Recent advances in the synthesis of ent-kaurane diterpenoids

Aculeanoids A-D, the second 17-nor fusicoccane diterpenoids with immunosuppressive activity from Aspergillus aculeatus

Chemical investigation on the secondary metabolites of Aspergillus aculeatus led to the identification of ten modified fusicoccane-type diterpenoids aculeanoids A-J (1-10). Their structures and absolute configurations were characterized by comprehensive spectroscopic analysis, DP4+ analysis, Mo2(OAc)4-induced ECD, single-crystal X-ray diffractions, and ECD calculations. Compounds 1-4 belong to a rare class of 17-nor fusicoccane diterpenoids, with only one previously reported example. Biologically, compounds 6, 7, and 10 exhibited significant immunosuppressive activities against con A-induced T cell proliferation with IC50 values ranging from 2.44 to 5.26 μM and LPS-induced B cell proliferation with IC50 values ranging from 4.18 to 5.78 μM, which provided more possibilities with the treatment of organ transplantation and various autoimmune diseases.

Functional identification of the diterpene synthases exploring the landscape of diterpene structural diversity in Isodon

The genus Isodon is recognized as a primary source for the production of ent-kaurane-type diterpenes and previous studies predominantly focused on ent-kaurene and miltiradiene as the most extensively investigated diterpenes in Isodon. The diversity of diterpene synthases within this genus has recently been acknowledged, while such studies have been largely restricted to single species. In this study, we systematically mined and functionally validated diterpene synthases from three Isodon species using transcriptomic and metabolomic analyses. We identified the expression profiles of genes associated with diterpene biosynthesis and integrated these data with metabolomic results to elucidate their roles within the diterpene biosynthetic pathway. By reconstructing the metabolic pathways in Escherichia coli, we functionally characterized 11 diterpene synthases and elucidated the biosynthetic pathways of several diterpene skeletons originated from nor or ent-CPP, including the previously reported skeletons like ent-kaurene and miltiradiene, as well as four skeletons (ent-13-epi-sandaracopimaradiene, ent-neoabietadiene, abieta-8(14)-en-13-ol synthase and sandaracopimaradiene) whose biogenesis was reported in Isodon genus for the first time. This study provides novel insights into the molecular basis underlying diterpene diversity in Isodon and establishes a valuable resource for the development of new bioactive molecules and potential drug lead compounds.

Geliboluols A-D: Kaurane-Type Diterpenoids from the Marine-Derived Rare Actinomycete Actinomadura geliboluensis

Four new kaurane-type diterpenoids, geliboluols A-D (1-4), along with one known analog (5), were isolated from the culture broth of the marine-derived rare actinomycete Actinomadura geliboluensis. The structures of compounds 1-4 were determined by spectroscopic analysis (HR-ESIMS, 1D, and 2D NMR), the MPA method, and by comparing their optical rotation values with those in the literature. The new compounds were evaluated for their cytotoxicity against seven blood cancer cell lines by a CellTiter-Glo (CTG) assay and six solid cancer cell lines by a sulforhodamine B (SRB) assay. Among the new compounds, compound 4 exhibited moderate cytotoxic activity against some blood cancer cell lines, with GI50 values ranging from 2.59 to 19.64 µM, and against solid cancer cell lines with GI50 values ranging from 4.34 to 7.23 µM.

Total Syntheses of Diepoxy-ent-Kaurane Diterpenoids Enabled by a Bridgehead-Enone-Initiated Intramolecular Cycloaddition

Here, we report the enantioselective total syntheses of four diepoxy-ent-kaurane diterpenoids including (-)-Macrocalin B, (-)-Acetyl-macrocalin B, and (-)-Isoadenolin A and the revised structure of (-)-Phyllostacin I, which hinges on the strategic design of a regioselective and stereospecific trapping of a highly reactive [3.2.1]-bridgehead enone intermediate via a tethered intramolecular Diels-Alder reaction. Combined experimental and computational studies demonstrated that the novel bridgehead-enone-initiated intramolecular cycloaddition could proceed in a stepwise diradical mechanism. Although the key step partially led to unexpected [2 + 2]-cycloaddition outcomes, we ultimately implemented an unprecedented TiIII-catalyzed cyclobutane ring-opening-annulation radical cascade to reassemble a keystone pentacyclic core. Coupled with a sequence of organized oxidation-state manipulations and an efficient late-stage assembly of the C-7,20 hemiketal bridge, our strategy would streamline the synthetic design of diepoxy-ent-kaurane diterpenoids and pave the way for their modular syntheses as well as highlight the powerful utility of [3.2.1]-bridgehead enone intermediates in the construction of structural complexity.

Ent-Kaurane Diterpenoids from Coffea Genus: An Update of Chemical Diversity and Biological Aspects

Coffee is one of the most important beverages in the world and is produced from Coffea spp. beans. Diterpenes with ent-kaurane backbones have been described in this genus, and substances such as cafestol and kahweol have been widely investigated, along with their derivatives and biological properties. Other coffee ent-kaurane diterpenoids have been reported with new perspectives on their biological activities. The aim of this review is to update the chemical diversity of ent-kaurane diterpenoids in green and roasted coffee, detailing each new compound and reporting its biological potential. A systematic review was performed using the bibliographic databases (SciFinder, Web of Science, ScienceDirect) and specific keywords such as "coffea diterpenes", "coffee diterpenes", "coffee ent-kaurane diterpenes" and "coffee diterpenoids". Only articles related to the isolation of coffee ent-kaurane compounds were considered. A total of 146 compounds were related to Coffea spp. since the first report in 1932. Different chemical skeletons were observed, and these compounds were grouped as furan-type, oxidation-type, rearrangement-type, lacton-type, and lactam-type, among others. In general, the new coffee diterpenoids showed potential as antidiabetic, antidiapogenic, α-glucosidade inhibition, antiplatelet activity, and Cav.3 inhibitors agents, revealing the possibilities for the design, discovery, and development of new drugs.

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

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

Synthesis of Bridged Five-Membered Ring Systems by Type II [3 + 2] Annulation of Allenylsilane-ene

The first type II intramolecular [3 + 2] annulation of allenylsilane-ene has been achieved, enabling diastereoselective and efficient construction of synthetically challenging bridged five-membered ring systems such as bicyclo[3.2.1]. This mild and direct process shows a broad substrate scope and is highly stereospecific. Particularly, this work represents the first stereoselective method for the direct synthesis of bicyclo[3.2.1] ring systems from acyclic precursors. Additionally, the first asymmetric total syntheses of (+)- and (-)-strepsesquitriol, and the efficient formation of the synthetically challenging tetracyclic core of pierisjaponol D are achieved by this type II [3 + 2] annulation reaction.

Advances in the Structures, Pharmacological Activities, and Biosynthesis of Plant Diterpenoids

More and more diterpenoids have attracted extensive attention due to the diverse chemical structures and excellent biological activities, and have been developed into clinical drugs or consumer products. The vast majority of diterpenoids are derived from plants. With the long-term development of plant medicinal materials, the natural resources of many plant diterpenoids are decreasing, and the biosynthetic mechanism of key active components has increasingly become a research hotspot. Using synthetic biology to engineer microorganisms into "cell factories" to produce the desired compounds is an essential means to solve these problems. In this review, we depict the plant-derived diterpenoids from chemical structure, biological activities, and biosynthetic pathways. We use representative plant diterpenes as examples to expound the research progress on their biosynthesis, and summarize the heterologous production of plant diterpenoids in microorganisms in recent years, hoping to lay the foundation for the development and application of plant diterpenoids in the future.

Total syntheses of (-)-macrocalyxoformins A and B and (-)-ludongnin C

The complex and diverse molecular architectures along with broad biological activities of ent-kauranoids natural products make them an excellent testing ground for the invention of synthetic methods and strategies. Recent efforts notwithstanding, synthetic access to the highly oxidized enmein-type ent-kauranoids still presents considerable challenges to synthetic chemists. Here, we report the enantioselective total syntheses of C-19 oxygenated enmein-type ent-kauranoids, including (-)-macrocalyxoformins A and B and (-)-ludongnin C, along with discussion and study of synthetic strategies. The enabling feature in our synthesis is a devised Ni-catalyzed decarboxylative cyclization/radical-polar crossover/C-acylation cascade that forges a THF ring concomitantly with the β-keto ester group. Mechanistic studies reveal that the C-acylation process in this cascade reaction is achieved through a carboxylation followed by an in situ esterification. Biological evaluation of these synthetic natural products reveals the indispensable role of the ketone on the D ring in their anti-tumor efficacy.

Recent Developments in the Syntheses of C-20-Oxygenated ent-Kaurane Diterpenoids

Ent-kaurane diterpenes are a large group of natural products, with more than 1,000 compounds since their discovery. Due to their excellent biological activities and complex polycyclic structures, these compounds have attracted organic synthesis chemists around the world to be devoted to achieve their total synthesis. At present, the isolated C-20-oxygenated ent-kaurane diterpenes are the most abundant of these natural products, reaching more than 350 in number. However, only total syntheses of 3,20-epoxy, 7,20-epoxy and 19,20-lactone ent-kaurane diterpenes have been reported. In this review, we elaborate the synthesis of these three types of C-20 oxygenated ent-kaurane natural products, discuss these synthetic strategies in detail, and provide good guidance and reference for the synthesis of other C-20 oxygenated compounds.

Natural Product-Inspired Molecules for Covalent Inhibition of SHP2 Tyrosine Phosphatase

Natural products have been playing indispensable roles in the development of lifesaving drug molecules. They are also valuable sources for covalent protein modifiers. However, they often are scarce in nature and have complex chemical structures, which are limiting their further biomedical development. Thus, natural product-inspired small molecules which still contain the essence of the parent natural products but are readily available and amenable for structural modification, are important and desirable in searching for lead compounds for various disease treatment. Inspired by the complex and diverse ent-kaurene diterpenoids with significant biological activities, we have created a synthetically accessible and focused covalent library by incorporating the bicyclo[3.2.1]octane α-methylene ketone, which is considered as the pharmacophore of ent-kaurene diterpenoids, as half of the structure, and replacing the other half with much less complex but more druglike scaffolds. From this library, we have identified and characterized selective covalent inhibitors of protein tyrosine phosphatase SHP2, an important anti-cancer therapeutic target. The success of this study demonstrated the importance of creating and evaluating natural product-inspired library as well as their application in targeting challenging disease targets.

Unveiling the Potential of Ent-Kaurane Diterpenoids: Multifaceted Natural Products for Drug Discovery

Natural products hold immense potential for drug discovery, yet many remain unexplored in vast libraries and databases. In an attempt to fill this gap and meet the growing demand for effective drugs, this study delves into the promising world of ent-kaurane diterpenoids, a class of natural products with huge therapeutic potential. With a dataset of 570 ent-kaurane diterpenoids obtained from the literature, we conducted an in silico analysis, evaluating their physicochemical, pharmacokinetic, and toxicological properties with a focus on their therapeutic implications. Notably, these natural compounds exhibit drug-like properties, aligning closely with those of FDA-approved drugs, indicating a high potential for drug development. The ranges of the physicochemical parameters were as follows: molecular weights-288.47 to 626.82 g/mol; number of heavy atoms-21 to 44; the number of hydrogen bond donors and acceptors-0 to 8 and 1 to 11, respectively; the number of rotatable bonds-0 to 11; fraction Csp3-0.65 to 1; and TPSA-20.23 to 189.53 Ų. Additionally, the majority of these molecules display favorable safety profiles, with only 0.70%, 1.40%, 0.70%, and 46.49% exhibiting mutagenic, tumorigenic, reproduction-enhancing, and irritant properties, respectively. Importantly, ent-kaurane diterpenoids exhibit promising biopharmaceutical properties. Their average lipophilicity is optimal for drug absorption, while over 99% are water-soluble, facilitating delivery. Further, 96.5% and 28.20% of these molecules exhibited intestinal and brain bioavailability, expanding their therapeutic reach. The predicted pharmacological activities of these compounds encompass a diverse range, including anticancer, immunosuppressant, chemoprotective, anti-hepatic, hepatoprotectant, anti-inflammation, antihyperthyroidism, and anti-hepatitis activities. This multi-targeted profile highlights ent-kaurane diterpenoids as highly promising candidates for further drug discovery endeavors.

Au-Catalyzed Asymmetric Polyene Cyclization and Its Application in the Total Synthesis of (+)-2-Ketoferruginol, (+)-Fleuryinol B, (+)-Salviol, and (-)-Erythroxylisin A

A catalytic asymmetric 1,3-acyloxy shift/polyene cyclization cascade has been achieved with good enantioselectivities under the catalysis of the chiral Au(I) reagent. The synthetic utility of this method has been showcased by the catalytic asymmetric total syntheses of (+)-2-ketoferruginol, (+)-fleuryinol B, and (+)-salviol. Notably, the first enantioselective total synthesis of (-)-erythroxylisin A has also been realized in 15 steps.

Ent-kaurane diterpenoids from the Annonaceae family: a review of research progress and call for further research

The Annonaceae is one of the plant families with members that are credited with numerous pharmacological functions. Among the group of compounds responsible for these bioactivities are the ent-kaurane diterpenoids. The ent-kauranes are a group of 20-Carbon, tetracyclic diterpenoids that are widely distributed in other plant families including the Annonaceae family. This mini-review focuses mainly on the ent-kaurane diterpenoids isolated from the Annonaceae family, delineates the various biological activities of these compounds, and highlights the research gaps that exist for further scientific scrutiny.

Diastereoselective Synthesis of trans-anti-Hydrophenanthrenes via Ti-mediated Radical Cyclization and Total Synthesis of Kamebanin

Ent-kaurenes consist of an ABC-ring based on a trans-anti-hydrophenanthrene skeleton and a D ring with an exomethylene. Highly oxygen-functionalized ent-kauren-15-ones have promising antiinflammatory pharmacological activity. In this study, we developed a novel diastereoselective synthesis of trans-anti-hydrophenanthrenes via a Ti-mediated reductive radical cyclization. We also demonstrated the applicability of this method by developing the first total synthesis of (±)-kamebanin (longest linear sequence; 17 steps, overall yield; 6.5 %). Furthermore, this synthesis provided a formal semi-pinacol rearrangement for the construction of the quaternary carbon at C8 and a novel Thorpe-Ziegler-type reaction for the construction of the D-ring.

Concise Synthesis of (-)-GA18 Methyl Ester

Gibberellins (GAs) are important plant hormones, but some of their family members are in extremely limited natural supply including GA18. Herein, we report a concise synthesis of (-)-GA18 methyl ester, a member of the C20 gibberellins, from commercially available and cheap andrographolide. Our synthesis features an intramolecular ene reaction to form the C ring, an oxidative cleavage followed by aldol condensation to realize a ring contraction and form the challenging trans-hydrindane (AB ring), and a photochemical [2+2] cycloaddition accompanied by a subsequent SmI2-mediated skeletal rearrangement to construct the methylenebicyclo[3.2.1]octanol moiety (CD ring).

Asymmetric Divergent Synthesis of ent-Kaurane-, ent-Atisane-, ent-Beyerane-, ent-Trachylobane-, and ent-Gibberellane-type Diterpenoids

A unified strategy toward asymmetric divergent syntheses of nine C8-ethano-bridged diterpenoids A1-A9 (candol A, powerol, sicanadiol, epi-candol A, atisirene, ent-atisan-16α-ol, 4-decarboxy-4-methyl-GA₁₂, trachinol, and ent-beyerane) has been developed based on late-stage transformations of common synthons having ent-kaurane and ent-trachylobane cores. The expeditious assembly of crucial advanced ent-kaurane- and ent-trachylobane-type building blocks is strategically explored through a regioselective and diastereoselective Fe-mediated hydrogen atom transfer (HAT) 6-exo-trig cyclization of the alkene/enone and 3-exo-trig cyclization of the alkene/ketone, showing the multi-reactivity of densely functionalized polycyclic substrates with π_C═C and π_C═O systems in HAT-initiated reactions. Following the rapid construction of five major structural skeletons (ent-kaurane-, ent-atisane-, ent-beyerane-, ent-trachylobane-, and ent-gibberellane-type), nine C8-ethano-bridged diterpenoids A1-A9 could be accessed in the longest linear 8 to 11 steps starting from readily available chiral γ-cyclogeraniol 1 and known chiral γ-substituted cyclohexenone 2, in which enantioselective total syntheses of candol A (A1, 8 steps), powerol (A2, 9 steps), sicanadiol (A3, 10 steps), epi-candol A (A4, 8 steps), ent-atisan-16α-ol (A6, 11 steps), and trachinol (A8, 10 steps) are achieved for the first time.

Phytochemical Profiling and Bio-Potentiality of Genus Scutellaria: Biomedical Approach

Scutellaria (Lamiaceae) comprises over 360 species. Based on its morphological structure of calyx, also known as Skullcap, it is herbaceous by habit and cosmopolitan by habitat. The species of Scutellaria are widely used in local communities as a natural remedy. The genus contributed over three hundred bioactive compounds mainly represented by flavonoids and phenols, chemical ingredients which serve as potential candidates for the therapy of various biological activities. Thus, the current review is an attempt to highlight the biological significance and its correlation to various isolated bioactive ingredients including flavonoids, terpenoids, phenols, alkaloids, and steroids. However, flavonoids were the dominant group observed. The findings of the Scutellaria reveal that due to its affluent basis of numerous chemical ingredients it has a diverse range of pharmacological potentials, such as antimicrobial, antioxidant, antifeedant, enzyme inhibition, anti-inflammatory, and analgesic significance. Currently, various bioactive ingredients have been investigated for various biological activities from the genus Scutellaria in vitro and in vivo. Furthermore, these data help us to highlight its biomedical application and to isolate the responsible compounds to produce innovative medications as an alternative to synthetic drugs.

Total Synthesis of the Alleged Structure of (+)-Fimbricalyxoid A

An enantioselective total synthesis of the alleged structure of (+)-fimbricalyxoid A is reported. The synthetic strategy features a pyridine-N-oxidate-mediated S_N2' reaction to introduce an oxygen functionality at position C3 of the A-ring and a sequential three-step process via the cleavage of the C-O bonds and hemiketalization to form the 3,20-oxybridge. With this strategy, the target molecule was synthesized in 19% overall yield and 12 steps from our previously synthesized cis-fused octahydrophenanthrene (+)-6.

Programmed Polyene Cyclization Enabled by Chromophore Disruption

A new polyene cyclization strategy exploiting β-ionyl derivatives was developed. Photoinduced deconjugation of the extended π-system within these chromophores unveils a contrathermodynamic polyene that engages in a Heck bicyclization to afford [4.4.1]-propellanes. This cascade improves upon the limited regioselectivity achieved using existing biomimetic tactics and tolerates both electron-rich and electron-deficient (hetero)aryl groups. The utility of this approach was demonstrated with the diverted total synthesis of taxodione and salviasperanol, two isomeric abietane diterpenes that were previously inaccessible along the same synthetic pathway.

Structures and Biological Activities of Polyacylated ent-Kaurane Diterpenoid Glycosides from the Aerial Parts of Inula hupehensis

Sixteen new (1-16) and three known (17-19) polyacylated ent-kaurane diterpenoid glycosides were isolated from the aerial parts of Inula hupehensis. The planar structures of 1-16 and their relative configurations were established on the basis of extensive spectroscopic analysis. The absolute configurations of all stereogenic centers for compounds 1 and 6 were determined by single-crystal X-ray diffraction experiments, and the absolute configurations of the other new compounds were assigned by chemical degradation and experimental ECD data. Antineuroinflammatory testing of all the isolates showed that compound 5 inhibited lipopolysaccharide-induced nitric oxide production in BV-2 microglial cells with an IC₅₀ value of 15.6 μM. In an α-glucosidase inhibitory assay, compound 13 exhibited a strong inhibitory effect with an IC₅₀ value of 32.8 μM, whereas the IC₅₀ value of the positive control, acarbose, was 387.8 μM.

Reductive Radical Annulation Strategy toward Bicyclo[3.2.1]octanes: Synthesis of ent-Kaurane and Beyerane Diterpenoids

Here we report a general [3 + 2] radical annulation that allows the facile construction of bicyclo[3.2.1]octane motifs in ent-kaurane- and beyerane-type diterpenoids. This radical annulation is difficult to control but was realized by harnessing an unprecedented and counterintuitive effect of TEMPO. Eleven natural products with a wide array of oxidation states are easily prepared, demonstrating the powerful utility of this straightforward synthetic strategy.

Electrochemical ODI-[5+2] Cascade for the Syntheses of Diversely Functionalized Bicyclo[3.2.1]octane Frameworks

A metal- and hypervalent iodine reagent-free electrochemical oxidative dearomatization-induced [5+2] cycloaddition/pinacol rearrangement cascade reaction was described. The electrosynthetic method showed strong tolerance for vinylphenols, ethynylphenols, and allenylphenols, which thus enabled the rapid assembly of diversely functionalized bicyclo[3.2.1]octanes in 41-95% yields and up to >20:1 dr. This protocol could be scaled up to gram amounts and should find wide application in complex natural product synthesis.