Total Synthesis of the Potent Anticancer Aglaia Metabolites (−)-Silvestrol and (−)-Episilvestrol and the Active Analogue (−)-4′-Desmethoxyepisilvestrol

Silvestrol, a potent anticancer agent with unfavourable pharmacokinetics: Current knowledge on its pharmacological properties and future directions for the development of novel drugs

Cancer remains a leading cause of death, with increasing incidence. Conventional treatments offer limited efficacy and cause significant side effects, hence novel drugs with improved pharmacological properties and safety are required. Silvestrol (SLV) is a flavagline derived from some plants of the Aglaia genus that has shown potent anticancer effects, warranting further study. Despite its efficacy in inhibiting the growth of several types of cancer cells, SLV is characterized by an unfavorable pharmacokinetics that hamper its use as a drug. A consistent research over the recent years has led to develop novel SLV derivatives with comparable pharmacodynamics and an ameliorated pharmacokinetic profile, demonstrating potential applications in the clinical management of cancer. This comprehensive review aims to highlight the most recent data available on SLV and its synthetic derivatives, addressing their pharmacological profile and therapeutic potential in cancer treatment. A systematic literature review of both in vitro and in vivo studies focusing on anticancer effects, pharmacodynamics, and pharmacokinetics of these compounds is presented. Overall, literature data highlight that rationale chemical modifications of SLV are critical for the development of novel drugs with high efficacy on a broad variety of cancers and improved bioavailability in vivo. Nevertheless, SLV analogues need to be further studied to better understand their mechanisms of action, which can be partially different to SLV. Furthermore, clinical research is still required to assess their efficacy in humans and their safety.

Halogenated Rocaglate Derivatives: Pan-antiviral Agents against Hepatitis E Virus and Emerging Viruses

The synthesis of a library of halogenated rocaglate derivatives belonging to the flavagline class of natural products, of which silvestrol is the most prominent example, is reported. Their antiviral activity and cytotoxicity profile against a wide range of pathogenic viruses, including hepatitis E, Chikungunya, Rift Valley Fever virus and SARS-CoV-2, were determined. The incorporation of halogen substituents at positions 4', 6 and 8 was shown to have a significant effect on the antiviral activity of rocaglates, some of which even showed enhanced activity compared to CR-31-B and silvestrol.

Total Synthesis and Biological Evaluation of Clavatadines C-E

We described herein the application of a convergent and protecting-group avoidant approach that led to the first total synthesis of the marine natural products clavatadine D (4) and E (5), and the second total synthesis of clavatadine C (3). In each case, a key amide-coupling afforded an immediate precursor of each natural product in a rapid manner from structurally similar western and eastern portions that derived from an ester of l-tyrosine and butane-1,4-diamine, respectively. A deprotection step free of detectable byproducts cleanly provided the remaining known members of the clavatadine family of natural products. Each total synthesis required five steps (longest linear sequence) with overall yields of 30-37%, 26-39%, and 28-50% for clavatadine C (3), D (4), and E (5), respectively. A screen of their potential anticancer activity against the NCI-60 cell line panel revealed cytotoxicity levels up to 38% across a broad spectrum of tumor types. Although clavatadine C (3) was relatively benign, clavatadine D (4) exhibited 20-38% growth inhibition against a wide array of cancer cell types including leukemia, non-small-cell lung, colon, ovarian, and breast. Clavatadine E (5) was active against two types of human brain tumors.

Comparative phytochemistry of flavaglines (= rocaglamides), a group of highly bioactive flavolignans from Aglaia species (Meliaceae)

Flavaglines are formed by cycloaddition of a flavonoid nucleus with a cinnamic acid moiety representing a typical chemical character of the genus Aglaia of the family Meliaceae. Based on biosynthetic considerations 148 derivatives are grouped together into three skeletal types representing 77 cyclopenta[b]benzofurans, 61 cyclopenta[bc]benzopyrans, and 10 benzo[b]oxepines. Apart from different hydroxy, methoxy, and methylenedioxy groups of the aromatic rings, important structural variation is created by different substitutions and stereochemistries of the central cyclopentane ring. Putrescine-derived bisamides constitute important building blocks occurring as cyclic 2-aminopyrrolidines or in an open-chained form, and are involved in the formation of pyrimidinone flavaglines. Regarding the central role of cinnamic acid in the formation of the basic skeleton, rocagloic acid represents a biosynthetic precursor from which aglafoline- and rocaglamide-type cyclopentabenzofurans can be derived, while those of the rocaglaol-type are the result of decarboxylation. Broad-based comparison revealed characteristic substitution trends which contribute as chemical markers to natural delimitation and grouping of taxonomically problematic Aglaia species. A wide variety of biological activities ranges from insecticidal, antifungal, antiprotozoal, and anti-inflammatory properties, especially to pronounced anticancer and antiviral activities. The high insecticidal activity of flavaglines is comparable with that of the well-known natural insecticide azadirachtin. Comparative feeding experiments informed about structure-activity relationships and exhibited different substitutions of the cyclopentane ring essential for insecticidal activity. Parallel studies on the antiproliferative activity of flavaglines in various tumor cell lines revealed similar structural prerequisites that let expect corresponding molecular mechanisms. An important structural modification with very high cytotoxic potency was found in the benzofuran silvestrol characterized by an unusual dioxanyloxy subunit. It possessed comparable cytotoxicity to that of the natural anticancer compounds paclitaxel (Taxol®) and camptothecin without effecting normal cells. The primary effect was the inhibition of protein synthesis by binding to the translation initiation factor eIF4A, an ATP-dependent DEAD-box RNA helicase. Flavaglines were also shown to bind to prohibitins (PHB) responsible for regulation of important signaling pathways, and to inhibit the transcriptional factor HSF1 deeply involved in metabolic programming, survival, and proliferation of cancer cells. Flavaglines were shown to be not only promising anticancer agents but gained now also high expectations as agents against emerging RNA viruses like SARS-CoV-2. Targeting the helicase eIF4A with flavaglines was recently described as pan-viral strategy for minimizing the impact of future RNA virus pandemics.

α-Ketol and α-iminol rearrangements in synthetic organic and biosynthetic reactions

In the presence of a suitable acid or base, α-hydroxyaldehydes, ketones, and imines can undergo isomerization that features the 1,2-shift of an alkyl or aryl group. In the process, the hydroxy group is converted to a carbonyl and the aldehyde/ketone or imine is converted to an alcohol or amine. Such α-ketol/α-iminol rearrangements are used in a wide variety of synthetic applications including asymmetric synthesis, tandem reactions, and the total synthesis and biosynthesis of natural products. This review explores the use of α-ketol rearrangements in these contexts over the past two decades.

Synthesis of cis-Oriented Vicinal Diphenylethylenes through a Lewis Acid-Promoted Annulation of Oxotriphenylhexanoates

This study explores the synthesis of cyclic cis-vicinal phenyl ethylenes from oxotriphenylhexanoates. The reaction is a BBr₃-promoted cyclization of 1,6-ketoesters (1) to five-membered diketo compounds (2). The synthesis is interesting as it constitutes one of the few examples of modular stereoselective synthesis of structures with a cis-oriented vicinal diphenylethylene. The core structure of 2 can be smoothly derivatized, which makes it a promising synthetic building block for further stereoselective synthetic applications.

The rocaglate CR-31-B (-) inhibits SARS-CoV-2 replication at non-cytotoxic, low nanomolar concentrations in vitro and ex vivo

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, a severe respiratory disease with varying clinical presentations and outcomes, and responsible for a major pandemic that started in early 2020. With no vaccines or effective antiviral treatments available, the quest for novel therapeutic solutions remains an urgent priority. Rocaglates, a class of plant-derived cyclopenta[b]benzofurans, exhibit broad-spectrum antiviral activity against multiple RNA viruses including coronaviruses. Specifically, rocaglates inhibit eukaryotic initiation factor 4A (eIF4A)-dependent mRNA translation initiation, resulting in strongly reduced viral RNA translation. Here, we assessed the antiviral activity of the synthetic rocaglate CR-31-B (-) against SARS-CoV-2 using both in vitro and ex vivo cell culture models. In Vero E6 cells, CR-31-B (-) inhibited SARS-CoV-2 replication with an EC₅₀ of ~1.8 nM. In primary human airway epithelial cells, CR-31-B (-) reduced viral titers to undetectable levels at a concentration of 100 nM. Reduced virus reproduction was accompanied by substantially reduced viral protein accumulation and replication/transcription complex formation. The data reveal a potent anti-SARS-CoV-2 activity by CR-31-B (-), corroborating previous results obtained for other coronaviruses and supporting the idea that rocaglates may be used in first-line antiviral intervention strategies against novel and emerging RNA virus outbreaks.

Phytochemistry and biological activities of Aglaia species

Aglaia is the largest genus in the Meliaceae family (also known as Mahagoni in Indonesia), consisting of over 150 species, of which 65 are indigenous to Indonesia. These species spread through the tropical regions, especially Southeast Asia as well as the Nothern part of Australia, and have been used in traditional medicine for the treatment of several diseases. However, preliminary chemical researches commenced in 1965, where dammarane-type triterpenoids, aglaiol was isolated, and the structure was determined by chemical reaction and spectroscopic methods. Several studies have been carried out on the stembark, bark, leaves, seeds and leaves in the last fifty five years, and about 291 metabolites have been isolated from the sesquiterpenoid, diterpenoid, triterpenoid, limonoid, steroid, lignan, and alkaloid groups, as well as flavagline, which known to be the largest. This specifically amounts to 34% of Aglaia species, reported to show cytotoxic and insecticidal potentials, and also the tendency for use as chemical markers for this species. The extracts and compounds obtained from Aglaia species are evaluated for potential biological activities, including cytotoxicity, insecticidal, anti-inflammatory, antifungal, molluscicidal, antituberculosis and antiviral effects. In addition, flavagline (rocaglamide) derivatives have been confirmed to exhibit exceptional cytotoxicity, and are, thus, considered lead compounds for further development. Therefore, the results support the concept of utilizing Aglaia species as a potential source for the production of biologically active compounds.

All-carbon [3 + 2] cycloaddition in natural product synthesis

Many natural products possess interesting medicinal properties that arise from their intriguing chemical structures. The highly-substituted carbocycle is one of the most common structural features in many structurally complicated natural products. However, the construction of highly-substituted, stereo-congested, five-membered carbocycles containing all-carbon quaternary center(s) is, at present, a distinct challenge in modern synthetic chemistry, which can be accessed through the all-carbon [3 + 2] cycloaddition. More importantly, the all-carbon [3 + 2] cycloaddition can forge vicinal all-carbon quaternary centers in a single step and has been demonstrated in the synthesis of complex natural products. In this review, we present the development of all-carbon [3 + 2] cycloadditions and illustrate their application in natural product synthesis reported in the last decade covering 2011-2020 (inclusive).

Targeting Protein Translation by Rocaglamide and Didesmethylrocaglamide to Treat MPNST and Other Sarcomas

Malignant peripheral nerve sheath tumors (MPNST) frequently overexpress eukaryotic initiation factor 4F components, and the eIF4A inhibitor silvestrol potently suppresses MPNST growth. However, silvestrol has suboptimal drug-like properties, including a bulky structure, poor oral bioavailability (<2%), sensitivity to MDR1 efflux, and pulmonary toxicity in dogs. We compared ten silvestrol-related rocaglates lacking the dioxanyl ring and found that didesmethylrocaglamide (DDR) and rocaglamide (Roc) had growth-inhibitory activity comparable with silvestrol. Structure-activity relationship analysis revealed that the dioxanyl ring present in silvestrol was dispensable for, but may enhance, cytotoxicity. Both DDR and Roc arrested MPNST cells at G2-M, increased the sub-G1 population, induced cleavage of caspases and PARP, and elevated the levels of the DNA-damage response marker γH2A.X, while decreasing the expression of AKT and ERK1/2, consistent with translation inhibition. Unlike silvestrol, DDR and Roc were not sensitive to MDR1 inhibition. Pharmacokinetic analysis confirmed that Roc had 50% oral bioavailability. Importantly, Roc, when administered intraperitoneally or orally, showed potent antitumor effects in an orthotopic MPNST mouse model and did not induce pulmonary toxicity in dogs as found with silvestrol. Treated tumors displayed degenerative changes and had more cleaved caspase-3-positive cells, indicative of increased apoptosis. Furthermore, Roc effectively suppressed the growth of osteosarcoma, Ewing sarcoma, and rhabdomyosarcoma cells and patient-derived xenografts. Both Roc- and DDR-treated sarcoma cells showed decreased levels of multiple oncogenic kinases, including insulin-like growth factor-1 receptor. The more favorable drug-like properties of DDR and Roc and the potent antitumor activity of Roc suggest that these rocaglamides could become viable treatments for MPNST and other sarcomas.

The Mitochondrial Apoptotic Effectors BAX/BAK Activate Caspase-3 and -7 to Trigger NLRP3 Inflammasome and Caspase-8 Driven IL-1β Activation

Intrinsic apoptosis resulting from BAX/BAK-mediated mitochondrial membrane damage is regarded as immunologically silent. We show here that in macrophages, BAX/BAK activation results in inhibitor of apoptosis (IAP) protein degradation to promote caspase-8-mediated activation of IL-1β. Furthermore, BAX/BAK signaling induces a parallel pathway to NLRP3 inflammasome-mediated caspase-1-dependent IL-1β maturation that requires potassium efflux. Remarkably, following BAX/BAK activation, the apoptotic executioner caspases, caspase-3 and -7, act upstream of both caspase-8 and NLRP3-induced IL-1β maturation and secretion. Conversely, the pyroptotic cell death effectors gasdermin D and gasdermin E are not essential for BAX/BAK-induced IL-1β release. These findings highlight that innate immune cells undergoing BAX/BAK-mediated apoptosis have the capacity to generate pro-inflammatory signals and provide an explanation as to why IL-1β activation is often associated with cellular stress, such as during chemotherapy.

Structurally Modified Cyclopenta[b]benzofuran Analogues Isolated from Aglaia perviridis

Four new cyclopenta[b]benzofuran derivatives based on an unprecedented carbon skeleton (1-4), with a dihydrofuran ring fused to dioxanyl and aryl rings, along with a new structural analogue (5) of 5‴-episilvestrol (episilvestrol, 7), were isolated from an aqueous extract of a large-scale re-collection of the roots of Aglaia perviridis collected in Vietnam. Compound 5 demonstrated mutarotation in solution due to the presence of a hydroxy group at C-2‴, leading to the isolation of a racemic mixture, despite being purified on a chiral-phase HPLC column. Silvestrol (6) and episilvestrol (7) were isolated from the most potently cytotoxic chloroform subfraction of the roots. All new structures were elucidated using 1D and 2D NMR, HRESIMS, IR, UV, and ECD spectroscopic data. Of the five newly isolated compounds, only compound 5 exhibited cytotoxic activity against a human colon cancer (HT-29) and human prostate cancer cell line (PC-3), with IC50 values of 2.3 μM in both cases. The isolated compounds (1-5) double the number of dioxanyl ring-containing rocaglate analogues reported to date from Aglaia species and present additional information on the structural requirements for cancer cell line cytotoxicity within this compound class.

Synthesis and antiproliferative activity of flavonoid triazolyl glycosides

Sixteen flavonoid triazolyl glycosides4–19were synthesized in good yieldsviaCu(I)-catalyzed azide-alkyne cycloadditions of terminal alkynes of flavonoids1–3with acetylated sugar azides followed by deacetylation with sodium methoxide in anhydrous methanol. The antiproliferative activity of the synthesized compounds against three human cancer cell lines (Hela, HCC1954 and SK-OV-3)in vitrowas evaluated. Flavonoids1,2and flavonoid triazolyl glycosides7,12,17exhibit potent antiproliferative activity against these cancer cell lines.

Total Synthesis of the Flavonoid Natural Product Houttuynoid A

The first total synthesis of the antiviral flavonoid houttuynoid A (1) has been achieved from aryl ketone 6 and benzofuran aldehyde 5 in nine linear steps. The C₆-C₃-C₆ structure of the flavonoid was synthesized by an I₂-catalyzed oxa-Michael addition of a chalcone intermediate, generated by the Claisen-Schmidt condensation of 5 and 6. This work provides a method for the synthesis of houttuynoids and provides a reference for the synthesis of the remaining members of the houttuynoid family.

Tunable ESIPT reaction and antioxidant activities of 3-hydroxyflavone and its derivatives by altering atomic electronegativity

3-HTF easiest to carry out ESIPT reaction would exhibit the highest efficient antioxidant activity among the three compounds.

The Search for Anticancer Agents from Tropical Plants

Many of the clinically used anticancer agents in Western medicine are derived from secondary metabolites found in terrestrial microbes, marine organisms, and higher plants, with additional compounds of this type being currently in clinical trials. If plants are taken specifically, it is generally agreed that the prospects of encountering enhanced small organic-molecule chemical diversity are better if tropical rather than temperate species are investigated in drug discovery efforts. Plant collection in tropical source countries requires considerable preparation and organization to conduct in a responsible manner that abides by the provisions of the 1992 Rio Convention of Biological Diversity and the 2010 Nagoya Protocol on Access to Genetic Resources. Correct taxonomic identifications and enhanced procedures for processing and documenting plant samples when collected in often difficult terrain are required. Phytochemical aspects of the work involve solvent fractionation, known compound dereplication, preliminary in vitro testing, and prioritization, leading to "activity-guided fractionation", compound structure determination, and analog development. Further evaluation of lead compounds requires solubility, formulation, preliminary pharmacokinetics, and in vivo testing in suitable models. Covering the work of the authors carried out in two sequential multidisciplinary, multi-institutional research projects, examples of very promising compounds discovered from plants acquired from Africa, Southeast Asia, the Americas, and the Caribbean region, and with potential anticancer activity will be mentioned. These include plant secondary metabolites of the diphyllin lignan, cyclopenta[b]benzofuran, triterpenoid, and tropane alkaloid types.

Total Synthesis of the Sesquiterpenoid Periconianone A Based on a Postulated Biogenesis

The first enantioselective total synthesis of the complex tricarbocyclic sesquiterpenoid periconianone A based on a postulated biogenesis is reported. Key elements of the synthetic route include the use of an isopropenyl group as a removable directing group for stereoselective synthesis, a sequence featuring a Rh-mediated O-H insertion/[3,3]-sigmatropic rearrangement and subsequent α-ketol rearrangement, and a late stage aldol reaction to furnish the complex cage-like framework.

Discovery of Anticancer Agents of Diverse Natural Origin

Recent progress is described in an ongoing collaborative multidisciplinary research project directed towards the purification, structural characterization, chemical modification, and biological evaluation of new potential natural product anticancer agents obtained from a diverse group of organisms, comprising tropical plants, aquatic and terrestrial cyanobacteria, and filamentous fungi. Information is provided on how these organisms are collected and processed. The types of bioassays are indicated in which initial extracts, chromatographic fractions, and purified isolated compounds of these acquisitions are tested. Several promising biologically active lead compounds from each major organism class investigated are described, and these may be seen to be representative of a very wide chemical diversity.

Cytotoxic Rocaglate Derivatives from Leaves of Aglaia perviridis

Rocaglates are a series of structurally complex secondary metabolites with considerable cytotoxicity that have been isolated from plants of the Aglaia genus (Meliaceae). A new rocaglate (aglapervirisin A, 1) and its eight new biosynthetic precursors of rocaglate (aglapervirisins B-J, 2-9) together with five known compounds, were isolated from the leaves of Aglaia perviridis. Their structures were elucidated based on a joint effort of spectroscopic methods [IR, UV, MS, ECD, 1D- and 2D-NMR, HRESIMS], chemical conversion and single-crystal X-ray diffraction. Among these isolates, three (1, 10-11) were silvestrols, a rare subtype rocaglates, exhibiting notable cytotoxicity against four human tumor cell lines, with IC50 values between 8.0 and 15.0 nM. Aglapervirisin A (1) induces cell cycle arrest at the G2/M-phase boundary at concentration 10 nM accompanied by reductions in the expression levels of Cdc2 and Cdc25C in HepG2 cells after 72h co-incubation, and further induces the apoptosis of HepG2 cells at concentrations over 160 nM.

Total synthesis of a biotinylated rocaglate: Selective targeting of the translation factors eIF4AI/II

The total synthesis of a biotinylated derivative of methyl rocaglate is described. This compound was accessed from synthetic methyl rocaglate (2) via formation of the propargyl amide and subsequent click reaction with a biotin azide. Affinity purification revealed that biotinylated rocaglate (8) and methyl rocaglate (2) bind with high specificity to translation factors eIF4AI/II. This remarkable selectivity is in line with that found for the more complex rocaglate silvestrol (3).

Synthesis of each enantiomer of rocaglamide by means of a palladium(0)-catalyzed Nazarov-type cyclization

A recently reported Pd(0)-catalyzed asymmetric Nazarov-type cyclization has been successfully applied in the key step of the first catalytic asymmetric total synthesis of (-)-rocaglamide (natural) and (+)-rocaglamide. The stereochemistry at the C3 position that controls the stereochemistry of all other stereocenters is determined in the cyclization step. This versatile and modular synthesis proceeds from simple reagents.

(3+2)-Cycloaddition Reactions of Oxyallyl Cations

The (3+2)-cycloaddition reaction involving oxyallyl cations has proven to be a versatile and efficient approach for the construction of five-membered carbo- and heterocycles, which are prevalent frameworks in natural products and pharmaceuticals. The following article will provide a brief summary of recent disclosures on this process featuring chemo-, regio- and diastereoselective oxyallyl cycloadditions with both electron-rich and electron-deficient 2π partners.

Delitschiapyrone A, a pyrone-naphthalenone adduct bearing a new pentacyclic ring system from the leaf-associated fungus Delitschia sp. FL1581

Delitschiapyrone A (1), an α-pyrone–naphthalenone adduct with an unprecedented pentacyclic ring system, was isolated from a solid culture of the leaf-associated fungus Delitschia sp. FL1581. The structure of 1 was elucidated by spectroscopic analysis and X-ray crystallography, and its absolute configuration was defined by experimental and calculated ECD. Biosynthetically, the unique 6/6/5/7/6 pentacyclic core of 1 may be formed by an intermolecular Diels–Alder-type addition of the precursors derived from (1′R)-2′,3′-dihydropyrenocine C (2) and 6-ethyl-2,7-dimethoxyjuglone (3) found to co-occur with 1 in this fungus.

Semisynthesis of Polymethoxyflavonoids from Naringin and Hesperidin

Polymethoxyflavonoids (PMFs) possess important biological activities, notably as anticancer agents. Semisynthesis of a series of PMFs were performed by glycoside hydrolysis, dehydrogenation, bromination, aromatic nucleophilic substitution, O-methylation, dimethyldioxirane oxidation and regioselective demethylation, starting from abundant and inexpensive natural sources naringin and hesperidin. A new synthetic method for selective methylation using CuBr catalysed and microwave-assisted reaction was developed, and the dimethyl dioxirane oxidation of flavones to flavonols was much improved. The new semisynthetic route has the advantages of easy availability of starting materials, simple operation and good yields.

Simplified silvestrol analogues with potent cytotoxic activity

The complex natural products silvestrol (1) and episilvestrol (2) are inhibitors of translation initiation through binding to the DEAD-box helicase eukaryotic initiation factor 4A (eIF4A). Both compounds are potently cytotoxic to cancer cells in vitro, and 1 has demonstrated efficacy in vivo in several xenograft cancer models. Here we show that 2 has limited plasma membrane permeability and is metabolized in liver microsomes in a manner consistent with that reported for 1. In addition, we have prepared a series of analogues of these compounds where the complex pseudo-sugar at C6 has been replaced with chemically simpler moieties to improve drug-likeness. Selected compounds from this work possess excellent activity in biochemical and cellular translation assays with potent activity against leukemia cell lines.

Evidence for a functionally relevant rocaglamide binding site on the eIF4A-RNA complex

Translation initiation is an emerging target in oncology and neurobiology indications. Naturally derived and synthetic rocaglamide scaffolds have been used to interrogate this pathway; however, there is uncertainty regarding their precise mechanism(s) of action. We exploited the genetic tractability of yeast to define the primary effect of both a natural and a synthetic rocaglamide in a cellular context and characterized the molecular target using biochemical studies and in silico modeling. Chemogenomic profiling and mutagenesis in yeast identified the eIF (eukaryotic Initiation Factor) 4A helicase homologue as the primary molecular target of rocaglamides and defined a discrete set of residues near the RNA binding motif that confer resistance to both compounds. Three of the eIF4A mutations were characterized regarding their functional consequences on activity and response to rocaglamide inhibition. These data support a model whereby rocaglamides stabilize an eIF4A-RNA interaction to either alter the level and/or impair the activity of the eIF4F complex. Furthermore, in silico modeling supports the annotation of a binding pocket delineated by the RNA substrate and the residues identified from our mutagenesis screen. As expected from the high degree of conservation of the eukaryotic translation pathway, these observations are consistent with previous observations in mammalian model systems. Importantly, we demonstrate that the chemically distinct silvestrol and synthetic rocaglamides share a common mechanism of action, which will be critical for optimization of physiologically stable derivatives. Finally, these data confirm the value of the rocaglamide scaffold for exploring the impact of translational modulation on disease.

Bioactive flavaglines and other constituents isolated from Aglaia perviridis

Eight new compounds, including two cyclopenta[b]benzopyran derivatives (1, 2), two cyclopenta[b]benzofuran derivatives (3, 4), three cycloartane triterpenoids (5-7), and an apocarotenoid (8), together with 16 known compounds, were isolated from the chloroform-soluble partitions of separate methanol extracts of a combination of the fruits, leaves, and twigs and of the roots of Aglaia perviridis collected in Vietnam. Isolation work was monitored using human colon cancer cells (HT-29) and facilitated with an LC/MS dereplication procedure. The structures of the new compounds (1-8) were determined on the basis of spectroscopic data interpretation. The Mosher ester method was employed to determine the absolute configurations of 5-7, and the absolute configuration of the 9,10-diol unit of compound 8 was established by a dimolybdenum tetraacetate [Mo2(AcO)4] induced circular dichroism procedure. Seven known rocaglate derivatives (9-15) exhibited significant cytotoxicity against the HT-29 cell line, with rocaglaol (9) being the most potent (ED50 0.0007 μM). The new compounds 2-4 were also active against this cell line, with ED50 values ranging from 0.46 to 4.7 μM. The cytotoxic compounds were evaluated against a normal colon cell line, CCD-112CoN. In addition, the new compound perviridicin B (2), three known rocaglate derivatives (9, 11, 12), and a known sesquiterpene, 2-oxaisodauc-5-en-12-al (17), showed significant NF-κB (p65) inhibitory activity in an ELISA assay.

Synthesis of biotinylated episilvestrol: highly selective targeting of the translation factors eIF4AI/II

Silvestrol (1) and episilvestrol (2) are protein synthesis inhibitors, and the former has shown efficacy in multiple mouse models of cancer; however, the selectivity of these potent cytotoxic natural products has not been described. Herein, it is demonstrated that eukaryotic initiation factors eIF4AI/II were the only proteins detected to bind silvestrol (1) and biotinylated episilvestrol (9) by affinity purification. Our study demonstrates the remarkable selectivity of these promising chemotherapeutics.

Any colour you like. Excited state and ground state proton transfer in flavonols and applications

The photoinduced and ground state proton transfer processes occurring in flavonols are responsible for their multi-wavelength emission. This peculiar behavior has touched on a wide range of research areas, ranging from biology to chemistry of materials leading, among others, to the development of fluorescent probes for physical and biophysical parameters, laser dyes, and wavelentgh shifting devices. This account aims to be a brief introduction to the multi-faceted applications of flavonols.

Synthetic silvestrol analogues as potent and selective protein synthesis inhibitors

Misregulation of protein translation plays a critical role in human cancer pathogenesis at many levels. Silvestrol, a cyclopenta[b]benzofuran natural product, blocks translation at the initiation step by interfering with assembly of the eIF4F translation complex. Silvestrol has a complex chemical structure whose functional group requirements have not been systematically investigated. Moreover, silvestrol has limited development potential due to poor druglike properties. Herein, we sought to develop a practical synthesis of key intermediates of silvestrol and explore structure-activity relationships around the C6 position. The ability of silvestrol and analogues to selectively inhibit the translation of proteins with high requirement on the translation-initiation machinery (i.e., complex 5'-untranslated region UTR) relative to simple 5'UTR was determined by a cellular reporter assay. Simplified analogues of silvestrol such as compounds 74 and 76 were shown to have similar cytotoxic potency and better ADME characteristics relative to those of silvestrol.

Translation inhibitors induce cell death by multiple mechanisms and Mcl-1 reduction is only a minor contributor

There is significant interest in treating cancers by blocking protein synthesis, to which hematological malignancies seem particularly sensitive. The translation elongation inhibitor homoharringtonine (Omacetaxine mepesuccinate) is undergoing clinical trials for chronic myeloid leukemia, whereas the translation initiation inhibitor silvestrol has shown promise in mouse models of cancer. Precisely how these compounds induce cell death is unclear, but reduction in Mcl-1, a labile pro-survival Bcl-2 family member, has been proposed to constitute the critical event. Moreover, the contribution of translation inhibitors to neutropenia and lymphopenia has not been precisely defined. Herein, we demonstrate that primary B cells and neutrophils are highly sensitive to translation inhibitors, which trigger the Bax/Bak-mediated apoptotic pathway. However, contrary to expectations, reduction of Mcl-1 did not significantly enhance cytotoxicity of these compounds, suggesting that it does not have a principal role and cautions that strong correlations do not always signify causality. On the other hand, the killing of T lymphocytes was less dependent on Bax and Bak, indicating that translation inhibitors can also induce cell death via alternative mechanisms. Indeed, loss of clonogenic survival proved to be independent of the Bax/Bak-mediated apoptosis altogether. Our findings warn of potential toxicity as these translation inhibitors are cytotoxic to many differentiated non-cycling cells.

Total synthesis of 2''',5'''-diepisilvestrol and its C1''' epimer: key structure activity relationships at C1''' and C2'''

The first total synthesis of the low-abundance natural product 2''',5'''-diepisilvestrol (4) is described. The key step involved a Mitsunobu coupling between cyclopenta[b]benzofuran phenol 7 and dioxane lactol 6. Deprotection then gave a 1:2.6 ratio of natural product 2''',5'''-diepisilvestrol (4) and its C1 epimer 1''',2''',5'''-triepisilvestrol (15) in 50% overall yield. An in vitro protein translation inhibition assay showed that 2''',5'''-diepisilvestrol (4) was considerably less active than episilvestrol (2), while the unnatural isomer 1''',2''',5'''-triepisilvestrol (15) was essentially inactive, showing that the configuration at C1''' and C2''' has a large effect on the biological activity.

Total synthesis of (±)-rocaglamide via oxidation-initiated Nazarov cyclization

This article describes the evolution of a Nazarov cyclization-based synthetic strategy targeting the anticancer, antiinflammatory, and insecticidal natural product (±)-rocaglamide. Initial pursuit of a polarized heteroaromatic Nazarov cyclization to construct the congested cyclopentane core revealed an unanticipated electronic bias in the pentadienyl cation. This reactivity was harnessed in a successful second-generation approach using an oxidation-initiated Nazarov cyclization of a heteroaryl alkoxyallene. Full details of these two approaches are given, as well as the characterization of undesired reaction pathways available to the Nazarov cyclization product. A sequence of experiments that led to an understanding of the unexpected reactivity of this key intermediate is described, which culminated in the successful total synthesis of (+)-rocaglamide.

Discovery of new anticancer agents from higher plants

Small organic molecules derived from higher plants have been one of the mainstays of cancer chemotherapy for approximately the past half a century. In the present review, selected single chemical entity natural products of plant origin and their semi-synthetic derivatives currently in clinical trials are featured as examples of new cancer chemotherapeutic drug candidates. Several more recently isolated compounds obtained from plants showing promising in vivo biological activity are also discussed in terms of their potential as anticancer agents, with many of these obtained from species that grow in tropical regions. Since extracts of only a relatively small proportion of the ca. 300,000 higher plants on earth have been screened biologically to date, bioactive compounds from plants should play an important role in future anticancer drug discovery efforts.

The synthesis of pennogenin 3-O-β-D-glucopyranosyl-(1 → 3)-[α-L-rhamnopyranosyl-(1 → 2)]-β-D-glucopyranoside

Pennogenin 3-O-β-D-glucopyranosyl-(1 → 3)-[α-L-rhamnopyranosyl-(1 → 2)]-β-D-glucopyranoside, a monodesmosidic saponin isolated from Paris polyphylla Smith var. yunnanensis with promised antitumor activities, was firstly synthesized from glucoside thiol via nine steps and with 27% overall yield.

Discovery of new anticancer agents from higher plants

Small organic molecules derived from higher plants have been one of the mainstays of cancer chemotherapy for approximately the past half a century. In the present review, selected single chemical entity natural products of plant origin and their semi-synthetic derivatives currently in clinical trials are featured as examples of new cancer chemotherapeutic drug candidates. Several more recently isolated compounds obtained from plants showing promising in vivo biological activity are also discussed in terms of their potential as anticancer agents, with many of these obtained from species that grow in tropical regions. Since extracts of only a relatively small proportion of the ca. 300,000 higher plants on earth have been screened biologically to date, bioactive compounds from plants should play an important role in future anticancer drug discovery efforts.

Synthesis of rocaglamide hydroxamates and related compounds as eukaryotic translation inhibitors: synthetic and biological studies

The rocaglates/rocaglamides are a class of natural products known to display potent anticancer activity. One such derivative, silvestrol, has shown activity comparable to taxol in certain settings. Here, we report the synthesis of various rocaglamide analogues and identification of a hydroxamate derivative (-)-9 having activity similar to silvestrol in vitro and ex vivo for inhibition of protein synthesis. We also show that (-)-9 synergizes with doxorubicin in vivo to reduce Eμ-Myc driven lymphomas.