Total synthesis and biological activity of neopeltolide and analogues

Neopeltolide and its synthetic derivatives: a promising new class of anticancer agents

Being the first or second cause of death worldwide, cancer represents the most significant clinical, social, and financial burden of any human illness. Despite recent progresses in cancer diagnosis and management, traditional cancer chemotherapies have shown several adverse side effects and loss of potency due to increased resistance. As a result, one of the current approaches is on with the search of bioactive anticancer compounds from natural sources. Neopeltolide is a marine-derived macrolide isolated from deep-water sponges collected off Jamaica's north coast. Its mechanism of action is still under research but represents a potentially promising novel drug for cancer therapy. In this review, we first illustrate the general structural characterization of neopeltolide, the semi-synthetic derivatives, and current medical applications. In addition, we reviewed its anticancer properties, primarily based on in vitro studies, and the possible clinical trials. Finally, we summarize the recent progress in the mechanism of antitumor action of neopeltolide. According to the information presented, we identified two principal challenges in the research, i) the effective dose which acts neopeltolide as an anticancer compound, and ii) to unequivocally establish the mechanism of action by which the compound exerts its antiproliferative effect.

Fukuyama reduction, Fukuyama coupling and Fukuyama-Mitsunobu alkylation: recent developments and synthetic applications

Fukuyama reaction for the synthesis of multifunctional aldehydes, secondary amines and ketones has gained considerable importance in synthetic organic chemistry because of mild reaction conditions. The use of thioesters in both Fukuyama aldehydes and ketones synthesis is highly attractive for organic chemists as they are easily accessible from corresponding carboxylic acids. Fukuyama-Mitsunobu reaction utilizes 2-nitrobenzenesulfonyl (Ns) for the protection/activation/deprotection of primary amines to afford secondary amines in good yields and high enantioselectivities. This review presents recent synthetic developments and applications of Fukuyama reaction for the synthesis of aldehydes, secondary amines and ketones.

Convergent Synthesis of 2-Oxazolone-4-carboxylates Esters by Reaction of Aldehydes with Ambivalent N-Cbz-α-Tosylglycinate Ester

N-Cbz-α-tosylglycinate ester was combined with aldehydes in a redox-neutral sequence leading to 2-oxazolone-4-carboxylates with high functional group tolerance. While the scope of the method was delineated to primary and secondary aliphatic aldehydes as well as aromatics, no racemization occurred with chiral aldehydes such as Garner's. Hitherto unknown, this process relies on the ambivalent role of N-Cbz-α-tosylglycinate ester acting as a pronucleophile.

Fluorescence-labeled neopeltolide derivatives for subcellular localization imaging

Design, synthesis and functional analysis of fluorescent derivatives of neopeltolide, an antiproliferative marine macrolide, are reported herein. Live cell imaging using the fluorescent derivatives showed rapid cellular uptake and localization within the endoplasmic reticulum as well as the mitochondria.

Design and Synthesis of Anti-Cancer Chimera Molecules Based on Marine Natural Products

In this paper, the chemical conjugation of marine natural products with other bioactive molecules for developing an advanced anti-cancer agent is described. Structural complexity and the extraordinary biological features of marine natural products have led to tremendous research in isolation, structural elucidation, synthesis, and pharmacological evaluation. In addition, this basic scientific achievement has made it possible to hybridize two or more biologically important skeletons into a single compound. The hybridization strategy has been used to identify further opportunities to overcome certain limitations, such as structural complexity, scarcity problems, poor solubility, severe toxicity, and weak potency of marine natural products for advanced development in drug discovery. Further, well-designed marine chimera molecules can function as a platform for target discovery or degradation. In this review, the design, synthesis, and biological evaluation of recent marine chimera molecules are presented.

Natural Product Neopeltolide as a Cytochrome bc1 Complex Inhibitor: Mechanism of Action and Structural Modification

The marine natural product neopeltolide was isolated from a deep-water sponge specimen of the family Neopeltidae. Neopeltolide has been proven to be a new type of inhibitor of the cytochrome bc₁ complex in the mitochondrial respiration chain. However, its detailed inhibition mechanism has remained unknown. In addition, neopeltolide is difficult to synthesize because of its very complex chemical structure. In the present work, the binding mode of neopeltolide was determined for the first time by integrating molecular docking, molecular dynamics simulations, and molecular mechanics Poisson-Boltzmann surface area calculations, which showed that neopeltolide is a Q_o site inhibitor of the bc₁ complex. Then, according to guidance via inhibitor-protein interaction analysis, structural modification was carried out with the aim to simplify the chemical structure of neopeltolide, leading to the synthesis of a series of new neopeltolide derivatives with much simpler chemical structures. The calculated binding energies (Δ G_cal) of the newly synthesized analogues correlated very well ( R² = 0.90) with their experimental binding free energies (Δ G_exp), which confirmed that the computational protocol was reliable. Compound 45, bearing a diphenyl ether fragment, was successfully designed and synthesized as the most potent candidate (IC₅₀ = 12 nM) against porcine succinate cytochrome c reductase. The molecular modeling results indicate that compound 45 formed a π-π interaction with Phe274 and two hydrogen bonds with Glu271 and His161. The present work provides a new starting point for future fungicide discovery to overcome the resistance that the existing bc₁ complex inhibitors are facing.

xCELLanalyzer: A Framework for the Analysis of Cellular Impedance Measurements for Mode of Action Discovery

Mode of action (MoA) identification of bioactive compounds is very often a challenging and time-consuming task. We used a label-free kinetic profiling method based on an impedance readout to monitor the time-dependent cellular response profiles for the interaction of bioactive natural products and other small molecules with mammalian cells. Such approaches have been rarely used so far due to the lack of data mining tools to properly capture the characteristics of the impedance curves. We developed a data analysis pipeline for the xCELLigence Real-Time Cell Analysis detection platform to process the data, assess and score their reproducibility, and provide rank-based MoA predictions for a reference set of 60 bioactive compounds. The method can reveal additional, previously unknown targets, as exemplified by the identification of tubulin-destabilizing activities of the RNA synthesis inhibitor actinomycin D and the effects on DNA replication of vioprolide A. The data analysis pipeline is based on the statistical programming language R and is available to the scientific community through a GitHub repository.

Conformation-guided analogue design identifies potential antimalarial compounds through inhibition of mitochondrial respiration

The synthesis of a 2-methyl-substituted analogue of the natural product, neopeltolide, is reported in an effort to analyze the importance of molecular conformation and ligand-target interactions in relation to biological activity. The methyl substitution was incorporated via highly diastereoselective ester enolate alkylation of a late-stage intermediate. Coupling of the oxazole sidechain provided 2-methyl-neopeltolide and synthetic neopeltolide via total synthesis. The substitution was shown to maintain the conformational preferences of its biologically active parent compound through computer modeling and NMR studies. Both compounds were shown to be potential antimalarial compounds through the inhibition of mitochondrial respiration in P. falciparum parasites.

Cobalt-Catalyzed Cross-Dehydrogenative C(sp2 )-C(sp3 ) Coupling of Oxazole/Thiazole with Ether or Cycloalkane

Direct C5-alkylation of oxazole/thiazole with ether or cycloalkane has been achieved through a cobalt-catalyzed cross-dehydrogenative coupling (CDC) process in moderate to good yields. This transformation represents the first C(sp² )-C(sp³ ) cross-coupling at the C5-position of the oxazole/thiazole via double C-H bond cleavages. Various functional groups on oxazole/thiazole substrates, as well as water and air, are well-tolerated with this concise and practical protocol, constituting straightforward access to heterocycles with great medicinal significance. A preliminary mechanism involving a radical process has also been proposed.

Target identification by image analysis

Covering: 1997 to the end of 2015Each biologically active compound induces phenotypic changes in target cells that are characteristic for its mode of action. These phenotypic alterations can be directly observed under the microscope or made visible by labelling structural elements or selected proteins of the cells with dyes. A comparison of the cellular phenotype induced by a compound of interest with the phenotypes of reference compounds with known cellular targets allows predicting its mode of action. While this approach has been successfully applied to the characterization of natural products based on a visual inspection of images, recent studies used automated microscopy and analysis software to increase speed and to reduce subjective interpretation. In this review, we give a general outline of the workflow for manual and automated image analysis, and we highlight natural products whose bacterial and eucaryotic targets could be identified through such approaches.

DDQ-promoted direct C5-alkylation of oxazoles with alkylboronic acids via palladium-catalysed C–H bond activation

The first protocol for the direct C5-alkylation of oxazoles through transition-metal-catalysed C(5)–H bond activation.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

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.

Contemporary Strategies for the Synthesis of Tetrahydropyran Derivatives: Application to Total Synthesis of Neopeltolide, a Marine Macrolide Natural Product

Tetrahydropyrans are structural motifs that are abundantly present in a range of biologically important marine natural products. As such, significant efforts have been paid to the development of efficient and versatile methods for the synthesis of tetrahydropyran derivatives. Neopeltolide, a potent antiproliferative marine natural product, has been an attractive target compound for synthetic chemists because of its complex structure comprised of a 14-membered macrolactone embedded with a tetrahydropyran ring, and twenty total and formal syntheses of this natural product have been reported so far. This review summarizes the total and formal syntheses of neopeltolide and its analogues, highlighting the synthetic strategies exploited for constructing the tetrahydropyran ring.

Discovery of cytochrome bc1 complex inhibitors inspired by the natural product karrikinolide

Novel and potent inhibitors targeting the cytochrome bc₁ complex were discovered from the natural product karrikinolide for the first time.

Diastereoseletive Transannular Oxa-Conjugate Addition Generates the 2,6-cis-Disubstituted Tetrahydropyran of Neopeltolide

Transannular 2,6-disubstituted pyrans, like the one found in the cytotoxic marine natural product neopeltolide, are a key functional group in many polyketides. While oxa-conjugate additions have been shown to provide direct and rapid access to tetrahydropyrans in acyclic neopeltolide intermediates, a transannular strategy for construction of this ring system in a macrocyclic core has not been investigated. In this study, we demonstrate that a transannular oxa-conjugate addition strategy is a viable approach to the construction of the bicyclic core of neopeltolide. We show that transannular addition occurs readily with an α,β-unsaturated ketone as the Michael acceptor and does not occur when an α,β-unsaturated ester is the Michael acceptor. Our data indicates that oxa-conjugate addition is reversible and that the stereochemical outcome can be under thermodynamic control. Using computational chemistry, we show that the lowest energy diastereomer is the desired cis-pyran found in neopeltolide, and we experimentally demonstrate that the trans and cis diastereomers are interconvertible under reaction conditions with the cis-pyran product predominating. This oxa-conjugate addition strategy should provide a viable route to accessing the fully elaborated macrocyclic core of neopeltolide.

Strategies and Methods for the Synthesis of Anticancer Natural Product Neopeltolide and its Analogs

Neopeltolide, isolated in 2007, with its novel structural features and potent anti cancer cell proliferation activity, has attracted a tremendous amount of synthetic efforts. This review briefly and chronologically summarizes each of the synthesis with the main focus on the strategies and methodologies for the construction of its cis-tetrahydropyran-containing macrolactone core.

Design and synthesis of analogues of natural products

In this article strategies for the design and synthesis of natural product analogues are summarized and illustrated with some selected examples.

Synthesis of tetrahydropyran/tetrahydrofuran-containing macrolides by palladium-catalyzed alkoxycarbonylative macrolactonizations

A novel Pd-catalyzed cascade alkoxycarbonylative macrolactonization to construct tetrahydropyran/tetrahydrofuran-containing bridged macrolactones in one step from alkendiols is described. Products with various ring sizes and substituents were obtained. Challenging macrolactones involving tertiary alcohols were synthesized smoothly as well. Mechanistically, experimental evidence to support a trans-oxypalladation step has been provided. The method was applied to the synthesis of potent anticancer compound 9-demethylneopeltolide.

Synthesis and biological evaluation of (+)-neopeltolide analogues: importance of the oxazole-containing side chain

We describe the synthesis and biological evaluation of (+)-neopeltolide analogues with structural modifications in the oxazole-containing side chain. Evaluation of the antiproliferative activity of newly synthesized analogues against A549 human lung adenocarcinoma cells and PANC-1 human pancreatic carcinoma cells have shown that the C19-C20 and C26-C27 double bonds within the oxazole-containing side chain and the terminal methyl carbamate group are essential for potent activity.

Sonogashira coupling in natural product synthesis

This review will focus on selected applications of Sonogashira coupling and subsequent transformations as key steps in the total synthesis of natural products.

Enantioselective total synthesis of macrolide (+)-neopeltolide

The asymmetric total synthesis of the anti-proliferative macrolide (+)-neopeltolide has been completed. The stereochemically defined trisubstituted tetrahydropyran ring was constructed via a catalytic hetero-Diels-Alder reaction creating two new chiral centers in a highly diastereoselective manner. The other key features of this synthesis included Brown's asymmetric allylation to install the requisite C-11 and C-13 stereocenters. The synthesis of the oxazole side chain consisted of a hydrozirconation of an alkynyl stannane to establish the Z stereochemistry, followed by a palladium catalyzed cross coupling to introduce the desired Z olefin in the oxazole side chain.

Iodine-catalyzed prins cyclization of homoallylic alcohols and aldehydes

The iodine-catalyzed Prins cyclization of homoallylic alcohols and aldehydes was investigated under metal-free conditions and without additives. Anhydrous conditions and inert atmosphere are not required. The reaction of 2-(3,4-dihydronaphthalen-1-yl)propan-1-ol and 21 aldehydes (aliphatic and aromatic) in CH₂Cl₂ in the presence of 5 mol % of iodine gave 1,4,5,6-tetrahydro-2H-benzo[f]isochromenes in 54%–86% yield. Under similar conditions, the Prins cyclization of six alcohols containing an endocyclic double bond (primary, secondary, or tertiary) led to dihydropyrans in 52%–91% yield. The acyclic homoallylic alcohols gave 4-iodo-tetrahydropyran in 29%–41% yield in the presence of 50 mol % of iodine. This type of substrate is the main limitation of the methodology. The relative configuration of the products was assigned by NMR and X-ray analysis. The mechanism and the ratio of the products are discussed, based on DFT calculations.

Concise synthesis and biological assessment of (+)-neopeltolide and a 16-member stereoisomer library of 8,9-dehydroneopeltolide: identification of pharmacophoric elements

We describe herein a concise synthesis of (+)-neopeltolide, a marine macrolide natural product that elicits a highly potent antiproliferative activity against several human cancer cell lines. Our synthesis exploited the powerful bond-forming ability and high functional group compatibility of olefin metathesis and esterification reactions to minimize manipulations of oxygen functionalities and to maximize synthetic convergency. Our findings include a chemoselective olefin cross-metathesis reaction directed by H-bonding, and a ring-closing metathesis conducted under non-high dilution conditions. Moreover, we developed a 16-member stereoisomer library of 8,9-dehydroneopeltolide to systematically explore the stereostructure-activity relationships. Assessment of the antiproliferative activity of the stereoisomers against A549 human lung adenocarcinoma, MCF-7 human breast adenocarcinoma, HT-1080 human fibrosarcoma, and P388 murine leukemia cell lines has revealed marked differences in potency between the stereoisomers. This study provides comprehensive insights into the structure-activity relationship of this important antiproliferative agent, leading to the identification of the pharmacophoric structural elements and the development of truncated analogues with nanomolar potency.

Stereoselective synthesis of the macrolactone core of (+)-neopeltolide

A stereoselective synthesis of the macrolactone core of the potent anticancer agent neopeltolide is disclosed. The key steps of the synthesis include asymmetric allylation using Krische' protocol, conjugate reduction using MacMillan's methodology, and an asymmetric hetero-Diels-Alder reaction using Jacobsen's catalyst. Substrate controlled diastereoselective 1,3-anti reduction of a keto alcohol, Luche reduction followed by Ireland-Claisen rearrangement, oxymercuration, and reductive lithiation are other key steps.

Synthesis and biological evaluation of neopeltolide and analogs

The synthesis of neopeltolide analogues that contain variations in the oxazole-containing side chain and in the macrolide core are reported along with the GI(50) values for these compounds against MCF-7, HCT-116, and p53 knockout HCT-116 cell lines. Although biological activity is sensitive to changes in the macrocycle and the side chain, several analogues displayed GI(50) values of <25 nM. Neopeltolide and several of the more potent analogues were significantly less potent against p53 knockout cells, suggesting that p53 plays an auxiliary role in the activity of these compounds.

Total synthesis of RNA-polymerase inhibitor ripostatin B and 15-deoxyripostatin A

Keep me skipped: a highly convergent total synthesis of ripostatin B, an inhibitor of the bacterial RNA polymerase, is described. The key steps to construct and avoid isomerization of the skipped triene are a double Stille cross-coupling reaction and a ring-closing metathesis. Furthermore, 15-deoxyripostatin A, a stable and conformationally locked analogue of ripostatin A, was prepared and tested in vivo.

Concise formal synthesis of (+)-neopeltolide

A concise formal synthesis of (+)-neopeltolide (1) has been accomplished. The synthesis demonstrated high atom efficiency employing only one step of functional group protection. Key steps involved iridium-catalyzed double asymmetric carbonyl allylation, palladium-catalyzed intramolecular alkoxycarbonylation, ruthenium-catalyzed olefin isomerization, and ring-closing metathesis.

The medicinal potential of promising marine macrolides with anticancer activity

Marine natural products have become a major source of new chemical entities in the discovery of potential anticancer agents that potently suppress various molecular targets. In particular, the marine macrolides, which include an array of novel biomolecules endowed with outstanding cytotoxic and/or antiproliferative activities, are a prominent class of marine natural products that offer continued promise for breakthroughs in anticancer research. Herein we highlight some recent studies of promising marine macrolides, paying particular attention to their discovery, anticancer activities, mechanisms of action, chemical synthesis, and representative analogues.

Assigning stereochemistry to single diastereoisomers by GIAO NMR calculation: the DP4 probability

GIAO NMR shift calculation has been applied to the challenging task of reliably assigning stereochemistry with quantifiable confidence when only one set of experimental data are available. We have compared several approaches for assigning a probability to each candidate structure and have tested the ability of these methods to distinguish up to 64 possible diastereoisomers of 117 different molecules, using NMR shifts obtained in rapid and computationally inexpensive single-point calculations on molecular mechanics geometries without time-consuming ab initio geometry optimization. We show that a probability analysis based on the errors in each (13)C or (1)H shift is significantly more successful at making correct assignments with high confidence than are probabilities based on the correlation coefficient and mean absolute error parameters. Our new probability measure, which we have termed DP4, complements the probabilities obtained from our previously developed CP3 parameter, which applies to the case of assigning a pair of diastereoisomers when one has both experimental data sets. We illustrate the application of DP4 to assigning the stereochemistry or structure of 21 natural products that were originally misassigned in the literature or that required extensive synthesis of diastereoisomers to establish their stereochemistry.