Properties of Configurationally Stable Atropoenantiomers in Macrocyclic Natural Products and the Chrysophaentin Family.

development of antibiotics, antineoplastics, and therapeutics for other diseases. Natural products are unique among all other small molecules in that they are produced by dedicated enzymatic assembly lines that are the protein products of biosynthetic gene clusters. As the products of chiral macromolecules, natural products have distinct three-dimensional shapes and stereochemistry is often encoded in their structures through the presence of stereocenters, or in the case of molecules that lack a stereocenter, the presence of an axis or plane of chirality. In the latter forms of chirality, if the barrier to rotation about the chiral axis or chiral plane is sufficiently high, stable conformers may exist allowing for isolation of discrete conformers, also known as atropisomers. Importantly, the diverse functions and biological activities of natural products are contingent upon their structures, stereochemistry and molecular shape. With continued innovation in methods for natural products discovery, synthetic chemistry, and analytical and computational tools, new insights into atropisomerism in natural products and related scaffolds are being made. As molecular complexity increases, more than one form of stereoisomerism may exist in a single compound (for example, point chirality, chiral axes, and chiral planes), sometimes creating atypical or noncanonical atropisomers, a term used to distinguish physically noninterconvertable atropisomers from typical atropisomers.Here we provide an account of the discovery and unusual structural and stereochemical features of the chrysophaentins, algal derived inhibitors of the bacterial cytoskeletal protein FtsZ and its associated protein partners. Eleven members of the chrysophaentin family have been discovered to date; seven of these are macrocyclic bis-bibenzyl ethers wherein the site of the ether linkage yields either a symmetrical or asymmetrical macrocyclic ring system. The asymmetrical ring system is highly strained and corresponds to the compounds having the most potent antimicrobial activity among the family. We review the structure elucidation and NMR properties that indicate restricted rotation between axes of two biaryl ethers, and the plane represented by the substituted 2-Z-butene bridge common to all of the macrocycles. Computational studies that corroborate high barriers to rotation about one representative plane, on the order of 20+ kcal/mol are presented. These barriers to rotation fix the conformation of the macrocycle into a bowl-like structure and suggest that an atropisomer should exist. Experimental evidence for atropisomerism is presented, consistent with computational predictions. These properties are discussed in the context of the total synthesis of 9-dechlorochrysophaenin A and its ring C isomers. Last, we discuss the implications for the presence of enantiomers in the biological activity and macrocyclization of the natural product.

Oxindole and Benzoxazinone Alkaloids from the Seeds of Persea americana (Avocado) and Their SIRT1 Stimulatory Activity.

Persea americana Mill. (Lauraceae), commonly known as avocado, is a well-known food because of its nutrition and health benefits. The seeds of avocado are major byproducts, and thus their phytochemicals and bioactivities have been of interest for study. The chemical components of avocado seeds were investigated by using UPLC-qTOF-MS/MS-based molecular networking, resulting in the isolation of seven new oxindole alkaloids (1-7) and two new benzoxazinone alkaloids (8 and 9). The chemical structures of the isolated compounds were identified by the analysis of NMR data in combination with computational approaches, including NMR and ECD calculations. Bioactivities of the isolated compounds toward silent information regulation 2 homologue-1 (SIRT1) in HEK293 cells were assessed. The results showed that compound 1 had the most potent effect on SIRT1 activation with an elevated NAD+/NADH ratio with potential for further investigation as an anti-aging agent.

Flavonostilbenes Isolated from the Stems of Rhamnoneuron balansae as Potential SIRT1 Activators.

The cumulative effects of cell damage result in aging, which gradually decreases human function in various aspects and leads to multiple age-related chronic diseases. To overcome the adverse effects of aging, silent mating type information regulation 2 homologue (SIRT1) activators are promising bioactive compounds that mimic calorie restriction to improve quality of life and prevent aging. In this study, 11 new flavonostilbenes (1-11) and three known compounds (12-14) were purified from stems of Rhamnoneuron balansae. The structures of the new compounds were determined using extensive data from spectroscopic methods, including NMR and HRESIMS. Their absolute configurations were deduced by ECD calculations with coupling constant analysis. All of the isolated new compounds (1-11) were evaluated for their effects on SIRT1 deacetylase activity, the NAD+/NADH ratio, and the AMP-activated protein kinase activation level in cell-based assays. The results showed that rhamnoneuronal D (1) exhibits promising biological activity in several in vitro models related to SIRT1 and suggest it is a potential natural-product-based antiaging agent.

Melicopteline A-E, Unusual Cyclopeptide Alkaloids with Antiviral Activity against Influenza A Virus from Melicope pteleifolia.

In the search for antiviral cyclopeptides against influenza A virus, five unprecedented Caryophyllaceae-type cyclopeptides (1-5) were isolated from the leaves of Melicope pteleifolia. Their chemical structures and absolute configurations were unambiguously determined by means of advanced Marfey's analysis and comprehensive spectroscopic analyses including two-dimensional nuclear magnetic resonance and MS/MS fragmentation. Interestingly, compounds 3-5 contain an unusual heterocycle, a 3a-hydroxypyrroloindole moiety, which was biosynthetically formed by a nucleophilic cyclization from the least abundant amino acid, tryptophan, precursor and has aroused a great interest in the aspect of chemical diversity and biological activity. All isolates (1-5) were evaluated for their protective effects against influenza A viruses H1N1 and H9N2 in MDCK cells. All isolated cyclopeptides exhibited strong anti-influenza activity, especially against H1N1. Compound 3 showed the most potent CPE inhibition effect, which was stronger than that of the positive control ribavirin against H1N1, with an EC50 (µM) of 2.57 ± 0.45 along with higher selectivity.

Rugonidines A-F, Diastereomeric 1,6-Dioxa-7,9-diazaspiro[4.5]dec-7-en-8-amines from the Leaves of Alchornea rugosa.

Rugonidines A-F (1-6), three pairs of novel configurationally semistable diastereomers featuring an unprecedented 1,6-dioxa-7,9-diazaspiro[4.5]dec-7-en-8-amine scaffold, were isolated from Alchornea rugosa based on MS/MS-based molecular networking analysis. Their structures were elucidated by NMR spectroscopy in combination with quantum-chemical calculations. Compounds 1-3 showed a significant increase in glucose uptake level in differentiated 3T3-L1 adipocytes using 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-d-glucose (2-NBDG) as a fluorescent-tagged glucose probe.

Triterpenoid saponins from the leaves and stems of Pericampylus glaucus and their insulin mimetic activities.

During an attempt to discover insulin mimetics, thirteen new triterpenoid saponins (1-13), including three phytolaccagenic acids (1, 2, and 12) and ten serjanic acids (3-11 and 13), as aglycones were isolated from a 70% ethanol extract of leaves and stems from Pericampylus glaucus. The chemical structures of compounds 1-13 were determined through spectroscopic data analysis, including NMR, IR, and HRESIMS. All isolated compounds (1-13) were evaluated using 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-d-glucose (2-NBDG) as a fluorescent-tagged glucose probe to determine their stimulatory effects on glucose uptake in differentiated 3 T3-L1 adipocyte cells. Consequently, four compounds (4, 7, 11, and 12) exhibited stimulatory effects on glucose uptake.

Glucose Uptake-Stimulating Galloyl Ester Triterpenoids from Castanopsis sieboldii.

Using molecular networking-guided isolation, three new galloyl ester triterpenoids (1-3), two new hexahydroxydiphenic acid-conjugated triterpenoids (6 and 7), and four known compounds (4, 5, 8, and 9) were isolated from the fruits and leaves of Castanopsis sieboldii. The chemical structures of 1-3, 6, and 7 were elucidated on the basis of interpreting their NMR, HRESIMS, and ECD spectra. All compounds (1-9) were evaluated for their glucose uptake-stimulating activities in differentiated adipocytes using 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-d-glucose as a fluorescent-tagged glucose probe. Compounds 2 and 9 resulted in a 1.5-fold increase in glucose uptake. Among them, compound 2 from the fruits showed an upregulation of p-AMPK/AMPK ratio in differentiated C2C12 myoblasts to support the mechanism proposed of glucose uptake stimulation.

Phenolic Constituents of the Roots of Rhamnoneuron balansae with Senolytic Activity.

With the advent of senolytic agents capable of selectively removing senescent cells in old tissues, the perception of age-associated diseases has been changing from being an inevitable to a preventable phenomenon of human life. In the search for materials with senolytic activity from natural products, six new flavonostilbenes (1-6), three new phenylethylchromanones (7-9), three new phenylethylchromones (10-12), and four known compounds (13-16) were isolated from the roots of Rhamnoneuron balansae. The chemical structures of these isolated compounds were determined based on the interpretation of spectroscopic data, including 1D and 2D NMR, ECD, and HRMS. The absolute configuration of compound 1 was also determined by a Mosher ester analysis and ECD calculations. Compounds 6-8 were shown to selectively destroy senescent cells, and the promoter activity of p16INK4A, a representative senescence marker, was reduced significantly by compound 6. The present results suggest the potential activity of flavonostilbene and phenylethylchromanone skeletons from R. balansae as new senolytics.

Oleanane Triterpenoids from the Leaves of Gymnema inodorum and Their Insulin Mimetic Activities.

During an effort to find insulin mimetic compounds, the leaves of Gymnema inodorum were shown to have a stimulatory effect on glucose uptake in 3T3-L1 adipocyte cells. Bioassay-guided fractionation on a 70% ethanol extract of G. inodorum was applied to yield two new (1 and 2) and two known (8 and 9) oleanane triterpenoids with a methyl anthranilate moiety together with five further new oleanane triterpenoids (3-7). The chemical structures of all isolates were determined based on their spectroscopic data, including IR, UV, NMR, and mass spectrometric analysis. The isolated compounds (1-9) were determined for their stimulatory activities on glucose uptake in differentiated 3T3-L1 adipocyte cells using 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-d-glucose (2-NBDG) as a fluorescent-tagged glucose probe. Three compounds (3, 5, and 9) showed stimulatory effects on the uptake of 2-NBDG in 3T3-L1 adipocyte cells. Chemicals with a methyl anthranilate moiety have been considered as crucial contributors of flavor odor in foods, and quantitative analysis showed the content of compound 8 to be 0.90 ± 0.01 mg/g of the total extract. These results suggest that the leaves of G. inodorum have the potential to be used as an antidiabetic functional food or tea.

Neuroprotective Effects of Triterpenoids from Camellia japonica against Amyloid ß-Induced Neuronal Damage.

Alzheimer's disease (AD), a neurocognitive impairment affecting human mental capacity, is related to the accumulation of amyloid-ß peptide (Aß) and the hyperphosphorylation of tau protein. In addition to modern therapies approved for AD treatment, natural products with antioxidant and anti-inflammatory properties have been studied for their potential to prevent AD pathogenesis. Six new noroleanane triterpenoids from the fruit peels of Camellia japonica were isolated, and their structures were determined by diverse spectroscopic methods. The neuroprotective effects of the six new compounds were tested against Aß-induced neurotoxicity and neuroinflammation in mouse hippocampal and microglial cells. In the model of HT22-transfected cells, compounds 1-4 showed strongly neuroprotective effects via antioxidant response element gene activation and decreased the level of glutamate uptake. Compounds 1-4 also appeared to have strong inhibitory effects on NO production in Aß1-42-transfected BV2 microglial cells. A docking simulation study was used to explain the inhibitory effects of compounds 1-4 on ß-secretase 1 (BACE1). Noroleanane triterpenoids 1-4 had potential neuroprotective and anti-inflammatory effects against Aß-induced neuronal damage. The structure-activity relationships of the 30 oleanane triterpenoids from C. japonica were assessed in a model of Aß1-42-transfected HT22 cells.

Lignan Dimers from Forsythia viridissima Roots and Their Antiviral Effects.

Six new dimeric lignans (1-6) and one new lignan glycoside (16) were isolated from Forsythia viridissima roots along with nine known lignans (7-15). Spectroscopic analyses and chemical methods were used to determine these new structures and their absolute configurations. Among these compounds, dimatairesinol (1) and viridissimaols A-E (2-6) were assigned as dimers of dibenzylbutyrolactone analogues. Furthermore, the isolated compounds were evaluated for their antiviral activities against coxsackievirus B3 (CVB3) and human rhinovirus 1B (HRV1B). In these tests, compounds 12 and 15 showed antiviral effects against CVB3 infection with IC50 values of 15.4 and 36.4 µM, respectively, while 2, 3, 8, and 9 showed activities against HRV1B with IC50 values of 45.7, 47.5, 13.0, and 43.2 µM, respectively.

Prenylated Phenolic Compounds from the Leaves of Sabia limoniacea and Their Antiviral Activities against Porcine Epidemic Diarrhea Virus.

Porcine epidemic diarrhea virus (PEDV), a serious swine epidemic, has been rampant in Asia since the 1990s. Despite the widespread use of PEDV vaccines, the occurrence of PEDV variants requires the discovery of new substances that inhibit these viruses. During a search for PEDV inhibitory materials from natural sources, seven new sabphenosides (1-7) and a new flavonoid (8), as well as eight known phenolic compounds (9-16), were obtained from the leaves of Sabia limoniacea. The structural determination of the new phenolic derivatives (1-8) was accomplished using comprehensive spectroscopic methods. Their absolute configurations were assigned by a combination of the ECD exciton chirality method following selective reduction and calculation of their ECD spectra. The bioactivities of the isolated compounds were measured based on their abilities to inhibit viral replication of PEDV. Among the test compounds, 15 and 16 exhibited the most promising antiviral activities, with IC50 values of 7.5 ± 0.7 µM and 8.0 ± 2.5 µM against PEDV replication. This study suggests that compounds 15 and 16 could serve as new scaffolds for the treatment of PEDV infection through the inhibition of PEDV replication.

Dereplication by High-Performance Liquid Chromatography (HPLC) with Quadrupole-Time-of-Flight Mass Spectroscopy (qTOF-MS) and Antiviral Activities of Phlorotannins from Ecklonia cava.

Ecklonia cava is edible seaweed that is found in Asian countries, such as Japan and Korea; and, its major components include fucoidan and phlorotannins. Phlorotannins that are isolated from E. cava are well-known to have an antioxidant effect and strong antiviral activity against porcine epidemic diarrhea virus (PEDV), which has a high mortality rate in piglets. In this study, the bioactive components were determined based on two different approaches: (i) bio-guided isolation using the antiviral activity against the H1N1 viral strain, which is a representative influenza virus that originates from swine and (ii) high-resolution mass spectrometry-based dereplication, including relative mass defects (RMDs) and HPLC-qTOFMS fragmentation analysis. The EC70 fraction showed the strongest antiviral activity and contained thirteen phlorotannins, which were predicted by dereplication. Ten compounds were directly isolated from E. cava extract and then identified. Moreover, the dereplication method allowed for the discovery of two new phlorotannins. The structures of these two isolated compounds were elucidated using NMR techniques and HPLC-qTOFMS fragmentation analysis. In addition, molecular modelling was applied to determine the absolute configurations of the two new compounds. The antiviral activities of seven major phlorotannins in active fraction were evaluated against two influenza A viral strains (H1N1 and H9N2). Six of the compounds showed moderate to strong effects on both of the viruses and phlorofucofuroeckol A (12), which showed an EC50 value of 13.48 ± 1.93 µM, is a potential active antiviral component of E. cava.

Insulin Mimetic Activity of 3,4- Seco and Hexanordammarane Triterpenoids Isolated from Gynostemma longipes.

As part of ongoing research to find new antidiabetic agents from medicinal plants, the chemical composition of Gynostemma longipes, an ethnomedicinal plant used to treat type 2 diabetes mellitus by local communities in Vietnam, was investigated. Ten new dammarane triterpenes, including two 3,4- seco-dammarane analogues, secolongipegenins S1 and S2 (1 and 2), a 3,4- seco-hexanordammarane, secolongipegenin S3 (3), two hexanordammarane glycosides, longipenosides ND1 and ND2 (4 and 5), and five other dammarane glycosides, longipenosides GL1-GL5 (6-10), were isolated from a 70% EtOH extract of the whole G. longipes plant. The structures of the new compounds were elucidated using diverse spectroscopic methods. All of the isolates were evaluated for their stimulatory activities on glucose uptake in differentiated 3T3-L1 adipocyte cells using 2-[ N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-d-glucose as a fluorescent-tagged glucose probe. The stimulant activities on glucose uptake by the test compounds were mediated via the activation of the AMPK pathway using differentiated mouse C2C12 skeletal myoblasts. Consequently, compounds 1, 2, and 4 enhanced glucose uptake and GLUT4 translocation significantly by regulating the AMPK signaling pathway.

Antiviral activity of CAVAC-1901, a combination of 3 standardized medicinal plants, against highly pathogenic influenza A virus in chickens.

Three different medicinal plants that consisted of the formulated mixture (CAVAC-1901) have been traditionally used for distinct medicinal purposes in different areas. Angelica dahurica has been used as an important ingredient of a prescription, Gumiganghwal-tang, for the common cold and influenza. Curcuma longa has been utilized for the treatment of asthma, and jaundice. Pinus densiflora (Korean red pine) has been used to improve memory and brain function for the treatment of vascular. Industrial livestock, which are characterized by dense breeding, are vulnerable to influenza infection, causing severe economic loss and social problems. However, there are no viable alternatives due to the risk of the occurrence of variants. Therefore, the aim of this study was to discover anti-influenza combinations of different medicinal plants with the concept of a multicomponent and multitarget (MCMT) strategy in traditional Chinese medicine (TCM). As part of a continuous project, 3 medicinal plants whose inhibitory activity against influenza A was previously reported at the compound level, and the inhibition of cytopathic effects (CPEs) by these formulated mixtures was evaluated against influenza A virus H1N1. A selected combination with an optimal ratio exhibiting synergistic activity was assessed for its antiviral activity in chickens against the highly pathogenic avian influenza (HPAI) H5N6. The selected combination (CAVAC-1901) showed potent inhibitory effects on the expression of neuraminidase and nucleoprotein, by RT-qPCR, Western blot, and immunofluorescence assays. The antiviral activity was more evident in chickens infected with H5N6. The sample-treated group (50 mg/kg/d) decreased mortality and virus titers in various organs. Our results indirectly suggest synergistic inhibitory activity of the combination of 3 different medicinal plants with different modes of action. Taken together, an optimally formulated mixture (CAVAC-1901) could serve as an effective alternative to current measures to minimize damage caused by HPAIs.

Unique guanidine-conjugated catechins from the leaves of Alchornea rugosa and their autophagy modulating activity.

Natural guanidines, molecules that contain the guanidine moiety, are structurally unique and often exhibit potent biological activities. A phytochemical investigation of the leaves of Alchornea rugosa (Lour.) Müll.Arg. by MS/MS-based molecular networking revealed eight undescribed guanidine-flavanol conjugates named rugonines A-H. The chemical structures of the isolated compounds were comprehensively elucidated by NMR spectroscopy, HRESIMS, and circular dichroism (CD) analysis. All isolated compounds were tested for autophagosome formation in HEK293 cells stably expressing GFP-LC3. The results revealed that compounds rugonines D-G showed potential autophagy inhibitory activity.

Linderanidins A-F: Rare oligomeric flavonoids with an unusual C-3-C-4 linkage from the roots of Lindera erythrocarpa and their inhibitory activities on autophagy.

Autophagy is a crucial process for maintaining cellular homeostasis by degrading and recycling unnecessary or damaged cellular components. In the process of exploring autophagy regulators in plants, unique nine oligomeric flavonoids linked by the bonding of C-3 and C-4, consisting of three pairs of biflavonoids, linderanidins A-C [(+)-1/(-)-1, (+)-2/(-)-2, and (+)-3/(-)-3], and three trimeric A-type proanthocyanidins, linderanidins D-F (4-6), were isolated from the roots of Lindera erythrocarpa. The structures and absolute configurations of these compounds were determined using various techniques, such as 1D and 2D NMR, mass spectrometry, X-ray crystallography, and electronic circular dichroism. All isolates were evaluated for their ability to regulate autophagy, and compounds (±)-1-(±)-3, (-)-1-(-)-3, (+)-1-(+)-3 and 4 were found to inhibit autophagy by blocking the fusion process between autophagosome and lysosome in HEK293 cells. This study suggests that unique oligomeric flavonoids possessing a C-3-C-4 linkage derived from the roots of L. erythrocarpa are potent autophagy inhibitors.

Insulin-mimetic activity of 23-glycosyl oleanane triterpenoids isolated from Gymnema latifolium.

Chemical investigation of the plant Gymnema latifolium led to the isolation of seven undescribed 23-glycosyl oleanane triterpenoids, gymlatinosides GLF1-GLF7, and two known compounds, gymnemosides D and E. The structures of the isolated compounds were elucidated using diverse spectroscopic methods. The extract of G. latifolium and all isolated compounds significantly enhanced 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose (2-NBDG) uptake into 3T3-L1 adipocytes at 20 µM. Among them, gymlatinosides GLF2 and gymlatinosides GLF4 showed particularly potent stimulatory effects on glucose uptake in a dose-dependent manner. Further investigation revealed that gymlatinosides GLF2 at 20 µM upregulated the expression of phosphorylated AMPK (p-AMPK). The results suggested that gymlatinosides GLF2 may enhance glucose uptake via regulating the AMPK signaling pathway.

Sesquiterpene lactone and its unique proaporphine hybrids from Magnolia grandiflora L. and their anti-inflammatory activity.

Two undescribed sesquiterpene lactone-proaporphine hybrid skeletons, two undescribed sesquiterpenes, and four known compounds were isolated from the aerial part of Magnolia grandiflora L. The structures of isolated compounds were unambiguously determined based on the interpretation of a combination of NMR spectroscopy, HRESIMS, DP4+ probability calculation of carbon data, X-ray crystallographic analyses, and ECD calculation. The isolated compounds were investigated for their anti-inflammatory activity against nitric oxide production and the protein expression of COX-2 in LPS-stimulated RAW 264.7 cells.

Inhibition of a broad range of SARS-CoV-2 variants by antiviral phytochemicals in hACE2 mice.

Although several vaccines and antiviral drugs against SARS-CoV-2 are currently available, control and prevention of COVID-19 through these interventions is limited due to inaccessibility and economic issues in some regions and countries. Moreover, incomplete viral clearance by ineffective therapeutics may lead to rapid genetic evolution, resulting in the emergence of new SARS-CoV-2 variants that may escape the host immune system as well as currently available COVID-19 vaccines. Here, we report that phytochemicals extracted from Chlorella spp. and Psidium guajava possess broad-spectrum antiviral activity against a range of SARS-CoV-2 variants. Through chromatography-based screening, we identified four bioactive compounds and subsequently demonstrated their potential antiviral activities in vivo. Interestingly, in hACE2 mice, treatment with these compounds significantly attenuates SARS-CoV-2-induced proinflammatory responses, demonstrating their potential anti-inflammatory activity. Collectively, our study suggests that phytochemicals from edible plants may be readily available therapeutics and prophylactics against multiple SARS-CoV-2 strains and variants.