Enantioselective Total Synthesis of Mandelalide A and Isomandelalide A: Discovery of a Cytotoxic Ring-Expanded Isomer

Synthesis of the C13-C27 Fragment of Madeirolide A Using Visible-Light-Promoted Radical Cyclization

The convergent synthesis of a fully elaborated C13-C27 fragment of madeirolide A has been achieved. The key features of the synthesis include the stereocontrolled construction of both the THF and THP rings via visible-light-induced iridium-catalyzed radical cyclization and the late-stage union of the two oxacyclic subunits through nickel-catalyzed decarboxylative cross-coupling.

A Quarter Century of Marine Biodiscovery in Algoa Bay, South Africa

Algoa Bay, the largest crenulate bay on the southeastern coast of South Africa, is currently one of the most well-studied marine ecosystems in southern Africa. A plethora of endemic marine invertebrates inhabits the benthic reefs on the western edge of the Bay in close proximity to South Africa's sixth largest city. Over the past 25 years, South African marine natural products chemists, together with international collaborators from the US National Cancer Institute and other US institutions, have focused their attention on Algoa Bay's benthic marine invertebrates as a potential source of new anticancer compounds. This review commemorates a quarter of a century of marine biodiscovery in Algoa Bay and presents the structures and bioactivities of 49 new and 36 known specialized metabolites isolated from two molluscs, eight ascidians, and six sponges. Thirty-nine of these compounds were cytotoxic to cancer cells in vitro with 20 exhibiting moderate to potent cytotoxicity. Six other compounds exhibited antimicrobial activity. Foremost among the potential anticancer compounds is mandelalide A (38) from the Algoa Bay ascidian Lissoclinum species.

The Marine-Derived Macrolactone Mandelalide A Is an Indirect Activator of AMPK

The mandelalides are complex macrolactone natural products with distinct macrocycle motifs and a bioactivity profile that is heavily influenced by compound glycosylation. Mandelalides A and B are direct inhibitors of mitochondrial ATP synthase (complex V) and therefore more toxic to mammalian cells with an oxidative metabolic phenotype. To provide further insight into the pharmacology of the mandelalides, we studied the AMP-activated protein kinase (AMPK) energy stress pathway and report that mandelalide A is an indirect activator of AMPK. Wild-type mouse embryonic fibroblasts (MEFs) and representative human non-small cell lung cancer (NSCLC) cells showed statistically significant increases in phospho-AMPK (Thr172) and phospho-ACC (Ser79) in response to mandelalide A. Mandelalide L, which also harbors an A-type macrocycle, induced similar increases in phospho-AMPK (Thr172) and phospho-ACC (Ser79) in U87-MG glioblastoma cells. In contrast, MEFs co-treated with an AMPK inhibitor (dorsomorphin), AMPKα-null MEFs, or NSCLC cells lacking liver kinase B1 (LKB1) lacked this activity. Mandelalide A was significantly more cytotoxic to AMPKα-null MEFs than wild-type cells, suggesting that AMPK activation serves as a protective response to mandelalide-induced depletion of cellular ATP. However, LKB1 status alone was not predictive of the antiproliferative effects of mandelalide A against NSCLC cells. When EGFR status was considered, erlotinib and mandelalide A showed strong cytotoxic synergy in combination against erlotinib-resistant 11-18 NSCLC cells but not against erlotinib-sensitive PC-9 cells. Finally, prolonged exposures rendered mandelalide A, a potent and efficacious cytotoxin, against a panel of human glioblastoma cell types regardless of the underlying metabolic phenotype of the cell. These results add biological relevance to the mandelalide series and provide the basis for their further pre-clinical evaluation as ATP synthase inhibitors and secondary activators of AMPK.

Catalytic Enantioselective Allylation of Acetylenic Aldehydes by Chiral Phosphoric Acid/Transition Metal Cooperative Catalysis: Formal Synthesis of Fostriecin

An enantioselective allylation of silyl-substituted acetylenic aldehydes by chiral phosphoric acid (CPA)/transition metal cooperative catalysis was developed. Enantioenriched homoallylic propargyl alcohols were obtained in good yields with excellent enantioselectivities (>99% ee) under mild conditions. Moreover, the shortest formal synthesis of fostriecin was achieved by the present enantioselective allylation protocol as the key step. The known intermediate of fostriecin reported by McDonald and co-worker was synthesized in only nine steps in 39% total yield.

Natural products and other inhibitors of F1FO ATP synthase

F₁F_O ATP synthase is responsible for the production of >95% of all ATP synthesis within the cell. Dysregulation of its expression, activity or localization is linked to various human diseases including cancer, diabetes, and Alzheimer's and Parkinson's disease. In addition, ATP synthase is a novel and viable drug target for the development of antimicrobials as evidenced by bedaquiline, which was approved in 2012 for the treatment of tuberculosis. Historically, natural products have been a rich source of ATP synthase inhibitors that help unravel the role of F₁F_O ATP synthase in cellular bioenergetics. During the last decade, new modulators of ATP synthase have been discovered through the isolation of novel natural products as well as through a ligand-based drug design process. In addition, new data has been obtained with regards to the structure and function of ATP synthase under physiological and pathological conditions. Crystal structure studies have provided a significant insight into the rotary function of the enzyme and may provide additional opportunities to design a new generation of inhibitors. This review provides an update on recently discovered ATP synthase modulators as well as an update on existing scaffolds.

Second-Generation Synthesis of the Northern Fragment of Mandelalide A: Role of π-Stacking on Sharpless Dihydroxylation of cis-Enynes

The development of a π-stacking-based approach for increased stereoselectivity in Sharpless asymmetric and diastereomeric dihydroxylation of cis-enynes is disclosed. The use of neighboring, electron-rich benzoate esters proved key to the success of this process. Density functional theory study suggests that the substrate benzoate ester group rigidifies the dihydroxylation transition states by forming a favorable π-stacking interaction in both Major-TS and Minor-TS. The energetic preference for the Major-TS was found in part because of the favorable eclipsing conformation of the alkene substituent as opposed to the disfavored bisecting conformation found in the Minor-TS. The application to a second-generation synthesis of the C15-C24 northern portion of mandelalide A is demonstrated.

Catalytic Enantioselective Allylations of Acetylenic Aldehydes via 2-Propanol-Mediated Reductive Coupling

Cyclometalated π-allyliridium C,O-benzoates modified by ( S)-SEGPHOS or ( S)-Cl,OMe-BIPHEP catalyze enantioselective 2-propanol-mediated reductive couplings of diverse nonmetallic allyl pronucleophiles with the acetylenic aldehyde TIPSC≡CCHO. Absolute stereochemistries of the resulting secondary homoallylic-propargylic alcohols were assigned using Rychnovsky's competing enantioselective conversion method.

Ascidian Toxins with Potential for Drug Development

Ascidians (tunicates) are invertebrate chordates, and prolific producers of a wide variety of biologically active secondary metabolites from cyclic peptides to aromatic alkaloids. Several of these compounds have properties which make them candidates for potential new drugs to treat diseases such as cancer. Many of these natural products are not produced by the ascidians themselves, rather by their associated symbionts. This review will focus mainly on the mechanism of action of important classes of cytotoxic molecules isolated from ascidians. These toxins affect DNA transcription, protein translation, drug efflux pumps, signaling pathways and the cytoskeleton. Two ascidian compounds have already found applications in the treatment of cancer and others are being investigated for their potential in cancer, neurodegenerative and other diseases.

Skeletal Optimization of Cytotoxic Lipidic Dialkynylcarbinols

In line with a recent study of the pharmacological potential of bioinspired synthetic acetylenic lipids, after identification of the terminal dialkynylcarbinol (DAC) and butadiynyl alkynylcarbinol (BAC) moieties as functional antitumor pharmacophoric units, this work specifically addresses the issue of carbon backbone length. A systematic variation of the aliphatic chain length was thus carried out in both the DAC and BAC series. The critical impact of the length of the lipidic skeleton was first confirmed in the racemic series, with the highest cytotoxic activity observed for C₁₇ to C₁₈ backbones. Enantiomerically enriched samples were prepared by asymmetric synthesis of the optimal C₁₈ DAC and C₁₇ BAC derivatives. Samples with upgraded enantiomeric purity were alternatively produced by enzymatic kinetic resolution. Eutomers possessing the S configuration displayed cytotoxicity IC₅₀ values as low as 15 nm against HCT116 cancer cells, the highest level of activity reached to date in this series.

Synthetic Access to the Mandelalide Family of Macrolides: Development of an Anion Relay Chemistry Strategy

The mandelalides comprise a family of structurally complex marine macrolides that display significant cytotoxicity against several human cancer cell lines. Presented here is a full account on the development of an Anion Relay Chemistry (ARC) strategy for the total synthesis of (-)-mandelalides A and L, the two most potent members of the mandelalide family. The design and implementation of a three-component type II ARC/cross-coupling protocol and a four-component type I ARC union permits rapid access respectively to the key tetrahydrofuran and tetrahydropyran structural motifs of these natural products. Other highlights of the synthesis include an osmium-catalyzed oxidative cyclization of an allylic 1,3-diol, a mild Yamaguchi esterification to unite the northern and southern hemispheres, and a late-stage Heck macrocyclization. Synthetic mandelalides A and L displayed potent cytotoxicity against human HeLa cervical cancer cells (EC50, 1.3 and 3.1 nM, respectively). This synthetic approach also provides access to several highly potent non-natural mandelalide analogs, including a biotin-tagged mandelalide probe for future biological investigation.

A Chimeric Styrene Monooxygenase with Increased Efficiency in Asymmetric Biocatalytic Epoxidation

The styrene monooxygenase (SMO) system from Pseudomonas sp. consists of two enzymes (StyA and StyB). StyB catalyses the reduction of FAD at the expense of NADH. After the transfer of FADH2 from StyB to StyA, reaction with O2 generates FAD-OOH, which is the epoxidising agent. The wastage of redox equivalents due to partial diffusive transfer of FADH2 , the insolubility of recombinant StyB and the impossibility of expressing StyA and StyB in a 1:1 molar ratio reduce the catalytic efficiency of the natural system. Herein we present a chimeric SMO (Fus-SMO) that was obtained by genetic fusion of StyA and StyB through a flexible linker. Thanks to a combination of: 1) balanced and improved expression levels of reductase and epoxidase units, and 2) intrinsically higher specific epoxidation activity of Fus-SMO in some cases, Escherichia coli cells expressing Fus-SMO possess about 50 % higher activity for the epoxidation of styrene derivatives than E. coli cells coexpressing StyA and StyB as discrete enzymes. The epoxidation activity of purified Fus-SMO was up to three times higher than that of the two-component StyA/StyB (1:1, molar ratio) system and up to 110 times higher than that of the natural fused SMO. Determination of coupling efficiency and study of the influence of O2 pressure were also performed. Finally, Fus-SMO and formate dehydrogenase were coexpressed in E. coli and applied as a self-sufficient biocatalytic system for epoxidation on greater than 500 mg scale.

Marine natural products

Covering: 2016. Previous review: Nat. Prod. Rep., 2017, 34, 235-294This review covers the literature published in 2016 for marine natural products (MNPs), with 757 citations (643 for the period January to December 2016) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1277 in 432 papers for 2016), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included.

Increased Biosynthetic Gene Dosage in a Genome-Reduced Defensive Bacterial Symbiont

Secondary metabolites, which are small-molecule organic compounds produced by living organisms, provide or inspire drugs for many different diseases. These natural products have evolved over millions of years to provide a survival benefit to the producing organism and often display potent biological activity with important therapeutic applications. For instance, defensive compounds in the environment may be cytotoxic to eukaryotic cells, a property exploitable for cancer treatment. Here, we describe the genome of an uncultured symbiotic bacterium that makes such a cytotoxic metabolite. This symbiont is losing genes that do not endow a selective advantage in a hospitable host environment. Secondary metabolism genes, however, are repeated multiple times in the genome, directly demonstrating their selective advantage. This finding shows the strength of selective forces in symbiotic relationships and suggests that uncultured bacteria in such relationships should be targeted for drug discovery efforts.

A symbiotic lifestyle frequently results in genome reduction in bacteria; the isolation of small populations promotes genetic drift and the fixation of deletions and deleterious mutations over time. Transitions in lifestyle, including host restriction or adaptation to an intracellular habitat, are thought to precipitate a wave of sequence degradation events and consequent proliferation of pseudogenes. We describe here a verrucomicrobial symbiont of the tunicate Lissoclinum sp. that appears to be undergoing such a transition, with low coding density and many identifiable pseudogenes. However, despite the overall drive toward genome reduction, this symbiont maintains seven copies of a large polyketide synthase (PKS) pathway for the mandelalides (mnd), cytotoxic compounds that likely constitute a chemical defense for the host. There is evidence of ongoing degradation in a small number of these repeats—including variable borders, internal deletions, and single nucleotide polymorphisms (SNPs). However, the gene dosage of most of the pathway is increased at least 5-fold. Correspondingly, this single pathway accounts for 19% of the genome by length and 25.8% of the coding capacity. This increased gene dosage in the face of generalized sequence degradation and genome reduction suggests that mnd genes are under strong purifying selection and are important to the symbiotic relationship.

IMPORTANCE Secondary metabolites, which are small-molecule organic compounds produced by living organisms, provide or inspire drugs for many different diseases. These natural products have evolved over millions of years to provide a survival benefit to the producing organism and often display potent biological activity with important therapeutic applications. For instance, defensive compounds in the environment may be cytotoxic to eukaryotic cells, a property exploitable for cancer treatment. Here, we describe the genome of an uncultured symbiotic bacterium that makes such a cytotoxic metabolite. This symbiont is losing genes that do not endow a selective advantage in a hospitable host environment. Secondary metabolism genes, however, are repeated multiple times in the genome, directly demonstrating their selective advantage. This finding shows the strength of selective forces in symbiotic relationships and suggests that uncultured bacteria in such relationships should be targeted for drug discovery efforts.

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New Mandelalides Expand a Macrolide Series of Mitochondrial Inhibitors

Mandelalides A-D (1-4) are macrocyclic polyketides known to have an unusual bioactivity profile influenced by compound glycosylation and growth phase of cultured cells. The isolation and characterization of additional natural congeners, mandelalides E-L (5-12), and the supply of synthetic compounds 1 and 12, as well as seco-mandelalide A methyl ester (13), have now facilitated mechanism of action and structure-activity relationship studies. Glycosylated mandelalides are effective inhibitors of aerobic respiration in living cells. Macrolides 1 and 2 inhibit mitochondrial function similar to oligomycin A and apoptolidin A, selective inhibitors of the mammalian ATP synthase (complex V). 1 inhibits ATP synthase activity from isolated mitochondria and triggers caspase-dependent apoptosis in HeLa cells, which are more sensitive to inhibition by 1 in the presence of the glycolysis inhibitor 2-deoxyglucose. Thus, mandelalide cytotoxicity depends on basal metabolic phenotype; cells with an oxidative phenotype are most likely to be inhibited by the mandelalides.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.

Stereoselective, Ag-Catalyzed Cyclizations To Access Polysubstituted Pyran Ring Systems: Synthesis of C1-C12 Subunit of Madeirolide A

The exploration into the scope of a silver-catalyzed cyclization (AgCC) of propargyl benzoates for accessing pyran ring systems has been reported. The impact of the degree of substitution, nature of the substitution on the carbon backbone/benzoate moiety, and stereochemistry has been evaluated. The application of this methodology to the synthesis of the C1-C12 southern fragment of madeirolide A is disclosed.

Total Synthesis of (-)-Mandelalide A Exploiting Anion Relay Chemistry (ARC): Identification of a Type II ARC/CuCN Cross-Coupling Protocol

Anion relay chemistry (ARC), an effective, multicomponent union tactic, was successfully employed for the total synthesis of the highly cytotoxic marine macrolide (-)-mandelalide A (1). The northern hemisphere was constructed via a new type II ARC/CuCN cross-coupling tactic, while the southern hemisphere was secured via a highly efficient four-component type I ARC union. Importantly, the synthesis of 1 showcases ARC as a rapid, scalable coupling strategy for the union of simple readily available building blocks to access diverse complex molecular fragments with excellent stereochemical control.

Augmentation of productivity in olefin cross-metathesis: maleic acid does the trick!

Why use the protected esters when the free acids result in better catalytic performances?