Plant Orthoesters

Isomer Differentiation by UHPLC-Q-Exactive-Orbitrap MS led to Enhanced Identification of Daphnane Diterpenoids in Daphne tangutica

INTRODUCTION

Liquid chromatography-mass spectrometry (LC-MS) has enhanced the rapid, accurate analysis of complex plant extracts, eliminating the need for extensive isolation. Tandem mass spectrometry (MS/MS) further enhances this process by providing detailed structural information. However, differentiating structural isomers remains a challenge due to their minor spectral and structural differences.

OBJECTIVE

This study aimed to extend the applicability of LC-MS/MS for the structural identification of daphnane diterpenoids, with a particular focus on distinguishing functional isomers.

METHODS

LC-MS analyses were performed using an UHPLC-Q-Exactive-Orbitrap MS. The MS conditions for distinguishing isomers were optimized using in-source CID and HCD modes with reference compounds. A qualitative analysis was then conducted on the extract of Daphne tangutica. The chemical structures of the detected daphnane diterpenoids were estimated by analyzing the fragmentation patterns in both the mass spectra and product ion spectra. These identifications were further validated by isolation and comparison with an in-house daphnane diterpenoid library.

RESULTS

By optimizing MS conditions, especially in the negative ion mode, it was possible to accurately distinguish structural isomers such as yuanhuajine and gniditrin. Qualitative analysis of D. tangutica identified a total of 28 daphnanes, including seven previously unreported compounds. Furthermore, a novel geometric isomer of gniditrin was isolated by conducting isolation on the crude diterpenoid fraction.

CONCLUSION

This study demonstrated that LC-MS/MS analysis can effectively distinguish functional isomers of daphnane diterpenoids, thereby enhancing the identification of daphnanes in plant extracts and highlighting its potential as a powerful tool for phytochemical analysis.

Anti-HIV diterpenoids from Daphne pseudomezereum

The plants of the genus Daphne (Thymelaeaceae) are known to be sources of anti-HIV diterpenoids. Therefore, the present study focused on investigating the anti-HIV diterpenoids in Daphne pseudomezereum, for which the isolation of diterpenoids has not been previously reported. The results showed that three previously undescribed daphnane diterpenoids, onishibarins A-C (1-3), along with seven known compounds (4-10) were isolated from the fruits of Daphne pseudomezereum. Their structures were established by comprehensive analysis of physicochemical and spectroscopic data. Evaluation of the anti-HIV activity and cytotoxicity of the isolated compounds showed that compounds 1, 9, and 10 exhibited potent anti-HIV activity at EC50 = 1.26, 0.84, and 0.78 nM, respectively, with cytotoxicity at IC50 > 5 μM.

Qualitative Analysis of Daphnane Diterpenoids in Various Parts of Daphne pontica L. by UHPLC-Q-Exactive-Orbitrap MS

INTRODUCTION

Daphne pontica L. is an evergreen shrub that is recorded as an anti-diarrheic plant in Turkish folk medicine. Previous studies on D. pontica have reported, albeit slightly, the isolation of daphnane diterpenoids, but no systematic phytochemical analysis of daphnane diterpenoids has been conducted.

OBJECTIVE

This study aimed to comprehensively investigate daphnane diterpenoids in the extracts from the different parts (stems, leaves, and fruits) of D. pontica.

METHODS

An ultra-high-performance liquid chromatography coupled with Q-Exactive hybrid quadrupole Orbitrap mass spectrometer (UHPLC-Q-Exactive-Orbitrap MS) was used for the qualitative analysis of D. pontica. The stems, leaves, and fruits of D. pontica were extracted with diethyl ether. Each extract was then pretreated by a solid phase extraction cartridge and subjected to LC-MS/MS analysis. Detected daphnane diterpenoids were tentatively identified by comparison with an in-house daphnane library, and their chemical structures were estimated in detail by MS/MS fragmentation evaluation.

RESULTS

A total of 33 kinds of daphnanes were identified from the different parts of D. pontica, and were classified into three subtypes: daphnane orthoester, polyhydroxy daphnane, and macrocyclic daphnane orthoester. Among them, six daphnanes were postulated to be previously unreported compounds based on MS/MS fragmentation elucidation. Furthermore, the three plant parts showed similar daphnane diterpenoid profiles, with the stems containing the most abundant daphnane diterpenoids.

CONCLUSION

This is the first study to perform qualitative analysis of daphnane diterpenoids systematically and comprehensively in different parts of D. pontica. The results revealed that D. pontica is a plant resource rich in a variety of daphnane diterpenoids.

Synthesis of dispiro-orthoester via an acetal oxo-carbenium ion

The first dispiro orthoester via a spiroacetal oxo-carbenium ion is presented. Oxidative dearomatization of phloretic esters results in a bifunctional electrophilic spiroacetal oxo-carbenium ion, which undergoes a double nucleophilic addition by diol delivering a range of unusual dispiro-orthoesters with an excellent diversity.

Daphnane diterpenoid orthoesters with an odd-numbered aliphatic side chain from Daphne pedunculata

Daphnane diterpenoids were recognized for their extensive range of potent biological activities. In the present study, phytochemical investigation including LC-MS/MS analysis resulted in the identification of five daphnane diterpenoid orthoesters (1-5). Among the five daphnane diterpenoids, two previously unreported compounds, daphnepedunins I and J (2 and 4) were isolated from Daphne pedunculata. The structure of new compounds was elucidated with extensive physicochemical and spectroscopic analyses. Their structure was characterized by the presence of an unusual odd-numbered aliphatic chain connected to an orthoester. The isolated compounds were evaluated for their anti-HIV activity against HIV-1 infection of MT4 cells, and the results indicated that compound 1 showed the most potent anti-HIV activity with an IC50 value of 0.82 nM.

Daphne genkwa: Ethnopharmacology, phytochemistry and pharmacology of an important traditional Chinese medicine

Daphne genkwa, as a traditional medicine, is widely distributed in China, Korea and Vietnam. In China, the dried flower buds of this plant are named "Yuanhua". It has the ability to effectively promote urination, eliminate phlegm and alleviate cough, eliminate parasites and cure of scabies, with a broad spectrum of pharmacological effects and considerable clinical efficacy. This paper provides a summary and classification of the main chemical constituents of D. genkwa based on a review of relevant domestic and foreign literature. It also outlines the current research status of traditional clinical usage, pharmacological effects, and toxicity of D. genkwa. The aim is to provide a theoretical basis for further study of D. genkwa and its potential new clinical applications.

Insecticidal Triterpenes in Meliaceae III: Plant Species, Molecules, and Activities in Munronia-Xylocarpus

Plants of the Meliaceae family have long attracted researchers' interest due to their various insecticidal activities, with triterpenes being the main active ingredients. In this paper, we discuss 93 triterpenoids with insecticidal activity from 37 insecticidal plant species of 15 genera (Munronia, Neobeguea, Pseudocedrela, Nymania, Quivisia, Ruagea, Dysoxylum, Soymida, Lansium, Sandoricum, Walsura, Trichilia, Swietenia, Turraea, and Xylocarpus) in the family Meliaceae. Among these genera, Trichilia deserves further research, with twelve species possessing insecticidal activity. The 93 insecticidal molecules included 27 ring-seco limonoids (comprising 1 ring A-seco group chemical, 1 ring B-seco group chemical, 5 ring D-seco group chemicals, 14 rings A,B-seco group chemicals, 5 rings B,D-seco group chemicals, and 1 rings A,B,D-seco group chemical), 22 ring-intact limonoids (comprising 5 cedrelone-class chemicals, 6 trichilin-class chemicals, 7 havanensin-class chemicals, 2 azadirone-class chemicals, 1 vilasinin-class chemical, and 1 other chemical), 33 2,30-linkage chemicals (comprising 25 mexicanolide-class chemicals and 8 phragmalin-class chemicals), 3 1,n-linkage-group chemicals, 3 onoceranoid-type triterpenoids, 2 apotirucallane-type terpenoids, 2 kokosanolide-type tetranortriterpenoids, and 1 cycloartane triterpene. In particular, 59 molecules showed antifeedant activity, 30 molecules exhibited poisonous effects, and 9 molecules possessed growth regulatory activity. Particularly, khayasin, beddomei lactone, 3β,24,25-trihydroxycycloartane, humilinolides A-E and methyl-2-hydroxy-3β-isobutyroxy-1-oxomeliac-8(30)-enate showed excellent insecticidal activities, which were comparable to that of azadirachtin and thus deserved more attention. Moreover, it was noteworthy that various chemicals (such as 12α-diacetoxywalsuranolide, 11β,12α-diacetoxycedrelone, 1α,7α,12α-triacetoxy-4α-carbomethoxy-11β-hydroxy-14β,15β-epoxyhavanensin, and 11-epi-21-hydroxytoonacilide, etc.) from Turraea showed excellent insecticidal activity. Specially, the insecticidal activity of khayasin from Neobeguea against the coconut leaf beetle were similar to that of rotenone. Therefore, it was a promising candidate insecticide for the control of the coconut leaf beetle.

COAP-Pd Catalyzed Asymmetric Formal [3+2] Cycloaddition for Optically Active Multistereogenic Spiro Cyclopentane-Indandiones Bearing Cyclic N-Sulfonyl Ketimine Skeletons

We reported a chiral oxamide-phosphine ligand (COAP-Ph)-Pd-catalyzed asymmetric [3+2] cycloaddition reaction between vinyl cyclopropane compounds derived from 1,3-indanedione and 2-vinylcyclopropane-1,1-dicarboxylates with cyclic sulfonyl 1-azadienes. The corresponding reactions provided a series of enantiomerically active spiro cyclopentane-indandione and cyclopentane structures bearing three consecutive stereogenic centers in good yields with good diastereo- and enantioselectivity. The COAP-Pd complex serves not only to promote generation of chiral π-allyl-palladium intermediates and induce the asymmetry of the reaction, but also depress the background reaction.

A Photoinduced Radical Cascade Cyclization for the Synthesis of Angularly Fused Tricyclic Compounds

A photoinduced electron transfer (PET)-triggered cascade reaction has been devised for the conversion of second-generation enol silyl ethers into angularly fused tricyclic scaffolds. Utilizing readily available and cost-effective DCA and phenanthrene as the catalytic systems, this cascade transformation is achieved with high efficiency. The reaction demonstrates a good substrate scope and excellent stereoselectivity, thereby enriching the realm of PET-induced cascade reactions. Additionally, the radical adducts generated through this process can serve as valuable subunits for the synthesis of complex molecules.

Azaphilone pigments from the marine-derived Penicillium sclerotium UJNMF 0503 and their neuroprotective potential against H2O2-induced cell apoptosis through modulating PI3K/Akt pathway

Azaphilones represent a particular group of fascinating pigments from fungal source, with easier industrialization and lower cost than the traditional plant-derived pigments, and they also display a wide range of pharmacological activities. Herein, 28 azaphilone analogs, including 12 new ones, were obtained from the fermentation culture of a marine fungus Penicillium sclerotium UJNMF 0503. Their structures were elucidated by MS, NMR and ECD analyses, together with NMR and ECD calculations and biogenetic considerations. Among them, compounds 1 and 2 feature an unusual natural benzo[d][1,3]dioxepine ring embedded with an orthoformate unit, while 3 and 4 represent the first azaphilone examples incorporating a novel rearranged 5/6 bicyclic core and a tetrahydropyran ring on the side chain, respectively. Our bioassays revealed that half of the isolates exhibited neuroprotective potential against H2O2-induced injury on RSC96 cells, while compound 13 displayed the best rescuing capacity toward the cell viability by blocking cellular apoptosis, which was likely achieved by upregulating the PI3K/Akt signaling pathway.

LC-MS identification, isolation, and structural elucidation of anti-HIV macrocyclic daphnane orthoesters from Edgeworthia chrysantha

The occurrence of macrocyclic daphnane orthoesters (MDOs) with a 1-alkyl group originating from a C14 aliphatic chain is extremely limited in the plant kingdom and has only been isolated from Edgeworthia chrysantha. In the present study, LC-ESI-MS/MS analysis was performed on different parts of E. chrysantha, including flower buds, flowers, leaves, and stems, and resulted in the identification of seven MDOs in all the four plant parts, including two previously unreported compounds 1 and 7. Further LC-MS guided isolation was carried out to afford compounds 1 and 7, and their structures were determined by various spectroscopic analyses. These compounds were also evaluated for anti-HIV activity, thus expanding insights into the structure-activity relationships for MDOs.

Daphnepedunins G and H, anti-HIV macrocyclic 3,4-seco-daphnane orthoesters from Daphne pedunculata

Daphnepedunins G (1) and H (2) with unusual macrocyclic 3,4-seco-daphnane orthoester structure were isolated from Daphne pedunculata. Their structures were determined by physicochemical and spectroscopic analyses combined with synthetic methods, including methyl esterification, derivatization reaction using a chiral anisotropic agent, and biomimetic conversion. Compounds 1 and 2 along with their methyl esters 1a and 2a were evaluated for anti-HIV activity, among which 1a and 2a exhibited potent activity with IC50 values of 1.08 and 1.17 μM, respectively.

Asymmetric Construction of the Core of C6, C7-Epoxy Daphnane Diterpenoid Orthoesters

Asymmetric construction of the core of C6, C7-epoxy daphnane diterpenoid orthoesters is developed through a convergent synthetic strategy. The salient features include a diastereoselective nucleophilic assembly of two bulky cyclic fragments, an oxidative cleavage/transesterification/aldol cascade to fashion the seven-membered ring, and a base-mediated transesterification/retro-aldol/aldol/epoxidation cascade to install the epoxy moiety with proper stereochemistry.

Tigliane and daphnane diterpenoids from Thymelaeaceae family: chemistry, biological activity, and potential in drug discovery

Tigliane and daphnane diterpenoids are characteristically distributed in plants of the Thymelaeaceae family as well as the Euphorbiaceae family and are structurally diverse due to the presence of polyoxygenated functionalities in the polycyclic skeleton. These diterpenoids are known as toxic components, while they have been shown to exhibit a wide variety of biological activities, such as anti-cancer, anti-HIV, and analgesic activity, and are attracting attention in the field of natural product drug discovery. This review focuses on naturally occurring tigliane and daphnane diterpenoids from plants of the Thymelaeaceae family and provides an overview of their chemical structure, distribution, isolation, structure determination, chemical synthesis, and biological activities, with a prime focus on the recent findings.

Guided Isolation of An Uncommon Cembranoid Orthoester, Sarcotortin A, and Three Skeletal Diverse Terpenoids from the Hainan Soft Coral Sarcophyton tortuosum Based on Molecular Networking Strategy

Applying the emerging molecular networking strategy, an uncommon cembranoid orthoester, sarcotortin A (1), featuring a 3/14/8/5-fused scaffold, an unusual eunicellane-type diterpenoid, sarcotorolide A (2), and two new biscembranoids, ximaolides M and N (7 and 8), along with nine known terpenoids 3-6 and 9-13 were isolated from the Hainan soft coral Sarcophyton tortuosum. The structure and absolute configuration of all new compounds were established by a combination of spectroscopic data, X-ray diffraction analysis, and/or quantum chemical computational approaches. The plausible biogenetic relationship among these skeletally different terpenoids was proposed and discussed. In in vitro bioassay, new compound 7 exhibited a remarkable inhibitory activity against protein tyrosine phosphatases 1B (PTP1B) with the IC₅₀ value of 8.06 μM. In addition, compounds 4 and 10 displayed significant inhibitory effects on lipopolysaccharide (LPS)-induced inflammatory responses in RAW264.7 macrophages cells with the IC₅₀ values of 19.13 and 16.45 μM, respectively. Compound 9 showed interesting cytotoxicity against H1975, MDA-MB231, A549, and H1299 cancer cell lines with IC₅₀ values of 31.59, 34.96, 43.87, and 27.93 μM, respectively.

Daphnepedunins A-F, Anti-HIV Macrocyclic Daphnane Orthoester Diterpenoids from Daphne pedunculata

From the whole plant of Daphne pedunculata, 12 macrocyclic daphnane diterpenoids, including six new compounds, daphnepedunins A-F (1-4, 9, and 10), were isolated. Their structures were elucidated by physiochemical and spectroscopic data analysis, the modified Mosher's method, and X-ray crystallography. The isolated compounds were evaluated for anti-HIV activity against HIV-1 infection in MT4 cells and showed significant anti-HIV activity with IC₅₀ values of 36.3-994 nM. A consideration of the anti-HIV activity of these compounds provided further insight into the structure-activity relationships of macrocyclic daphnane diterpenoids.

Bioactivity-guided isolation of anti-inflammatory limonins from Chukrasia tabularis

Chukrasia tabularis is an economically important tree and widely cultured in the southeast of China. Its barks, leaves, and fruits are consumed as a traditional medicine and perceived as a valuable source for bioactive limonin compounds. The extracts from root barks of C. tabularis showed significant anti-inflammatory effect. The aim of this research was to explore the material basis of C. tabularis anti-inflammatory activity, and to purify and identify anti-inflammatory active ingredients. By a bioassay-guided isolation of dichloromethane fraction obtained two novel phragmalin limonins, Chukrasitin D and E (1 and 2), together with 12 known limonins (3-14). The chemical structure of these compounds is determined on the basis of extensive spectral analysis and chemical reactivity. In addition, the activities of these isolated limonins on the production of nitric oxide (NO), tumor necrosis factor alpha (TNF-α), and nuclear factor kappa B (NF-κB) in RAW264.7 cells induced by lipopolysaccharide (LPS) were evaluated. Limonins 1 and 2 indicated significant anti-inflammatory activity with IC50 values of 6.24 and 6.13 μM. Compound 1 notably inhibited the production of NF-κB, TNF-α and interleukin 6 (IL-6) in macrophages. The present results suggest that the root barks of C. tabularis exhibited anti-inflammatory effect and the limonins may be responsible for this activity.

Anti-HIV Macrocyclic Daphnane Orthoesters with an Unusual Macrocyclic Ring from Edgeworthia chrysantha

Edgeworthianins A-E (1-5) were isolated from Edgeworthia chrysantha as a class of macrocyclic daphnane orthoesters with an unusual macrocyclic ring formed from a C14 aliphatic chain. Their structures were elucidated by extensive physicochemical and spectroscopic analyses. Compounds 2, 4, and 5 exhibited potent anti-HIV activity against HIV-1 infection of MT4 cells with EC₅₀ values of 29.3, 8.4, and 2.9 nM, respectively. These compounds broaden the findings of the structure-activity relationship of macrocyclic daphnane orthoesters for further anti-HIV drug development.

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.

Total Synthesis of Resiniferatoxin

From both structural and functional perspectives, the large family of daphnane diterpene orthoesters (DDOs) represent a truly remarkable class of natural products. As potent lead compounds for the treatment of pain, neurodegeneration, HIV/AIDS, and cancer, their medicinal potential continues to be heavily investigated, yet synthetic routes to DDO natural products remain rare. Herein we report a distinct approach to this class of complex diterpenes, highlighted by a 15-step total synthesis of the flagship DDO, resiniferatoxin.

HPLC-Based Purification and Isolation of Potent Anti-HIV and Latency Reversing Daphnane Diterpenes from the Medicinal Plant Gnidia sericocephala (Thymelaeaceae)

Despite the success of combination antiretroviral therapy (cART), HIV persists in low- and middle-income countries (LMIC) due to emerging drug resistance and insufficient drug accessibility. Furthermore, cART does not target latently-infected CD4+ T cells, which represent a major barrier to HIV eradication. The “shock and kill” therapeutic approach aims to reactivate provirus expression in latently-infected cells in the presence of cART and target virus-expressing cells for elimination. An attractive therapeutic prototype in LMICs would therefore be capable of simultaneously inhibiting viral replication and inducing latency reversal. Here we report that Gnidia sericocephala, which is used by traditional health practitioners in South Africa for HIV/AIDS management to supplement cART, contains at least four daphnane-type compounds (yuanhuacine A (1), yuanhuacine as part of a mixture (2), yuanhuajine (3), and gniditrin (4)) that inhibit viral replication and/or reverse HIV latency. For example, 1 and 2 inhibit HIV replication in peripheral blood mononuclear cells (PBMC) by >80% at 0.08 µg/mL, while 1 further inhibits a subtype C virus in PBMC with a half-maximal effective concentration (EC50) of 0.03 µM without cytotoxicity. Both 1 and 2 also reverse HIV latency in vitro consistent with protein kinase C activation but at 16.7-fold lower concentrations than the control prostratin. Both 1 and 2 also reverse latency in primary CD4+ T cells from cART-suppressed donors with HIV similar to prostratin but at 6.7-fold lower concentrations. These results highlight G. sericocephala and components 1 and 2 as anti-HIV agents for improving cART efficacy and supporting HIV cure efforts in resource-limited regions.

Highly efficient construction of an oxa-[3.2.1]octane-embedded 5-7-6 tricyclic carbon skeleton and ring-opening of the bridged ring via C-O bond cleavage

We report herein a highly efficient strategy for construction of a bridged oxa-[3.2.1]octane-embedded 5–7–6 tricyclic carbon skeleton through [3 + 2] IMCC (intramolecular [3 + 2] cross-cycloaddition), and the substituents and/or stereochemistries on C-4, C-6, C-7 and C-10 fully match those in the rhamnofolane, tigliane and daphnane diterpenoids. Furthermore, ring-opening of the bridged oxa-[3.2.1]octane via C–O bond cleavage was also successfully achieved.

We reported a highly efficient construction of an oxa-[3.2.1]octane-embedded 5–7–6 tricyclic carbon skeleton with a full match of the substituents and stereochemistries on C-4/-6/-7/-10 with those in the rhamnofolane/tigliane/daphnane diterpenoids.

Phytochemical analysis of Daphne pontica L. stems with their pro-apoptotic properties against DU-145 and LNCaP prostate cancer cells

BACKGROUND

Daphne pontica is an endemic plant grown wild in the North part of Iran, with anticancer activities.

OBJECTIVES

This study aims to analyze the phytochemistry and screen the cytotoxic activity of new bioactive compounds against a panel of cancer cells, in addition to proapototic properties against prostate cancer cells.

METHOD

Purification procedure was done using repeated column chromatographies by MPLC and HPLC systems. The structures were elucidated by the NMR and exact mass spectroscopy, stereochemistry by NOESY, and absolute configuration by electronic circular dichroism (ECD) spectra. Cytotoxicity was done against DU 145, LNCaP, HeLa, MCF-7, and MDA-MB 231 cells by standard MTT assay. An annexin V/PI assay was performed to measure the type of death following treatment with these compounds for 48 h, followed by the caspase-3 activity test.

RESULTS

In this study, one new dilignan named lignopontin A (9), in addition to 13 known compounds including two phenolic acids (3, 5), one flavanone (6), one bis flavonoid (1), one cumarin glycoside (2), one mono (4) and two dicumarins (10, 11), two lignans (7, 8), and three daphnane diterpenoids (12-14) were isolated for the first time from D. pontica stems. Complete spectral data of compound 12, named as 6,7α-epoxy-5β-hydroxy-9,13,14-ortho-(4,2E)-pentadeca-2,4-diene-1-yl)-resiniferonol, and compound 14, named as 6,7α-epoxy-5β-hydroxy-9,3,14-ortho-(2,4E)-pentadeca-2,4-di-1-yl)-resiniferonol-12β-yl-acetate are reported for the first time. In the MTT assay of newly described compounds against a panel of cancer cells, compounds 9, 12, and 14 possessed moderate to potent cytotoxicity against prostate, breast, and cervical cancer cells in a dose-dependent manner. Flow cytometry analysis against prostate cancer cells indicated that the cytotoxicity of compounds 12 and 14 was due to their ability to induce apoptosis. In the case of compound 9, in Du 145 cells, cell death was mainly through apoptosis. In contrast, LNCaP cells showed both apoptosis and necrotic cell death, predominated by necrosis at the higher concentrations. Caspase-3 activity confirmed apoptosis observed in these compounds through the caspase pathway in prostate cancer cells.

CONCLUSION

D. pontica is a new source of dimeric phenolic compounds, including bisflavonoids, phenylpropanoid-cumarin adduct, and dilignans, as well as daphnane diterpenoids with resiniferonol core with long-chain orthoester moieties. In cytotoxicity screening, compounds 9, 12, and 14 inhibited the growth of DU-145 and LNCaP cells in a dose-dependent manner with IC₅₀ varied from 0.9 - 27.3 and 25.2 - 87.4 μM, respectively. Among them, 9 exhibited selective growth inhibition against DU 145 treated cells. LNCaP cells demonstrated the highest sensitivity to treatment with compound 12.

Total Synthesis of Resiniferatoxin Enabled by Photocatalytic Decarboxylative Radical Cyclization

Resiniferatoxin (1) is a complex daphnane diterpenoid with a highly oxygenated 5/7/6-membered ABC-ring system. Here we report a new synthetic route to 1 that requires 27 steps from a starting d-ribose derivative. The carbon spacer and A-ring are sequentially attached to the C-ring by radical allylation and Stille coupling reactions, respectively. An Ir(III)-catalyzed photoinduced decarboxylative radical reaction then forged the sterically hindered bond between the tetra- and trisubstituted carbons to cyclize the central seven-membered B-ring.

Phytochemistry and Pharmacological Activities of the Diterpenoids from the Genus Daphne

There are abundant natural diterpenoids in the plants of the genus Daphne from the Thymelaeaceae family, featuring a 5/7/6-tricyclic ring system and usually with an orthoester group. So far, a total of 135 diterpenoids has been isolated from the species of the genus Daphne, which could be further classified into three main types according to the substitution pattern of ring A and oxygen-containing functions at ring B. A variety of studies have demonstrated that these compounds exert a wide range of bioactivities both in vitro and in vivo including anticancer, anti-inflammatory, anti-HIV, antifertility, neurotrophic, and cholesterol-lowering effects, which is reviewed herein. Meanwhile, the fascinating structure-activity relationship is also concluded in this review in the hope of providing an easy access to available information for the synthesis and optimization of efficient drugs.

Unified Total Syntheses of Rhamnofolane, Tigliane, and Daphnane Diterpenoids

Rhamnofolane, tigliane, and daphnane diterpenoids are structurally complex natural products with multiple oxygen functionalities, making them synthetically challenging. While these diterpenoids share a 5/7/6-trans-fused ring system (ABC-ring), the three-carbon substitutions at the C13- and C14-positions on the C-ring and appending oxygen functional groups differ among them, accounting for the disparate biological activities of these natural products. Here, we developed a new, unified strategy for expeditious total syntheses of five representative members of these three families, crotophorbolone (1), langduin A (2), prostratin (3), resiniferatoxin (4), and tinyatoxin (5). Retrosynthetically, 1-5 were simplified into their common ABC-ring 6 by detaching the three-carbon units and the oxygen-appended groups. Intermediate 6 with six stereocenters was assembled from four achiral fragments in 12 steps by integrating three powerful transformations, as follows: (i) asymmetric Diels-Alder reaction to induce formation of the C-ring; (ii) π-allyl Stille coupling reaction to set the trisubstituted E-olefin of the B-ring; and (iii) Eu(fod)₃-promoted 7-endo cyclization of the B-ring via the generation of a bridgehead radical. Then 6 was diversified into 1-5 by selective installation of the different functional groups. Attachment of the C14-β-isopropenyl and isopropyl groups led to 1 and 2, respectively, while oxidative acetoxylation and C13,14-β-dimethylcyclopropane formation gave rise to 3. Finally, formation of an α-oriented caged orthoester by C13-stereochemical inversion and esterification with two different homovanillic acids delivered 4 and 5 with a C13-β-isopropenyl group. This unified synthetic route to 1-5 required only 16-20 total steps, demonstrating the exceptional efficiency of the present strategy.

Identification of anti-HIV macrocyclic daphnane orthoesters from Wikstroemia ligustrina by LC-MS analysis and phytochemical investigation

Macrocyclic daphnane orthoesters (MDOs) have attracted significant research interest for the drug discovery to cure HIV infection based on the "Shock and Kill" strategy. In the present study, the first chemical study on Wikstroemia ligustrina (Thymelaeaceae) was carried out by LC-MS analysis and phytochemical investigation. Nine daphnane diterpenoids (1-9) including seven MDOs were detected by LC-MS analysis. Further phytochemical investigation resulted in the isolation and structural elucidation of five daphnanes (1, 2, 5, 8, and 9) with potent anti-HIV activity. Taking the isolated MDO (1) as a model compound, the MS/MS fragmentation pathway was also elucidated.

Production of C20, C30 and C40 terpenes in the engineered phototrophic bacterium Rhodobacter capsulatus

Terpenes constitute one of the largest groups of secondary metabolites that are used, for example, as food-additives, fragrances or pharmaceuticals. Due to the formation of an intracytoplasmic membrane system and an efficient intrinsic tetraterpene pathway, the phototrophic α-proteobacterium Rhodobacter capsulatus offers favorable properties for the production of hydrophobic terpenes. However, research efforts have largely focused on sesquiterpene production. Recently, we have developed modular tools allowing to engineer the biosynthesis of terpene precursors. These tools were now applied to boost the biosynthesis of the diterpene casbene, the triterpene squalene and the tetraterpene β-carotene in R. capsulatus SB1003. Selected enzymes of the intrinsic isoprenoid pathway and the heterologous mevalonate (MVA) pathway were co-expressed together with the respective terpene synthases in various combinations. Remarkably, co-expression of genes ispA, idi and dxs enhanced the synthesis of casbene and β-carotene. In contrast, co-expression of precursor biosynthetic genes with the squalene synthase from Arabidopsis thaliana reduced squalene titers. Therefore, we further employed four alternative pro- and eukaryotic squalene synthases. Here, the synthase from Methylococcus capsulatus enabled highest product levels of 90 mg/L squalene upon co-expression with ispA. In summary, we demonstrate the applicability of R. capsulatus for the heterologous production of diverse terpene classes and provide relevant insights for further development of such platforms.

A neoclerodane orthoester and other new neoclerodane diterpenoids from Teucrium yemense chemistry and effect on secretion of insulin

Teucrium yemense, a medicinal plant commonly grown in Saudi Arabia and Yemen, is traditionally used to treat infections, kidney diseases, rheumatism, and diabetes. Extraction of the dried aerial parts of the plant with methanol, followed by further extraction with butanol and chromatography, gave twenty novel neoclerodanes. Their structures, relative configurations and some conformations were determined by MS and 1-D and 2-D NMR techniques. Most were fairly conventional but one contained an unusual stable orthoester, one had its (C-16)-(C-13)-(C-14)-(C-15) (tetrahydro)furan unit present as a succinic anhydride and one had a rearranged carbon skeleton resulting from ring-contraction to give a central octahydroindene bicyclic core, rather than the usual decalin. Mechanisms are proposed for the biosynthetic formation of the orthoester and for the ring-contraction. Four novel neoclerodanes increased the glucose-triggered release of insulin from isolated murine pancreatic islets by more than the standard drug tolbutamide, showing that they are potential leads for the development of new anti-diabetic drugs.

Daphnane-type diterpenes from genus Daphne and their anti-tumor activity

Daphnane-type diterpenenoids are the major biologically active constituents in the genus Daphne. We find that there are about 101 Daphnane-type diterpenes in this genus, most of those compounds show different degrees of inhibitory effect on various cancer cell. Some of them have been studied in depth and the potent molecular mechanisms might be associated with modulation of different cell-signaling pathways. In addition, some compounds of this type also can inhibit the synthesis of protein and DNA. Absolutely, the anti-tumor activity of Daphnane-type diterpenes is worthy of attention. Unfortunately, most of the current research on the activity of these compounds is focused on simple drug efficacy, and its in-depth mechanism research is far from enough. On the other point of view, there still exists wide growing space on the depth of these compounds.

Synthesis of Enynic and Allenic Orthoesters via Defluoromethoxylation of 2-Trifluoromethyl-1,3-enynes

In this protocol, the chemoselective defluoromethoxylation reactions of 2-trifluoromethyl-1,3-enynes were developed. The enynic and allenic orthoesters were selectively produced in good to excellent yields via multiple substitution processes under mild reaction conditions, respectively. The enynic orthoester products were proved capable of acting as efficient "platform molecules" to access various functionalized allenyl compounds.

New Truxinic and Truxillic Acid Sucrose Diesters From the Leaves of Trigonostemon honbaensis

A new δ-truxinic acid sucrose diester and a new ε-truxillic acid sucrose diester (named trigohonbanosides E and F) were isolated from the leaves of Trigonostemon honbaensis. Their chemical structures were determined by extensive analysis of their HR-ESI-MS and NMR spectra. At a concentration of 20 µM, trigohonbanosides E and F exhibited weak inhibitory effects on NO production in LPS-activated RAW264.7 cells with inhibitory percentages of 22.7% ± 1.1% and 18.5% ± 1.4%, respectively.

Balancing skeleton and functional groups in total syntheses of complex natural products: a case study of tigliane, daphnane and ingenane diterpenoids

Total synthesis of natural products has greatly contributed to natural product research, organic synthesis and drug discovery and development. However, in most cases, the efficiency of total synthesis is far from sufficient for direct practical industrial application. Thus, designing a concise and efficient synthetic route with balanced efforts between building the complex skeleton and introducing functional groups is highly desirable. In this critical review, we first present an introduction of this issue and a philosophical framework that cover possible synthetic approaches. Next, we have chosen the biogenetically closely related, biologically important and synthetically extremely challenging natural products, tiglianes, daphnanes and ingenanes as the particular case for the discussion, since in the past 40 years many synthetic approaches have been reported. The successes and pitfalls included therefore serve as the basis to draw some conclusions that may inspire future development in this area.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Isolation, Structural Elucidation, and Anti-HIV Activity of Daphnane Diterpenoids from Daphne odora

Five new [daphneodorins D-H (1, 5, and 10-12)] and seven known daphnane diterpenoids (2-4 and 6-9) were isolated from Daphne odora. The structures of the new compounds were elucidated by extensive physicochemical and spectroscopic analysis. The isolated compounds were evaluated for their anti-HIV activity against HIV-1 infection of MT4 cells. Nine daphnane diterpenoid orthoesters (1-9) showed potent anti-HIV activity with EC₅₀ values of 1.5-7.7 nM.

Genkwadaphnin inhibits growth and invasion in hepatocellular carcinoma by blocking DHCR24-mediated cholesterol biosynthesis and lipid rafts formation

BACKGROUND

The liver is the central organ for cholesterol homoeostasis, and its dysfunction might cause liver pathological alterations including hepatocellular carcinomas (HCCs). 3β-hydroxysteroid-Δ24 reductase (DHCR24), a crucial enzyme of cholesterol biosynthetic pathway, is involved in lipid rafts formation. Genkwadaphnin (GD) is a daphnane diterpene isolated from the flower buds of Daphne genkwa Siebold et Zuccarini (Thymelaeaceae).

METHODS

We evaluated in vitro and in vivo effect of GD using HCC cells and BALB/c nude mice. Microarray assays were used to identify the differential genes by GD. DHCR24 expression and activity, cholesterol level, lipid rafts structure and the role of DHCR24 in human HCC specimens were tested by various molecular biology techniques.

RESULTS

High expression of DHCR24 in human HCC specimens was correlated with poor clinical outcome. Interfering DHCR24 altered growth and migration of HCC cells. GD inhibited growth and metastasis of HCC cells both in vivo and in vitro. GD suppressed DHCR24 expression and activity, as well as DHCR24-mediated cholesterol biosynthesis and lipid rafts formation, then further inhibited HCC cell invasion and migration.

CONCLUSIONS

Our data suggest that DHCR24-mediated cholesterol metabolism might be an effective therapeutic strategy in HCC, and natural product GD might be a promising agent for HCC therapy.

Mexicanolide-type limonoids from the twigs and leaves of Cipadessa baccifera

Twelve previously undescribed mexicanolide-type limonoids, including two pairs of isomers, together with seven known analogues were isolated from the twigs and leaves of Cipadessa baccifera. Their structures were determined by extensive spectroscopic methods and electronic circular dichroism (ECD) calculations. Structural variations mainly occurred at the attachment of C-3 and the carbon residues linked to C-17. 21-deoxo-23-oxofebrifugin A and 3-O-detigloyl-3-O-isobutyryl-21-deoxo-23-oxofebrifugin A are two rare naturally occurring mexicanolide-type limonoids bearing an α,β-unsaturated-γ-lactone motif at C-17. Moreover, cipaferen R is the first degraded tetranortriterpenoid derivative featuring an unique acetyl group at C-17. Some isolated compounds were evaluated for nematicidal, antifungal, cytotoxic (against five human cancer cell lines), and acetylcholinesterase inhibitory activities. No nematicidal and antifungal activities were observed, yet 3-O-detigloyl-3-O-isobutyrylfebrifugin A, febrifugin A, febrifugin, and khaysin T exhibited moderate cytotoxic activity against the tested cells with IC₅₀ values ranging from 18.56 ± 0.27 to 38.00 ± 0.85 μM, and 3-O-detigloyl-3-O-isobutyrylfebrifugin A, granatumin E, khaysin T, and 2'S-cipadesin A showed moderate inhibitory activities against acetylcholinesterase (AChE) at 50 μM.

Applications of alkyl orthoesters as valuable substrates in organic transformations, focusing on reaction media

In this review we focus on applications of alkyl orthoesters as valuable and efficient substrates to perform various classes of two-component and multi-component organic reactions. The article has classified them according to two aspects, which are: (i) a focus on the reaction medium (solvent-free conditions, aqueous media, and organic solvents); and (ii) an examination of product structures. Reaction accomplishment under solvent-free conditions is an eco-friendly process with the absence of volatile toxic solvents, which puts it in line with green chemistry goals. Water is an interesting choice in organic transformations due to its inexpensiveness and safety. The authors hope their assessment will help chemists to attain new approaches for utilizing alkyl orthoesters in various organic synthetic methods. The review covers the corresponding literature up to the beginning of 2020.

Daphnanes diterpenes from the latex of Hura crepitans L. And activity against human colorectal cancer cells Caco-2

Hura crepitans (Euphorbiaceae) is a tree from South America that produces an irritant latex used as a fish poison. A bio-guided fractionation of an ethanolic extract of the latex led to the isolation and structural identification of three known daphnane-type diterpenes (1-3) including huratoxin (1), together with two new analogs (4, 5). Compound 1 was found to exhibit significant and selective cell growth inhibition against the colorectal cancer cell line Caco-2, with morphological modifications suggesting formations mimicking the intestinal crypt architecture. The underlying mechanism of 1 was further investigated, in comparison with 12-O-tetradecanoylphorbol-13-acetate (TPA), revealing two different mechanisms.

A review on Pimelea poisoning of livestock

Pimelea poisoning of cattle, historically known as St. George Disease or Marree Disease, is a prevailing issue in arid grazing regions of inland Australia. Ingestion of the toxic native Pimelea species that contain the secondary metabolite simplexin, a diterpene orthoester with potent protein kinase C activity, induces diarrhoea, characteristic oedema and potentially fatal right-sided heart failure in cattle. Outbreaks of toxic Pimelea in the grazing field depend on seasonal prevalence. However, all stages of the plant carry the toxin, from seeds, juvenile plants to dead plant material. Livestock generally avoid consuming green Pimelea plants and only consume toxic Pimelea when pasture is minimal or where Pimelea growing through grass tussocks results in inadvertent ingestion. Our knowledge base of Pimelea poisoning has greatly improved with past research, yet the health hazards for livestock grazing in Pimelea affected pastures remains a significant issue whilst the ongoing search to develop effective strategies to mitigate poisoning continues. The goal of this review is to collate historical and recent research giving an overview of the current understandings of Pimelea poisoning, the toxin, its toxic effects and progress made towards remedies to alleviate the effects of Pimelea intoxication.

Total synthesis of crotophorbolone

As a natural diterpenoid, crotophorbolone possesses a challenging trans,trans-5/7/6 framework decorated with six contiguous stereogenic centers and is structurally and biogenetically related to tigliane-type diterpenoids with intriguing bioactivities such as phorbol and prostratin. Based on the convergent strategy, we completed an eighteen-step total synthesis of crotophorbolone starting from (−)-carvone and (+)-dimethyl-2,3-O-isopropylidene-l-tartrate. The key elements of the synthesis involve expedient installation of the six-membered ring and the five-membered ring with multiple functional groups at an early stage, cyclization of the seven-membered ring through alkenylation of the ketone between the five-membered ring and the six-membered ring, functional group-sensitive ring-closing metathesis and final selective introduction of hydroxyls at C₂₀ and C₄.

Convergent total synthesis of crotophorbolone was accomplished in 18 longest linear steps. Observation of unexpected thermodynamic stability of a cis,trans-5/7/6 tricycle would benefit synthetic design of tigliane- and daphnane-related diterpenoids.

Daphnane Diterpenoids from Trigonostemon lii and Inhibition Activities Against HIV-1

Natural products are the important source for the discovery of more potent anti-HIV agents. In this study, six daphnane diterpenoids including three unreported structures were isolated from Trigonostemon lii, which showed significant activities against HIV-1 strains replication in the nanomolar/picomolar range. Meanwhile, these diterpenoids significantly inhibited the fusion of H9/HIV-1 IIIB cells with uninfected C8166 cells, with the EC_50s from 1.06 to 8.73 ng/mL, and did not show any inhibition activities against HIV-1 reverse transcriptase. Moreover, all of the diterpenoids shows significant inhibitions against T20-resistan HIV-1 strains, PNL4-3gp41(36G)V38E, N42S and pNL4-3gp41(36G)V38A, N42T. The results revealed that the six diterpenoids could be a new type of potential lead candidate as an HIV entry inhibitor, particularly for those infected by T20-resistant variants.

Daphneodorins A-C, Anti-HIV Gnidimacrin Related Macrocyclic Daphnane Orthoesters from Daphne odora

Three novel gnidimacrin related macrocyclic daphnanes (GMDs), daphneodorins A-C (2-4), were isolated from Daphne odora Thunb., together with gnidimacrin (1). Their structures were established by extensive physicochemical and spectroscopic analyses. Compounds 2 and 3 potently inhibited HIV-1 replication at subnanomolar concentrations (EC50 0.16 and 0.25 nM, respectively). Compounds 2-4 represent a novel type of GMDs that are highly oxygenated on the macrocyclic ring, suggesting good potential for anti-HIV drug development by further chemical modification.

Facile synthesis of the daphnane and tigliane framework by semi-flow tube-based-bubbling photooxidation and diastereoselective conjugate addition

The common [5-7-6] tricyclic framework in tigliane and daphnane natural diterpenes containing five contiguous stereocenters has been synthesized in 9 steps from readily available starting materials in a completely stereocontrolled manner.