Total synthesis of (+)-zampanolide

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Electrochemical Azo-free Mitsunobu-type Reaction

The classic chemical Mitsunobu reaction suffers from the need of excess alcohol activation reagents and the generation of significant by-products. Efforts to overcome these limitations have resulted in numerous creative solutions, but the substrate scope of these catalytic processes remains limited. Here we report an electrochemical Mitsunobu-type reaction, which features azo-free alcohol activation and broad substrate scope. This user-friendly technology allows a vast collection of heterocycles as the nucleophile, which can couple with a series of chiral cyclic and acyclic alcohols in moderate to high yields and excellent ee's. This practical reaction is scalable, chemoselective, uses simple Electrasyn setup with inexpensive electrodes and requires no precaution to exclude air and moisture. The synthetic utility is further demonstrated on the structural modification of diverse bioactive natural products and pharmaceutical derivatives and its straightforward application in a multiple-step synthesis of a drug candidate.

Construction of chiral N,O-hemiaminals via a copper-catalyzed enantioselective Michael/N-hemiacetalization cascade reaction

An efficient Michael/N-hemiacetalization cascade reaction of 5-aminoisoxazoles with β,γ-unsaturated α-ketoesters was developed under the catalysis of a chiral copper complex. A series of optically pure six-membered ring N,O-hemiaminals were obtained with excellent yields (up to 96% yield) and high enantioselectivities (up to 98% ee). The possible transition state was supported by DFT calculations and thereby the corresponding mechanism was proposed.

Synthesis of Morpholine-Based Analogues of (-)-Zampanolide and Their Biological Activity

We describe the convergent synthesis of three prototypical examples of a new class of analogues of the complex, cytotoxic marine macrolide (-)-zampanolide that incorporate an embedded N-substituted morpholine moiety in place of the natural tetrahydropyran ring. The final construction of the macrolactone core was based on a high-yielding intramolecular HWE olefination, while the hemiaminal-linked side chain was elaborated through a stereoselective, BINAL-H-mediated addition of (Z,E)-sorbamide to a macrocyclic aldehyde precursor. The synthesis of the common functionalized morpholine building block involved two consecutive epoxide openings with tosylamide and the product of the first opening reaction, respectively, as nucleophiles. Of the three morpholino-zampanolides investigated, the N-acetyl and the N-benzoyl derivatives both exhibited nanomolar antiproliferative activity, thus being essentially equipotent with the natural product. In contrast, the activity of the N-tosyl derivative was significantly reduced.

Enantioselective Reaction of 2H-Azirines with Oxazol-5-(4H)-ones Catalyzed by Cinchona Alkaloid Sulfonamide Catalysts

The enantioselective reaction of 2H-azirines with oxazol-5-(4H)-ones (oxazolones) using a cinchona alkaloid sulfonamide catalyst has been developed. The reaction proceeded at the C-2 position of oxazolones to afford products with consecutive tetrasubstituted stereogenic centers in high yield with high diastereo- and enantioselectivity. The obtained aziridines were converted into various chiral compounds without loss of enantiopurity.

Studies toward the Synthesis of an Oxazole-Based Analog of (-)-Zampanolide

Studies are described toward the synthesis of an oxazole-based analog of (-)-zampanolide (2). Construction of (-)-dactylolide analog 22 was achieved via alcohol 5 and acid 4 through esterification and Horner-Wadsworth-Emmons (HWE)-based macrocyclization; however, attempts to attach (Z,E)-sorbamide to 22 proved unsuccessful. The C(8)-C(9) double bond of the macrocycle was prone to migration into conjugation with the oxazole ring, which may generally limit the usefulness of zampanolide analogs with aromatic moieties as tetrahydropyran replacements.

Enantioselective organocatalytic amination of 2-perfluoroalkyl-oxazol-5(2H)-ones towards the synthesis of chiral N,O-aminals with perfluoroalkyl and amino groups

The first highly enantioselective amination at the C-2 position of oxazol-5(2H)-ones has been presented. Two efficient relay asymmetric transformation processes were successfully utilized to synthesize chiral N,O-aminals with a quaternary center.

The Progress of the Anticancer Agents Related to the Microtubules Target

Anticancer drugs based on the microtubules target are potent mitotic spindle poison agents, which interact directly with the microtubules, and were classified as microtubule-stabilizing agents and microtubule-destabilizing agents. Researchers have worked tremendously towards the improvements of anticancer drugs, in terms of improving the efficacy, solubility and reducing the side effects, which brought about advancement in chemotherapy. In this review, we focused on describing the discovery, structures and functions of the microtubules as well as the progress of anticancer agents related to the microtubules, which will provide adequate references for researchers.

A Comparative Synthetic Strategy Perspective on α-Amino Acid- and Non-Amino Acid-Derived Synthons towards Total Syntheses of Selected Natural Macrolides

Macrocyclic alkaloids (macrolides) and cyclopeptides have an immense range of applications in drug discovery research because of their natural abundance and potential biological and physicochemical properties. Presently, more than 100 approved drugs or clinical drug candidates contain macrocyclic scaffolds as the biologically active component. This review provides an interesting perspective about the use of amino acid-derived chiral pools versus other methods derived from miscellaneous synthons towards the total synthesis of non-peptidic macrolides. The synthetic routes and the key strategies involved in the total syntheses of ten natural macrolides have been discussed. Both the amino acid-derived and non-amino acid-derived synthetic routes have been illustrated to present a comparative study between the two approaches.

Synthesis, conformational preferences, and biological activity of conformational analogues of the microtubule-stabilizing agents, (-)-zampanolide and (-)-dactylolide

Zampanolide and dactylolide are microtubule-stabilizing polyketides possessing potent cytotoxicity towards a variety of cancer cell lines. Using our understanding of the conformational preferences of the macrolide core in both natural products, we hypothesized that analogues lacking the C17-methyl group would maintain the necessary conformation for bioactivity while reducing the number of synthetic manipulations necessary for their synthesis. Analogues 3, 4 and 5 were prepared via total synthesis, and their conformational preferences were determined through computational and high-field NMR studies. While no observable activities were present in dactylolide analogues 3 and 4, zampanolide analogue 5 exhibited sub-micromolar cytotoxicity. Herein, we describe these efforts towards understanding the structure- and conformation-activity relationships of dactylolide and zampanolide.

Anti-tubulin agents of natural origin: Targeting taxol, vinca, and colchicine binding domains

Microtubules are a protein which is made of α- and β-heterodimer. It is one of the main components of the cell which play a vital role in cell division especially in G2/M-phase. It exists in equilibrium dynamic of polymerization and depolymerization of α- and β-heterodimer. It is one of the best targets for developing anti-cancer drugs. Various natural occurring molecules are well known for their anti-tubulin effect such as vinca, paclitaxel, combretastatin, colchicine etc. These microtubule-targeted drugs are acted through two processes (i) inhibiting depolymerization of tubulin (tubulin stabilizing agents) and (ii) inhibiting polymerization of tubulin (tubulin destabilizing agents). Now days, various binding domains have been explore through which these molecules are binding to tubulin but the three major binding domain of tubulin are taxol, vinca and colchicine binding domain. The present article mainly focus on the classification of various naturally occurring compounds on the basis of their inhibition processes (depolymerization and polymerization) and the site of interaction (targets taxol, vinca and colchicine binding domain) which has been hitherto reported. By placing all the naturally occurring taxol, vinca and colchicine binding site analogues at one place makes a better understanding of the tubulin interactions with known natural tubulin binders that would helps in the discovery of new and potent natural, semi-synthetic and synthetic analogues for treating cancer.

Chemo- and regioselective reactions of 5-bromo enones/enaminones with pyrazoles

Reaction of 5-bromo enones with pyrazoles provided a series of unexpected N,O-aminal derivatives, through a 1,4-conjugated addition at the β-carbon of the 5-bromo enones instead of the expected nucleophilic substitution of the bromine. This reaction also furnished the 1,3-regioisomer of the pyrazole. A similar reaction of pyrazoles using 5-bromo enaminones furnished only N-alkylated pyrazoles-with high regioselectivity and at good yields-through nucleophilic substitution of the bromine.

Zampanolides B-E from the Marine Sponge Cacospongia mycofijiensis: Potent Cytotoxic Macrolides with Microtubule-Stabilizing Activity

Four new compounds (2-5) structurally related to the microtubule-stabilizing agent (-)-zampanolide (1) have been isolated from the Tongan marine sponge Cacospongia mycofijiensis. Three of these new structures, zampanolides B-D (2-4), exhibit nanomolar cytotoxicity toward the HL-60 cell line, are antimitotic, and induce in vitro tubulin polymerization at levels comparable to 1. Zampanolide E (5), saturated at C-8/C-9, was significantly less potent and does not stabilize purified tubulin, even at 10-fold higher concentrations. The structural differences across these compounds reveal a plasticity of the zampanolide pharmacophore. While unsaturation is required at Δ⁸, the configuration of this alkene and those of Δ⁴ and Δ^4' have little effect on tubulin polymerization. The first natural co-occurrence of 1 and (-)-dactylolide (6) from the same sponge extract is also noted.

An amine protecting group deprotectable under nearly neutral oxidative conditions

The 1,3-dithiane-based dM-Dmoc group was studied for the protection of amino groups. Protection was achieved under mild conditions for aliphatic amines, and under highly reactive conditions for the less reactive arylamines. Moderate to excellent yields were obtained. Deprotection was performed by oxidation followed by treating with a weak base. The yields were good to excellent. The new amino protecting group offers a different dimension of orthogonality in reference to the commonly used amino protecting groups in terms of deprotection conditions. It is expected to allow a collection of transformations to be carried out on the protected substrates that are unattainable using any known protecting groups.

The Curtius rearrangement: mechanistic insight and recent applications in natural product syntheses

The Curtius rearrangement is a versatile reaction in which a carboxylic acid can be converted to an isocyanate through an acyl azide intermediate under mild conditions. The resulting stable isocyanate can then be readily transformed into a variety of amines and amine derivatives including urethanes and ureas. There have been wide-ranging applications of the Curtius rearrangement in the synthesis of natural products and their derivatives. Also, this reaction has been extensively utilized in the synthesis and application of a variety of biomolecules. In this review, we present mechanistic studies, chemical methodologies and reagents for the synthesis of isocyanates from carboxylic acids, the conversion of isocyanates to amines and amine derivatives, and their applications in the synthesis of bioactive natural products and their congeners.

Recent advances in microtubule-stabilizing agents

Highly dynamic mitotic spindle microtubules are superb therapeutic targets for a group of chemically diverse and clinically successful anticancer drugs. Microtubule-targeted drugs disrupt microtubule dynamics in distinct ways, and they are primarily classified into two groups: microtubule destabilizing agents (MDAs), such as vinblastine, colchicine, and combretastatin-A4, and microtubule stabilizing agents (MSAs), such as paclitaxel and epothilones. Systematic discovery and development of new MSAs have been aided by extensive research on paclitaxel, yielding a large number of promising anticancer compounds. This review focuses on the natural sources, structural features, mechanisms of action, structure-activity relationship (SAR) and chemical synthesis of MSAs. These MSAs mainly include paclitaxel, taccalonolides, epothilones, FR182877 (cyclostreptin), dictyostatin, discodermolide, eleutherobin and sarcodictyins, zampanolide, dactylolide, laulimalides, peloruside and ceratamines from natural sources, as well as small molecular microtubule stabilizers obtained via chemical synthesis. Then we discuss the application prospect and development of these anticancer compounds.

Achmatowicz Reaction and its Application in the Syntheses of Bioactive Molecules

Substituted pyranones and tetrahydropyrans are structural subunits of many bioactive natural products. Considerable efforts are devoted toward the chemical synthesis of these natural products due to their therapeutic potential as well as low natural abundance. These embedded pyranones and tetrahydropyran structural motifs have been the subject of synthetic interest over the years. While there are methods available for the syntheses of these subunits, there are issues related to regio and stereochemical outcomes, as well as versatility and compatibility of reaction conditions and functional group tolerance. The Achmatowicz reaction, an oxidative ring enlargement of furyl alcohol, was developed in the 1970s. The reaction provides a unique entry to a variety of pyranone derivatives from functionalized furanyl alcohols. These pyranones provide convenient access to substituted tetrahydropyran derivatives. This review outlines general approaches to the synthesis of tetrahydropyrans, covering general mechanistic aspects of the Achmatowicz reaction or rearrangement with an overview of the reagents utilized for the Achmatowicz reaction. The review then focuses on the synthesis of functionalized tetrahydropyrans and pyranones and their applications in the synthesis of natural products and medicinal agents.

Microtubule-stabilizing agents: New drug discovery and cancer therapy

Microtubule-stabilizing agents (MSAs) have been highly successful in the treatment of cancer in the past 20years. To date, three classes of MSAs have entered the clinical trial stage or have been approved for clinical anticancer chemotherapy, and more than 10 classes of novel structural MSAs have been derived from natural resources. The microtubule typically contains two MSA-binding sites: the taxoid site and the laulimalide/peloruside site. All defined MSAs are known to bind at either of these sites, with subtle but significant differences. MSAs with different binding sites may produce a synergistic effect. Although having been extensively applied in the clinical setting, paclitaxel and other approved MSAs still pose many challenges such as multidrug resistance, low bioavailability, poor solubility, high toxicity, and low passage through the blood-brain barrier. A variety of studies focus on the structure-activity relationship in order to improve the pharmaceutical properties of these agents. Here, the mechanisms of action, advancements in pharmacological research, and clinical developments of defined MSAs during the past decade are discussed. The latest discovered MSAs are also briefly introduced in this review. The increasing number of natural MSAs indicates the potential discovery of more novel, natural MSAs with different structural bases, which will further promote the development of anticancer chemotherapy.

Preparation of conjugated dienoates with Bestmann ylide: Towards the synthesis of zampanolide and dactylolide using a facile linchpin approach

Bestmann ylide [(triphenylphosphoranylidene)ketene] acts as a chemical linchpin that links nucleophilic entities, such as alcohols or amines, with carbonyl moieties to produce unsaturated esters and amides, respectively. In this work, the formation of α,β,γ,δ-unsaturated esters (dienoates) is achieved through the coupling of Bestmann ylide, an alcohol and an α,β-unsaturated aldehyde. Primary and secondary alcohols, including allylic alcohols, are suitable substrates; the newly formed alkene has an E-geometry. Strategically, this represents a highly efficient route to unsaturated polyketide derivatives. A linchpin approach to the synthesis of a major fragment of the natural products zampanolide and dactylolide is investigated using Bestmann ylide to link the C16-C20 alcohol with the C3-C8 aldehyde fragment.

One-step protecting-group-free synthesis of azepinomycin in water

We report an efficient, atom economical general acid-base catalyzed one-step multi-gram synthesis of azepinomycin from commercially available compounds in water. We propose that the described pH-dependent Amadori rearrangement, which couples an amino-imidazole and simple sugar, is of importance as a potential step toward predisposed purine nucleotide synthesis at the origins of life.

Organocatalytic regioselective asymmetric Michael addition of azlactones to o-hydroxy chalcone derivatives

The regioselective and enantioselective Michael addition between azlactones and o-hydroxy chalcone derivatives is reported. Enantiomerically enriched N,O-aminals with two continuous stereogenic centers are exclusively obtained in moderate to good yields with excellent diastereoselectivities and good to excellent enantioselectivities. The experimental results show that an o-hydroxy group on the cinnamenyl motif of chalcone derivatives plays a crucial role at the reaction sites for the regioselective Michael addition. In addition, circular dichroism (CD) spectroscopy and density functional theory (DFT) are used to investigate the absolute configuration of N,O-aminals and the corresponding transition-state structures.

Zampanolide and dactylolide: cytotoxic tubulin-assembly agents and promising anticancer leads

Zampanolide is a marine natural macrolide and a recent addition to the family of microtubule-stabilizing cytotoxic agents. Zampanolide exhibits unique effects on tubulin assembly and is more potent than paclitaxel against several multi-drug resistant cancer cell lines. A high-resolution crystal structure of αβ-tubulin in complex with zampanolide explains how taxane-site microtubule-stabilizing agents promote microtubule assemble and stability. This review provides an overview of current developments of zampanolide and its related but less potent analogue dactylolide, covering their natural sources and isolation, structure and conformation, cytotoxic potential, structure-activity studies, mechanism of action, and syntheses.

Size-controlled synthesis of mesoporous palladium nanoparticles as highly active and stable electrocatalysts

We report a solution phase synthesis of monodispersed mesoporous Pd nanoparticles (MPNs) with narrow particle size distributions.

The vinylogous Mukaiyama aldol reaction (VMAR) in natural product synthesis

This review will provide an overview on the recent developments of polyketide synthesis using the vinylogous Mukaiyama aldol reaction for the construction of advanced intermediates. In general, four different motifs can be constructed efficiently using the recent developments of asymmetric variants of this strategy.

Conformational preferences of zampanolide and dactylolide

The solution conformation behavior of the macrolide core of microtubule-stabilizing agents (-)-zampanolide and (-)-dactylolide has been determined through a combination of high-field NMR experiments and computational modeling. Taken together, the results demonstrate that in solution both molecules exist as a mixture of three interconverting conformational families, one of which bears strong resemblance to zampanolide's tubulin-bound conformation.

Stereoselective halocyclization of alkenes with N-acyl hemiaminal nucleophiles

Halocyclization of alkenes was realized using N-acylhemiaminal nucleophiles. High diastereoselectivity could be achieved for the formation of three stereogenic centers in this halogen-mediated cyclization reaction. We also demonstrated that enantioselective bromocyclization of alkenes using N-acylhemiaminal nucleophiles was possible.

DFT study on the mechanism and stereochemistry of the Petasis-Ferrier rearrangements

The Petasis-Ferrier rearrangement is a very important and useful reaction for the synthesis of multifunctional tetrahydrofurans and tetrahydropyrans from easily synthesized enol acetals. Here we report our DFT investigation of the detailed reaction mechanism of the Petasis-Ferrier rearrangement, proposing that the active promoting species in this reaction is the cationic aluminum species, instead of the usually considered neutral Lewis acid (this will give very high activation energies and cannot explain why the Petasis-Ferrier rearrangements usually take place at low temperature or under mild conditions). Calculations indicated that the mechanisms of the Petasis-Ferrier rearrangements for the formations of five- and six-membered rings are different. Formation of five-membered tetrahydrofuranone is stepwise with C-O bond cleavage to generate an oxocarbenium enolate intermediate, which then undergoes an aldol-type reaction to give the desired cyclized oxacycle. In contrast, the formation of six-membered tetrahydropyranone is a concerted and asynchronous process with the C-O bond breakage and aldol-type C-C bond formation occurring simultaneously. A DFT understanding of why the catalytic versions of the Petasis-Ferrier rearrangements cannot be realized when using R2Al(+) as the active promoting species has also been discussed. In addition, DFT calculations were used to reveal the origins of the stereochemistry observed in the Petasis-Ferrier rearrangements.

Microtubule stabilizing agents as potential treatment for Alzheimer's disease and related neurodegenerative tauopathies

The microtubule (MT) associated protein tau, which is highly expressed in the axons of neurons, is an endogenous MT-stabilizing agent that plays an important role in axonal transport. Loss of MT-stabilizing tau function, caused by misfolding, hyperphosphorylation, and sequestration of tau into insoluble aggregates, leads to axonal transport deficits with neuropathological consequences. Several in vitro and preclinical in vivo studies have shown that MT-stabilizing drugs can be utilized to compensate for the loss of tau function and to maintain/restore effective axonal transport. These findings indicate that MT-stabilizing compounds hold considerable promise for the treatment of Alzheimer disease and related tauopathies. The present article provides a synopsis of the key findings demonstrating the therapeutic potential of MT-stabilizing drugs in the context of neurodegenerative tauopathies, as well as an overview of the different classes of MT-stabilizing compounds.

Isolation, biological activity, synthesis, and medicinal chemistry of the pederin/mycalamide family of natural products

This review highlights the broad range of science that has arisen from the isolation of pederin, the mycalamides, theopederins, and onnamides, and psymberin. Specific topics include structure determination, biological activity, synthesis, and analog preparation and analysis.

Total Synthesis of Potent Antitumor Macrolide, (-)-Zampanolide: An Oxidative Intramolecular Cyclization-Based Strategy

A detailed account of the enantioselective total synthesis of (-)-zampanolide, a macrolide marine natural product with high anti-cancer activity is described. For synthesis of the 4-methylene tetrahydropyran unit of (-)-zampanolide, initially, we relied upon an oxidative C-H activation of an alkenyl ether and intramolecular cyclization to provide the substituted tetrahydropyran ring. However, this strategy was unsuccessful. Subsequently, we found that a cinnamyl ether is critical for the successful oxidative intramolecular cyclization reaction. The synthesis also features a cross metathesis reaction to construct a tri-substituted olefin, a ring-closing metathesis to form a highly functionalized macrolactone and a chiral phosphoric acid promoted N-acyl aminal formation to furnish (-)-zampanolide stereoselectively and in good yield. The synthetic (-)-zampanolide had effects on cultured cells and on tubulin assembly consistent with properties reported for the natural product.

Toward an enantioselective synthesis of (-)-zampanolide: preparation of the C9-C20 region

Progress toward the synthesis of the microtubule-stabilizing agent, (-)-zampanolide, is reported. Construction of the 2,6-cis-tetrahydropyran ring was accomplished utilizing ether transfer methodology in conjunction with an intramolecular radical cyclization reaction. Efficient installation of the C16-C20 side chain relied on a one-pot cross-metathesis/olefination sequence, Sharpless epoxidation, and selective reduction of a vinyl epoxide.

Synthesis of Functionalized 4-Methylenetetrahydropyrans by Oxidative Activation of Cinnamyl or Benzyl Ethers

Oxidative activation of benzyl or cinnamyl ether bearing allylsilane derivatives using a catalytic amount of DDQ and 2 equivalents of CAN in the presence of PPTS provided functionalized 4-methylenetetrahydropyrans in good yields and excellent diastereoselectivity. The reaction could be applied to the synthesis of a variety of substituted tetrahydropyran derivatives.