(+)-epi-Epoformin, a Phytotoxic Fungal Cyclohexenepoxide: Structure Activity Relationships

Natural epoxyquinoids: isolation, biological activity and synthesis. An update

Epoxyquinoids are of continuing interest due to their wide natural distribution and diverse biological activities, including, but not limited to, antibacterial, antifungal, anticancer, enzyme inhibitory, and others. The last review on their total synthesis was published in 2017. Since then, almost 100 articles have been published on their isolation from nature and their biological profile. In addition, the review specifically considers synthesis, including total and enantioselective, as well as the development of shorter approaches for the construction of epoxyquinoids with complex chemical architecture. Thus, this review focuses on progress in this area in order to stimulate further research.

An Ecotoxicological Evaluation of Four Fungal Metabolites with Potential Application as Biocides for the Conservation of Cultural Heritage

Biocides based on chemical synthetic compounds have been commonly used to counteract damages caused by microorganisms on stone cultural heritage. However, in the last few years, the use of commercial and traditional biocides has been banned and/or limited due to their dangerous profile for the environment, as well as human and animal health. Natural products could be used as suitable alternatives for cultural heritage purposes, as they have low toxicity and stability compared with synthetic pesticides. Even if most of the investigated solutions have already shown promising results, their efficiency, ecotoxicological, and chemical features are poorly investigated. In this manuscript, we aimed to evaluate the ecotoxicological profile of four fungal metabolites-namely, cavoxin, epi-epoformin, seiridin, and sphaeropsidone-with potential antimicrobial properties for monumental artworks. A battery of ecotoxicological tests using Aliivibrio fischeri (bacterium), Raphidocelis subcapitata (alga), Raphanus sativus L. (macrophyte), Daphnia magna (crustacean), and Caenorhabditis elegans (nematode) revealed a relative lower toxicity of these compounds, especially when compared with Preventol^® and Rocima^®, commercial biocides mainly used for the conservation of cultural heritage.

In Vitro and In Vivo Toxicity Evaluation of Natural Products with Potential Applications as Biopesticides

The use of natural products in agriculture as pesticides has been strongly advocated. However, it is necessary to assess their toxicity to ensure their safe use. In the present study, mammalian cell lines and fish models of the zebrafish (Danio rerio) and medaka (Oryzias latipes) have been used to investigate the toxic effects of ten natural products which have potential applications as biopesticides. The fungal metabolites cavoxin, epi-epoformin, papyracillic acid, seiridin and sphaeropsidone, together with the plant compounds inuloxins A and C and ungeremine, showed no toxic effects in mammalian cells and zebrafish embryos. Conversely, cyclopaldic and α-costic acids, produced by Seiridium cupressi and Dittrichia viscosa, respectively, caused significant mortality in zebrafish and medaka embryos as a result of yolk coagulation. However, both compounds showed little effect in zebrafish or mammalian cell lines in culture, thus highlighting the importance of the fish embryotoxicity test in the assessment of environmental impact. Given the embryotoxicity of α-costic acid and cyclopaldic acid, their use as biopesticides is not recommended. Further ecotoxicological studies are needed to evaluate the potential applications of the other compounds.

A Study on the Phytotoxic Potential of the Seasoning Herb Marjoram (Origanum majorana L.) Leaves

In the search of new alternatives for weed control, spices appear as an option with great potential. They are rich in bioactive natural products and edible, which might minimize toxicity hazard. Marjoram (Origanum majorana L.) is an aromatic herb that has been widely employed as a seasoning herb in Mediterranean countries. Although marjoram boasts a plethora of therapeutic properties (painkiller, antibiotic, treatment for intestinal disorders, etc.), the potential for its extracts for weed control is still to be more thoroughly explored. In order to determine their phytotoxic potential, marjoram leaves were subjected to different bioguided extraction processes, using water, ethyl acetate, acetone or methanol. The most active extract (acetone) was sequentially fractionated to identify its most active compounds. This fractionation led to the isolation and identification of 25 compounds that were classified as monoterpenes, diterpenes or flavonoids. Among them, a new compound named majoradiol and several compounds are described in marjoram for the first time. The phytotoxicity of the major compounds to etiolated wheat coleoptiles was compared against that of the commercial herbicide (Logran^®), with similar or higher activity in some cases. These results confirm the extraordinary potential of the extracts from this edible plant to develop safer and more environmentally friendly herbicides.

Activity of Some Plant and Fungal Metabolites towards Aedes albopictus (Diptera, Culicidae)

Aedes albopictus (Skuse) is a widespread mosquito, a vector of important human arboviruses, including Chikungunya, Dengue and Zika. It is an extremely difficult species to control even for the onset of resistances to chemicals insecticides, therefore ecofriendly products are urgently needed. In this study, the activity of Amaryllidaceae alkaloids and some of their semisynthetic derivatives, of 2-methoxy-1,4-naphthoquinone and two analogues, of cyclopaldic acid and epi-epoformin on the survival and development of Ae. albopictus larvae was evaluated. First-instar larval exposure for 24 and 48 h to cyclopaldic acid, resulted in mortality mean per-centage of 82.4 and 96.9 respectively; 1,2-O,O-diacetyllycorine 48h post-treatment caused 84.7% mortality. Larval and pupal duration were proved to decrease significantly when larvae were exposed to cyclopaldic acid, 1,2-O,O-diacetyllycorine and N-methyllycorine iodide. The mean number of third-instar larvae surviving to 2-methyl-1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone and 2-methoxy-1,4-naphthoquinone was significantly lower than the number of correspondent control larvae over the time. This study indicated that 1,2-O,O'-diacetyllycorine, N-methyllycorine iodide, cyclopaldic acid and 1,4-naphthoquinone structural derivatives have good potential for developing bioinsecticides for mosquito control programs. The obtained results are of general interest due to the global importance of the seri-ous human diseases such a vector is able to spread.

Allelopathic Effect of Quercetin, a Flavonoid from Fagopyrum esculentum Roots in the Radicle Growth of Phelipanche ramosa: Quercetin Natural and Semisynthetic Analogues Were Used for a Structure-Activity Relationship Investigation

Allelopathic potential of buckwheat roots on the radicle growth of the broomrape weed species Orobanche cumana and Phelipanche ramosa was studied. Buckwheat root exudates induced a significant growth inhibition in P. ramosa radicles but radicles of O. cumana were not affected. Among the metabolites present in the root organic extract we identified the flavonol quercetin and the stilbene p-coumaric acid methyl ester with only quercetin showing inhibitory effect on P. ramosa. The activity of quercetin was compared with other two similar flavanoids, the flavone apigenin and the dihydroflavanol 3-O-acetylpadmatin extracted respectively from Lavandula stoechas and Dittrichia viscosa plants. In this comparative assay only 3-O-acetylpadmatin besides quercetin, showed inhibition activity of radicle growth while apigenin was inactive. These results indicated that the presence of two ortho-free hydroxy groups of C ring, like catechol, could be an important feature to impart activity while the carbon skeleton of B ring and substituents of both A and B rings are not essential. Besides reduction of radicle growth, haustorium induction was observed at the tip of P. ramosa radicles treated with quercetin which swelled and a layer of papillae was formed. Activity of quercetin on haustorium induction in P. ramosa was assayed in comparison with the known haustorium-inducing factor 2,6-dimethoxy-p-benzoquinone (DMBQ) and a three partial methyl ether derivatives semisynthetized from quercetin. Results indicated that P. ramosa haustorium was induced by DMBQ at concentrations of 1–0.5 mM and quercetin and its derivatives at concentration range 0.1–0.05 mM.

Fungal Metabolites with Antagonistic Activity against Fungi of Lithic Substrata

Fungi are among the biotic agents that can cause deterioration of building stones and cultural heritage. The most common methods used to control fungal spread and growth are based on chemical pesticides. However, the massive use of these synthetic chemicals produces heavy environmental pollution and risk to human and animal health. Furthermore, their use is time dependent and relies on the repetition of treatments, which increases the possibility of altering building stones and culture heritage through environmental contamination. One alternative is the use of natural products with high antifungal activity, which can result in reduced toxicity and deterioration of archeological remains. Recently, three fungal strains, namely Aspergillus niger, Alternaria alternata and Fusarium oxysporum, were isolated as damaging agents from the external tuff wall of the Roman remains "Villa of Poppea" in Oplontis, Naples, Italy. In this manuscript, three selected fungal metabolites, namely cyclopaldic acid, cavoxin and epi-epoformin, produced by fungi pathogenic for forest plants, were evaluated as potential antifungal compounds against the above fungi. Cavoxin and epi-epoformin showed antifungal activity against Asperigillus niger and Fusarium oxysporum, while cyclopaldic acid showed no activity when tested on the three fungi. The same antifungal activity was observed in vitro experiments on infected stones of the Neapolitan yellow tuff (NYT), a volcanic lithotype widely diffused in the archeological sites of Campania, Italy. This study represents a first step in the use of these two fungal metabolites to allow better preservation of artworks and to guarantee the conditions suitable for their conservation.

Inuloxin E, a New Seco-Eudesmanolide Isolated from Dittrichia viscosa, Stimulating Orobanche cumana Seed Germination

A new sesquiterpenoid belonging to the subgroup seco-eudesmanolides and named inuloxin E was isolated from Dittrichia viscosa, together with the already known sesquiterpenoids inuloxins A–D and α-costic acid. Inuloxin E was characterized by spectroscopic data (essentially NMR and ESI MS) as 3-methylene-6-(1-methyl-4-oxo-pentyl)-3a,4,7,7a-tetrahydro-3H-benzofuran-2-one. Its relative configuration was determined by comparison with the closely related inuloxin D and chemical conversion of inuloxin E into inuloxin D and by the observed significant correlation in the NOESY spectrum. Both inuloxins D and E induced germination of the parasitic weed Orobanche cumana, but were inactive on the seeds of Orobanche minor and Phelipanche ramosa. The germination activity of some hemisynthetic esters of inuloxin D was also investigated.

Easy Access to Alkoxy, Amino, Carbamoyl, Hydroxy, and Thiol Derivatives of Sesquiterpene Lactones and Evaluation of Their Bioactivity on Parasitic Weeds

It has been hypothesized that the α-methylene-γ-lactone moiety of sesquiterpene lactones is a key unit for their bioactivity. As a consequence, modifications of these compounds have been focused on this fragment. In the work reported here, two sesquiterpene lactones, namely, dehydrocostuslactone and β-cyclocostunolide, a eudesmanolide obtained by controlled cyclization of costunolide, were chosen for modification by Michael addition at C-13. On applying this reaction to both compounds, it was possible to introduce the functional groups alkoxy, amino, carbamoyl, hydroxy, and thiol to give products in good to high yields, depending on the base and solvent employed. In particular, the introduction of a thiol group at C-13 in both compounds was achieved with outstanding yields (>90%) and this is unprecedented for these sesquiterpene lactones. The bioactivities of the products were evaluated on etiolated wheat coleoptile elongation and germination of seeds of parasitic weeds, with significant activity observed on Orobanche cumana and Phelipanche ramosa. The structure-activity relationships are discussed.

Recent advances in allelopathy for weed control: from knowledge to applications

Allelopathy is the biological phenomenon of chemical interactions between living organisms in the ecosystem, and must be taken into account in addressing pest and weed problems in future sustainable agriculture. Allelopathy is a multidisciplinary science, but in some cases, aspects of its chemistry are overlooked, despite the need for a deep knowledge of the chemical structural characteristics of allelochemicals to facilitate the design of new herbicides. This review is focused on the most important advances in allelopathy, paying particular attention to the design and development of phenolic compounds, terpenoids and alkaloids as herbicides. The isolation of allelochemicals is mainly addressed, but other aspects such as the analysis and activities of derivatives or analogs are also covered. Furthermore, the use of allelopathy in the fight against parasitic plants is included. The past 12 years have been a prolific period for publications on allelopathy. This critical review discusses future research areas in this field and the state of the art is analyzed from the chemist's perspective. © 2019 Society of Chemical Industry.

Lentiquinones A, B, and C, Phytotoxic Anthraquinone Derivatives Isolated from Ascochyta lentis, a Pathogen of Lentil

A strain of the pathogenic fungus Ascochyta lentis isolated from lentil ( Lens culinaris) was studied to ascertain its capability to produce bioactive metabolites. From the culture filtrates were found three new anthraquinone derivatives, named lentiquinones A (1), B (2), and C (3), and the known lentisone. From the mycelium, four known analogues were identified, namely pachybasin (in larger amount), ω-hydroxypachybasin, 1,7-dihydroxy-3-methylanthracene-9,10-dione, and phomarin. Lentiquinones A-C were characterized by spectroscopic methods as 3,4,6-trihydroxy-8-methyl-2 H-benzo[ g]chromene-5,10-dione, 2,3,4,5,10-pentahydroxy-7-methyl-3,4,4a,10-tetrahydroanthracen-9(2 H)-one, and its 2-epimer, respectively, and the relative configuration of the two latter compounds was deduced by X-ray diffraction data analysis. The absolute configuration of lentiquinones B and C was determined as (2 R,3 S,4 S,4a S,10 R) and (2 S,3 S,4 S,4a S,10 R), respectively, by electronic circular dichroism (ECD) in solution and solid state, and TDDFT calculations. When tested by using different bioassays, the novel compounds showed interesting activities. In particular, applied to punctured leaves of host and nonhost plants, the three new compounds and lentisone caused severe necrosis, with lentiquinone A being the most active among the new metabolites. On cress ( Lepidium sativum), this latter compound proved to be particularly active in inhibiting root elongation. On Lemna minor all the compounds reduced the content of chlorophyll, with 1,7-dihyroxy-3-methylanthracene-9,10-dione being the most active. The new compounds, together with lentisone, proved to have antibiotic properties.