Drychampones A–C: Three Meroterpenoids from Dryopteris championii

Undescribed phloroglucinol derivatives with antiviral activities from Dryopteris atrata (Wall. Ex Kunze) Ching

Nine undescribed phloroglucinol derivatives (dryatraols A-I) with five different backbones and three known dimeric acylphloroglucinols were isolated from the rhizome of Dryopteris atrata (Wall. Ex Kunze) Ching (Dryopteridaceae). Dryatraol A contains an unprecedented carbon skeleton-a butyrylphloroglucinol and a rulepidanol-type sesquiterpene are linked via a furan ring to form a 6/5/6/6 ring system. Dryatraols B and C are the first examples of monomeric phloroglucinols coupled with the aristolane-type sesquiterpene through the C-C bond. Dryatraol D features a rare spiro [benzofuran-2',5″-furan] backbone. Dryatraols E-I are five undescribed adducts with a butyrylphloroglucinol or filicinic acid incorporated into the germacrene-type sesquiterpene via a pyran ring. These undescribed structures were determined by comprehensively analysing the spectroscopic data, X-ray diffraction results, and electronic circular dichroism calculations. The result of in vitro antiviral activity evaluation indicated that dryatraol C displayed the strongest antiviral effect against both respiratory syncytial virus and influenza A virus (H1N1), with IC₅₀ values of 11.9 μM and 5.5 μM, respectively. Dryatraols F-H exhibited considerable inhibitory activity against herpes simplex virus type 1 (HSV-1), with IC₅₀ values ranging from 2.6 to 6.3 μM. Analysis of the inhibitory mechanism using a time-of-addition assay revealed that dryatraol G may inhibit the replication of HSV-1 by interfering with the late stage of the viral life cycle.

Demystifying racemic natural products in the homochiral world

Natural products possess structural complexity, diversity and chirality with attractive functions and biological activities that have significantly impacted drug discovery initiatives. Chiral natural products are abundant in nature but rarely occur as racemates. The occurrence of natural products as racemates is very intriguing from a biosynthetic point of view; as enzymes are chiral molecules, enzymatic reactions generating natural products should be stereospecific and lead to single-enantiomer products. Despite several reports in the literature describing racemic mixtures of stereoisomers isolated from natural sources, there has not been a comprehensive review of these intriguing racemic natural products. The discovery of many more natural racemates and their potential enzymatic sources in recent years allows us to describe the distribution and chemical diversity of this 'class of natural products' to enrich discussions on biosynthesis. In this Review, we describe the chemical classes, occurrence and distribution of pairs of enantiomers in nature and provide insights about recent advances in analytical methods used for their characterization. Special emphasis is on the biosynthesis, including plausible enzymatic and non-enzymatic formation of natural racemates, and their pharmacological significance.

Biological activities of meroterpenoids isolated from different sources

Meroterpenoids are natural products synthesized by unicellular organisms such as bacteria and multicellular organisms such as fungi, plants, and animals, including those of marine origin. Structurally, these compounds exhibit a wide diversity depending upon the origin and the biosynthetic pathway they emerge from. This diversity in structural features imparts a wide spectrum of biological activity to meroterpenoids. Based on the biosynthetic pathway of origin, these compounds are either polyketide-terpenoids or non-polyketide terpenoids. The recent surge of interest in meroterpenoids has led to a systematic screening of these compounds for many biological actions. Different meroterpenoids have been recorded for a broad range of operations, such as anti-cholinesterase, COX-2 inhibitory, anti-leishmanial, anti-diabetic, anti-oxidative, anti-inflammatory, anti-neoplastic, anti-bacterial, antimalarial, anti-viral, anti-obesity, and insecticidal activity. Meroterpenoids also possess inhibitory activity against the expression of nitric oxide, TNF- α, and other inflammatory mediators. These compounds also show renal protective, cardioprotective, and neuroprotective activities. The present review includes literature from 1999 to date and discusses 590 biologically active meroterpenoids, of which 231 are from fungal sources, 212 are from various species of plants, and 147 are from marine sources such as algae and sponges.

Six Unusual Meroterpenoids from the Leaves of Psidium guajava L. and Their PTP1B Inhibitory Activities

Six unusual meroterpenoids, psidiguajadiol A-J (1-6), and three known meroterpenoids (7-9) were isolated from the leaves of Psidium guajava L. Compounds 2-6 represent the first examples of 6/8-formyl-5,7-dihydroxy-4-phenylchromane-coupled sesquiterpenoids. The structures of the undescribed compounds, including their absolute configurations, were elucidated by spectroscopic analyses, X-ray diffraction, and computational calculations. Compounds 3, 4, and 6 exhibited inhibitory activities against PTP1B with IC₅₀ values of 9.83, 18.52, and 16.87 μM, respectively. In light of these findings, we performed molecular docking studies to predict their inhibition mechanisms at the atomic level.

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.

Meroterpenoids from the leaves of Psidium guajava (guava) cultivated in Korea using MS/MS-based molecular networking

MS/MS-based molecular networking showed differences in the chemical profiles, especially the terpenoid-coupled-phloroglucinol clusters, of Psidium guajava grown in Jeju Island of South Korea ("Jejuguava"), Vietnam and China. A chemical investigation of the 95% EtOH extract of Jejuguava leaves revealed meroterpenoids characterized by a dihydropyran ring junction between an acylphloroglucinol structure and terpenoid, and named jejuguajavones A-J (1-10). Compounds (±)-8-(±)-10 are racemic mixtures that were separated using a chiral HPLC column. The chemical structures of all the isolated compounds (1-10) were determined by analyzing the spectroscopic data and performing electronic circular dichroism calculations. Among the isolates, compounds 1-4 exhibit inhibitory activity against the protein tyrosine phosphatase 1B (PTP1B) enzyme, and this result was confirmed by molecular docking simulations.

Potent and Non-Cytotoxic Antibacterial Compounds Against Methicillin-Resistant Staphylococcus aureus Isolated from Psiloxylon mauritianum, A Medicinal Plant from Reunion Island

With the occurrence of antibiotic-resistant Staphylococcus aureus strains, identification of new anti-staphylococcal drugs has become a necessity. It has long been demonstrated that plants are a large and diverse source of antibacterial compounds. Psiloxylon mauritianum, an endemic medicinal plant from Reunion Island, was chemically investigated for its reported biological activity against S. aureus. Aspidin VB, a phloroglucinol derivative never before described, together with Aspidin BB, were first isolated from the ethyl acetate extract of P. mauritianum leaves. Their structures were elucidated from spectroscopic data. Aspidin VB exhibited strong antibacterial activity against standard and methicillin-resistant S. aureus strains, with a minimal inhibition concentration (MIC) of 0.25 μg/mL, and no cytotoxicity was observed at 10-5 M in MRC5 cells. Due to its biological activities, Aspidin VB appears to be a good natural lead in the fight against S. aureus.

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.

Non-volatile natural products in plant glandular trichomes: chemistry, biological activities and biosynthesis

The investigation methods, chemistry, bioactivities, and biosynthesis of non-volatile natural products involving 489 compounds in plant glandular trichomes are reviewed.

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.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as kanamienamide from the marine cyanobacterium Moorea bouillonii.