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

Analgesic Grayanane-Derived Diterpenoids from the Flowers of Rhododendron molle

Ten new grayanane-derived diterpenoids, rhodomollein LVII-LXVI (1-10), along with the known compound rhodomollein XLIII (11), were isolated from the flowers of Rhododendron molle. Their structures were elucidated by detailed spectroscopic analysis, X-ray diffraction crystallography, and ECD calculations. Rhodomollein LVII-LIX (1-3) are the first-discovered 3-O-(E)-p-coumaroylquinic acid, nicotinic acid, and 2-furoic acid derivatives of grayanane diterpenoids, respectively. In an acetic acid-induced writhing test, compounds 1 and 3 demonstrated significant antinociceptive effects with writhing inhibition rates of 77.2% and 71.5%, respectively, at a dose of 0.2 mg/kg. Compound 1 was found to be twice as potent as morphine, exhibiting significantly lower toxicity (LD50 = 130.90 mg/kg, i.p.) compared to rhodojaponin VI (LD50 = 1.79 mg/kg, i.p.).

Discovery of ent-kaurane diterpenoid glucosides as potent analgesics from the leaves of Pieris formosa

To search for structurally novel analgesics from Ericaceae plants, the leaves of Pieris formosa collected at Yichang, Hubei, China, were phytochemically investigated for the first time. A total of fifteen ent-kaurane diterpene glucosides (1-15) including twelve new ones, named forminosides A-L (1-12), were isolated. Their structures were elucidated by comprehensive spectroscopic data analyses, quantum chemical calculations (13C NMR and ECD calculations and DP4+ analysis), and chemical methods. The absolute configures of 1-3, 5-8, 11, and 13 were further determined by single-crystal X-ray diffraction analysis. Forminoside A (1) represents the first 3α-(β-d-glucopyranosyloxy)-11,16-epoxy-ent-kaurane diterpenoid bearing a unique 12-oxa-pentacyclo[9.3.3.01,10.04,9.013,16]heptadecane core. Forminoside J (10) is the first 17-nor-ent-kaurane type diterpenoid from Ericaceae family, while forminoside L (12) represents the first example of 4,5-seco-ent-kaurane diterpenoid glycoside bearing an unusual α-hydroxyl-α,β-unsaturated ketone block. Notably, the structure of mollisside A was revised to 3β-(β-d-glucopyranosyloxy)-16β,17-dihydroxy-ent-kaurane based on the NMR and single-crystal X-ray diffraction data analysis of forminoside C (3). All the isolates 1-15 showed potent analgesic activity in the HOAc-induced writhing test in mice. Among them, compounds 1-3, 5-12, and 15 exhibited significant analgesic effects at a dose of 5.0 mg/kg with the inhibition rates over 50%. Compounds 1, 5, 7, and 9-12 still displayed significant analgesic effects with the inhibition rates exceeding 50% at a lower dose of 1.0 mg/kg. Forminosides J (10) and L (12) still showed significant analgesic potency even at a lower dose of 0.2 mg/kg, comparable to that of the positive control, morphine. This is first report of the analgesic activity of 11,16-epoxy-ent-kaurane diterpenoid. A preliminary structure-activity relationship was explored, providing new clues to design novel analgesics based on the ent-kaurane and related diterpenoids.

Dauresorcinols A and B, two pairs of merosesquiterpenoid enantiomers with new carbon skeletons from Rhododendron dauricum

Five pairs of new merosesquiterpenoid enantiomers, named dauresorcinols A-E (1-5), were isolated from the leaves of Rhododendron dauricum. Their structures were elucidated by comprehensive spectroscopic data analysis, quantum chemical calculations, Rh2(OCOCF3)4-induced ECD, and single-crystal X-ray diffraction analysis. Dauresorcinols A (1) and B (2) possess two new merosesquiterpene skeletons bearing an unprecedented 2,6,7,10,14-pentamethyl-11-oxatetracyclo[8.8.0.02,7.012,17]octadecane and a caged 15-isohexyl-1,5,15-trimethyl-2,10-dioxatetracyclo[7.4.1.111,14.03,8]pentadecane motif, respectively. Plausible biosynthetic pathways of 1-5 are proposed involving key oxa-electrocyclization and Wagner-Meerwein rearrangement reactions. (+)/(-)-1 and 3-5 showed potent α-glucosidase inhibitory activity, 3 to 22 times stronger than acarbose, an antidiabetic drug targeting α-glucosidase. Docking results provide a basis to design and develop merosesquiterpenoids as potent α-glycosidase inhibitors.

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.

The Diverse Mycorrizal Morphology of Rhododendron dauricum, the Fungal Communities Structure and Dynamics from the Mycorrhizosphere

It is generally believed that mycorrhiza is a microecosystem composed of mycorrhizal fungi, host plants and other microscopic organisms. The mycorrhiza of Rhododendron dauricum is more complex and the diverse morphology of our investigated results displays both typical ericoid mycorrhizal characteristics and ectomycorrhizal traits. The characteristics of ectendoomycorrhiza, where mycelial invade from the outside into the root cells, have also been observed. In order to further clarify the mycorrhizal fungi members and other fungal communities of R. dauricum mycorrhiza, and explore the effects of vegetation and soil biological factors on their community structure, we selected two woodlands in the northeast of China as samples-one is a mixed forest of R. dauricum and Quercus mongolica, and the other a mixed forest of R. dauricum, Q. mongolica, and Pinus densiflor. The sampling time was during the local growing season, from June to September. High-throughput sequencing yielded a total of 3020 fungal amplicon sequence variants (ASVs), which were based on sequencing of the internal transcribed spacer ribosomal RNA (ITS rRNA) via the Illumina NovaSeq platform. In the different habitats of R. dauricum, there are differences in the diversity of fungi obtained from mycorrhizal niches, and specifically the mycorrhizal fungal community structure in the complex vegetation of mixed forests, where R. dauricum is found, exhibits greater stability, with relatively minor changes over time. Soil fungi are identified as the primary source of fungi within the mycorrhizal niche, and the abundance of mycorrhizal fungi from mycorrhizal niches in R. dauricum is significantly influenced by soil pH, organic matter, and available nitrogen. The relationship between soil fungi and mycorrhizal fungi from mycorrhizal niches is simultaneously found to be intricate, while the genus Hydnellum emerges as a central genus among mycorrhizal fungi from mycorrhizal niches. However, there is currently a substantial gap in the foundational research of this genus, including the fact that mycorrhizal fungi from mycorrhizal niches have, compared to fungi present in the soil, proven to be more sensitive to changes in soil moisture.

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.