Biomimetic Total Synthesis of ent-Penilactone A and Penilactone B

gem-Difluoro-Masked o-Quinone Methides Generated by Photocatalytic Radical (3+3) Annulation and Their (4+1) Cycloaddition with Sulfur Ylides

A visible light-promoted radical (3+3) annulation of vinyldiazo compounds and bromodifluoromethyl alkynyl ketones for the construction of gem-difluoro-masked o-quinone methides (o-QMs) is described. The reactivity of this new type of o-QM precursor is demonstrated by its (4+1) cycloaddition with sulfur ylides, affording monofluorinated aromatic benzofurans by the elimination of HBr without external oxidants.

Natural Acetogenins, Chatenaytrienins-1, -2, -3 and -4, Mitochondrial Potential Uncouplers and Autophagy Inducers-Promising Anticancer Agents

The present paper details the complete stereoselective synthesis of four natural acetogenins, chatenaytrienins-1, -2, -3 and -4, previously isolated from the roots of fruit trees of the family Annonaceae (A. nutans and A. muricata), as an inseparable mixture. The novel organometallic reactions, developed by the authors, of Ti-catalyzed cross-cyclomagnesiation of O-containing and aliphatic allenes using available Grignard reagents were applied at the key stage of synthesis. We have studied the biological activity of the synthesized individual chatenaytrienins-1, -2, -3 and -4 in vitro, including their cytotoxicity in a panel of tumor lines and their ability to induce apoptosis, affect the cell cycle and mitochondria, and activate the main apoptotic signaling pathways in the cell, applying modern approaches of flow cytometry and multiplex analysis with Luminex xMAP technology. It has been shown that chatenaytrienins affect mitochondria by uncoupling the processes of mitochondrial respiration, causing the accumulation of ROS ions, followed by the initiation of apoptosis. The most likely mechanism for the death of cortical neurons from the consumption of tea from the seeds of Annona fruit is long-term chronic hypoxia, which leads to the development of an atypical form of Parkinson's disease that is characteristic of the indigenous inhabitants of Guam and New Caledonia.

Natural Products from Chilean and Antarctic Marine Fungi and Their Biomedical Relevance

Fungi are a prolific source of bioactive molecules. During the past few decades, many bioactive natural products have been isolated from marine fungi. Chile is a country with 6435 Km of coastline along the Pacific Ocean and houses a unique fungal biodiversity. This review summarizes the field of fungal natural products isolated from Antarctic and Chilean marine environments and their biological activities.

Reactivity of quinone methides with carbenes generated from α-diazocarbonyl compounds and related compounds

Over the years, quinone methides have broadly been applied in synthesis and biological systems for synthesizing heterocyclic compounds and biologically active molecules. In this feature article, we have discussed the novel and uncovered reactivity of o-quinone methides, p-quinone methides, aza-o-quinone methides, and indolyl-2-methides with carbenes generated from α-diazocarbonyl compounds and related compounds. Two in situ-generated transient intermediates undergo cycloannulation reactions, metathesis-type reactions, 1,6-conjugate addition reactions, cyclopropanation reactions, and many other transformations to access nitrogen- and oxygen-containing heterocyclic compounds and beyond. The reactivity of quinone methides and carbenes is observed in various metal catalysts, Brønsted-acids, Lewis acids, phase transfer catalysts, additives, and visible-light-induced transformations.

Biosynthesis of Fungal Natural Products Involving Two Separate Pathway Crosstalk

Fungal natural products (NPs) usually possess complicated structures, exhibit satisfactory bioactivities, and are an outstanding source of drug leads, such as the cholesterol-lowering drug lovastatin and the immunosuppressive drug mycophenolic acid. The fungal NPs biosynthetic genes are always arranged within one single biosynthetic gene cluster (BGC). However, a rare but fascinating phenomenon that a crosstalk between two separate BGCs is indispensable to some fungal dimeric NPs biosynthesis has attracted increasing attention. The hybridization of two separate BGCs not only increases the structural complexity and chemical diversity of fungal NPs, but also expands the scope of bioactivities. More importantly, the underlying mechanism for this hybridization process is poorly understood and needs further exploration, especially the determination of BGCs for each building block construction and the identification of enzyme(s) catalyzing the two biosynthetic precursors coupling processes such as Diels–Alder cycloaddition and Michael addition. In this review, we summarized the fungal NPs produced by functional crosstalk of two discrete BGCs, and highlighted their biosynthetic processes, which might shed new light on genome mining for fungal NPs with unprecedented frameworks, and provide valuable insights into the investigation of mysterious biosynthetic mechanisms of fungal dimeric NPs which are constructed by collaboration of two separate BGCs.

Inhibitory Potentiality of Secondary Metabolites Extracted from Marine Fungus Target on Avian Influenza Virus-A Subtype H5N8 (Neuraminidase) and H5N1 (Nucleoprotein): A Rational Virtual Screening

Highly contagious avian influenza virus’ (AIV) subtypes, including H5N1 and H5N8 are considered as serious threats for poultry industry. Despite its severity, treatment and mitigation attempts are fall into baffling. Though a few approved anti-influenza medications are available, the M2 channel blockers amantadine and rimantadine, as well as the neuraminidase inhibitor oseltamivir are being less effective due to widespread drug resistance. To cope up with these circumstances, scientists have found nucleoprotein as a novice drug targeting site for H5N1. Hence, the current study used a rational screening method to find the best candidates for nucleoprotein inhibitors of H5N1 subtype and neuraminidase inhibitors for H5N8 subtype against pathogenic AIV. Finding the best candidates, molecular docking method and computational pharmacokinetics and pharmacology was developed to estimate the potential of the multi-targeting fungal-derived natural compounds for the development of drug. Chevalone E compound was found as the best inhibitor for both nucleoprotein and neuraminidase of H5N1 and H5N8 subtypes respectively, whereas, Brevione F and Brocazine-A for nucleoprotein with Penilactone-A and Aspergifuranone for neuraminidase. In case of drug prediction, the study recommends Estramustine and Iloprost against both nucleoprotein and neuraminidase. Besides these, Butorphanol, Desvenlafaxine, Zidovudine and Nadolol are the best drug candidates for nucleoprotein inhibitors, meanwhile, Sitaxentan, Ergoloid mesylate, Capecitabine and Fenoterol act as speculated candidates against neuraminidase.

Seawater fungi-derived compound screening to identify novel small molecules against dengue virus NS5 methyltransferase and NS2B/NS3 protease

Dengue fever is a virus spread by mosquitoes that has no effective treatment or vaccination. Several dengue cases combined with the current COVID-19 pandemic, exacerbates this problem. Two proteins, NS5 methyltransferase and NS2B/NS3 primary protease complexes, are crucial for dengue viral replication and are the target sites for antiviral development. Thus, this study screened published literature and identified 162 marine fungus-derived compounds with active bioavailability. Following Lipinski's rules and antiviral property prediction, 41 compounds were selected for docking with NS5 methyltransferase and NS2B/NS3 protease (PDB ID: 6IZZ and 2FOM) to evaluate compounds that could stop the action of dengue viral protein complexes. To find the best candidates, computational ADME, toxicity, and drug target prediction were performed to estimate the potential of the multi-targeting fungal-derived natural compounds. Analyzing the result from 41 compounds, Chevalone E (−13.5 kcal/mol), Sterolic acid (−10.3 kcal/mol) showed higher binding energy against dengue NS2B/NS3 protease; meanwhile, Chevalone E (−12.0 kcal/mol), Brevione K (−7.4 kcal/mol), had greater binding affinity against NS5 methyltransferase. Consequently, this study suggests that Chevalone E is an effective inhibitor of NS5 methyltransferase and NS2B/NS3 protease. Ligand-based virtual screening from DrugBank was utilized to predict biologically active small compounds against dengue virus NS2B/NS3 major protease and NS5 methyltransferase. Both licensed medications, estramustine (DB01196) and quinestrol (DB04575), were found to be similar to Chevalone E, with prediction scores of 0.818 and 0.856, respectively. In addition, cholic acid (DB02659), acitretin (DB00459), and mupirocin (DB00410) are similar to Sterolic acid, zidovudine (DB00495), imipenem (DB01598), and nadolol (DB01203) are similar to Brocazine A, and budesonide (DB01222) and colchicine (DB01394) are related to Brevione K. These findings suggest that these could be feasible dengue virus treatment options, meaning that more research is needed.

Divergent Asymmetric Reactions of ortho-Quinone Methides with α-Thiocyanato Indanones for the Synthesis of Spiro- and Fused-Indanones

Reported in this work is a water triggered chemo-divergent enantioselective spiro-annulation and cascade reaction of ortho-quinone methides (o-QMs) with α-thiocyanato indanones catalyzed by a chiral organic base. In the case of spiro-annulation, the use of trace amount of water as additive is critical to achieve high enantioselectivity (up to 96 % ee). We found that a cascade reaction was enabled by just tuning the ratio of water in solvent. Accordingly, two new highly efficient asymmetric reactions for the divergent synthesis of spiro- and fused-indanone scaffolds with excellent enantioselectivities (up to 99 % ee) were developed. Mechanistic investigations suggest that interfacial hydrogen bonding may play an important role in achieving the switchable reaction pathways.

Access to molecular complexity. Multicomponent reactions involving five or more components

The evaluation of the significance of a chemical transformation addresses many factors, including such important characteristics as the number of chemical bonds formed in one step, the reaction time, labour intensity, the cost of reactants and catalysts and so on. The amount of waste produced in the reaction has also gained increasing importance in recent years. Multicomponent reactions (MCRs) occupy a special place as a synthetic tool in modern organic chemistry. These reactions allow the synthesis of target products with complex structures, minimizing labour costs. This review summarizes the literature on multicomponent reactions involving five or more components. The data in the review are classified according to the number of reactants participating in the reaction and the types of reactions. It is worth noting that in some cases, these transformations can be a part of a domino process, making this classification difficult, if not impossible. The structural diversity of the reaction products greatly increases with increasing number of components involved in the MCR, which becomes virtually unlimited when using combinations of MCRs. This review highlights the main trends of past decades in the field of MCRs. The last two decades have witnessed an explosive growth in the number of publications in this area of chemistry.

The bibliography includes 309 references.

Harnessing the biocatalytic potential of iron- and α-ketoglutarate-dependent dioxygenases in natural product total synthesis

Covering: up to the end of 2019Iron- and α-ketoglutarate-dependent dioxygenases (Fe/αKGs) represent a versatile and intriguing enzyme family by virtue of their ability to directly functionalize unactivated C-H bonds at the cost of αKG and O2. Fe/αKGs play an important role in the biosynthesis of natural products, valuable biologically active secondary metabolites frequently pursued as drug leads. The field of natural product total synthesis seeks to contruct these molecules as effeciently as possible, although natural products continue to challenge chemists due to their intricate structural complexity. Chemoenzymatic approaches seek to remedy the shortcomings of traditional synthetic methodology by combining Nature's biosynthetic machinery with traditional chemical methods to efficiently construct natural products. Although other oxygenase families have been widely employed for this purpose, Fe/αKGs remain underutilized. The following review will cover recent chemoenzymatic total syntheses involving Fe/αKG enzymes. Additionally, related information involving natural product biosynthesis, methods development, and non-chemoenzymatic total syntheses will be discussed to inform retrosynthetic logic and synthetic design.

Chemoreactive-Inspired Discovery of Influenza A Virus Dual Inhibitor to Block Hemagglutinin-Mediated Adsorption and Membrane Fusion

Owing to the emergence of drug resistance and high morbidity and mortality, the need for novel anti-influenza A virus (IAV) drugs with divergent targets is highly sought after. Herein, we reveal the discovery of an anti-IAV agent as a dual inhibitor to block hemagglutinin-mediated adsorption and membrane fusion using a chemoreactive ortho-quinone methide (o-QM) equivalent. Based on the o-QM equivalent nonenzymatically multipotent behavior, we created a series of clavatol-derived pseudo-natural products and found that penindolone (PND), a new diclavatol indole adduct, exhibited potent and broad-spectrum anti-IAV activities with low risk of inducing drug resistance. Distinct from current anti-IAV drugs, PND possesses a novel scaffold and is the first IAV inhibitor targeting both HA1 and HA2 subunits of virus hemagglutinin to dually block the IAV adsorption and membrane fusion process. More importantly, intranasal and oral administration of PND can protect mice against IAV-induced death and weight loss, superior to the effects of the clinical drug oseltamivir. Thus, the use of chemoreactive intermediates could expand our understanding of chemical diversity and aid in the development of anti-IAV drugs with novel targets.

Rational Engineered C-Acyltransferase Transforms Sterically Demanding Acyl Donors

The biocatalytic Friedel-Crafts acylation has been identified recently for the acetylation of resorcinol using activated acetic acid esters for the synthesis of acetophenone derivatives catalyzed by an acyltransferase. Because the wild-type enzyme is limited to acetic and propionic derivatives as the substrate, variants were designed to extend the substrate scope of this enzyme. By rational protein engineering, the key residue in the active site was identified which can be replaced to allow binding of bulkier acyl moieties. The single-point variant F148V enabled the transformation of previously inaccessible medium chain length alkyl and alkoxyalkyl carboxylic esters as donor substrates with up to 99% conversion and up to >99% isolated yield.

Increasing Structural Diversity of Natural Products by Michael Addition with ortho-Quinone Methide as the Acceptor

The active form of clavatol, ortho-quinone methide, can be generated from hydroxyclavatol in an aqueous system and used as a highly reactive intermediate for coupling with diverse natural products under very mild conditions. These include flavonoids, hydroxynaphthalenes, coumarins, xanthones, anthraquinones, phloroglucinols, phenolic acids, indole derivatives, tyrosine analogues, and quinolines. The clavatol moiety was mainly attached via C-C bonds to the ortho- or para-positions of phenolic hydroxyl/amino groups and the C2-position of the indole ring.

Chemoenzymatic o-Quinone Methide Formation

Generation of reactive intermediates and interception of these fleeting species under physiological conditions is a common strategy employed by Nature to build molecular complexity. However, selective formation of these species under mild conditions using classical synthetic techniques is an outstanding challenge. Here, we demonstrate the utility of biocatalysis in generating o-quinone methide intermediates with precise chemoselectivity under mild, aqueous conditions. Specifically, α-ketoglutarate-dependent non-heme iron enzymes, CitB and ClaD, are employed to selectively modify benzylic C-H bonds of o-cresol substrates. In this transformation, biocatalytic hydroxylation of a benzylic C-H bond affords a benzylic alcohol product which, under the aqueous reaction conditions, is in equilibrium with the corresponding o-quinone methide. o-Quinone methide interception by a nucleophile or a dienophile allows for one-pot conversion of benzylic C-H bonds into C-C, C-N, C-O, and C-S bonds in chemoenzymatic cascades on preparative scale. The chemoselectivity and mild nature of this platform is showcased here by the selective modification of peptides and chemoenzymatic synthesis of the chroman natural product (-)-xyloketal D.

Ti-Catalyzed Cross-Cyclomagnesiation of 1,2-Dienes in the Total Z,Z,Z-Stereoselective Synthesis of Natural Acetogenin-Chatenaytrienin-1

The first total synthesis of natural acetogenin, chatenaytrienin-1, was performed in 10 steps and in 41% overall yield using cross-cyclomagnesiation of (6Z)-heptadeca-1,2,6-triene and trideca-11,12-dien-1-ol tetrahydropyran acetal with EtMgBr in the presence of Mg metal and the Cp₂TiCl₂ catalyst (10 mol %) as the key step of the synthesis.

A visible light photoredox catalyzed carbon radical-mediated generation of ortho-quinone methides for 2,3-dihydrobenzofuran synthesis

A visible light photoredox-catalyzed carbon radical-mediated strategy for in situ formation of ortho-quinone methides from 2-vinyl phenols towards 2,3-dihydrobenzofuran synthesis is described.

An organic-base catalyzed asymmetric 1,4-addition of tritylthiol to in situ generated aza-o-quinone methides at the H2O/DCM interface

Highly enantioselective organocatalytic Michael addition of tritylthiol to N-o-QM intermediates generated in situ at the H₂O/DCM interface is presented.

Iodine-Mediated Direct Generation of o-Quinone Methides at Room Temperature: A Facile Protocol for the Synthesis of ortho-Hydroxybenzyl Thioethers

An iodine-mediated preparation of ortho-quinone methides (o-QMs) from ortho-hydroxybenzyl alcohols by a C-O bond scission strategy is described. The in situ generated o-QMs were then employed for C-S bond formation by thio-Michael addition of thiols to provide the ortho-hydroxybenzyl thioethers (o-HBT) in moderate to excellent yields.

Base-mediated diastereoselective [4 + 3] annulation of in situ generated ortho-quinone methides with C,N-cyclic azomethine imines

An efficient [4 + 3] annulation of 2-(1-tosylalkyl)phenols with C,N-cyclic azomethine imines via in situ generation of ortho-quinone methides (o-QMs) under mild basic reaction conditions is disclosed, furnishing biologically interesting seven-membered heterocyclic compounds with moderate to good yields and excellent diastereoselectivities. A gram-scale reaction is performed to demonstrate the potential in industrial application and two transition states are proposed to rationalize the outstanding diastereoselectivity.

Secondary Metabolites from Polar Organisms

Polar organisms have been found to develop unique defences against the extreme environment environment, leading to the biosynthesis of novel molecules with diverse bioactivities. This review covers the 219 novel natural products described since 2001, from the Arctic and the Antarctic microoganisms, lichen, moss and marine faunas. The structures of the new compounds and details of the source organism, along with any relevant biological activities are presented. Where reported, synthetic and biosynthetic studies on the polar metabolites have also been included.

Short Route to the Total Synthesis of Natural Muricadienin and Investigation of Its Cytotoxic Properties

An original synthesis of the acetogenin muricadienin, the bioprecursor of solamin, has been developed. The key step in the five-step 41% overall yield synthesis is the catalytic cross-cyclomagnesiation reaction of functionally substituted 1,2-dienes with EtMgBr in the presence of Cp2TiCl2 and magnesium metal. It has been demonstrated for the first time that muricadienin exhibits a moderate in vitro inhibitory activity against topoisomerases I and IIα, key cell cycle enzymes. Using flow cytometry, muricadienin was shown to have high cytotoxicity toward the HEK293 kidney cancer cells (IC50 0.39 μM).

The Mosaic of Rottlerin

The first total synthesis of rottlerin is described. The methodology allows the development of potential novel protein kinase C δ (PKCδ) analogues for better treatment of various diseases. Kamalachalcone A and dimeric rottlerin were synthesized in a very practical and economical way using FeCl3 as a catalyst.

The domestication of ortho-quinone methides

An ortho-quinone methide (o-QM) is a highly reactive chemical motif harnessed by nature for a variety of purposes. Given its extraordinary reactivity and biological importance, it is surprising how few applications within organic synthesis exist. We speculate that their widespread use has been slowed by the complications that surround the preparation of their precursors, the harsh generation methods, and the omission of this stratagem from computer databases due to its ephemeral nature.

About a decade ago, we discovered a mild anionic triggering procedure to generate transitory o-QMs at low temperature from readily available salicylaldehydes, particularly OBoc derivatives. This novel reaction cascade included both the o-QM formation and the subsequent consumption reaction. The overall transformation was initiated by the addition of the organometallic reagent, usually a Grignard reagent, which resulted in the formation of a benzyloxy alkoxide. Boc migration from the neighboring phenol produced a magnesium phenoxide that we supposed underwent β-elimination of the transferred Boc residue to form an o-QM for immediate further reactions. Moreover, the cascade proved controllable through careful manipulation of metallic and temperature levers so that it could be paused, stopped, or restarted at various intermediates and stages. This new level of domestication enabled us to deploy o-QMs for the first time in a range of applications including diastereocontrolled reactions.

This sequence ultimately could be performed in either multipot or single pot processes. The subsequent reaction of the fleeting o-QM intermediates included the 1,4-conjugate additions that led to unbranched or branched ortho-alkyl substituted phenols and Diels–Alder reactions that provided 4-unsubstituted or 4-substituted benzopyrans and chroman ketals. The latter cycloadducts were obtained for the first time with outstanding diastereocontrol. In addition, the steric effects of the newly created stereocenters in subsequent reactions of chroman ketals and acetals were studied and proved predictable. Through the use of a chiral auxiliary, Diels–Alder products were deployed in numerous enantioselective reactions including several complex natural products syntheses. In this Account, we summarize our efforts, which we hope have contributed to the synthetic renaissance for this venerable species.

Recent advances in deep-sea natural products

Covering: 2009 to 2013. This review covers the 188 novel marine natural products described since 2008, from deep-water (50->5000 m) marine fauna including bryozoa, chordata, cnidaria, echinodermata, microorganisms, mollusca and porifera. The structures of the new compounds and details of the source organism, depth of collection and country of origin are presented, along with any relevant biological activities of the metabolites. Where reported, synthetic studies on the deep-sea natural products have also been included. Most strikingly, 75% of the compounds were reported to possess bioactivity, with almost half exhibiting low micromolar cytotoxicity towards a range of human cancer cell lines, along with a significant increase in the number of microbial deep-sea natural products reported.

Peniphenones A-D from the mangrove fungus Penicillium dipodomyicola HN4-3A as inhibitors of Mycobacterium tuberculosis phosphatase MptpB

A pair of unusual benzannulated 6,6-spiroketal enantiomers [(-)-1 and (+)-1] and three new biogenetically related compounds (2-4), together with two known related analogues (5 and 6), have been isolated from a mangrove fungus, Penicillium dipodomyicola HN4-3A. Their structures were elucidated on the basis of spectroscopic analysis (1D and 2D NMR data) and X-ray crystallography. The absolute configurations of enantiomers (-)-1 and (+)-1 were determined using quantum chemical calculations of the electronic circular dichroic (ECD) spectra. Compounds 2 and 3 exhibited strong inhibitory activity against Mycobacterium tuberculosis protein tyrosine phosphatase B (MptpB) with IC50 values of 0.16±0.02 and 1.37±0.05 μM, respectively.

Recent advances in deep-sea natural products

Review of deep-sea natural products covering the five-year period 2009–2013.