Pepluanols C–D, Two Diterpenoids with Two Skeletons from Euphorbia peplus

Total Synthesis of the Euphorbia Diterpenoid Pepluacetal

The first total synthesis of the Euphorbia diterpenoid pepluacetal is disclosed in both racemic and chiral fashions. The synthesis strategically relies on a photo-induced Wolff rearrangement/lactonization cascade (WRLC) reaction to access the cyclobutane moiety, a ring-closing metathesis/cyclopropanation sequence to rapidly forge the 7-3 bicyclic system, and a late-stage Rh-catalyzed transannular carbenoid insertion to C(sp3)-H bond followed by a Baeyer-Villiger oxidation and ring-opening manipulations to install the side chain. The synthetic route demonstrates excellent stereochemical control on the non-classical concave-face bond formation, remote traceless stereochemical relay and high scalability to provide 20 mg of (+)-pepluacetal.

Diterpenoids from Euphorbia peplus possessing cytotoxic and anti-inflammatory activities

Phytochemical investigation into the medium polar fraction of the ethanol extract of Euphorbia peplus led to the identification of 32 diterpenoids with five structural types. Compounds 1-5 and 7-11 are reported for the first time, while the configuration of 6,7-epoxy group of 6 was revised to be β-oriented. Compounds 1-5 feature a rare structural variation of the double bond at Δ1 migrating to Δ1(10) in the tigliane-type diterpenoid family. Biologically, compound 21 was found to be the only one to show moderate cytotoxic activity, associated with the presence of a benzoyloxy residue at C-16. Besides, compounds 4, 8, 12, 13, 16, and 19 show significant inhibitory activities against NO production induced by LPS in RAW264.7 macrophage cells, with IC50 values within 2-5 μM. Structure-activity relationship (SAR) analysis revealed that the ingenane-type diterpenoids have the best anti-inflammatory activity, and the esterification at 3-OH or 5-OH is crucial. Further biological researches demonstrated that 13, the predominant metabolite in this plant, exerts anti-inflammatory effects by blocking the activation of NF-κB and MAPK signaling pathways.

Discovery of Ingenane Diterpenoids from Euphorbia hylonoma as Antiadipogenic Agents

Thirteen new Euphorbia diterpenoids, euphylonanes A-M (1-13), and eight known ones were isolated from the whole plants of Euphorbia hylonoma. Compounds 1 and 2 are two rearranged ingenanes bearing a rare 6/6/7/3-fused ring system. Compound 3 represents the first example of a 9,10-epoxy tigliane, while 4-21 are typical ingenanes varying with substituents. Structures were elucidated using a combination of spectroscopic, computational, and chemical methods. Most ingenanes exerted a significant antiadipogenic effect in 3T3-L1 adipocytes, among which 4 was the most active with an EC50 value of 0.60 ± 0.27 μM. Mechanistic study revealed that 4 inhibited the adipogenesis and lipogenesis in adipocytes via activation of the AMPK signaling pathway.

Rearrangement of the Tetra- and Tricyclic Skeletons of Pepluanol B to Access the Core Structures of Tigliane- and Myrsinane-Type Euphorbia Diterpenes

Pepluanol B is a new Euphorbia diterpene with an unprecedented tetracyclic backbone. However, its biogenetic relationship with known Euphorbia diterpenes is unclear. We report herein that its β-hydroxyl ketone motif could undergo a base-promoted retro-aldol/aldol process in two pathways and afford the skeletons of tigliane- and myrsinane-type Euphorbia diterpenes through the formation of the C8-C14 and C7-C13 bonds, respectively. The retro-aldol/aldol cascade indicates that pepluanol B is possibly a biosynthetic precursor of lathyranes and other relevant dipterpenes.

Diterpenoids from Euphorbia fischeriana with Kv1.3 Inhibitory Activity

Euphorbia diterpenoids possess inhibitory effects of Kv1.3 ion channel, but most of this research has focused on diterpenoids with jatrophane-related or ingenane-related skeletons. In the present study, nine undescribed (1-9) and 16 known (10-25) diterpenoids, based on jatrophane, lathyrane, ingenane, abietane, and atisane skeletons, were identified from the methanol extract of the aerial parts of Euphorbia fischeriana. The structures were established by analysis of the spectroscopic data as well as by single-crystal X-ray diffraction analysis. Among the isolated diterpenoids, macrocyclic jatrophanes and lathyranes exerted Kv1.3 blocking activity. Compound 8 exhibited good selectivity on the inhibition of the Kv 1.3 channel rather than hERG channel, with a selectivity index over 7.0. The selective activity of lathyrane diterpenoids indicates that macrocyclic diterpenoids have the potential to be further investigated as therapeutic agents for the treatment of autoimmune diseases.

Unprecedented diterpene skeletons isolated from vascular plants in the last twenty years (2001-2021)

Every year there are hundreds of reports about the isolation of undescribed terpenoids based on novel functionalizations of known carbocyclic skeletons series. However, on some occasions the compounds obtained have a carbocyclic skeleton that does not correspond with the series established, in these peculiar opportunities, in addition to finding an undescribed natural product, is obtained an unprecedented carbocyclic skeleton, whose biogenesis must necessarily involve other additional steps that explain its formation. This review accounts for the reports of seventy-nine unprecedented diterpene skeletons (corresponding to one-hundred-three undescribed diterpenoids) isolated from vascular plants in the last two decades. According to the genus, Euphorbia and Salvia are the most prolific in reports of unprecedented diterpene skeletons with a total of twenty, and nine skeletons, respectively. If the findings are expressed in terms of the family, Euphorbiaceae and Lamiaceae have the highest number of reports of undescribed diterpene skeletons, with twenty-seven and twenty-two, respectively. Finally, fifty-three skeletons are derived from higher diterpenoids (2-12, 68, 69, 86, 104-109, 158-161, 186, 189, 222, 250-255, 285-298, 403-404, 415, 416, and 436), twenty are derived from lower diterpenoids (135, 136, 192-194, 225-229, 363-370, 397, and 425), and six (96, 97, 147, 148, 205, and 206) are derived from skeletons whose biogenesis has not yet been established, or at least, cannot be formally included within the groups mentioned above. This article comprehensively highlights the hypothetical biosynthetic pathway for each of the one-hundred-three undescribed compounds with unprecedented diterpene skeletons and summarizes their most significant biological activities.

Phytochemical and pharmacological review of diterpenoids from the genus Euphorbia Linn (2012-2021)

ETHNOPHARMACOLOGICAL RELEVANCE

Euphorbia is one of the major genera in angiosperms, which is widely distributed all over the world, including Asia, Africa and Central and South America. The roots or tubers of Euphorbia are famous for medicinal purposes, especially in China. Many of them, such as Euphorbia pekinensis Rupr, Euphorbia fischeriana Steud and Euphorbia Kansui S.L.Liou ex S.B.Ho. . are used as Chinese herbal medicines.

AIM OF THE STUDY

This paper reviews the diterpenoids isolated from the genus Euphorbia species and the pharmacological activities of these compounds to evaluate its traditional use and potential future development.

MATERIALS AND METHODS

Information on the studies of the genus Euphorbia Linn was collected from scientific journals, books and reports via library and electronic data search (Scifinder, Web of Science, PubMed, Elsevier, Scopus, Google Scholar, Springer, Science Direct, Wiley, ACS, CNKI and Kew Plants of the Word Online). Meanwhile, it was also obtained from published works of material medica, folk records, ethnophmacological literatures, Ph.D. and Masters dissertations.

RESULTS

Known as the main constituents of the genus Euphorbia Linn, Diterpenoids possess many pharmacological properties such as anti-inflammation, antiviral activities and cytotoxicity. To date, various types of diterpenoids were identified from this genus, including isopimarane, rosane, abietane, ent-kaurane, ent-atisane. cembrane, casbane, lathyrane, myrsinane, jatropholane, tigliane, ingenane, jatrophane, paraliane, pepluane, and euphoractin.

CONCLUSIONS

This review describes 14 types of diterpenoid isolated from 45 Euphorbia species from 2012 to 2021, a total of 615 compounds. Among them, mainly include jatrophane (171), lathyrane (92), myrsinane (62), abietane (70), ent-atisane (36), ent-kaurane (7), tigliane (26) and ingenane (19). The possible biological pathways of these compounds were presumed. At the same time, more than 10 biological activities of these compounds were summarized, such as anti-inflammation, antiviral activities and cytotoxicity.

Euphorbia diterpenoids: isolation, structure, bioactivity, biosynthesis, and synthesis (2013-2021)

Covering: 2013 to 2021As the characteristic metabolites of Euphorbia plants, Euphorbia diterpenoids have always been a hot topic in related science communities due to their intriguing structures and broad bioactivities. In this review, we intent to provide an in-depth and extensive coverage of Euphorbia diterpenoids reported from 2013 to the end of 2021, including 997 new Euphorbia diterpenoids and 78 known ones with latest progress. Multiple aspects will be summarized, including their occurrences, chemical structures, bioactivities, and syntheses, in which the structure-activity relationship and biosynthesis of this class will be discussed for the first time.

Enantioselective Total Synthesis of (+)-Pepluanol A

Herein, we report an enantioselective and convergent total synthesis of (+)-pepluanol A, a structurally intriguing Euphorbia diterpenoid natural product featuring a 5/6/7/3-fused tetracyclic skeleton, from known building blocks in 11 steps. The successful strategy relies on a phenyl selenide-mediated Morita-Baylis-Hillman type reaction as a connective step, forging the precursor for the key intramolecular Diels-Alder reaction to construct the congested 5/6/7-tricyclic framework. A diastereoconvergent cascade starting with an acid-induced removal of the C1-MOM protecting group followed by a retro-aldol/aldol reaction resulted in the formation of a single diastereomer. This stereoconvergency allowed for the successful substrate-controlled diastereoselective cyclopropanation of an advanced intermediate to establish the full carboskeleton of (+)-pepluanol A (1).

Euphorstranoids A and B, two highly rearranged ingenane diterpenoids from Euphorbia stracheyi: structural elucidation, chemical transformation, and lipid-lowering activity

Euphorstranoids A (1) and B (2), two highly rearranged ingenane diterpenoids with an unusual 5/6/7/3 carbon ring system, were isolated from Euphorbia stracheyi.

Diterpenoids with Rearranged 9(10→11)-abeo-10,12-Cyclojatrophane Skeleton and the First (15S)-Jatrophane from Euphorbia helioscopia: Structural Elucidation, Biomimetic Conversion, and Their Immunosuppressive Effects

Two novel diterpenoids, one with a rearranged trans,trans-fused tricyclo[10.3.0.0^4,6]pentadecane framework (1) and the other with an unprecedented 15S configuration (2), were isolated from Euphorbia helioscopia. Their structures were elucidated by extensive analysis of HR-ESI-MS, NMR, quantum-chemical calculation, and X-ray crystallographic data. Biosynthetically, 1 has a unique "cyclopropane-shift-like" biogenesis involving an oxa-di-π-methane (ODPM) rearrangement, which inspired us to accomplish the biomimetic conversion of 3 to 1. Moreover, compound 1 displayed a potent immunosuppressive effect by inhibiting Kv1.3 voltage-gated channels.

Diterpenoids from the genus Euphorbia: Structure and biological activity (2013-2019)

Euphorbiaceae is one of the largest families of higher plants, including 7500 species, and many of them are used as medicines in China. From 2013 to 2019, a total of 455 previously undescribed diterpenoids were isolated from 53 species of Euphorbia, and some skeleton types were first discovered from the genus Euphorbia. Most of the diterpenoids isolated from Euphorbia spp. have been tested for their biological activity, and some of them were first reported for Euphorbia diterpenoids in recent years, such as neuroprotection, antimalarial activity and inhibition of osteoclast formation. In this review, we summarize all the isolated diterpenoids from the genus Euphorbia according to their skeleton types, classify all these diterpenoids into 26 normal classes and 37 novel skeleton types, and summarize their biological activity.

Ten-Step Asymmetric Total Synthesis of (+)-Pepluanol A

The first asymmetric synthesis of pepluanol A (1) is presented. The synthesis route is very concise (10 steps) and features a Curtin-Hammett-driven stereoconvergent intramolecular Diels-Alder reaction. A Nozaki-Hiyama-Kishi reaction comprises the connective step, bringing together the seven-membered enone system bearing the dienophile and the diene in the side chain. Subsequent stereoconvergent IMDA reaction furnishes the carboskeleton of the natural product in only 7 steps. The reactions were carried out on a gram scale up to an advanced intermediate and including the stereoconvergent intramolecular Diels-Alder reaction.

Bond reactivity indices approach analysis of the [2+2] cycloaddition of jatrophane skeleton diterpenoids from Euphorbia gaditana Coss to tetracyclic gaditanone

The reaction mechanism of the intramolecular [2 + 2] cycloaddition from a jatrophane precursor to the gaditanane skeleton, an unprecedented 5/6/4/6-fused tetracyclic ring framework recently isolated from Euphorbia spp., was studied using the bond reactivity indices approach. Furthermore, six diterpenoids, including three undescribed jatrophanes isolated from E. gaditana Coss, were described. The structures of these compounds were deduced by a combination of 2D NMR spectroscopy and ECD data analysis.

Macrocyclic diterpenoids from the seeds of Euphorbia peplus with potential activity in inducing lysosomal biogenesis

The first phytochemical investigation of the seeds of Euphorbia peplus led to the isolation and characterization of five new (1-5), named euphopepluanones A-E, and five known diterpenoids (6-10). Their structures were established by extensive spectroscopic analysis and X-ray crystallographic experiments. Euphopepluanones A-E (1-3) feature a very rare 5/11/5-tricyclic skeleton, and euphopepluanones D-E (4-5) represent the first report of lathyrane type diterpenoids found in E. peplus. The new compounds 1-5 were assessed for their activities to induce lysosomal biogenesis through LysoTracker Red staining, in which compounds 1 and 3 could significantly induce lysosomal biogenesis. In addition, compounds 1 and 3 could promote the nuclear translocation of TFEB, a master transcriptional factor of lysosomal genes, indicating that compounds 1 and 3 induced lysosomal biogenesis through activation of TFEB.

Euphnerins A and B, Diterpenoids with a 5/6/6 Rearranged Spirocyclic Carbon Skeleton from the Stems of Euphorbia neriifolia

Euphnerins A (1) and B (2), two extremely modified diterpenoids possessing an unprecedented 5/6/6 rearranged spirocyclic carbon skeleton, and a biosynthetically related known diterpenoid (3) were purified from the stems of Euphorbia neriifolia. Their structures were identified by NMR experiments and X-ray diffraction analysis, as well as experimental and calculated electronic circular dichroism data comparison. A putative biosynthetic relationship of 1 and 2 with their presumed precursor 3 is proposed. Compound 1 showed NO inhibitory effects in lipopolysaccharide-stimulated BV-2 cells with an IC₅₀ value of 22.4 μM.

Eupulcherol A, a triterpenoid with a new carbon skeleton from Euphorbia pulcherrima, and its anti-Alzheimer's disease bioactivity

Eupulcherol A (1), a novel triterpenoid with an unprecedented carbon skeleton, was isolated from Euphorbia pulcherrima.

Survey of natural products reported by Asian research groups in 2018

The new natural products reported in 2018 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2018 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Macrocyclic Diterpenoids from Euphorbia helioscopia and Their Potential Anti-inflammatory Activity

Guided by ¹H NMR spectroscopic experiments using the aromatic protons as probes, 11 macrocyclic diterpenes (1-11) were isolated from the aerial parts of Euphorbia helioscopia. Their full three-dimensional structures, including absolute configurations, were established unambiguously by spectroscopic analysis and single-crystal X-ray crystallographic experiments. Among the isolated compounds, compound 1 is the third member thus far of a rare class of Euphorbia diterpenes featuring an unusual 5/10 fused ring system, and 2-4 are new jatrophane diterpenes. Based on the NMR data of the jatrophane diterpenes obtained in this study as well as those with crystallographic structures reported in the literature, the correlations of the chemical shifts of the relevant carbons and the configurations of C-2, C-13, and C-14 of their flexible macrocyclic ring were considered. Moreover, the anti-inflammatory activities of 1-11 were investigated by monitoring their inhibitory effects on nitric oxide production in lipopolysaccharide-stimulated RAW 264.7 cells. Compound 1 showed an IC₅₀ of 7.4 ± 0.6 μM, which might be related to the regulation of the NF-κB signaling pathway by suppressing the translocation of the p65 subunit and the consequent reduction of IL-6 and TNF-α secretions.

NO inhibitory diterpenoids as potential anti-inflammatory agents from Euphorbia antiquorum

Two new ent-atisane-type diterpenoids (1 and 2), three new lathyrane-type diterpenoids (3-5), and seven known analogues (6-12) were isolated from Euphorbia antiquorum. The structures of these diterpenoids were established by analysis of their NMR, MS, and electronic circular dichroism data. The anti-inflammatory activities were evaluated biologically and compounds 1, 4, 7, 8, and 10 displayed strong NO inhibitory effects with IC₅₀ values less than 40 μM. The potential anti-inflammatory mechanism was also investigated using molecular docking and Western blotting.

New e:b-Friedo-Hopane Type Triterpenoids from Euphorbia peplus with Simiarendiol Possessing Significant Cytostatic Activity against HeLa Cells by Induction of Apoptosis and S/G2 Cell Cycle Arrest

Seven rare e:b-friedo-hopane-type triterpenoids including four new (1–4) and three known (5–7) ones with 5 being first reported as a natural product, together with five other known triterpenoids (8–12), were isolated from the nonpolar fractions of the ethanolic extract of Euphorbia peplus. Structural assignments for these compounds were based on spectroscopic analyses and quantum chemical computation method. The structural variations for the C-21 isopropyl group, including dehydrogenation (1 and 3) and hydroxylation at C-22 (simiarendiol, 2), were the first cases among e:b-friedo-hopane-type triterpenoids. Simiarendiol (2) bearing a 22-OH showed significant cytostatic activity against HeLa and A549 human tumor cell lines with IC₅₀ values of 3.93 ± 0.10 and 7.90 ± 0.31 μM, respectively. The DAPI staining and flow cytometric analysis revealed that simiarendiol (2) effectively induced cell apoptosis and arrested cell cycle at the S/G2 phases in a dose-dependent manner in HeLa cells.

Bioactive Diterpenoids from the Stems of Euphorbia royleana

Two ingenane- (1 and 2), two ent-atisane- (3 and 4), two ent-kaurane- (5 and 6), two ent-abietane- (7 and 8), and one ent-isopimarane-type (9) diterpenoid and 12 known analogues have been isolated from the methanolic extract of the stems of Euphorbia royleana. Their structures, including absolute configurations, were determined by extensive spectroscopic methods and ECD data analysis. The nitric oxide inhibitory activities of those diterpenoids were examined biologically in lipopolysaccharide-stimulated BV-2 cells, with compounds 1, 2, 5-7, 10, and 12 having IC₅₀ values lower than 40 μM. Molecular docking was used to investigated the possible mechanism of compounds 1, 2, 5-7, 10, and 12.

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 pepluanol C from Euphorbia peplus.