Occurrence, biological activities and synthesis of kaurane diterpenes and their glycosides

Asymmetric synthesis of (+)-17-epi-methoxy-kauran-3-one through tandem oxidative polycyclization-pinacol process

A synthesis of (+)-17-epi-methoxy-kauran-3-one, an O-methylated isomer of the natural diterpene 17-hydroxy-kauran-3-one, has been achieved. The strategy is based on a diastereoselective oxidative polycyclization-pinacol tandem process consisting of transforming a functionalized phenol into a compact and complex tetracycle, which represents the main core of kaurane family members. The synthesis also includes an enantioselective Yamamoto's allylation, a diastereoselective Ru-catalyzed hydrocyanation, a ring-closing metathesis and a reductive isomerization process as key steps. The structure of our synthetic substrate was determined through comparison with an O-methylated derivative of the natural compound.

Engineering Aspergillus nidulans for heterologous ent-kaurene and gamma-terpinene production

Terpenes are a large and varied group of natural products with a wide array of bioactivities and applications. The chemical production of industrially relevant terpenes can be expensive and time-consuming due to the structural complexity of these compounds. Here, we studied Aspergillus nidulans as a heterologous host for monoterpene and diterpene production. Previously, we identified a novel diterpene gene cluster in A. nidulans and showed that overexpression of the cluster-specific transcription factor (pbcR) led to ent-pimara-8(14),15-diene (PD) production. We report further characterization of the A. nidulans PD synthase gene (pbcA). In A. nidulans, overexpression of pbcA resulted in PD production, while deletion of pbcA abolished PD production. Overexpression of Fusarium fujikuroi ent-kaurene synthase (cps/ks) and Citrus unshiu gamma-terpinene synthase resulted in ent-kaurene and gamma-terpinene production, respectively. A. nidulans is a fungal model organism and a close relative to other industrially relevant Aspergillus species. A. nidulans is a known producer of many secondary metabolites, but its ability to produce heterologous monoterpene and diterpene compounds has not been characterized. Here, we show that A. nidulans is capable of heterologous terpene production and thus has potential as a production host for industrially relevant compounds. The genetic engineering principles reported here could also be applied to other Aspergilli.

ent-Kaurane-based regio- and stereoselective inverse electron demand hetero-Diels-Alder reactions: synthesis of dihydropyran-fused diterpenoids

A mild and concise approach for the construction of a 3,4-dihydro-2H-pyran ring integrated into the A-ring of the natural product oridonin using an optimized inverse electron demand hetero-Diels-Alder (IED HDA) reaction is reported herein. A self-dimerization of the exocyclic enone installed in the A-ring through a homo-HDA reaction was identified to exclusively give a dimeric ent-kaurane diterpenoid with the spirochroman core. Moreover, efficient cross-HDA cycloadditions of this enone with various vinyl ethers or vinyl sulfides, instead of its own homo-HDA dimerization, were achieved in a regio- and stereoselective manner, thus providing access to novel dihydropyran-fused diterpenoids as potential anticancer agents to overcome chemoresistance.

The enantioselective construction of tetracyclic diterpene skeletons with Friedel-Crafts alkylation and palladium-catalyzed cycloalkenylation reactions

Due to the profound extent to which natural products inspire medicinal chemists in drug discovery, there is demand for innovative syntheses of these often complex materials. This article describes the synthesis of tricarbocyclic natural product architectures through an extension of the enantioselective Birch-Cope sequence with intramolecular Friedel-Crafts alkylation reactions. Additionally, palladium-catalyzed enol silane cycloalkenylation of the tricarbocyclic structures afforded the challenging bicyclo[3.2.1]octane C/D ring system found in the gibberellins and the ent-kauranes, two natural products with diverse medicinal value. In the case of the ent-kaurane derivative, an unprecedented alkene rearrangement converted four alkene isomers to one final product.

Anti-allodynic and Anti-hyperalgesic effects of an ethanolic extract and xylopic acid from the fruits of Xylopia aethiopica in murine models of neuropathic pain

Background:

Fruit extracts of Xylopia aethiopica are used traditionally in the management of pain disorders including headache and neuralgia. An animal model of vincristine-induced sensory neuropathy was developed after repeated intraperitoneal injection in rats and used in the present work to study the effects of the ethanolic extract of X. aethiopica (XAE) and its diterpene xylopic acid (XA) in vincristine-induced neuropathic pain.

Materials and Methods:

Vincristine (0.1 mg kg^-1 day^-1) was administered during two cycles of five consecutive days to induce chemotherapy-induced neuropathic pain. Static tactile anti-allodynic, anti-hyperalgesic, and cold anti-allodynic effects of XAE (30-300 mg kg^-1) and XA (10-100 mg kg^-1) were assessed using Von Frey filaments of bending forces of 4, 8, and 15 g, the Randall-Selitto paw pressure test, and cold water (4.5°C), respectively.

Results:

Administration of vincristine caused the development of allodynia and hyperalgesia with no significant motor deficit, spontaneous pain, and foot deformity. XAE (30-300 mg kg^-1) and XA (10-100 mg kg^-1) exhibited anti-hyperalgesic, tactile, and cold anti-allodynic properties with XA exhibiting greater potency than XAE. Pregabalin (10-100 mg kg^-1) used as control produced similar effect.

Conclusion:

These findings establish the anti-allodynic and anti-hyperalgesic effects of the ethanolic fruit XAE and its major diterpene XA in vincristine-induced neuropathtic pain.

Specific 12α-hydroxylation of grandiflorenic acid by permeabilised fungus Fusarium graminearum

Biotransformation of grandiflorenic acid by permeabilised fungus Fusarium graminearum to yield its hydroxylation derivative, 12α-hydroxygrandiflorenic acid, was studied. The biotransformed product was isolated by column chromatography and its structure was determined by mass spectrum and nuclear magnetic resonance analysis. Grandiflorenic acid was efficiently metabolised by the fungus. After 72 h, the substrate was almost completely converted into the product.

Steviol glycosides: chemical diversity, metabolism, and function

Steviol glycosides are a group of highly sweet diterpene glycosides discovered in only a few plant species, most notably the Paraguayan shrub Stevia rebaudiana. During the past few decades, the nutritional and pharmacological benefits of these secondary metabolites have become increasingly apparent. While these properties are now widely recognized, many aspects related to their in vivo biochemistry and metabolism and their relationship to the overall plant physiology of S. rebaudiana are not yet understood. Furthermore, the large size of the steviol glycoside pool commonly found within S. rebaudiana leaves implies a significant metabolic investment and poses questions regarding the benefits S. rebaudiana might gain from their accumulation. The current review intends to thoroughly discuss the available knowledge on these issues.

Synthesis, cytotoxicity and antiplasmodial activity of novel ent-kaurane derivatives

This paper reports on the syntheses and spectrometric characterisation of eleven novel ent-kaurane diterpenoids, including a complete set of (1)H, (13)C NMR and crystallographic data for two novel ent-kaurane diepoxides. Moreover, the antineoplastic cytotoxicity for kaurenoic acid and the majority of ent-kaurane derivatives were assessed in vitro against a panel of fourteen cancer cell lines, of which allylic alcohols were shown to be the most active compounds. The good in vitro antimalarial activity and the higher selectivity index values observed for some ent-kaurane epoxides against the chloroquine-resistant W2 clone of Plasmodium falciparum indicate that this class of natural products may provide new hits for the development of antimalarial drugs.

Analgesic effects of an ethanol extract of the fruits of Xylopia aethiopica (Dunal) A. Rich (Annonaceae) and the major constituent, xylopic acid in murine models

Background:

Fruit extracts of Xylopia aethiopica are used traditionally in the management of pain disorders including rheumatism, headache, colic pain, and neuralgia. Little pharmacological data exists in scientific literature of the effect of the fruit extract and its major diterpene, xylopic acid, on pain. The present study evaluated the analgesic properties of the ethanol extract of X. aethiopica (XAE) and xylopic acid (XA), in murine models.

Materials and Methods:

XAE and XA were assessed in chemical (acetic acid-induced abdominal writhing and formalin tests), thermal (Tail-flick and Hargreaves thermal hyperalgesia tests), and mechanical (Randall-Selitto paw pressure test) pain models.

Results:

XAE and XA exhibited significant analgesic activity in all the pain models used. XAE (30-300 mg kg^-1, p.o.) and XA (10-100 mg kg^-1, p.o.) inhibited acetic acid-induced visceral nociception, formalin- induced paw pain (both neurogenic and inflammatory), thermal pain as well as carrageenan-induced mechanical and thermal hyperalgesia in animals. Morphine (1-10 mg kg^-1, i.p.) and diclofenac (1-10 mg kg^-1, i.p.), used as controls, exhibited similar anti-nociceptive activities. XAE and XA did not induce tolerance to their respective anti-nociceptive effects in the formalin test after chronic administration. Morphine tolerance did not also cross-generalize to the analgesic effects of XAE or XA.

Conclusions:

These findings establish the analgesic properties of the ethanol fruit extract of X. aethiopica and its major diterpene, xylopic acid.

Geopyxins A-E, ent-kaurane diterpenoids from endolichenic fungal strains Geopyxis aff. majalis and Geopyxis sp. AZ0066: structure-activity relationships of geopyxins and their analogues

Four new ent-kaurane diterpenoids, geopyxins A-D (1-4), were isolated from Geopyxis aff. majalis, a fungus occurring in the lichen Pseudevernia intensa, whereas Geopyxis sp. AZ0066 inhabiting the same host afforded two new ent-kaurane diterpenoids, geopyxins E and F (5 and 6), together with 1 and 3. The structures of 1-6 were established on the basis of their spectroscopic data, while the absolute configurations were assigned using modified Mosher's ester method. Methylation of 1-3, 5, and 6 gave their corresponding methyl esters 7-11. On acetylation, 1 and 7 yielded their corresponding monoacetates 12 and 14 and diacetates 13 and 15. All compounds were evaluated for their cytotoxic and heat-shock induction activities. Compounds 2, 7-10, 12, 14, and 15 showed cytotoxic activity in the low micromolar range against all five cancer cell lines tested, but only compounds 7-9, 14, and 15 were found to activate the heat-shock response at similar concentrations. From a preliminary structure-activity perspective, the electrophilic α,β-unsaturated ketone carbonyl motif present in all compounds except 6 and 11 was found to be necessary but not sufficient for both cytotoxicity and heat-shock activation.

Diterpenoids from Euphorbia neriifolia

Nine diterpenoids were isolated from the stems of Euphorbia neriifolia Linn. with their structures established by 1D and 2D NMR techniques. Relative stereochemistries were determined by ROESY, and single-crystal X-ray experiments.

Oxygenated compounds from the bioconversion of isostevic acid and their inhibition of TNF-α and COX-2 expressions in LPS-stimulated RAW 264.7 cells

Fourteen oxygenated compounds were isolated from the preparative-scale biotransformation of isostevic acid (ent-beyeran-19-oic acid). Incubation of it with Aspergillus niger BCRC 32720 produced eight metabolites, four with Bacillus megaterium ATCC 14581, and another four with Mortierella isabellina ATCC 38063. In addition to their structural elucidation by NMR spectroscopy and HRMS, structures of four of these were further confirmed by X-ray diffraction studies. Real-time reverse transcription PCR analysis found that 15 of these compounds displayed significant in vitro anti-inflammatory activity in lipopolysaccharide-stimulated RAW 264.7 macrophages by reducing the levels of both TNF-α and COX-2 mRNA relative to control cells stimulated by LPS alone. The activity of one metabolite was similar to that of dexamethasone in inhibiting the expression of TNF-α mRNA, while all test compounds except two of them were more potent than dexamethasone in inhibiting the expression of the COX-2 mRNA.

Kaurenoic acid from pulp of Annona cherimolia in regard to Annonaceae-induced Parkinsonism

Guadeloupean Parkinsonism has been linked epidemiologically to the consumption of Annonaceae fruits. These were proposed to be etiological agents for sporadic atypical Parkinsonism worldwide, because of their content of neurotoxins such as isoquinolinic alkaloids and Annonaceous acetogenins. The pulp of Annona cherimolia Mill. from Spain was screened for these toxic molecules using Matrix-Assisted Laser Desorption Ionisation - Time of Flight mass spectrometry (MALDI-TOF MS) and it was found not to be a source of exposure. However, kaurenoic acid, a diterpene considered to be cytotoxic, was detected in high amounts (66 mg/fresh fruit). Treatment of rat embryonic striatal primary cultures, up to a high concentration (50 µM), did not cause neuronal death nor astrogliosis, suggesting that this molecule is not at risk of implication in human neurodegenerative diseases.

Antitrypanasomal activity of novel benzaldehyde-thiosemicarbazone derivatives from kaurenoic acid

A series of new thiosemicarbazones derived from natural diterpene kaurenoic acid were synthesized and tested against the epimastigote forms of Trypanosoma cruzi to evaluate their antitrypanosomal potential. Seven of the synthesized thiosemicarbazones were more active than kaurenoic acid with IC₅₀ values between 2-24.0 μM. The o-nitro-benzaldehyde-thiosemicarbazone derivative was the most active compound with IC₅₀ of 2.0 μM. The results show that the structural modifications accomplished enhanced the antitrypanosomal activity of these compounds. Besides, the thiocyanate, thiosemicarbazide and the p- methyl, p-methoxy, p-dimethylamine, m-nitro and o-chlorobenzaldehyde-thiosemicarbazone derivatives displayed lower toxicity for LLMCK₂ cells than kaurenoic acid, exhibing an IC₅₀ of 59.5 μM.

iso-Kaurenoic acid from Wedelia paludosa D.C

A recent reinvestigation of aerial parts of Wedelia paludosa D.C. is described and reports, for the first time, the isolation of iso-kaurenoic acid from this species.

ent-Kaurane glycosides from Tricalysia okelensis

Tricalysiosides V and W, two new ent-kaurane glycosides with an acylated disaccharide moiety at the C-3 position, were isolated from the roots of Tricalysia okelensis and their structures established by spectroscopic and chemical methods as ent-kauran-3alpha,16alpha,17-triol-19-al 3-O-[5-O-vanilloyl-beta-D-apiopyranosyl(1-->6)]-beta-D-glucopyranoside (1) and ent-kauran-3alpha,16alpha,17-triol-19-al 3-O-[5-O-E-sinapoyl-beta-D-apiopyranosyl(1-->6)]-beta-D-glucopyranoside (2).

Hydroxylation of the diterpenes ent-kaur-16-en-19-oic and ent-beyer-15-en-19-oic acids by the fungus Aspergillus niger

The diterpenes ent-kaur-16-en-19-oic acid (1) and ent-beyer-15-en-19-oic acid (2) are the major constituents of a spasmolytic diterpenic mixture obtained from the roots of Viguiera hypargyrea, a Mexican medicinal plant. Microbial transformation of 1 and 2 was performed with Aspergillus niger. Two metabolites, ent-7alpha,11beta-dihydroxy-kaur-16-en-19-oic acid (4) and ent-1beta,7alpha-dihydroxy-kaur-16-en-19-oic acid (5), were isolated from the incubation of 1, and one metabolite, ent-1beta,7alpha-dihydroxy-beyer-15-en-19-oic acid (6), was isolated in high yield (40%) from 2. The structures were elucidated on the basis of spectroscopic analyses and confirmed by X-ray crystallographic studies. Compounds 1-4 and 6 and methyl ester derivatives 4a and 6a were evaluated for their ability to inhibit the electrically induced contraction of guinea-pig ileum. Compounds 1, 3, 4, 4a and 5 were significantly active. These results showed that dihydroxylation of 1 at 7beta, 11alpha-, and 1alpha, 7beta-positions resulted in a loss of potency.