Diterpene Biosynthesis in Actinomycetes: Studies on Cattleyene Synthase and Phomopsene Synthase

Three diterpene synthases from actinomycetes have been studied. The first enzyme from Streptomyces cattleya produced the novel compound cattleyene. The other two enzymes from Nocardia testacea and Nocardia rhamnosiphila were identified as phomopsene synthases. The cyclisation mechanism of cattleyene synthase and the EIMS fragmentation mechanism of its product were extensively studied by incubation experiments with isotopically labelled precursors. Oxidative transformations expanded the chemical space of these unique diterpenes.     

Two Diterpene Synthases from Chryseobacterium: Chryseodiene Synthase and Wanjudiene Synthase

Two bacterial diterpene synthases (DTSs) from Chryseobacterium were characterised. The first enzyme yielded the new compound chryseodiene that closely resembles the known fusicoccane diterpenes from fungi, but its experimentally and computationally studied cyclisation mechanism is fundamentally different to the mechanism of fusicoccadiene synthase. The second enzyme produced wanjudiene, a diterpene hydrocarbon with a new skeleton, besides traces of the enantiomer of bonnadiene that was recently discovered from Allokutzneria albata.     

Increasing complexity of a diterpene synthase reaction with a single residue switch

Terpene synthases often catalyze complex reactions involving intricate series of carbocation intermediates. The resulting, generally cyclical, structures provide initial hydrocarbon frameworks that underlie the astonishing structural diversity of the enormous class of terpenoid natural products (>50,000 known), and these enzymes often mediate the committed step in their particular biosynthetic pathway. Accordingly, how terpene synthases specify product outcome has drawn a great deal of attention. In previous work, we have shown that mutational introduction of a hydroxyl group at specific positions within diterpene synthase active sites can "short circuit" complex cyclization and/or rearrangement reactions, resulting in the production of "simpler"' diterpenes. Here we demonstrate that the converse change, substitution of an Ile for Thr at the relevant position in a native pimaradiene synthase, leads to a dramatic increase in reaction complexity. Product outcome is shifted from the tricyclic pimaradiene to a rearranged tetracycle, aphidicol-15-ene. Thus, the nature of the residue at this position acts as a true switch for product outcome. In addition, the ability of aliphatic residue substitution to enable a more complex reaction emphasizes the importance of substrate conformation imposed by a largely inert active site. Furthermore, the profound plasticity of diterpene synthases exemplified by this single residue switch for product outcome is consistent with the screening/diversity-oriented hypothesis of natural products metabolism.     

Ruptenes: A Family of Terpene Analogs Give Insight into Cyclisation Mechanisms by Cascade Disruption

The terpenoid substrate analogs (7R)-6,7-dihydrogeranylgeranyl diphosphate (6,7-dihydro-GGPP) and (7R)-6,7-dihydrogeranylfarnesyl diphosphate (6,7-dihydro-GFPP) were synthesised from (S)-citronellol and enzymatically converted with nine diterpene and two sesterterpene synthases, respectively. In two cases the substrate analogs were converted into diterpenes in cyclisation reactions corresponding to those observed for the native substrate GGPP, while the cyclisation cascade was disrupted or redirected in the other nine cases, leading to products that were named ruptenes. Several of the isolated ruptenes represent deprotonation products of cationic intermediates that are analogs of the intermediates proposed along the cyclisation cascades for the native substrates GGPP or GFPP, thus giving insights into the complex reaction mechanisms of terpene synthase mediated biosynthesis.     

Enzymatic synthesis of an indole diterpene by an oxidosqualene cyclase: mechanistic, biosynthetic, and phylogenetic implications

Petromindole (1) is an unusual indole diterpene that bears a triterpene-like carbon skeleton, suggesting biogenesis from 3-(omega-oxido-geranylgeranyl)indole (4). We found that lupeol synthase (LUP1) from Arabidopsis thaliana cyclizes 4 to 1. Chiral HPLC comparisons of racemic 1 (from biomimetic cyclization of N-pivaloyl-4) with the LUP1 product and authentic 1 established the absolute stereochemistry of petromindole (3S) as that of cyclic triterpenes. Quantum mechanical calculations and conformational analysis of intermediates in the cyclization of 4 to 1 indicated that petromindole biosynthesis differs fundamentally from that of other indole diterpenes. This analysis revealed that radarins also originate from cyclization of 4 but undergo a backbone rearrangement rather than annulation to indole. The combined results support our hypothesis that native fungal petromindole synthase evolved from a pentacyclic triterpene synthase distant from most other indole diterpene synthases.     

Biomimetic reactions of some macrocyclic diterpenes

Some macrocyclic diterpenes derived from euphohelioscopin A and euphornin have been stereoselectively converted into the jatropholane- and daphnane-type compounds and the known tricyclic compound which has been treated with phCOCl - pyridine to afford euphohelionone, the new type of diterpene isolated from euphorbia helioscopia L.     

A facile route for the synthesis of limonidilactone analogues from andrographolide

Labdane diterpenes are an important class of natural products with a wide variety of biological properties. (−)-Limonidilactone is a labdane diterpene with a novel skeleton isolated from Vitex limonifolia. A new semi-synthetic route from andrographolide, the major constituent of Andrographis paniculata, has been described towards the synthesis of novel labdanes having a limonidilactone type skeleton.     

A single residue switch converts abietadiene synthase into a pimaradiene specific cyclase

Terpene synthases often catalyze complex cyclization reactions that typically represent the committed step in particular biosynthetic pathways, leading to great interest in their enzymatic mechanisms. We have recently demonstrated that substitution of a specific Ile with Thr was sufficient to "short circuit" the complex cyclization reaction normally catalyzed by ent-kaurene synthases to instead produce ent-pimaradiene. Here we report that the complex cyclization/rearrangement reaction catalyzed by abietadiene synthase can be similarly cut short to produce pimaradienes by an analogous Ser for Ala change, albeit with a slight shift in active site location to accommodate the difference in substrate stereochemistry. This result has mechanistic implications for enzymatic catalysis of abietadiene cyclization, and terpene synthases more broadly. Furthermore, these defined single residue switches may be useful in engineering product outcome in diterpene synthases more generally.     

Further Diterpenes from the Seeds of Caesalpinia minax Hance

Three new cassane‐type diterpenes, neocaesalpin AF ( 1 ), neocaesalpin AG ( 2 ), and neocaesalpin AH ( 3 ), were isolated from the seeds of Caesalpinia minax Hance . Their structures were elucidated on the basis of extensive spectroscopic analyses. The partially hydrogenated lactone unit in a tetracyclic cassane diterpene in 2 is rarely encountered in the cassane diterpenes isolated from the genus Caesalpinia. All compounds were evaluated for their in vitro cytotoxic activities against HCT‐8 and MCF‐7 human cancer cell lines.     

Studies on Chemical Constituents of Salix Matsudana

Two new phytane‐type diterpene‐γ‐lactones named Hanliuin III ( 3 ) and Hanliuin IV ( 4 ), together with two previously reported diterpenes, six known flavonoids and five other known compounds were isolated from Salix matsudana. The structures of two new compounds were deduced from spectroscopic methods and 2D NMR techniques.     

Microwave-assisted gold(I) catalyzed pyran ring opening in brevifloralactone: synthesis of the hawtriwaic acid core

An efficient microwave-assisted Au(I) catalyzed cleavage of the pyran ring of brevifloralactone is described. We report one of the first combination reactions of gold and microwave irradiation. In base to this reaction, nine new ent-clerodane diterpene derivatives have been obtained by partial synthesis from brevifloralactone, a naturally occurring clerodane-type diterpene isolated in large quantities from the aerial part of Salvia breviflora. The clerodane diterpenes have very interesting biological activities and the semisynthetic approach described here represents an alternative to obtain them from other major diterpenes isolated from natural sources. The structures of these compounds were stablished from their physical and spectroscopic data.     

Resin diterpenes from Austrocedrus chilensis

Seventeen diterpenes belonging to the labdane, abietane and isopimarane skeleton classes were isolated from the resin of the Chilean gymnosperm Austrocedrus chilensis and identified by spectroscopic and spectrometric methods. The diterpene 12-oxo-labda-8(17),13E-dien-19 oic acid is reported for the first time as a natural product and 14 diterpenes are reported for the first time for the species.     

Two Diterpene Synthases for Spiroalbatene and Cembrene A from Allokutzneria albata

Two bacterial diterpene synthases from the actinomycete Allokutzneria albata were investigated, resulting in the identification of the structurally unprecedented compound spiroalbatene from the first and cembrene A from the second enzyme. Both enzymes were thoroughly investigated in terms of their mechanisms by isotope labeling experiments, site‐directed mutagenesis, and variation of the metal cofactors and pH value. For spiroalbatene synthase, the pH‐ and Mn²⁺‐dependent formation of the side product thunbergol was observed, which is biosynthetically linked to spiroalbatene.     

Quinone derivatives by chemical transformations of 16-hydroxycarnosol from Salvia species

The known diterpenes 12,16-epoxycarnosol (2), isotanshinone II (6), and (+)-neocryptotanshinone (8) were obtained by partial synthesis from 16-hydroxycarnosol (1), a C-16 hydroxylated abietatriene diterpene isolated in relative abundance from the aerial part of Salvia mellifera GREENE. The physical and spectroscopic data of these semisynthetic diterpenes were identical to those given for the natural ones in the literature. These abietane diterpenes have very interesting biological activities and the semisynthetic approach described here represents an alternative to obtain them from other major diterpenes isolated from Salvia species. Additionally, seven new semisynthetic diterpene analogues, 11,14-dioxo-12,16-epoxy-8,12-abietadien-20,7beta-olide (3), 11,14-dioxo-12,16-epoxy-8,12,15(16)-abietatrien-20,7beta-olide (4), 15,16-didehydro-12,16-epoxycarnosol (5), 1-oxoisotanshinone II (7), 16-hydroxycolumbaridione (9), 12,16-diacetoxycolumbaridione (10), and 14-methoxy-12,16-epoxycarnosol (13), were obtained from 1. The structures of the new compounds were established based on their spectroscopic data.     

松香烷内酯型大戟二萜化合物的研究进展

松香烷内酯是大戟属植物中提取的一种典型二萜类化合物, 其基本结构为四环二萜, 有多种药理活性, 特别是有较强的抗肿瘤活性. 对国内外已经报道的松香烷内酯型大戟二萜化合物结构进行了归纳分类, 同时综述了近年来国内外文献对此类化合物的抗肿瘤药理活性、作用机制以及合成研究的进展情况, 总结和讨论了此类化合物的构效关系, 评述了这类化合物作为抗肿瘤药物的开发潜力, 提出了目前存在的问题与不足, 并对其发展前景和研究方向做出了预测和展望.     

Seven-membered vibsane-type diterpenes with a 5,10-cis relationship from Viburnum awabuki

New five seven-membered vibsane-type diterpenes named 5-epi-vibsanin C, 5-epi-vibsanin H, 5-epi-vibsanin K, 18-O-methyl-5-epi-vibsanin K and 5-epi-vibsanin E have been isolated from the leaves of Viburnum awabuki (Caplifoliaceae). Their structures have been elucidated by analyses of spectroscopic data and comparison of their spectral data with those of the previously known seven-membered vibsane-type diterpenes. The occurrence of these seven-membered vibsane-type diterpenes with a cis relationship on the C-5 and C-10 positions in nature have been predicted by conformational analysis of vibsanin B, an eleven-membered vibsane-type diterpene. Vibsanin C, 5-epi-vibsanin C and 5-epi-vibsanin H exhibited moderate cytotoxic activities on KB cells.     

New tri- and tetracyclic diterpenes from Euphorbia villosa

Three new tetracyclic diterpenes were isolated from the chloroform-soluble extract of Euphorbia villosa, together with one new and one known lathyrane diterpene. The structures were elucidated by means of various spectroscopic methods, including HREI-MS, HRFAB-MS, UV, and 1D and 2D NMR techniques. Spectral analyses revealed that two of the tetracyclic compounds contain the rare 5-6-6-4 fused ring system, while the third has a 5-6-7-3 fused diterpene core. Such diterpene skeletons have previously been found only in euphoractines A-E isolated from Euphorbia micractina. As a new structural feature, the diterpene framework described here has a C-2 epimer configuration. The new lathyrane diterpene is a diester of a hitherto unknown polyfunctional parent alcohol.     

Rubesanolides A and B: diterpenoids from Isodon rubescens

From the medicinal plant Isodon rubescens, we isolated two novel diterpenes, rubesanolides A (1) and B (2). The compounds contain a unique β-lactone subgroup. This is the first discovery for a natural diterpene having rings A, B, and C in chair, boat, and twist-chair conformations, respectively. The structures were elucidated by analysis of spectroscopic data, and the absolute configuration of 1 was determined by X-ray diffraction.     

New rearranged clerodane diterpenes from Tinospora baenzigeri.

Baenzigeroside B, a new rearranged clerodane diterpene glucoside, was isolated from the stems of Tinospora baenzigeri. The aglycone of baenzigeroside B, baenzigeride B (isolated as its acetate), was found together with baenzigeride A, baenzigerosides A and B in the leaves of the same plant. Baenzigeride B and baenzigeroside B are the first examples of a new class of rearranged clerodane diterpenes. The possible biogenesis of the compounds is discussed.     

Determination of Free Diterpenes in Green and Roasted Coffees

The three coffee diterpenes cafestol, kahweol, and 16-O-methylcafestol are mostly esterified with fatty acids. Little has been published about the diterpenes occurring in the free form. By means of gel permeation chromatography on Bio Beads S-X3, it is now possible to simultaneously analyze and quantify the small amounts of these compounds by RP-HPLC. In this way, free kahweol was first proved to be an ingredient of Robusta coffee. Various Arabica and Robusta coffees – both green and roasted – were investigated. Free diterpenes were found in green coffees in amounts below 200 mg/kg dry matter. In comparison to the respective total diterpene content determined by the same HPLC method after saponification of the coffee oil, the proportion of free diterpenes ranged from 0.7 to 3.5 %. During the roasting process, the three uncombined diterpenes behaved similarly: free 16-O-methylcafestol, cafestol as well as kahweol were degraded with increasing roasting temperature.