The reaction was successfully applied to a short protecting group-free total synthesis of radulanin A, a natural 2,5-dihydrobenzoxepine. Moreover, the strong herbicidal potential of this natural product is demonstrated for the first time.

Marine sponges of the genus Stelletta as promising drug sources: chemical and biological aspects

Marine sponges of the genus Stelletta are well known as rich sources of diverse and complex biologically relevant natural products, including alkaloids, terpenoids, peptides, lipids, and steroids. Some of these metabolites, with novel structures and promising biological activities, have attracted a lot of attention from chemists seeking to perform their total synthesis in parallel to intensive biological studies towards new drug leads. In this review, we summarized the distribution of the chemically investigated Stelletta sponges, the isolation, synthesis and biological activities of their secondary metabolites, covering the literature from 1982 to early 2018.

Multifaceted Study on a Cytochalasin Scaffold: Lessons on Reactivity, Multidentate Catalysis, and Anticancer Properties

An intramolecular Diels-Alder (IMDA) reaction efficiently accelerated by Schreiner's thiourea is reported, to build a functionalized cytochalasin scaffold (periconiasin series) for biological purposes. DFT calculation highlighted a unique multidentate cooperative hydrogen bonding in this catalysis. The deprotection end game afforded a collection of diverse structures and showed the peculiar reactivity of the Diels-Alder cycloadducts upon functionalization. Biological studies revealed strong cytotoxicity of a few compounds on breast cancer cell lines while actin polymerization is preserved.

Inhibition of Phytophthora species, agents of cocoa black pod disease, by secondary metabolites of Trichoderma species

Cocoa production is affected by the black pod disease caused by several Phytophthora species that bring, about each year, an estimated loss of 44% of world production. Chemical control remains expensive and poses an enormous risk of poisoning for the users and the environment. Biocontrol by using antagonistic microorganisms has become an alternative to the integrated control strategy against this disease. Trichoderma viride T7, T. harzanium T40, and T. asperellum T54, which showed in vivo and in vitro antagonistic activity against P. palmivora, were cultured and mycelia extracted. Inhibition activity of crude extracts was determined, and then organic compounds were isolated and characterized. The in vitro effect of each compound on the conidia germination and mycelia growth of four P. palmivora, two P. megakaria, and one P. capsici was evaluated. T. viride that displayed best activities produced two active metabolites, viridin and gliovirin, against P. palmivora and P. megakaria strains. However, no activity against P. capsici was observed. Besides being active separately, these two compounds have a synergistic effect for both inhibitions, mycelia growth and conidia germination. These results provide the basis for the development of a low-impact pesticide based on a mixture of viridin and gliovirine.

An Integrative Approach to Decipher the Chemical Antagonism between the Competing Endophytes Paraconiothyrium variabile and Bacillus subtilis

An integrative approach combining traditional natural products chemistry, molecular networking, and mass spectrometry imaging has been undertaken to decipher the molecular dialogue between the fungus Paraconiothyrium variabile and the bacterium Bacillus subtilis, which were isolated as endophytes from the conifer Cephalotaxus harringtonia and are characterized by a strong and mutual antibiosis. From this study, we highlight that bacterial surfactins and a fungal tetronic acid are involved in such competition and that the fungus is able to hydrolyze surfactins to fight against the bacterial partner.

Harnessing the potential diversity of resinic diterpenes through visible light-induced sensitized oxygenation coupled to Kornblum–DeLaMare and Hock reactions

A biomimetic procedure for the late functionalization of resinic acids is reported, implementing photooxygenation by singlet oxygen, using visible light and a photosensitized, combined to the Kornblum–DeLaMare reaction or the Hock rearrangement.

Antimicrobial Oligophenalenone Dimers from the Soil Fungus Talaromyces stipitatus

New polyketide-derived oligophenalenone dimers, 9a-epi-bacillisporin E (1) and bacillisporins F-H (2-5), along with the known bacillisporin A (6), were isolated from the fungus Talaromyces stipitatus. Their structures and absolute configurations were determined on the basis of spectroscopic analyses, electronic circular dichroism, and GIAO NMR shift calculation followed by DP4 analysis. The antimicrobial activity of these compounds was evaluated against a panel of human pathogenic bacteria. Among them, bacillisporin H (5) exhibited antimicrobial activity together with modest cytotoxicity against HeLa cells.

Variation in cyanogenic compounds concentration within a Heliconius butterfly community: does mimicry explain everything?

Background

Aposematic species advertise their unpalatability using warning signals such as striking coloration. Given that predators need to sample aposematic prey to learn that they are unprofitable, prey with similar warning signals share the cost of predator learning. This reduction in predation risk drives evolutionary convergence of warning signals among chemically defended prey (Müllerian mimicry). Whether such warning signal convergence is associated to similar defence levels among co-mimics is still an open question that has rarely been tested in wild populations. We quantified variation in cyanide-based (CN) chemical protection in wild caught individuals of eight aposematic Heliconius butterfly species belonging to four sympatric mimicry rings. We then tested for correlations between chemical protection and ecological species-specific traits.

Results

We report significant differences in CN concentrations both within and between sympatric species, even when accounting for the phylogeny, and within and between mimicry rings, even after considering inter-specific variation. We found significant correlations between CN concentration and both hostplant specialization and gregarious behaviour in adults and larvae. However, differences in CN concentrations were not significantly linked to mimicry ring abundance, although the two most toxic species did belong to the rarest mimicry ring.

Conclusions

Our results suggest that mimicry can explain the variation in the levels of chemical defence to a certain extent, although other ecological factors are also relevant to the evolution of such variability.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0843-5) contains supplementary material, which is available to authorized users.

First Total Synthesis, Structure Revision, and Natural History of the Smallest Cytochalasin: (+)-Periconiasin G

The total synthesis of the smallest cytochalasin isolated so far, periconiasin G, which bears a seven-membered ring in lieu of the usual macrocycle, has been performed from both enantiomers of citronellal, relying on an intramolecular Diels-Alder reaction in favor of the natural endo stereochemistry. We show that, among the four synthesized stereoisomers, including the exo isomers, the one matching the NMR data of the natural product was not that assigned in the original report, imposing structure revision. The natural product, previously isolated from a plant-mutualistic fungus, was biologically investigated taking into account its natural history, showing significant effects against the phytopathogenic fungus Botrytis cinerea and thus opening new opportunities in combating this pest.

3-Acylated tetramic and tetronic acids as natural metal binders: myth or reality?

Covering: up to 20153-Acylated tetramic and tetronic acids are characterized by a low pKa and are likely to be deprotonated under physiological conditions. In addition, their structure makes them excellent chelators of metallic cations. We will discuss the significance of these chemical properties with regard to the biological properties and mechanisms of action of these compounds, highlighting the importance of considering them as salts or chelates for biological purposes, rather than acids.

Talaroketals A and B, unusual bis(oxaphenalenone) spiro and fused ketals from the soil fungus Talaromyces stipitatus ATCC 10500

Talaroketals A and B, unusual bis(oxaphenalenone) spiro and fused ketals were isolated from the soil fungusTalaromyces stipitatusATCC 10500.

Unexpected talaroenamine derivatives and an undescribed polyester from the fungus Talaromyces stipitatus ATCC10500

Chemical investigation of the fungus Talaromyces stipitatus ATCC 10500, whose genome has been sequenced, led to the isolation of four undescribed talaroenamines B-E along with the known talaroenamine A. Their structures were elucidated on the basis of spectroscopic studies including mass spectrometry, extensive 2D NMR and electronic circular dichroism (ECD). Interestingly, talaroenamine A had previously been isolated from the strain of T. stipitatus Δtrop C, a strain knocked out for the gene encoding a non-heme Fe(II)-dependent dioxygenase catalyzing the oxidative ring expansion leading to the tropolone, but never from a wild-type strain. All talaroenamines were evaluated for their antiplasmodial activity and Talaroenamine D exhibited the best inhibition against the chloroquine-resistant Plasmodium falciparum (FcB1 strain) without noticeable toxicity on HeLa and preadipose cell lines. In the course of the chemical investigation of T. stipitatus, an undescribed polyester was also isolated and its absolute configuration was determined by hydrolysis and transesterification followed by gas chromatography on chiral column.

Ring-closing enyne metathesis of terminal alkynes with propargylic hindrance

The ring closing enyne metathesis of substrates with propargylic hindrance was investigated, revealing the successful combination of the Stewart-Grubbs catalysts and microwave heating sometimes up to 170 °C for oxacycles. Medium-sized rings were obtained from terminal alkynes previously reputed as reluctant substrates. This unmatched combination was applied to the synthesis of carbocycles and oxacycles. In addition, this is the first report on the use of the Stewart Grubbs catalyst in ring closing enyne metatheses.

Direct biosynthetic cyclization of a distorted paracyclophane highlighted by double isotopic labelling of l-tyrosine

The biosynthesis of pyrrocidines was investigated using a double (¹⁸O,¹³C) labelling of l-tyrosine. It shows that the phenolic ¹⁸O is incorporated during aryl ether bond formation.

The fungal leaf endophyte Paraconiothyrium variabile specifically metabolizes the host-plant metabolome for its own benefit

Fungal endophytes live inside plant tissues and some have been found to provide benefits to their host. Nevertheless, their ecological impact is not adequately understood. Considering the fact that endophytes are continuously interacting with their hosts, it is conceivable that both partners have substantial influence on each other's metabolic processes. In this context, we have investigated the action of the endophytic fungus Paraconiothyrium variabile, isolated from the leaves of Cephalotaxus harringtonia, on the secondary metabolome of the host-plant. The alteration of the leaf compounds by the fungus was monitored through metabolomic approaches followed by structural characterization of the altered products. Out of more than a thousand molecules present in the crude extract of the plant leaf, we have observed a specific biotransformation of glycosylated flavonoids by the endophyte. In all cases it led to the production of the corresponding aglycone via deglycosylation. The deglycosylated flavonoids turned out to display significant beneficial effects on the hyphal growth of germinated spores. Our finding, along with the known allelopathic role of flavonoids, illustrates the chemical cooperation underlying the mutualistic relationship between the plant and the endophyte.

Chemically unprecedented biocatalytic (AuaG) retro-[2,3]-Wittig rearrangement: a new insight into aurachin B biosynthesis

AuaG is flavin-dependent monooxygenase responsible for the conversion of aurachin C to aurachin B, a reaction thought to resemble semipinacol migration of the farnesyl substituent. A study of the substrate tolerance of AuaG reveals that it has the peculiar ability to oxidise short-chain analogues of aurachin D. Unexpectedly, a novel retro-[2,3]-Wittig rearrangement was observed with an isoprenyl substrate analogue, thus leading to the 1,1-dimethylallyl ether. Additionally, we found that saturated-chain analogues of N-oxidised aurachin C were not transformed by the C3→C4 semipinacol reaction, as might have been expected for such substrates. Based on this and the unique retro-[2,3]-Wittig rearrangement, we discuss an alternative biosynthetic route for the conversion of aurachin C to aurachin B.

Transition metal-promoted biomimetic steps in total syntheses

Important biomimetic steps in natural product synthesis have been promoted by transition metals, as exemplified by this beautiful ruthenium-catalyzed rearrangement of an endoperoxide into elysiapyrone A. Such reactions are supposed to occur during the biosynthesis, yet under different catalysis conditions.

Synthesis and biological activities of the respiratory chain inhibitor aurachin D and new ring versus chain analogues

Aurachins are myxobacterial 3-farnesyl-4(1H)-quinolone derived compounds initially described as respiratory chain inhibitors, more specifically as inhibitors of various cytochrome complexes. They are also known as potent antibiotic compounds. We describe herein the first synthesis of aurachin D through a key Conrad–Limpach reaction. The same strategy was used to reach some ring as opposed to chain analogues, allowing for the description of structure–activity relationships. Biological screening of the analogues showed antiparasitic, cytotoxic, antibacterial and antifungal activities, and depletion of the mitochondrial membrane potential. The strongest activity was found on Plasmodium falciparum with a selectivity index of 345, compared to Vero cells, for the natural product and its geranyl analogue. The loss of mitochondrial membrane potential induced by aurachins in human U-2 OS osteosarcoma cells was studied, showing the best activity for aurachin D and a naphthalene analogue, yet without totally explaining the observed cytotoxic activity of the compounds. Finally, a synthetic entry is given to the complete carboheterocyclic core of aurachin H through the N-oxidation/epoxidation of aurachin D and a shorter chain analogue, followed by subsequent biomimetic cyclization.

Hirsutellones and beyond: figuring out the biological and synthetic logics toward chemical complexity in fungal PKS-NRPS compounds

Covering: up to early 2013. Fungal polyketides and their hybrid non ribosomal peptide derivatives are characterized by often striking structural features and biological activities. Their diversity and their complexity arise from highly organized and programmable biosynthetic pathways and have been challenged by many synthetic chemists. This review will conceptually illustrate how complexity can be generated, starting from a general biosynthetic purpose (the fundaments of PKS-NRPS assembly lines) and finally showing how the particular class of hirsutellone compounds has emerged from such processes in relation to post-elongation and secondary tailoring events. Synthetic efforts to produce these natural products will be described with a special emphasis on complexity-generating strategies and steps. Thus, the biosynthetic and synthetic works will be analyzed in a continuous flow, focusing on both the logic of Nature and organic chemists.

Guaiane sesquiterpenes from Biscogniauxia nummularia featuring potent antigerminative activity

Xylaranone, a previously unreported guaiane sesquiterpene along with the known terpenoid xylaranol B and the two mellein derivatives 3,5-dimethyl-8-methoxy-3,4-dihydroisocoumarin and 3,5-dimethyl-8-hydroxy-3,4-dihydroisocoumarin were isolated from Biscogniauxia nummularia. Pogostol was also isolated from this fungus, and in light of our spectroscopic data, its structure was revised and corrected. This fungus, which was isolated as an endophyte from the plum yew Cephalotaxus harringtonia, is also suspected of being a pathogen. Interestingly, we report here the potent antigerminative activity of xylaranone and xylaranol B against seeds of Raphanus sativus at concentrations comparable to glyphosate, a commonly used herbicide. This effect suggests a role for these metabolites in the latent fungal pathogenesis of B. nummularia.

Asymmetric synthesis of the oxygenated polycyclic system of (+)-harringtonolide

A straightforward asymmetric synthesis of the cage oxygenated structure of (+)-harringtonolide has been accomplished for the first time. The key steps involved (i) a templated stereoselective IMDA reaction to build a highly functionalized cyclohexene ring D, (ii) functionalization of the cycloadduct, (iii) ring-closing metathesis providing the five-membered ring C, and finally (iv) a challenging one-step cascade cyclization of an epoxy-alcohol toward the target structure, whose mechanism was investigated.