Guajavadimer A, a Dimeric Caryophyllene-Derived Meroterpenoid with a New Carbon Skeleton from the Leaves of Psidium guajava

Psidium guajava: An Insight into Ethnomedicinal Uses, Phytochemistry, and Pharmacology

BACKGROUND

Psidium guajava (guava) is widely distributed in tropical and subtropical regions and adapted to various environmental conditions. Guava is an important economic fruit widely used as food and folk medicine. It contains flavonoids, alkaloids, tannins, triterpenoids, reducing sugars, essential oils, carotenoids, polyphenols, etc. The presence of triterpenoid acids such as guavacoumaric, ursolic, jacoumaric, guajavanoic, guavenoic, and Asiatic acids helps to develop novel drugs against various diseases. It is used traditionally for medicinal purposes, mainly for antioxidant, antimicrobial, antispasmodic, antidiabetic, anticancer, antiallergy, anti-inflammatory, and hepato-protective properties.

OBJECTIVE

The systematic literature study aims to summarize its botanical description, phytochemicals, pharmacological activities, and clinical trials. This review focuses on the plant's chemical composition and scientific approaches to human welfare.

METHODS

A systematic literature search was done on Psidium guajava through previous literature and online databases such as Google Scholar, Pubmed, Science Direct, etc., to explain its ethnomedicinal uses, phytochemistry, and pharmacological applications.

RESULTS

Previous literature studies of Psidium guajava suggest it can serve as antioxidant, antimicrobial, antispasmodic, antidiabetic, anticancer, anti-allergy, anti-inflammatory, and hepatoprotective effects. Successful clinical trials performed on the plant extracts against infantile rotaviral enteritis and infectious gastroenteritis showed future directions to work with the plant for clinical applications.

CONCLUSION

In this review, an attempt is made to show all literature studied, especially in phytochemistry, pharmacology, clinical trials and uses as traditional folk medicine around the world. The leaves have been used by folklore over the years to treat various ailments such as skin ulcers, diarrhoea, vaginal irritation, cough, conjunctivitis, etc. Further studies are required to explore more therapeutic remedies and to develop new medicines for future perspectives.

Total Synthesis of Guajavadimer A via Lewis Acid-Catalyzed Cascade Double Hetero-Diels-Alder Reactions

The first total synthesis of guajavadimer A, a dimeric caryophyllene-derived meroterpenoid featuring an unprecedented 4-9-6-6-6-9-4-fused ring system, is reported. Key to the approach is the construction of the pyrano[4,3,2-de]chromene core via a cascade of double hetero-Diels-Alder reactions. Practically, a 4-substituted-2,6-dihydroxybenzaldehyde dimethyl acetal serves as an effective surrogate for ortho-quinone methide, which is generated from the corresponding aldehyde and trimethyl orthoformate, with β-caryophyllene undergoing cycloaddition to generate pyrano[4,3,2-de]chromene derivatives with excellent regioselectivity and stereoselectivity in one pot under mild conditions.

Psidium guajava L. An Incalculable but Underexplored Food Crop: Its Phytochemistry, Ethnopharmacology, and Industrial Applications

Psidium guajava L. (guava) is a small tree known for its fruit flavor that is cultivated almost around the globe in tropical areas. Its fruit is amazingly rich in antioxidants, vitamin C, potassium, and dietary fiber. In different parts of the world, this plant holds a special place with respect to fruit and nutritional items. Pharmacological research has shown that this plant has more potential than just a fruit source; it also has beneficial effects against a variety of chronic diseases due to its rich nutritional and phytochemical profile. The primary goal of this document is to provide an updated overview of Psidium guajava L. and its bioactive secondary metabolites, as well as their availability for further study, with a focus on the health benefits and potential industrial applications. There have been several studies conducted on Psidium guajava L. in relation to its use in the pharmaceutical industry. However, its clinical efficacy and applications are still debatable. Therefore, in this review a detailed study with respect to phytochemistry of the plant through modern instruments such as GC and LC-MS has been discussed. The biological activities of secondary metabolites isolated from this plant have been extensively discussed. In order to perform long-term clinical trials to learn more about their effectiveness as drugs and applications for various health benefits, a structure activity relationship has been established. Based on the literature, it is concluded that this plant has a wide variety of biopharmaceutical applications. As a whole, this article calls for long-term clinical trials to obtain a greater understanding of how it can be used to treat different diseases.

Bioinspired enantioselective total syntheses of antibacterial callistrilones enabled by double SN2' cascade

A bioinspired, catalytic approach to the enantioselective total syntheses of antibacterial callistrilones A, C-E and 13-epi-callistrilone E natural products containing an unprecedented, sterically compact [1]benzofuro-[2,3-a]xanthene 6/6/6/5/6/3-fused hexacyclic skeleton is described. The key features of the synthesis include a highly regio- and diastereoselective double S_N2' cascade reaction, Lewis acid catalysed Michael addition and late stage diastereoselective epoxide formation from the sterically hindered β-face of the alkene as the key steps.

Meroterpenoids with unknown skeletons from the leaves of Psidium guajava including one anti-inflammatory and anticoagulant compound: psidial F

Four unknown meroterpenoids named as psidials D-G (1-4) together with 5 known compounds (5-9) had been obtained from the leaves of Psidium guajava. Their absolute structures were elucidated by spectral and calculated methods. Psidials DF (1-3) represented unknown carbon skeleton of the 3,5-diformylbenzyl phloroglucinol-coupled sesquiterpenoid. The possible biosynthetic pathway for 1-3 was postulated. In the bioactivity assay, psidial F (3) was found to possess anti-inflammatory and anticoagulant activities.

Dibenzocyclooctadiene lignans from Kadsura coccinea alleviate APAP-induced hepatotoxicity via oxidative stress inhibition and activating the Nrf2 pathway in vitro

Phytochemical investigation on the roots of Kadsura coccinea led to the isolation five previously unknown dibenzocyclooctadiene lignans, named heilaohusuins A-E (1-5). Their structures determined by NMR spectroscopy, HR-ESI-MS, and ECD spectra. Hepatoprotection effects of a series of dibenzocyclooctadiene derivatives (1-68) were investigated against acetaminophen (APAP) induced HepG2 cells. Compounds 2, 10, 13, 21, 32, 41, 46, and 49 showed remarkable protective effects, increasing the viabilities to > 52.2% (bicyclol, 52.1 ± 1.3%) at 10 μM. The structure-activity relationships (SAR) for hepatoprotective activity were summarized, according to the activity results of dibenzocyclooctadiene derivatives. Furthermore, we found that one new dibenzocyclooctadiene lignan heilaohusuin B attenuates hepatotoxicity, the mechanism might be closely correlated with oxidative stress inhibition via activating the Nrf2 pathway.

Meroterpenoids: A Comprehensive Update Insight on Structural Diversity and Biology

Meroterpenoids are secondary metabolites formed due to mixed biosynthetic pathways which are produced in part from a terpenoid co-substrate. These mixed biosynthetically hybrid compounds are widely produced by bacteria, algae, plants, and animals. Notably amazing chemical diversity is generated among meroterpenoids via a combination of terpenoid scaffolds with polyketides, alkaloids, phenols, and amino acids. This review deals with the isolation, chemical diversity, and biological effects of 452 new meroterpenoids reported from natural sources from January 2016 to December 2020. Most of the meroterpenoids possess antimicrobial, cytotoxic, antioxidant, anti-inflammatory, antiviral, enzyme inhibitory, and immunosupressive effects.

Chemical constituents of Psidium guajava leaves and their antibacterial activity

Seven undescribed compounds, namely (+)-psiflavanone A, (-)-psiflavanone A, (+)-psiflavanone B, (-)-psiflavanone B, (+)-psiketide, (-)-psiketide, and psidinone, were isolated and chirally separated from the 95%-ethanol extract of Psidium guajava leaves. (+)/(-)-Psiflavanone A and (+)/(-)-psiflavanone B are two pairs of rare enantiomeric C-benzoylated flavanones isolated from the title plant for the first time. (+)/(-)-Psiketide are a pair of enantiomeric aromatic polyketides, and psidinone is the first example of a C₁₈ phenanthrenediquinone with an angle-type 6/6/6 tricyclic skeleton. Their structures were identified by extensive analysis of HRESIMS, UV, IR, NMR, and calculated electronic circular dichroism (ECD) data. Particularly, the structures of (+)-psiflavanone A, (-)-psiflavanone A, (+)-psiflavanone B, and psidinone were further confirmed by X-ray diffraction. Psidinone exhibited antibacterial activity against Staphylococcus aureus, Staphylococcus epidermidis, and Mycobacterium Smegmatis mc² 155, with minimum inhibitory concentrations (MICs) of 16, 8, and 0.5 μM, respectively. These results not only highlight the chemical diversity and biological activity of P. guajava but are also expected to pave way for its further expansion to other applications in future.

Heilaohuguosus A-S from the fruits of Kadsura coccinea and their hepatoprotective activity

Phytochemical investigations on the fresh fruits of Kadsura coccinea (Lem.) A. C. Sm. have led to the isolation of fourteen undescribed 2,2'-cyclolignans named heilaohuguosus A-N, four undescribed aryltetrahydronaphthalene lignans, heilaohuguosus O-R and one tetrahydrofuran lignan, heilaohuguosu S, with twenty-seven previously described lignan analogues. Their structures and absolute configurations of heilaohuguosus A-S were established by spectroscopic methods including 1D and 2D-NMR techniques and CD experiments. All isolated compounds were evaluated for their hepatoprotective activity against APAP-induced toxicity in HepG-2 cells, four 2,2'-cyclolignans, heilaohuguosus A and L, tiegusanin I and kadsuphilol I showed good hepatoprotective activities against APAP toxicity in HepG-2 cells with cell survival rates of 53.5 ± 1.7%, 55.2 ± 1.2%, 52.5 ± 2.4%, and 54.0 ± 2.2% (positive control bicyclol, 52.1 ± 1.3%) at 10 μM, respectively.

Chemical Characterization and Hepatoprotective Effects of a Standardized Triterpenoid-Enriched Guava Leaf Extract

Nutraceutical/pharmaceutical agents capable of maintaining redox and inflammation homeostasis are considered as candidates for the prevention and/or treatment of liver diseases. Psidium guajava (commonly known as guava) leaf is a commercially available functional food that has been reported to possess hepatoprotective property. However, the hepatoprotective constituents in guava leaf are not known. In the current study, a standardized triterpenoid-enriched extract of guava leaves (TGL) was developed. A new ursolic acid derivative, namely 2α,3β,6β,23,30-pentahydroxyurs-11,13(18)-dien-28,20β-olide (1), and 23 known triterpenoids were isolated and identified from TGL. The hepatoprotective effects of TGL were evaluated through a model using acetaminophen (APAP)-exposed C57BL/6 male mice. Pretreatment of TGL (75 and 150 mg/kg) restored the mice hepatic architecture, improved the serum ALT and AST levels, and reduced the hepatic ROS and MDA contents. Further molecular mechanistic study revealed that TGL modulated Nrf2 and MAPK signaling pathways to alleviate APAP-induced oxidative and inflammatory stress in liver. In addition, the new compound 1 from TGL showed protective effects against APAP-induced cytotoxicity via activation of the Nrf2 pathway in HepG2 cells. Overall, this is the first report on the hepatoprotective effects of a standardized triterpenoid-enriched extract of guava leaves, which supports its potential nutraceutical application in liver disease management.

Psidguajones A and B, a pair of complex meroterpenoid epimers from the leaves of Psidium guajava

Psidguajones A and B, a pair of dimeric sesquiterpene-based meroterpenoid epimers were isolated from the leaves of Psidium guajava for the first time. Their structures were confirmed by comprehensive spectroscopic...

Silver(i)-catalyzed novel ipso-cycloaddition and retro-Friedel–Crafts reaction of ortho-hydroxyphenyl-substituted para-quinone methides

The Lewis acid AgPF6 was found to catalyze novel [2 + 2] ipso-cycloaddition and retro-Friedel–Crafts reaction of o-hydroxyphenyl-substituted p-quinone methides in the presence of H2O, affording the polysubstituted triarylmethanes in up to 97% yields.

A biomimetic synthesis-enabled stereochemical assignment of rhodotomentones A and B, two unusual caryophyllene-derived meroterpenoids from Rhodomyrtus tomentosa

Rhodotomentones A and B (1 and 2), two unusual caryophyllene-derived meroterpenoids (CDMTs) featuring a rare 6/6/9/4/6/6 hexacyclic ring system, along with their biogenetically-related CDMTs 7 and 12–15, were isolated from Rhodomyrtus tomentosa.

Progress in the Chemistry of Macrocyclic Meroterpenoids

In the last decade, the chemistry of meroterpenoids—conjugated molecules formed from isoprenyl fragments through biosynthetic pathways—has developed rapidly. The class includes some natural metabolites and fully synthetic fragments formed through nonbiological synthesis. In the field of synthetic receptors, a range of structures can be achieved by combining fragments of different classes of organic compounds into one hybrid macrocyclic platform which retains the properties of these fragments. This review discusses the successes in the synthesis and practical application of both natural and synthetic macrocycles. Among the natural macrocyclic meroterpenoids, special attention is paid to isoprenylated flavonoids and phenols, isoprenoid lipids, prenylated amino acids and alkaloids, and isoprenylpolyketides. Among the synthetic macrocyclic meroterpenoids obtained by combining the “classical” macrocyclic platforms, those based on cyclodextrins, together with meta- and paracyclophanes incorporating terpenoid fragments, and meroterpenoids obtained by macrocyclization of several terpene derivatives are considered. In addition, issues related to biomedical activity, processes of self-association and aggregation, and the formation of host–guest complexes of various classes of macrocyclic merotenoids are discussed in detail.

The First Racemic Total Syntheses of the Antiplasmodials Watsonianones A and B and Corymbone B

The first biomimetic total syntheses of three biologically meaningful acylphloroglucinols, watsonianones A and B and corymbone B, with potent antiplasmodial activity, were performed. Their total syntheses were carried out through a diversity-oriented synthetic strategy from congener 2,2,4,4-tetramethyl-6-(3-methylbutylidene)cyclohexane-1,3,5-trione with high step efficiency. The spontaneous enolization/air oxidation of the precursor 2,2,4,4-tetramethyl-6-(3-methylbutylidene)cyclohexane-1,3,5-trione through a singlet O₂-induced Diels-Alder reaction pathway to assemble the key biosynthetic peroxide intermediate is also discussed.

Meroterpene-Like α-Glucosidase Inhibitors Based on Biomimetic Reactions Starting from β-Caryophyllene

BACKGROUND

Natural meroterpenes derived from phloroglucinols and β-caryophyllene have shown high inhibitory activity against α-glucosidase or cancer cells, however, the chemical diversity of this type of skeletons in Nature is limited.

METHODS

To expand the chemical space and explore their inhibitory activities against α-glucosidase (EC 3.2.1.20), we employed β-caryophyllene and some natural moieties (4-hydroxycoumarins, lawsone or syncarpic acid) to synthesize new types of meroterpene-like skeletons. All the products (including side products) were isolated and characterized by NMR, HR-MS, and ECD.

RESULTS

In total, 17 products (representing seven scaffolds) were generated through a one-pot procedure. Most products (12 compounds) showed more potential activity (IC₅₀ < 25 μM) than the positive controls (acarbose and genistein, IC₅₀ 58.19, and 54.74 μM, respectively). Compound 7 exhibited the most potent inhibition of α-glucosidase (IC₅₀ 3.56 μM) in a mixed-type manner. The CD analysis indicated that compound 7 could bind to α-glucosidase and influence the enzyme's secondary structure.

CONCLUSIONS

Compound 7 could serve as a new type of template compound to develop α-glucosidase inhibitors. Full investigation of a biomimic reaction can be used as a concise strategy to explore diverse natural-like skeletons and search for novel lead compounds.

Natural disesquiterpenoids: an update

This review highlights the progress on the isolation, bioactivity, biogenesis and total synthesis of dimeric sesquiterpenoids since 2010.

Hepatoprotective Tetrahydrobenzocyclooctabenzofuranone Lignans from Kadsura longipedunculata

Three new tetrahydrobenzocyclooctabenzofuranone lignan glucosides, longipedunculatins A-C (1-3), a new dibenzocyclooctadiene lignan glucoside, longipedunculatin D (4), a new dibenzocyclooctadiene lignan (5), five new tetrahydrobenzocyclooctabenzofuranone lignans (6-10), and two new simple lignans (11, 12) were isolated from the roots of Kadsura longipedunculata. Their structures and absolute configurations were established using a combination of MS, NMR, and experimental and calculated electronic circular dichroism data. Compound 7 showed moderate hepatoprotective activity against N-acetyl-p-aminophenol-induced toxicity in HepG2 cells with a cell survival rate at 10 μM of 50.8%. Compounds 2, 7, and 12 showed significant in vitro inhibitory effects with an inhibition rate of 55.1%, 74.9%, and 89.8% on nitric oxide production assays at 10 μM.

Biomimetic Synthesis of Psiguajdianone Guided Discovery of the Meroterpenoids from Psidium guajava

Psiguajdianone (1), a novel caryophyllene-derived meroterpenoid dimer, was isolated from Psidium guajava. The structure of 1 was determined by X-ray analysis and confirmed by total synthesis. Our synthetic strategy involves biomimetic cascade Knoevenagel condensation/hetero-Diels-Alder reaction and dimerization. Notably, the caryophyllene-derived meroterpenoids obtained during our synthesis were first identified as artifacts in the laboratory, and five of them were proven to be natural products present in the plant. Moreover, these compounds show significant anti-inflammatory activity.

New Acylated Phenolic Glycosides with ROS-Scavenging Activity from Psidium guajava Leaves

Reactive oxygen species and subsequent oxidative stress are reported to play important roles in chronic metabolic diseases. Plant-derived polyphenols, especially food-derived phenolics, have attracted a lot of attention due to their potential usage against oxidative stress-related diseases. The leaf of Psidium guajava (known as guava) is regarded as a good resource of polyphenols and its products are commercially available in Japan as functional foods against multiple chronic metabolism disorders. In the course of finding novel polyphenols with antioxidative activities from guava leaf, 11 acylated phenolic glycosides (1-11), including 5 new oleuropeic acid-conjugated phenolic glycosides, named guajanosides A-E (1, 2, and 5-7), along with 17 known meroterpenoides (12-28), were isolated and identified. Their structures were determined by spectroscopic data analysis, chemical degradation, and acid hydrolysis. Compounds 1, 2, and 5-11 displayed potent reactive oxygen species-scavenging activity in lipopolysaccharide-stimulated RAW 264.7 macrophage cells. Western blot revealed that compound 6 markedly increased the expression levels of nuclear factor-erythroid 2-related factor 2 (Nrf2), NAD(P)H quinone dehydrogenase 1 (NQO1), and the glutamate-cysteine ligase catalytic subunit. The current study revealed the presence of oleuropeic acid-derived phenolic glycosides in guava leaf and highlighted the potential usage of this type of phenolics against oxidative stress-related metabolic diseases via activation of the Nrf2 signaling pathway.

Enzymatic Intermolecular Hetero-Diels-Alder Reaction in the Biosynthesis of Tropolonic Sesquiterpenes

Diels-Alder reactions are among the most powerful synthetic transformations to construct complex natural products. Despite that increasing of enzymatic intramolecular Diels-Alder reactions have been discovered, natural intermolecular Diels-Alderases are rarely described. Here, we report an intermolecular hetero-Diels-Alder reaction in the biosynthesis of tropolonic sesquiterpenes and functionally characterize EupfF as the first fungal intermolecular hetero-Diels-Alderase. We demonstrate that EupfF catalyzed the dehydration of a hydroxymethyl-containing tropolone (5) to generate a reactive tropolone o-quinone methide (6) and might further stereoselectively control the subsequent intermolecular hetero-Diels-Alder reaction with (1E,4E,8Z)-humulenol (8) to produce enantiomerically pure neosetophomone B (1). Our results reveal the biosynthetic pathway of 1 and expand the repertoire of activities of Diels-Alder cyclases.

Isolation and Synthesis of Novel Meroterpenoids from Rhodomyrtus tomentosa: Investigation of a Reactive Enetrione Intermediate

Rhodomyrtusials A-C, the first examples of triketone-sesquiterpene meroterpenoids featuring a unique 6/5/5/9/4 fused pentacyclic ring system were isolated from Rhodomyrtus tomentosa, along with several biogenetically-related dihydropyran isomers. Two bis-furans and one dihydropyran isomer showed acetylcholinesterase (AChE) inhibitory activity. Structures of the isolates were unambiguously established by a combination of spectroscopic data, ECD analysis, and total synthesis. Bioinspired total syntheses of six isolates were achieved in six steps utilizing a reactive enetrione intermediate generated in situ from a readily available hydroxy-endoperoxide precursor.

Biomimetic total syntheses of chromane meroterpenoids, guadials B and C, guapsidial A and psiguajadial D

The first biomimetic total syntheses of chromane meroterpenoids, guadials B and C, guapsidial A and psiguajadial D have been completed. The key synthetic transformation involves an efficient and high yielding hetero-Diels-Alder reaction. The two structurally isomeric natural products, guadials B and C, were obtained from a common o-quinone methide in the separate reactions with α-pinene and β-pinene, respectively. The two regioisomeric natural products, guapsidial A and psiguajadial D, were achieved in a single chemical operation.

Isolation, Structure Elucidation, and Total Synthesis of Myrtuspirone A from Myrtus communis

A pair of enantiomeric triketone-phloroglucinol hybrids, (+)- and (-)-myrtuspirone A (1), featuring an unprecedented 3-isopropyl-3 H-spiro[benzofuran-2,1'-cyclohexane] backbone, were isolated from the leaves of Myrtus communis. The absolute configuration of each enantiomer of 1 was determined by X-ray diffraction and chemical calculations. Furthermore, the gram-scale total syntheses of (±)-1 and (-)-1 were conducted in four steps using a Michael- N-iodosuccinimide (NIS)-mediated (3 + 2)-annulation reaction. Both (+)- and (-)-1 exhibited antibacterial activities against Gram-positive bacteria including multidrug-resistant strains.

Fischernolides A–D, four novel diterpene-based meroterpenoid scaffolds with antitumor activities from Euphorbia fischeriana

Fischernolides A–D (1–4), representing the rare 28-carbon meroterpenoid skeleton of abietane and an acylphloroglucinol conjugate, bearing an α-furanone or α-pyrone ring, were isolated and identified from the roots of Euphorbia fischeriana.

Littordials A–E, novel formyl-phloroglucinol-β-caryophyllene meroterpenoids from the leaves of Psidium littorale

Littordials A–E (1–5), featuring unusual acyl phloroglucinol units, were isolated from the leaves of Psidium littorale. 2, 3 and 5 exhibited significant cytotoxic activities on MDA-MB-231 and B16 cell lines.

Therapeutic Potential of Plants and Plant Derived Phytochemicals against Acetaminophen-Induced Liver Injury

Acetaminophen (APAP), which is also known as paracetamol or N-acetyl-p-aminophenol is a safe and potent drug for fever, pain and inflammation when used at its normal therapeutic doses. It is available as over-the-counter drug and used by all the age groups. The overdose results in acute liver failure that often requires liver transplantation. Current clinical therapy for APAP-induced liver toxicity is the administration of N-acetyl-cysteine (NAC), a sulphydryl compound an approved drug which acts by replenishing cellular glutathione (GSH) stores in the liver. Over the past five decades, several studies indicate that the safety and efficacy of herbal extracts or plant derived compounds that are used either as monotherapy or as an adjunct therapy along with conventional medicines for hepatotoxicity have shown favorable responses. Phytochemicals mitigate necrotic cell death and protect against APAP-induced liver toxicityby restoring cellular antioxidant defense system, limiting oxidative stress and subsequently protecting mitochondrial dysfunction and inflammation. Recent experimental evidences indicat that these phytochemicals also regulate differential gene expression to modulate various cellular pathways that are implicated in cellular protection. Therefore, in this review, we highlight the role of the phytochemicals, which are shown to be efficacious in clinically relevant APAP-induced hepatotoxicity experimental models. In this review, we have made comprehensive attempt to delineate the molecular mechanism and the cellular targets that are modulated by the phytochemicals to mediate the cytoprotective effect against APAP-induced hepatotoxicity. In this review, we have also defined the challenges and scope of phytochemicals to be developed as drugs to target APAP-induced hepatotoxicity.

Computationally Assisted Assignment of the Kadsuraols, a Class of Chemopreventive Agents for the Control of Liver Cancer

Kadsuraols A-C (1-3), which are tetrahydrocyclobutaphenanthrofuranone-type lignans with a new carbon skeleton comprising a four-membered ring across C-1'-C-8, have been isolated from the roots of Kadsura longipedunculata. Their structures and absolute configurations were unambiguously determined using nuclear magnetic resonance, X-ray diffraction crystallography, DP4+ calculations, and computed and experimental electronic circular dichroism spectra. Kadsuraol C (3) exhibited hepatoprotective activity against N-acetyl- p-aminophenol (APAP)-induced toxicity. The compounds showed no cytotoxicity at 10 μM in a zone assay.

Eucalyptusdimers A-C, Dimeric Phloroglucinol-Phellandrene Meroterpenoids from Eucalyptus robusta

Eucalyptusdimers A-C, three dimeric phellandrene-derived meroterpenoids featuring an unprecedented, fused skeleton between two phellandrene and two acylphloroglucinol subunits, along with one biogenetically related intermediate, (±)-eucalyprobusone A, were isolated from the fruits of Eucalyptus robusta. Their structures and absolute configurations were elucidated using spectroscopic data, X-ray crystallography, and electronic circular dichroism analysis. The isolated meroterpenoids were evaluated for their anti-inflammatory, acetylcholinesterase inhibitory, and protein tyrosine phosphatase 1B inhibitory effects.

Phloroglucinol Derivatives with Unusual Skeletons from Cleistocalyx operculatus and Their in Vitro Antiviral Activity

Four novel phloroglucinol derivatives (1-4) featuring a 2,4-dimethyl-cinnamyl-phloroglucinol moiety, along with their putative biosynthetic precursors 5 and 6, were isolated from the leaves of Cleistocalyx operculatus. Compounds 1 and 2 are two pairs of new enantiomeric phloroglucinol dimers possessing an unprecedented polycyclic skeleton with a highly functionalized dihydropyrano[3,2- d]xanthene tetracyclic core. Compounds 3 and 4 are two new phloroglucinol-terpene adducts (PTAs) with a novel carbon skeleton. The structures of 1-4 including their absolute configurations were unambiguously accomplished by combination of extensive spectroscopic analyses, X-ray crystallography, and quantum chemical ECD calculations. A hypothetical biosynthetic pathway for 1-4 was also proposed. Compound 1 exhibited a promising in vitro antiherpes simplex virus type-1 (HSV-1) effect.

Hepatoprotective Dibenzocyclooctadiene and Tetrahydrobenzocyclooctabenzofuranone Lignans from Kadsura longipedunculata

Five new dibenzocyclooctadiene lignans, longipedlignans A-E (1-5), five new tetrahydrobenzocyclooctabenzofuranones (6-10), and 18 known analogues (11-28) were isolated from the roots of Kadsura longipedunculata. Compounds 6-10 are new spirobenzofuranoid-dibenzocyclooctadiene-type lignans. Their structures and absolute configurations were established using a combination of MS, NMR, and electronic circular dichroism data. Spirobenzofuranoids 6 and 15 showed moderate hepatoprotective activity against N-acetyl- p-aminophenol-induced toxicity in HepG2 cells with cell survival rates at 10 μM of 52.2% and 50.2%, respectively.

Natural Products Research in China From 2015 to 2016

This review covers the literature published by chemists from China during the 2015-2016 on natural products (NPs), with 1,985 citations referring to 6,944 new compounds isolated from marine or terrestrial microorganisms, plants, and animals. The emphasis is on 730 new compounds with a novel skeleton or/and significant bioactivity, together with their source organism and country of origin.

Survey of natural products reported by Asian research groups in 2016

The new natural products reported in peer-reviewed articles in 2016 in journals with good reputations were reviewed and analyzed. The advances that Asian research groups made in the field of natural products chemistry in 2016 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on structural classification.

Meroterpenoids with Antitumor Activities from Guava (Psidium guajava)

Psidium guajava L., a species native to South America, has been widely cultivated in the tropical and subtropical areas of China for its popular fruits. The preliminary analysis by liquid chromatography-ultraviolet (LC-UV) indicated the presence of meroterpenoids in the fruits of P. guajava (guava). Subsequent fractionation of the petroleum ether extract resulted in the identification of two new meroterpenoids, psiguajavadials A (1) and B (2), together with 14 previously described meroterpenoids (3-16). Their structures were fully elucidated by comprehensive spectroscopic techniques and theoretical calculations. All of the meroterpenoids showed cytotoxicities against five human cancer cell lines, with guajadial B (12) being the most effective having an IC₅₀ value of 150 nM toward A549 cells. Furthermore, biochemical topoisomerase I (Top1) assay revealed that psiguajavadial A (1), psiguajavadial B (2), guajadial B (12), guajadial C (14), and guajadial F (16) acted as Top1 catalytic inhibitors and delayed Top1 poison-mediated DNA damage. The flow cytometric analysis indicated that the new meroterpenoids psiguajavadials A (1) and B (2) could induce apoptosis of HCT116 cells. These data suggest that meroterpenoids from guava fruit could be used for the development of antitumor agents.

Psiguajadials A-K: Unusual Psidium Meroterpenoids as Phosphodiesterase-4 Inhibitors from the Leaves of Psidium guajava

Bioassay-guided fractionation of the ethanolic extract of the leaves of Psidium guajava led to the isolation of 11 new Psidium meroterpenoids, psiguajadials A–K (1–11), along with 17 known ones (12–28). Their structures and absolute configurations were elucidated by spectroscopic methods and comparison of experimental and calculated ECD. Compounds 1 and 2 represent two unprecedented skeletons of 3,5-diformyl-benzyl phloroglucinol-coupled sesquiterpenoid, while 3 is the first example of Psidium meroterpenoids coupling via an oxepane ring. Putative biosynthetic pathways towards 1 and 2 are proposed. Compounds 1–13 and 16–26 exhibited moderate inhibitory activities against phosphodiesterase-4 (PDE4), a drug target for asthma and chronic obstructive pulmonary disease, with IC₅₀ values in the range of 1.34–7.26 μM.

Assignment of Absolute Configuration of a New Hepatoprotective Schiartane-Type Nortriterpenoid Using X-Ray Diffraction

A new schiartane-type nortriterpenoid, micrandilactone H was isolated from Kadsuralongipedunculata Finet et Gagnep. Its 2D (two dimension) structure was elucidated by NMR spectroscopic analysis, and it is similar to that of Kadnanolactones H and the absolute configuration was established through X-ray diffraction and ECD data analysis. This represents the first complete assignment of the absolute configuration of a schiartane-type nortriterpenoid by X-ray diffraction and the ECD method. Micrandilactone H showed moderate hepatoprotective activity against N-acetyl-p-aminophenol (APAP)-induced toxicity in HepG2 cells with cell survival rates of 56.84% at 10 μM.

Meroterpenoids with New Skeletons from Myrtus communis and Structure Revision of Myrtucommulone K

Five sesquiterpene-based meroterpenoids with three kinds of new skeletons [1, 2, 3, (+)-4, and (-)-4] were isolated from the leaves of Myrtus communis. Compound 1 featured a new carbon skeleton with an unprecedented octahydrospiro[bicyclo[7.2.0]undecane-2,2'-chromene] tetracyclic ring system, which possessed two preferred conformations detected by variable-temperature NMR spectroscopy experiments. In addition, the structure of reported myrtucommulone K was revised to be compound 3. The plausible biosynthetic pathways of these meroterpenoids and their cytotoxicities are discussed.

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