Discovery of Kalmane Diterpenoids as Potent Analgesics from the Flowers of Rhododendron dauricum

Phellintremulins A-C, antinociceptive sesquiterpenoids from the medicinal fungus Phellinus tremulae

Phellintremulin A (1), a rearranged sesquiterpenoid with an unprecedented bicyclic backbone, and two previously unreported illudane-type sesquiterpenoids, namely phellintremulin B (2) and phellintremulin C (3), together with two known analogues (±)‒4 and (±)‒5, were isolated from cultures of the medicinal fungus Phellinus tremulae. Their structures and absolute configurations were established by means of spectroscopic data and HRESIMS analyses, as well as ECD and NMR calculations. A plausible biogenesis for 1 was discussed. The electrophysiological experiments showed that phellintremulins (A‒C) can inhibit Nav current in DRG neuron cells at 10 μM, with percentage inhibitions of 23.2%, 49.3%, and 31.7%, respectively. The antinociceptive activities of phellintremulins (A‒C) were evaluated via the acetic acid-induced writhing test in mice at a dose of 3 mg/kg. They showed significant antinociceptive effects with percentages of inhibition of 43.8%, 54.4%, and 50.6%, respectively, and phellintremulin B and C expressed more potent analgesic effect than lidocaine.

Forging the Tetracyclic Core Framework of Rhodomolleins XIV and XLII: A Ring-Distortion Approach

A ring distortion approach for the synthesis of an advanced intermediate en route to rhodomolleins XIV and XLII was described, which led to successful construction of the 5/8/5/5 tetracyclic core framework of the kalmane diterpenoids. Key steps of the strategy include an oxidative dearomatization-induced (ODI)-Diels-Alder cycloaddition, a Dowd-Beckwith rearrangement, and a bioinspired Wagner-Meerwein rearrangement.

Recent Advances in Grayanane Diterpenes: Isolation, Structural Diversity, and Bioactivities from Ericaceae Family (2018-2024)

Diterpenes represent one of the most diverse and structurally complex families of natural products. Among the myriad of diterpenoids, grayanane diterpenes are particularly notable. These terpenes are characterized by their unique 5/7/6/5 tetracyclic system and are exclusive to the Ericaceae family of plants. Renowned for their complex structures and broad spectrum of bioactivities, grayanane diterpenes have become a primary focus in extensive phytochemical and pharmacological research. Recent studies, spanning from 2018 to January 2024, have reported a series of new grayanane diterpenes with unprecedented carbon skeletons. These compounds exhibit various biological properties, including analgesic, antifeedant, anti-inflammatory, and inhibition of protein tyrosine phosphatase 1B (PTP1B). This paper delves into the discovery of 193 newly identified grayanoids, representing 15 distinct carbon skeletons within the Ericaceae family. The study of grayanane diterpenes is not only a deep dive into the complexities of natural product chemistry but also an investigation into potential therapeutic applications. Their unique structures and diverse biological actions make them promising candidates for drug discovery and medicinal applications. The review encompasses their occurrence, distribution, structural features, and biological activities, providing invaluable insights for future pharmacological explorations and research.

Novel Flavonol Alkaloids in Green Tea: Synthesis, Detection, and Anti-Alzheimer's Disease Effect in a Transgenic Caenorhabditis elegans CL4176 Model

Novel N-ethy-2-pyrrolidinone-substituted flavonols, myricetin alkaloids A-C (1-3), quercetin alkaloids A-C (4a, 4b, and 5), and kaempferol alkaloids A and B (6 and 7), were prepared from thermal reaction products of myricetin, quercetin, kaempferol─l-theanine, respectively. We used HPLC-ESI-HRMS/MS to detect 1-7 in 14 cultivars of green tea and found that they were all present in "Shuchazao," "Longjing 43", "Fudingdabai", and "Zhongcha 108" green teas. The structures of 1-4 and 6 were determined by extensive 1D and 2D NMR spectroscopies. These flavonol alkaloids along with their skeletal flavonols were assessed for anti-Alzheimer's disease effect based on molecular docking, acetylcholinesterase inhibition, and the transgenic Caenorhabditis elegans CL4176 model. Compound 7 strongly binds to the protein amyloid β (Aβ1-42) through hydrogen bonds (BE: -9.5 kcal/mol, Ki: 114.3 nM). Compound 3 (100 μM) is the strongest one in significantly extending the mean lifespan (13.4 ± 0.5 d, 43.0% promotion), delaying the Aβ1-42-induced paralysis (PT50: 40.7 ± 1.9 h, 17.1% promotion), enhancing the locomotion (140.0% promotion at 48 h), and alleviating glutamic acid (Glu)-induced neurotoxicity (153.5% promotion at 48 h) of CL4176 worms (p < 0.0001).

Discovery of highly functionalized grayanane diterpenoids from the flowers of Rhododendron molle as potent analgesics

A systematical investigation on the chemical constituents of the flowers of Rhododendron molle (Ericaceae) led to the isolation and characterization of thirty-eight highly functionalized grayanane diterpenoids (1-38), including twelve novel analogues molleblossomins A-L (1-12). Their structures were elucidated by comprehensive methods, including 1D and 2D NMR analysis, calculated ECD, 13C NMR calculations with DP4+ probability analysis, and single crystal X-ray diffraction. Molleblossomins A (1), B (2), and E (5) are the first representatives of 2β,3β:9β,10β-diepoxygrayanane, 2,3-epoxygrayan-9(11)-ene, and 5,9-epoxygrayan-1(10),2(3)-diene diterpenoids, respectively. Molleblossomins G (7) and H (8) represent the first examples of 1,3-dioxolane-grayanane conjugates furnished with the acetaldehyde and 4-hydroxylbenzylidene acetal moieties, respectively. All grayanane diterpenoids 1-38 were screened for their analgesic activities in the acetic acid-induced writhing model, and all of them exhibited significant analgesic activities. Diterpenoids 6, 13, 14, 17, 20, and 25 showed more potent analgesic effects than morphine at a lower dose of 0.2 mg/kg, with the inhibition rates of 51.4%, 68.2%, 94.1%, 66.9%, 97.7%, and 60.0%, respectively. More importantly, even at the lowest dose of 0.04 mg/kg, rhodomollein X (14), rhodojaponin VI (20), and rhodojaponin VII (22) still significantly reduced the number of writhes in the acetic acid-induced pain model with the percentages of 61.7%, 85.8%, and 64.6%, respectively. The structure-activity relationship was summarized and might provide some hints to design novel analgesics based on the functionalized grayanane diterpenoids.

Structurally diverse analgesic diterpenoids from the flowers of Rhododendron molle

Thirteen diterpenoids (1-13), classified into four structurally diverse carbon skeletons, including 1,5-seco-kalmane (1 and 6), grayanane (2-11), kalmane (12), and rhodomollane (13), were isolated from the flowers extract of Rhododendron molle. Among them, rhodomollinols A - E (1-5) were five new diterpenoids and their structures were elucidated by extensive spectroscopic methods including HRESIMS, UV, IR, 1D and 2D NMR, as well as quantum ECD calculations. Rhodomollinol A (1) is the first representative of a 6-deoxy-1,5-seco-kalmane diterpenoid. The abnormal NMR phenomenon of the presence of only 9 carbon resonances instead of 20 carbons in the 13C NMR spectrum of 1 was observed and elucidated by the quantum NMR calculations. All diterpenoids 1-13 showed significant analgesic activities in an acetic acid-induced writhing model. It's the first time to report the analgesic activity of a rhodomollane-type diterpenoid. At a dose of 1.0 mg/kg, diterpenoids 1-3, 6, 8, 9, and 12 reduced the writhe numbers with inhibition rates over 50%, and 9 exhibited stronger analgesic activity with a writhe inhibition rate of 89.7% than that of the positive control morphine. Importantly, even at the lowest dose of 0.04 mg/kg, rhodomollinols A (1) and B (2), rhodomollein X (7), and 2-O-methylrhodojaponin VI (9) still showed more potent analgesic effects than morphine with the writhe inhibition rates of 51.8%, 48.0%, 61.7%, and 60.0%, respectively. A preliminary structure-activity relationship might provide some clues to design potential analgesics on the basis of structurally diverse Ericaceae diterpenoids.

An updated review of the genus Rhododendron since 2010: Traditional uses, phytochemistry, and pharmacology

Rhododendron, the largest genus of Ericaceae, consists of approximately 1000 species that are widely distributed in Europe, Asia, and North America but mainly exist in Asia. Rhododendron plants have not only good ornamental and economic value but also significant medicinal potential. In China, many Rhododendron plants are used as traditional Chinese medicine or ethnic medicine for the treatment of respiratory diseases, pain, bleeding and inflammation. Rhododendron is known for its abundant metabolites, especially diterpenoids. In the past 13 years, a total of 610 chemical constituents were reported from Rhododendron plants, including 222 diterpenoids, 122 triterpenoids, 103 meroterpenoids, 71 flavonoids and 92 other constituents (lignans, phenylpropanoids, phenolic acids, monoterpenoids, sesquiterpenoids, coumarins, steroids, fatty acids). Moreover, the bioactivities of various extracts and isolates, both in vitro and in vivo, were also investigated. Our review summarized the research progress of Rhododendron regarding traditional uses, phytochemistry and pharmacology in the past 13 years (2010 to December 2022), which will provide new insight for prompting further research on Rhododendron application and drug development.

Traditional uses, phytochemistry, pharmacology, toxicology, and quality control of Rhododendron dauricum L. leaves: A comprehensive review

ETHNOPHARMACOLOGICAL RELEVANCE

Rhododendron dauricum L. is a traditional herb mainly distributed in the northeast China, Mongolia, Korea Peninsula, and Russia Far East. The dried leaves of Rhododendron dauricum L. (LRD), generally known "Man Shan Hong" have been traditionally applied as folk medicines to treat fever, copious phlegm, asthma, acute and chronic bronchitis, sore throat, dysentery, diabetes mellitus, cancer, and hypertension. To date, no comprehensive review on R. dauricum leaves has been published.

AIM OF THE STUDY

Recent progresses in traditional use, phytochemistry, pharmacology, toxicology, and quality control of R. dauricum leaves are systematically presented and critically evaluated in order to provide scientifical basis for its reasonable utilization and further study.

MATERIALS AND METHODS

All information about R. dauricum leaves were retrieved from internet scientific databases including Sci-Finder, Web of Science, PubMed, CNKI, Google Scholar, Elsevier, Wiley, ACS publications, SpringerLink, and the Chinese Pharmacopoeia between 1970 and 2022. Plant names were validated by "The Plant List" (http://www.theplantlist.org/).

RESULTS

So far, 114 structurally diverse compounds have been isolated and identified from LRD, mainly including flavonoids, diterpenoids, triterpenoids, meroterpenoids, phenols, and 54 volatile components were identified from the essential oils of LRD. Among these, flavonoids are considered as characteristic components and major bioactive phytochemicals. The crude extracts and compounds from LRD have been reported to possess broad pharmacological effects including antitussive and expectorant, anti-inflammatory, anti-HIV, antibacterial, and cytotoxic effects, etc. CONCLUSIONS: As a traditional herb medicine, LRD have been used popularly. On the one hand, traditional uses of LRD provide valuable directions for current research; on the other hand, modern phytochemical and pharmacological studies verify the traditional uses to make its reasonable utilization. However, several defects such as active components determination, in vivo and clinical pharmacological evaluation, toxicology assessment, and quality control of LRD need further study.

Highly oxygenated grayanane diterpenoids with structural diversity from the flowers of Rhododendron dauricum and their analgesic activities

Twenty-eight grayanane diterpenoids (1-28) including 13 new ones, named daublossomins A-M (1-13), and two new natural products, 3-O-acetylgrayanotoxin II (14) and 10-epi-grayanotoxin III (15), were isolated from the flowers of Rhododendron dauricum L. (Ericaceae). Their structures were elucidated by means of comprehensive spectroscopic methods and quantum chemical calculations (¹³C NMR-DP4+ analysis and calculated ECD), and the absolute configurations of ten grayanane diterpenoids 1, 4, 5, 7, 8, 22, 23, 25, 27, and 28 were determined by X-ray crystallographic analysis. Daublossomin A (1) represents the first example of an 11,16-epoxygrayan-6-one diterpenoid. Daublossomins B (2) and C (3) are the first examples of 9β,10β-epoxygrayanane diterpenoids, and daublossomin I (9) is the second conjugated grayan-1(5),6(7),9(10)-triene diterpenoid. Compounds 1-11 and 13-27 were evaluated for their analgesic activities in the HOAc-induced writhing test in mice, and 1-8, 10, 11, 13, 15, 17, 18, 22-24, and 26 exhibited significant analgesic effects at a dose of 5.0 mg/kg (inhibition rates > 50%). Among them, daublossomins A (1) and F (6) still showed potent analgesic activity even at a lower dose of 0.2 mg/kg with the inhibition rates of 54.4% and 55.2%, respectively. Grayanotoxin III (20) showed more potent analgesic activities than the positive control, morphine, at a dose of 0.04 mg/kg. A preliminary structure-activity relationship for the analgesic grayanane diterpenoids was discussed, providing some useful clues to design and develop structurally novel potent analgesics.

Structurally diverse alkaloids with nine frameworks from Zephyranthes candida and their acetylcholinesterase inhibitory and anti-inflammatory activities

Twenty-six structurally diverse Amaryllidaceae alkaloids, including ten undescribed compounds named zephyranines A-I and 6-O-ethylnerinine, two undescribed natural products zephyranthine-6-one and 3-O-deacetyl-sternbergine, were isolated from whole plants of Zephyranthes candida. Their structures were determined by HRESIMS, 1D and 2D NMR, CD data analysis, NMR and ECD calculations, and single-crystal X-ray diffraction analysis. All structures were classified into nine framework types: 10b,11-seco-crinine, graciline, crinine, homolycorine, trisphaeridine, lycorine, galasine, tazettine, and belladine. Zephyranine A represents the first naturally occurring 10b,11-seco-crinine type alkaloid, and zephyranine B is the sixth graciline type alkaloid. 6-O-ethylnerinine is an artifact from the extraction and isolation. All isolates were evaluated for their acetylcholinesterase (AChE) inhibitory and anti-inflammatory activities. Zephyranines A, G, and H exhibited moderate AChE inhibitory activities, with IC₅₀ values of 8.2, 39.0, and 10.8 μM, respectively. Zephyranine B, haemanthamine, haemanthidine, 11-hydroxyvittatine, and 8-demethoxy-10-O-methylhostasine exhibited potent anti-inflammatory activity on the LPS-induced NO production in RAW264.7 mouse macrophages with IC₅₀ values of 21.3, 4.6, 12.2, 5.6, and 17.4 μM, respectively. Structure-activity-relationship analysis and docking studies indicated that interactions with the key Trp286 and Tyr337 residues are required for potent AChE inhibitors.

Anti-infammatory scalemic chromanoids and chromenoids from Rhododendron dauricum

Four pairs of undescribed chromane and chromene meroterpenoid scalemic mixtures (1a/1b-4a/4b), together with three pairs of known chromane meroterpenoid ones (5a/5b-7a/7b) were isolated from the twigs and leaves of Rhododendron dauricum L. Among them, 1a/1b-3a/3b and 5a/5b-7a/7b were the chromane ones derived from an intramolecular [2 + 2] cyclic addition of their respective chromene precursors, forming a 6/6/6/4 and 6/6/5/4 ring fused scaffold. The absolute configurations of the chiral center at C-15 of 2a/2b were determined by Snatzke's method, and comparing the experimental and calculated electronic circular dichroism (ECD) data. The inhibitory effects of the isolated compounds were tested against lipopolysaccharide (LPS)-induced nitric oxide production in RAW264.7 macrophage cells to evaluate their anti-inflammatory activity. Compounds 4a, 4b and 6a displayed inhibitory effects on nitric oxide (NO) production, and compound 4b exhibited the obvious anti-inflammatory activity, with an IC₅₀ value of 6.91 ± 0.97 μM, by downregulating nuclear factor kappa B (NF-κB) and reducing the expression of inducible nitric oxide synthase (iNOS) in LPS-induced RAW264.7 cells. These results intimated that 4b could be used as a leading compound to develop anti-inflammatory drugs and is worthy of further investigated.