Chirality Sensing of Cryptochiral Guests with Prism[n]arenes.

Optical chirality sensing has gained significant attention in recent years. Within this field, the quest for stereodynamic chiroptical probes capable of detecting cryptochiral guests presents a formidable challenge. Macrocycles exhibiting planar chirality have emerged as promising candidates for amplifying the chirality of cryptochiral guests. In this study, we demonstrate that the formation of host-guest complexes between cryptochiral molecules and planar chiral prismarenes triggers electronic circular dichroism (ECD) signals via host-guest complexation-induced chirality amplification. The absolute configuration of the most stable chiral macrocyclic host-guest complex has been established by resorting to both exciton model and DFT computations. Furthermore, we demonstrated that this supramolecular chirality sensing system can be employed to determine the enantiomeric composition of scalemic mixtures by measuring the ECD bands intensity. The information described here opens the way for the use of prismarenes as stereodynamic probes for sensing of cryptochiral guests.

Palladium Complexes of N-Methylcorroles.

Alkylation of one of the inner-core nitrogen atoms is one possible approach to obtain dianionic corrole ligands, suitable for the coordination of divalent metal ions, such as PdII . Inner-core N-methylation can be obtained by treating the corrole with CH3 I, but the reaction conditions should be optimized to limit the formation of the dimethylated derivative. Two regioisomers, the N-21 and the N-22 methyl derivatives are obtained from the reaction, with the first product achieved in a higher amount. Structural characterization of the reaction products evidenced the distortion induced by the introduction of the methyl groups; the N-methylcorroles are chiral compounds, and the enantiomers were separated by chromatography, with their absolute configuration assigned by ECD computation. Palladium insertion was achieved in the case of monosubstituted corroles, but not with the dimethylated macrocycle; X-ray characterization of the complexes showed the distortion of the macrocycles. The Pd complexes do not show luminescence emission, but are able to produce singlet oxygen upon irradiation. The PdII complexes were also inserted in human serum albumin (HSA) and dispersed in water; in this case, the protein protects the corroles from photobleaching, and a switch from the type II to the type I mechanism in reactive oxygen species (ROS) production is observed.

(±)-3-Deoxyradicinin Induces Stomata Opening and Chloroplast Oxidative Stress in Tomato (Solanum lycopersicum L.).

Radicinin is a phytotoxic dihydropyranopyran-4,5-dione isolated from the culture filtrates of Cochliobolus australiensis, a phytopathogenic fungus of the invasive weed buffelgrass (Cenchrus ciliaris). Radicinin proved to have interesting potential as a natural herbicide. Being interested in elucidating the mechanism of action and considering radicinin is produced in small quantities by C. australiensis, we opted to use (±)-3-deoxyradicinin, a synthetic analogue of radicinin that is available in larger quantities and shows radicinin-like phytotoxic activities. To obtain information about subcellular targets and mechanism(s) of action of the toxin, the study was carried out by using tomato (Solanum lycopersicum L.), which, apart from its economic relevance, has become a model plant species for physiological and molecular studies. Results of biochemical assays showed that (±)-3-deoxyradicinin administration to leaves induced chlorosis, ion leakage, hydrogen peroxide production, and membrane lipid peroxidation. Remarkably, the compound determined the uncontrolled opening of stomata, which, in turn, resulted in plant wilting. Confocal microscopy analysis of protoplasts treated with (±)-3-deoxyradicinin ascertained that the toxin targeted chloroplasts, eliciting an overproduction of reactive singlet oxygen species. This oxidative stress status was related by qRT-PCR experiments to the activation of transcription of genes of a chloroplast-specific pathway of programmed cell death.

Mitochondrial Role in Intrinsic Apoptosis Induced by a New Synthesized Chalcone in Hepatocellular Carcinoma Cells.

Hepatocellular carcinoma (HCC) is the most common type of liver cancer and the fourth cause of cancer-related deaths worldwide. Presently, a few drugs are available for HCC treatment and prevention, including both natural and synthetic compounds. In this study, a new chalcone, (E)-1-(2,4,6-triethoxyphenyl)-3-(3,4,5-trimethoxyphenyl)prop-2-en-1-one (ETTC), was synthesized and its effects and mechanisms of action over human hepatoma cells were investigated. Cytotoxic activity was revealed in HCC cells, while no effects were observed in normal hepatocytes. In HCC cells, ETTC caused subG1 cell cycle arrest and apoptosis, characterized by nuclear fragmentation. The activation of caspases 3/7 and 9, the increase in pro-apoptotic BAX, and the decrease in anti-apoptotic BCL-2 suggest the activation of the intrinsic pathway of apoptosis. ETTC mitochondrial targeting is confirmed by the reduction in mitochondrial membrane potential and Complex I activity together with levels of superoxide anion increasing. Our outcomes prove the potential mitochondria-mediated antitumor effect of newly synthesized chalcone ETTC in HCC.

Synthesis and Stereochemical Characterization of a Novel Chiral α-Tetrazole Binaphthylazepine Organocatalyst.

A novel α-tetrazole-substituted 1,1'-binaphthylazepine chiral catalyst has been synthesized and its absolute configuration has been determined by DFT computational analysis of the vibrational circular dichroism (VCD) spectrum of its precursor. The VCD analysis, carried out through the model averaging method, allowed to assign the absolute configuration of a benzylic stereocenter in the presence of a chiral binaphthyl moiety. The 1,1'-binaphthylazepine tetrazole and the nitrile its immediate synthetic precursor, have been preliminarily tested as chiral organocatalysts in the asymmetric intramolecular oxa-Michael cyclization of 2-hydroxy chalcones for the synthesis of chiral flavanones obtaining low enantioselectivity.

In Vitro Effects of Fungal Phytotoxins on Cancer Cell Viability: First Insight into Structure Activity Relationship of a Potent Metabolite of Cochliobolus australiensis Radicinin.

Natural compounds have always represented an important source for new drugs. Although fungi represent one such viable source, to date, no fungal metabolite has been marketed as an anticancer drug. Based on our work with phytotoxins as potential chemical scaffolds and our recent findings involving three phytopathogenic fungi, i.e., Cochliobolus australiensis, Kalmusia variispora and Hymenoscyphus fraxineus, herein, we evaluate the in vitro anti-cancer activity of the metabolites of these fungi by MTT assays on three cancer cell models harboring various resistance levels to chemotherapeutic drugs. Radicinin, a phytotoxic dihydropyranopyran-4,5-dione produced by Cochliobolus australiensis, with great potential for the biocontrol of the invasive weed buffelgrass (Cenchrus ciliaris), showed significant anticancer activity in the micromolar range. Furthermore, a SAR study was carried out using radicinin, some natural analogues and hemisynthetic derivatives prepared by synthetic methods developed as part of work aimed at the potential application of these molecules as bioherbicides. This investigation opens new avenues for the design and synthesis of novel radicinin analogues as potential anticancer agents.

Diplofuranoxin, a disubstituted dihydrofuranone, was produced together with sphaeropsidin A and epi-sphaeropsidone by Diplodia subglobosa, an emerging ash (Fraxinus excelsior L.) pathogen in Europe.

An undescribed disubstituted dihydrofuranone, named diplofuranoxin, was isolated, together with the six well known metabolites sphaeropsidins A and C, epi-sphaeropsidone, mellein and cis- and trans-4-hydroxymelleins, from the fungal species Diplodia subglobosa, an emerging pathogen involved in the ash dieback aetiology in Europe. Currently, the disease represents the main threat to European ash heritage and the wood associated industry. Diplofuranoxin, was characterized essentially by NMR and HRESIMS spectra as (3Z)-3-(2,3-dihydroxybutylidene)-5-methyldihydrofuran-2(3H)-one. Its relative and absolute configuration was determined by joining NOESY NMR experiments and computational analysis of electronic circular dichroism spectrum. All the metabolites were screened for phytotoxic, antioomycetes and zootoxic activities and only sphaeropsidin A and epi-sphaeropsidone were active in two out of three bioassays performed. In addition, sphaeropsidin A completely inhibited mycelium growth of Phytophthora cambivora, whereas the inhibition rate of epi-sphaeropsidone was less than 50% at the higher concentration used. Both metabolites were inactive in the Artemia salina assay. Results obtained in this study have allowed to characterize for the first time the main metabolites produced in vitro by D. subglobosa and to increase the knowledge on the metabolic profile of Botryosphaeriaceae for a correct taxonomic classification of the strains belonging to this family.

Computational Approaches and Use of Chiroptical Probes in the Absolute Configuration Assignment to Natural Products by ECD Spectroscopy: A 1,2,3-Trihydroxy-p-menthane as a Case Study.

In this study, the computational analysis of electronic circular dichroism (ECD) spectra and the employment of biphenyl chiroptical probes were compared in the absolute configuration assignment of (-)-1α,2α,3ß-trihydroxy-p-menthane (1), taken as a representative example of a UV-transparent chiral natural product. The usefulness of chiroptical probes in the configurational assignments of natural products and their complementarity to the computational protocols is herein highlighted. The biphenyl probe approach proves to be straightforward, reliable, and suitable for conformationally mobile and ECD silent compounds, not treatable by computational analysis of chiroptical data.

Pinofuranoxins A and B, Bioactive Trisubstituted Furanones Produced by the Invasive Pathogen Diplodia sapinea.

Two new bioactive trisubstituted furanones, named pinofuranoxins A and B (1 and 2), were isolated from Diplodia sapinea, a worldwide conifer pathogen causing severe disease. Pinofuranoxins A and B were characterized essentially by NMR and HRESIMS spectra, and their relative and absolute configurations were assigned by NOESY experiments and computational analyses of electronic circular dichroism spectra. They induced necrotic lesions on Hedera helix L., Phaseolus vulgaris L., and Quercus ilex L. Compound 1 completely inhibited the growth of Athelia rolfsii and Phytophthora cambivora, while 2 showed antioomycetes activity against P. cambivora. In the Artemia salina assay both toxins showed activity inducing larval mortality.

Argyrotoxins A-C, a trisubstituted dihydroisobenzofuranone, a tetrasubstituted 2-hydroxyethylbenzamide and a tetrasubstitutedphenyl trisubstitutedbutyl ether produced by Alternaria argyroxiphii, the causal agent of leaf spot on African mahogany trees (Khaya senegalensis).

Three previously undescribed metabolites named argyrotoxins A-C, were isolated, together with the well known porritoxinol, its closely related phthalide, a phthalide derivative, zinniol, alternariol and its 4-methyl ether from Alternaria argyroxiphii E.G. Simmons & Aragaki, the causal agent of leaf spot on African mahogany trees, Khaya senegalensis A. Juss. (Meliaceae). The known compounds were identified comparing their physical and spectroscopic properties to those previously reported in literature. Argyrotoxins A-C were characterized essentially by NMR (1H, 13C, COSY, HSQC, HMBC and NOESY NMR spectra) and HRESIMS spectra as 4-(7-methoxy-6-methyl-3-oxo-1,3-dihydro-isobenzofuran-5-yloxy)-2-methyl-butyric acid, 5-but-2-enyloxy-N-(2-hydroxyethyl)-2-hydroxymethyl-3-methoxy-4-methyl-benzamide and 1-(5-(hydroxymethyl)-3-methoxy-4-(methoxymethyl)-2-methylphenoxy)-3-methylbutane-2,3-diol, respectively. The absolute configuration of argyrotoxin A was determined through electronic circular dichroism, by applying the biphenyl chiroptical probe approach. The phytoxicity of all metabolites isolated was evaluated by leaf puncture assay at concentration of 1 mg/mL. Zinniol proved to be the most active compound causing necrotic lesions on young leaves of Hedera elix L., Phaseolus vulgaris L. and Quercus ilex L. Argirotoxins A and B were found active, to a minor extent, on Phaseolus vulgaris L. leaves, while porritoxinol exhibited activity on holm oak leaves. The other secondary metabolites herein reported for A. argyroxiphii were inactive.

Absolute configuration of seco-eudesmanolide inuloxin D from experimental and predicted chiroptical studies of its 4-O-acetyl derivative.

Sesquitepenoids inuloxins A-D, belonging to different subgroups, were isolated from Dittrichia viscosa and showed potential biocontrol of some parasitic plants as Pelipanche, Orobanche, and Cuscuta species. The absolute configurations of the first three inuloxins A-C were previously determined by using experimental and computational chiroptical spectroscopic methods. The absolute configuration of inuloxin D remains to be established. The bioactive inuloxin E, closely related to inuloxin D, was recently isolated from the same plant organic extract. The same relative configuration of inuloxin D was assigned to inuloxin E by comparison of their NMR spectroscopic data. The absolute configurations of inuloxin D and inuloxin E are suggested in this work by analysis of the experimental and predicted chiroptical properties of the 4-O-acetyl derivative of inuloxin D.

α-Costic acid, a plant sesquiterpene with acaricidal activity against Varroa destructor parasitizing the honey bee.

The organic extract of the aerial parts of Dittrichia viscosa, a perennial native plant of the Mediterranean basin, showed a significant acaricidal activity against Varroa destructor, the parasite mite of Apis mellifera, commonly called honey bee. Among the metabolites isolated from the organic extract of this Asteraceae, α-costic acid showed to be one of the compounds responsible for the toxic activity exhibited by the crude plant extract on this parasite mite species. In addition to the toxic effect a clear acaricidal response has been recorded when the parasitic mite was exposed to 1 mg/mL concentration of α-costic acid while no effects have been showed on honey bees using the same compound at the same concentration. This finding suggests a potential use of α-costic acid to control Varroa mites. The possibility to reliably achieve absolute configuration of α-costic acid by DFT computational analysis of chiroptical spectra has been also demonstrated.†.

Porphyrins Through the Looking Glass: Spectroscopic and Mechanistic Insights in Supramolecular Chirogenesis of New Self-Assembled Porphyrin Derivatives.

The solvent driven aggregation of porphyrin derivatives, covalently linked to a L- or D-prolinate enantiomer, results in the stereospecific formation of species featuring remarkable supramolecular chirality, as a consequence of reading and amplification of the stereochemical information stored in the proline-appended group. Spectroscopic, kinetic, and topographic SEM studies gave important information on the aggregation processes, and on the structures of the final chiral architectures. The results obtained may be the seeds for the construction of stereoselective sensors aiming at the detection, for example, of novel emergent pollutants from agrochemical, food, and pharmaceutical industry.

Drophiobiolins A and B, Bioactive Ophiobolan Sestertepenoids Produced by Dreschslera gigantea.

Two new bioactive ophiobolan sestertepenoids, named drophiobiolins A and B (1 and 2) were isolated from Drechslera gigantea, a fungus proposed as a mycoherbicide for biocontrol of Digitaria sanguinalis. They were isolated together with ophiobolin A, the main metabolite, 6-epi-ophiobolin A, 3-anhydro-6-epi-ophiobolin A, and ophiobolin I. Drophiobolins A and B were characterized by NMR, HRESIMS, and chemical methods as 7-hydroxy-7-(6-hydroxy-6-methylheptan-2-yl)-1,9a-dimethyl-3-oxo-3,3a,6,6a,7,8,9,9a,10,10a-decahydrodicyclopenta [a,d][8]annulene-4-carbaldehyde and 6-(hydroxymethyl)-3',9,10a-trimethyl-5'-(2-methylprop-1-en-1-yl)-3a,4,4',5',10,10a-hexahydro-1H,3'H-spiro[dicyclopenta[a,d] [8]annulene-3,2'-furan]-5,7(2H,9aH)-dione. The relative configuration of drophiobolins A and B, which did not afford crystals suitable for X-ray analysis, was determined by NOESY experiments, while the absolute configuration was assigned by comparison of their experimental and TDDFT calculated electronic circular dichroism (ECD) spectra. The phytotoxic activity of drophiobolins A and B was tested by leaf-puncture assay on cultivated (Lycopersicon esculentum L.), as well as on host (Digitaria sanguinalis L.) and nonhost (Chenopodium album L.) weed plants, compared to that of ophiobolin A. Both of the newly identified ophiobolins showed significant phytotoxicity. Drophiobolins A and B exhibited cytotoxicity against Hela B cells with an IC50 value of 10 µM. However, they had a lesser or no effect against Hacat, H1299, and A431 cells when compared to that of ophiobolin A.

Further secondary metabolites produced by the fungus Pyricularia grisea isolated from buffelgrass (Cenchrus ciliaris).

The fungal pathogen Pyricularia grisea has been studied to evaluate its production of phytotoxins for the biocontrol of the buffelgrass (Cenchrus ciliaris L.) weed. A first investigation allowed to isolate several new and known phytotoxic metabolites. However, the further investigation on the organic extract obtained from the fungus liquid culture showed the presence of other metabolites possibly contributing to its phytotoxicity. Thus, four known metabolites were isolated and identified by spectroscopic (nuclear magnetic resonance [NMR] and high-resolution electrospray ionization mass spectrometry [HRESIMS]) methods as dihydropyriculol (1), epi-dihydropyriculol (2), 3-methoxy-6,8-dihydroxy-3-methyl-3,4-dihydroisocoumarin (3), and (R)-mevalonolactone (4). The absolute configuration of 1-3 was determined for the first time by a computational analysis of their electronic circular dichroism (ECD) spectra. When the isolated compounds were bioassayed at a concentration of 5 × 10-3 M in a buffelgrass coleoptile and radicle elongation test no toxicity was detected. On the contrary, compounds 1 and 3 showed a significant stimulating effect of radical elongation. Furthermore, the difference in growth stimulation between 1 and its epimer 2 highlights the tight relationship between absolute configuration and biological activity of these fungal metabolites.

Stoechanones A and B, Phytotoxic Copaane Sesquiterpenoids Isolated from Lavandula stoechas with Potential Herbicidal Activity against Amaranthus retroflexus.

From the organic extract of Lavandula stoechas, a Mediterranean native plant species, two new phytotoxic copaane sesquiterpenoids were isolated and named stoechanones A and B (1 and 2). They were obtained together with the methyl esters of caffeic and p-coumaric acids and the flavonoid apigenin (3-5, respectively). The structures of stoechanones A and B were determined by spectroscopic (essentially 1D and 2D 1H and 13C NMR and HRESIMS) and chemical methods, and they were characterized as 9,10-dihydroxy-8-isopropyl-1,5-dimethyltricyclo[4.4.0.02.7]dec-4-en-3-one and its 9-O-acetyl derivative. Their relative configurations were assigned by NOESY experiments, and the absolute configurations by comparison of the experimental and DFT-computed ECD spectra. When assayed through Petri dish bioassays, both stoechanones A and B showed phytotoxic effects against seed germination and seedling growth of Amaranthus retroflexus, strongly inhibiting seed germination percentage and radicle and hypocotyl lengths of seedlings. Owing to the herbicidal activity toward A. retroflexus, these two new tricyclic sesquiterpenoids could be proposed and developed as natural bioherbicides in order to increase the control of this problematic weed in the future.

Amplification of the chiroptical response of UV-transparent amines and alcohols by N-phthalimide derivatization enabling absolute configuration determination through ECD computational analysis.

The stereoselective transformation of chiral UV-transparent amines and alcohols to phthalimides has proved to be a simple and efficient method to enhance the chiroptical response of these substrates allowing their reliable absolute configuration determination by computational analysis of ECD spectra. Such a transformation also leads to a significant reduction in the molecular conformational flexibility thus simplifying the conformational analysis required by the computational treatment. The method described herein thus allows the absolute configuration assignment to these challenging substrates to be much easier and reliable.

Absolute Configuration Assignment to Chiral Natural Products by Biphenyl Chiroptical Probes: The Case of the Phytotoxins Colletochlorin A and Agropyrenol.

The application of flexible biphenyls as chiroptical probes for the absolute configuration assignment to chiral natural products is described. The method is straightforward and reliable and can be applied to conformationally mobile and ECD silent compounds, not treatable by computational analysis of chiroptical data. By this approach, the (6'R) absolute configuration of the phytotoxin colletochlorin A (1) was confirmed, while the absolute configuration of the phytotoxin agropyrenol (2), previously assigned by the NMR Mosher method, was revised and assigned as (3'S,4'S). Moreover, with the biphenyl method the configurational assignment can be obtained simply by the sign of a diagnostic Cotton effect at 250 nm in the ECD spectrum, thus allowing application without the need of advanced knowledge of chiroptical spectroscopy and computational protocols.

Synthesis and Herbicidal Activity Against Buffelgrass (Cenchrus ciliaris) of (±)-3-deoxyradicinin.

A novel synthetic strategy for obtainment of (±)-3-deoxyradicinin (2) is reported. This synthetic methodology is more efficient than those previously reported in the literature and also shows higher versatility towards the introduction of different side-chains at both C-7 and C-2. The obtained compound (±)-2 shows phytotoxicity against the grass-weed buffelgrass comparable to that of the natural phytotoxin radicinin (1). Therefore, (±)-2 can constitute a more practical synthetic alternative to 1 as bioherbicide for buffelgrass control.

Assignment Through Chiroptical Methods of The Absolute Configuration of Fungal Dihydropyranpyran-4-5-Diones Phytotoxins, Potential Herbicides for Buffelgrass (Cenchrus ciliaris) Biocontrol.

Radicinin and cochliotoxin (1 and 2) two phytotoxic pyranpyran-4,5-diones were isolated together with their close metabolites 3-epi-radicinin, radicinol, and its 3-epimer (3-5), from the culture filtrates of Cochliobolus australiensis, a fungus proposed as mycoherbcide for biocontrol of buffelgrass, a very noxious and dangerous weed. The absolute configuration of cochliotoxin was determined by chiroptical Optical Rotatory Dispersion (ORD), Electronic Circular Dichroism (ECD), and Vibrational Circular Dichroism (VCD)) and computational methods. The same methods were used to confirm that of radicinin, radicinol and their 3-epimers, previously determined with chemical, spectroscopic and ECD methods.