Stereoselective synthesis of (-)-acanthoic acid

Development of a Synthetic Platform for Ent-Pimaranes Reveals their Potential as Novel Non-Redox Active Ferroptosis Inhibitors

We present a comprehensive account on the evolution of a synthetic platform for a subfamily of ent-pimaranes. For the most complex member, norflickinflimiod C, five distinct strategies relying on either cationic or radical polyene cyclizations to construct the requisite tricyclic carbon scaffold were explored. Insights from early and late stage oxidative and reductive dearomatization studies ultimately led to a mild, rhodium-catalyzed arene hydrogenation for the final synthetic route. A Sharpless asymmetric dihydroxylation was found to be suitable to render the platform enantioselective and diversification of a late-stage key intermediate culminated in the total synthesis of eight ent-pimaranes in 11-16 steps. These compounds were found to inhibit the formation of pro-inflammatory leukotrienes and other 5-lipoxygenase products. Notably, three ent-pimaranes exhibited low micromolar, non-redox active ferroptosis inhibition with remarkable structural specificity.

Asymmetric synthesis of the fully functionalized six-membered A-ring of siphonol A

A synthetic study toward the construction of the fully functionalized six-membered A-ring of siphonol A is described. The salient features include the introduction of a six-membered ring system through a HWE reaction, the construction of a stereocenter at C5 via a hetero-Diels-Alder reaction, and the installation of the fully functionalized six-membered A-ring of siphonol A through photolytic decarboxylation.

Fragment Coupling Approach to Diaporthein B

Pimarane diterpenes are produced by a diverse array of plants, fungi, and bacteria. Many members of this family possess antimicrobial and antiproliferative activities. The pimarane diterpenes are characterized by a tricyclic carbon scaffold comprising three fused six-membered rings and at least three quaternary centers. Here, we describe two convergent, fragment-based strategies toward the synthesis of diaporthein B (3), one of the most highly oxidized pimarane diterpenes. The first approach provided access to the tricyclic carbon scaffold of the target and featured a highly diastereoselective fragment coupling, a novel carbonylative Stille cross-coupling to directly access an α-hydroxyketone from a vinyl iodide, and a tandem aldol cyclization-deprotection cascade. The second route utilized a diastereoselective 1,4-addition of a silyloxyfuran to an unsaturated ketone, followed by an epoxidation-ring opening sequence, to access a highly oxidized intermediate containing two elaborated cyclohexane rings. The chemistry developed herein may ultimately be useful in an eventual synthesis of this class of natural products.

Acanthoic acid, unique potential pimaradiene diterpene isolated from Acanthopanax koreanum Nakai (Araliaceae): A review on its pharmacology, molecular mechanism, and structural modification

Acanthoic acid (AA) is a pimaradiene diterpene isolated from the root bark of Acanthopanax koreanum Nakai (Araliaceae) with a wide range of pharmacological activities, including anti-cancer, anti-inflammatory, anti-diabetes, liver protection, gastrointestinal protection, and cardiovascular protection. In addition, AA promotes its pharmacological effects by targeting liver X receptors (LXRs), nuclear factor-kappa B (NF-κB), Toll-Like Receptor 4 (TLR4) and IL-1 receptor-associated kinase (IRAK) signaling pathways, or AMP-activated protein kinase (AMPK) signaling pathway, etc. Also, some studies focus on the structural modification of AA to improve its pharmacological activities. The review summarizes the pharmacological activities, molecular mechanism, and the structural modification of AA, which might supply information for the development of AA in the future.

Acanthoic acid suppresses lipin1/2 via TLR4 and IRAK4 signalling pathways in EtOH- and lipopolysaccharide-induced hepatic lipogenesis

Objectives

In alcoholic liver disease, alcohol and lipopolysaccharide (LPS) are major stimulation factors of hepatic lipogenesis. Our objective was to determine the protective mechanism of acanthoic acid (AA) in EtOH- and LPS-induced hepatic lipogenesis.

Methods

HSC-T6 cells were treated with ethanol (200 mm) plus LPS (1 μg/ml) for 1 h, followed by AA (10 or 20 μm) for another 6 h. C57BL/6 mice were pretreated with of AA (20 and 40 mg/kg) or equal volume of saline and then exposed to three doses of ethanol (5 g/kg body weight) within 24 h. The mice were sacrificed at 6 h after the last ethanol dosing.

Key findings

Acanthoic acid significantly decreased the expressions of α-SMA, collagen-I, SREBP-1, and lipin1/2 induced, also decreased fat droplets caused by EtOH/LPS. AA treatment decreased the protein expressions of TLR4, CD14, IRAK4, TRAF3, p-TAK1 and NF-κB increased by EtOH/LPS on HSC cells. Results in vivo were consistent with results in vitro.

Conclusions

Our data demonstrated that AA might modulate hepatic fibrosis and lipid deposition in HSC-T6 cell stimulated with ethanol combined with LPS by decreasing lipin1/2 via TLR4 and IRAK4 signalling pathways, and AA might be considered as a potential therapeutic candidate for alcoholic liver disease.

Synthetic Strategies Toward the Decalin Motif of Maklamicin and Related Spirotetronates

Herein we describe a scalable approach to the decalin moiety of maklamicin. Key to the synthesis is an intramolecular Diels-Alder (IMDA) reaction that proceeds via an endo-axial transition state to generate the desired stereochemistry. We explored the diastereoselectivity of the IMDA reaction as a function of both chiral catalysis and acyclic precursor stereochemistry.

Anti-inflammatory actions of acanthoic acid-related diterpenes involve activation of the PI3K p110γ/δ subunits and inhibition of NF-κB

The effect of acanthoic acid analogs on the response to proinflammatory challenge was investigated. Some pimarane diterpenes are known activators of the LXRαβ nuclear receptors, but we show here that they also exert a rapid, potent, and selective activation of the p110γ and p110δ subunits of PI3K. Combination of these effects results in an important attenuation of the global transcriptional response to LPS in macrophages. PI3K/Akt activation leads to inhibition of the LPS-dependent stimulation of IKK/NF-κB and p38 and ERK MAPKs. Macrophages from LXRαβ-deficient mice exhibited an inhibition of these pathways similar to the corresponding wild-type cells. Silencing or inhibition of p110γ/δ suppressed the effect of these diterpenes (DTPs) on IKK/NF-κB and MAPKs signaling. Taken together, these data show a multitarget anti-inflammatory mechanism by these DTPs including a selective activation of PI3K isoenzymes.

Formal synthesis of (-)-englerin A and cytotoxicity studies of truncated englerins

An efficient formal synthesis of (-)-englerin A (1) is reported. The target molecule is a recently isolated guaiane sesquiterpene that possesses highly potent and selective activity against renal cancer cell-lines. Our enantioselective strategy involved the construction of the BC ring system of compound 1 through a Rh(II)-catalyzed [4+3] cycloaddition reaction followed by subsequent attachment of the A ring through an intramolecular aldol condensation reaction. As such, this strategy allows the synthesis of truncated englerins. Evaluation of these analogues with the A498 renal cancer cell-line suggested that the A ring of englerin is crucial to its antiproliferative activity. Moreover, evaluation of these analogues led to the identification of potent growth-inhibitors of CEM cells with GI(50) values in the range 1-3 μM.

Enantioselective synthesis of the ABC ring motif of norzoanthamine based on asymmetric Robinson annulation reactions

An enantioselective strategy for the synthesis of tetracyclic motif 5, representing the northern fragment of norzoanthamine, is presented. Key to the strategy is the use of two asymmetric Robinson annulation reactions that produce the tricyclic ABC ring system with excellent stereoselectivity. Further functionalization at the periphery of the C ring produces compound 5 containing six contiguous stereocenters of the natural product.

Synthesis of (-)-callicarpenal, a potent arthropod-repellent

Callicarpenal (1), a natural terpenoid isolated from American beautyberry (Callicarpa americana), has shown significant repellent activities against mosquitoes, ticks and imported fire ants. Here we report our efficient synthetic approach to this natural product, and preliminary results of the mosquito biting-deterrent effects of callicarpenal as well as its synthetic precursors and related C(8)-epimers. The synthetic strategy allows rapid access to various epimers and analogues of the natural product that can be used to explore its structure-activity relationship and optimize its biological properties.

Acanthoic Acid

The title compound [systematic name: (1R,4aR,7S,8aS,10aS)-1,4a,7-trimethyl-7-vinyl-1,2,3,4,4a,6,7,8,8a,9,10,10a-dodeca­hydro­phenanthrene-1-carb­oxy­lic acid], C₂₀H₃₀O₂, is a pimarane-type diterpene extracted from Croton oblongifolius. There are two independent mol­ecules in the asymmetric unit. In both of these, the six-membered rings A, B and C adopt chair, boat and half-chair conformations, respectively. Rings A and B are trans-fused. The two mol­ecules in the asymmetric unit form O—H⋯O hydrogen-bonded R₂²(8) dimers. The absolute configuration was assigned on the basis of the published literature on analogous structures.

The biology and chemistry of the zoanthamine alkaloids

Marine natural products have long played an important role in natural products chemistry and drug discovery. Mirroring the rich variety and complicated interactions of the marine environment, the substances isolated from sea creatures tend to be incredibly diverse in both molecular structure and biological activity. The natural products isolated from the polyps of marine zoanthids are no exception. The zoanthamine alkaloids, the first of which were isolated over 20 years ago, are of particular interest to the synthetic community because they feature a novel structural framework and exhibit a broad range of biological activities. In this Review, we summarize the major contributions to understanding the zoanthamine natural products with regard to their isolation and structure determination, as well as studies on their biological activity and total synthesis.

Diterpenoid, steroid, and triterpenoid agonists of liver X receptors from diversified terrestrial plants and marine sources

It has been demonstrated that liver X receptors (LXR) play a significant role in cholesterol homeostasis. Agonists of LXR are expected to increase cellular cholesterol efflux, lower LDL, and raise HDL levels. Screening of a natural product library of plant extracts using a LXR-SPA binding assay and bioassay-guided fractionation of a number of plant and marine gorgonian extracts led to the isolation of a number of active compounds. These included acanthoic acid (1) and alcohol (2), viperidone (3), polycarpol (4), rosacea acid (5), a cycloartane derivative (6), a new cycloartane analogue (7), betulinic acid (8), and gorgostane derivatives (9, 10, and 11). Of these compounds, 1, 4, and 11 exhibited potent binding affinity for alpha-receptor with IC(50) values of 0.25, 0.12, and 0.07 microM, respectively. Functionally they also showed strong coactivator association stimulation for LXRalpha receptor with EC50 values of 0.18, 0.03, and 0.05 microM, respectively. They also exhibited 15-, 8-, and 13-fold induction of the alpha-receptor in a transactivation assay in HEK-293 cells, respectively. In general these compounds were selective for the LXR alpha-receptor over the beta-receptor in all assays and were much better stimulators of the alpha-receptor than the endogenous steroid ligands.

A new family of synthetic diterpenes that regulates cytokine synthesis by inhibiting IkappaBalpha phosphorylation

The synthesis and the biological evaluation of a new family diterpenes are presented. The synthetic studies were inspired by the structural framework of acanthoic acid (1) and yielded a family of compounds that were evaluated as anti-inflammatory agents. Among them, compounds 2, 10, 12, and 16 exhibited a very low nonspecific cytotoxicity and inhibited the synthesis of TNF-alpha with greater than 65 % efficacy at low micromolar concentrations. Cytokine-specificity studies revealed that these compounds also inhibited the synthesis of the proinflammatory cytokines IL-1beta and IL-6, while inhibition of IL-1ra and IL-8 synthesis was marginal and only occurred at high concentrations. Further studies, through EMSA and Western blot analyses, indicated that these compounds decreased the extent of phosphorylation of IkappaBalpha; this suggests that they exert their anti-inflammatory profile by inhibiting NF-kappaB-mediated cytokine synthesis. These findings imply that these diterpenes represent promising leads for the development of novel anti-inflammatory agents.

Diastereoselectivities Realized in the Amino Acid Catalyzed Aldol Cyclizations of Triketo Acetonides of Differing Ring Size

A study designed to assess the diastereoselectivity of the intramolecular aldol reaction of two differently sized monocyclic 1,3-diketones bearing a chiral, oxygenated side chain has been undertaken. The cyclizations were brought about under catalysis by pyrrolidine, a series of D- and L-amino acids including proline, and several proline derivatives. The levels of selectivity were found to be consistently higher with the six-membered ring system than its cycloheptane counterpart.

Correction of the structure of a new sesquiterpene from Cistus creticus ssp. creticus

In an attempt to identify the structure of a sesquiterpene from Cistus creticus ssp. creticus proposed in the literature as 1,1,4a,6-tetramethyl-5-methylene-1,2,3,4,4alpha,5,8,8alpha-octahydronaphthalene, the synthesis of its cis isomer 2 was carried out in 11 steps and 9.5% yield. Comparison of the spectra of 2 and those reported earlier for the synthetic trans isomer 1 with the spectral profile of the isolated natural product indicated that the latter was not compatible with either 1 or 2. The correct structure was assigned, by detailed spectroscopic analysis of the natural product, as 6-isopropenyl-4,4a-dimethyl-1,2,3,4,4a,5,6,7-octahydronaphthalene (3).

Synthesis of a novel family of diterpenes and their evaluation as anti-inflammatory agents

The synthesis and biological evaluation of a new family of diterpenes, represented by structures 2 and 3, is presented. These compounds constitute isomeric analogues of acanthoic acid (1) and were examined as potent anti-inflammatory agents. Among them, methyl ester 12 exhibited a low non-specific cytotoxicity, inhibited TNF-alpha synthesis and displayed good specificity in suppressing cytokine expression.

Enantioselective synthesis of the antiinflammatory agent (-)-acanthoic acid

An enantioselective synthesis of the potent antiinflammatory agent (-)-acanthoic acid (1) is described. The successful strategy departs from (-)-Wieland-Miescher ketone (10), which is readily available in both enantiomeric forms and constitutes the starting point toward a fully functionalized AB ring system of 1. Conditions were developed for a regioselective double alkylation at the C4 center of the A ring, which produced compound 32 as a single stereoisomer. Construction of the C ring of 1 was accomplished via a Diels-Alder reaction between sulfur-containing diene 43 and methacrolein (36), which after desulfurization and further functionalization yielded synthetic acanthoic acid. The described synthesis confirms the proposed stereochemistry of the natural product and represents a fully stereocontrolled entry into an underexplored class of biologically active diterpenes.