Microtubule-stabilizing marine metabolite laulimalide and its derivatives: synthetic approaches and antitumor activity

Asymmetric Olefin Isomerization via Photoredox Catalytic Hydrogen Atom Transfer and Enantioselective Protonation

Asymmetric olefin isomerization can be appreciated as an ideal synthetic approach to access valuable enantioenriched C═C-containing molecules due to the excellent atom economy. Nonetheless, its occurrence usually requires a thermodynamic advantage, namely, a higher stability of the product to the substrate. It has thus led to rather limited examples of success. Herein, we report a photoredox catalytic hydrogen atom transfer (HAT) and enantioselective protonation strategy for the challenging asymmetric olefin isomerization. As a paradigm, by establishing a dual catalyst system involving a visible light photosensitizer DPZ and a chiral phosphoric acid, with the assistance of N-hydroxyimide to perform HAT, a wide array of allylic azaarene derivatives, featuring α-tertiary carbon stereocenters and β-C═C bonds, was synthesized with high yields, ees, and E/Z ratios starting from the conjugated α-substituted alkenylazaarene E/Z-mixtures. The good compatibility of assembling deuterium on stereocenters by using inexpensive D2O as a deuterium source further underscores the broad applicability and promising utility of this strategy. Moreover, mechanistic studies have provided clear insights into its challenges in terms of reactivity and enantioselectivity. The exploration will robustly inspire the development of thermodynamically unfavorable asymmetric olefin isomerizations.

Re-evaluation of the Fijianolide/Laulimalide Chemotype Suggests an Alternate Mechanism of Action for C-15/C-20 Analogs

Herein, we report on naturally derived microtubule stabilizers with activity against triple negative breast cancer (TNBC) cell lines, including paclitaxel, fijianolide B/laulimalide (3), fijianolide B di-acetate (4), and two new semisynthetic analogs of 3, which include fijianolide J (5) and fijianolide L (6). Similar to paclitaxel, compound 3 demonstrated classic microtubule stabilizing activity with potent (GI₅₀ = 0.7-17 nM) antiproliferative efficacy among the five molecularly distinct TNBC cell lines. Alternatively, compounds 5 or 6, generated from oxidation of C-20 or C-15 and C-20 respectively, resulted in a unique profile with reduced potency (GI₅₀ = 4-9 μM), but improved efficacy in some lines, suggesting a distinct mechanism of action. The C-15, C-20 di-acetate, and dioxo modifications on 4 and 6 resulted in compounds devoid of classic microtubule stabilizing activity in biochemical assays. While 4 also had no detectable effect on cellular microtubules, 6 promoted a reorganization of the cytoskeleton resulting in an accumulation of microtubules at the cell periphery. Compound 5, with a single C-20 oxo substitution, displayed a mixed phenotype, sharing properties of 3 and 6. These results demonstrate the importance of the C-15/C-20 chiral centers, which appear to be required for the potent microtubule stabilizing activity of this chemotype and that oxidation of these sites promotes unanticipated cytoskeletal alterations that are distinct from classic microtubule stabilization, likely through a distinct mechanism of action.

Formal [5+1] annulation reactions of dielectrophilic peroxides: facile access to functionalized dihydropyrans

Peroxides, functioning as unique five-atom bielectrophilic synthons, enable the new [5+1] annulation reactions to access dihydropyrans in high yields.

A General Diastereoselective Strategy for Both cis- and trans-2,6-Disubstituted Tetrahydropyrans: Formal Total Synthesis of (+)-Muconin

A protocol for general diastereoselective tandem dihydroxylation followed by S_N2 cyclization was developed for the convenient and efficient synthesis of cis- and trans-2,6-disubstituted tetrahydropyrans from ζ-mesyloxy α,β-unsaturated esters. The application of this novel method was demonstrated through the concise formal synthesis of (+)-muconin, a nonclassical acetogenin, with sequential THP-THF ring formation.

Early investigational tubulin inhibitors as novel cancer therapeutics

INTRODUCTION

Microtubules represent one of the most logical and strategic molecular targets amongst the current targets for chemotherapy, alongside DNA. In the past decade, tubulin inhibitors as cancer therapeutics have been an area of focus due to the improved understanding and biological relevance of microtubules in cellular functions. Fueled by the objective of developing novel chemotherapeutics and with the aim of establishing the benefits of tubulin inhibition, several clinical trials have been conducted with others ongoing.

AREA COVERED

At present, the antitubulin development pipeline contains an armful of agents under clinical investigation. This review focuses on novel tubulin inhibitors as cancer therapeutics. The article covers the agents which have completed the phase II studies along with the agents demonstrating promising results in phase I studies.

EXPERT OPINION

Countless clinical trials evaluating the efficacy, safety and pharmacokinetics of novel tubulin inhibitors highlights the scientific efforts being paid to establish their candidature as cancer therapeutics. Colchicine binding site inhibitors as vascular disrupting agents (VDAs) and new taxanes appear to be the most likely agents for future clinical interest. Numerous agents have demonstrated clinical benefits in terms of efficacy and survival in phase I and II studies. However conclusive benefits can only be ascertained on the basis of phase III studies.

Applications of oxazolidinones as chiral auxiliaries in the asymmetric alkylation reaction applied to total synthesis

In this review, a number of applications of chiral oxazolidinones in the asymmetric alkylation reaction applied to total synthesis are described.

Ester coupling reactions – an enduring challenge in the chemical synthesis of bioactive natural products

In this review we investigate the use of complex ester fragment couplings within natural product total syntheses. Using examples from the literature up to 2014 we illustrate the state-of-the-art as well as the challenges within this area of organic synthesis.

Tubulin inhibitors: a patent review

INTRODUCTION

Microtubules play an important role in several cellular processes, particularly in the formation of the mitotic spindle during the process of mitosis. These highly dynamic mitotic-spindle microtubules have become a successful target of cancer therapy. Microtubule-targeting agents, such as vinca alkaloids and taxanes, were used in clinic over 50 years. In past decades, development of new antimicrotubule agents that possess different structure and binding sites of tubulin has shown potent activity against the proliferation of various cancer cells, as well as in multidrug-resistant cancers. Interestingly, many of these agents represent an attractive ability that targeting the tumor blood vessels results in tumor vascular disruption. Therefore, exploring new agents and strategies may provide more effective therapeutic options in the related treatment of cancer.

AREAS COVERED

In past few years, there are many chemical compounds that successfully interferes the microtubules and display antitumor effect. In these, published compounds supply the fresh minds in modification of present drugs and new insights into the development of tubulin inhibitors.

EXPERT OPINION

This article arranges the microtubule-targeting agents that have published in patent in recent years. It may help in the investigation of new tubulin binding site and development of novel drug candidate in the future.

Macrolide-based microtubule-stabilizing agents - chemistry and structure-activity relationships

This article provides an overview on the chemistry and structure-activity relationships of macrolide-based microtubule-stabilizing agents. The primary focus will be on the total synthesis or examples thereof, but a brief summary of the current state of knowledge on the structure-activity relationships of epothilones, laulimalide, dictyostatin, and peloruside A will also be given. This macrolide class of compounds, over the last decade, has become the subject of growing interest due to their ability to inhibit human cancer cell proliferation through a taxol-like mechanism of action.

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.

Total synthesis of laulimalide: synthesis of the northern and southern fragments

The first stage in the development of a synthetic route for the total synthesis of laulimalide (1) is described. Our retrosynthetic analysis envisioned a novel macrocyclization route to the natural product by using a Ru-catalyzed alkene-alkyne coupling. This would be preceded by an esterification of the C19 hydroxyl group, joining together two equally sized synthons, the northern fragment 7 and the southern fragment 8. Our first generation approach to the northern fragment entailed a key sequential Ru/Pd coupling sequence to assemble the dihydropyran. The key reactions proceeded smoothly, but the inability to achieve a key olefin migration led to the development of an alternative route based on an asymmetric dinuclear Zn-catalyzed aldol reaction of a hydroxyl acylpyrrole. This key reaction led to the desired diol adduct 66 with excellent syn/anti selectivity (10:1), and allowed for the successful completion of the northern fragment 7. The key step for the synthesis of the southern fragment was a chemoselective Rh-catalyzed cycloisomerization reaction to form the dihydropyran ring from a diyne precursor. This reaction proved to be selective for the formation of a six-membered ring, over a seven. The use of an electron-deficient bidentate phosphine allowed for the reaction to proceed with a reduced catalyst loading.

Role of the cytoskeleton in formation and maintenance of angiogenic sprouts

Angiogenesis is the formation of new blood vessels from pre-existing structures, and is a key step in tissue and organ development, wound healing and pathological events. Changes in cell shape orchestrated by the cytoskeleton are integral to accomplishing the various steps of angiogenesis, and an intact cytoskeleton is also critical for maintaining newly formed structures. This review focuses on how the 3 main cytoskeletal elements--microfilaments, microtubules, and intermediate filaments--regulate the formation and maintenance of angiogenic sprouts. Multiple classes of compounds target microtubules and microfilaments, revealing much about the role of actin and tubulin and their associated molecules in angiogenic sprout formation and maintenance. In contrast, intermediate filaments are much less studied, yet intriguing evidence suggests a vital, but unresolved, role in angiogenic sprouting. This review discusses evidence for regulatory molecules and pharmacological compounds that affect actin, microtubule and intermediate filament dynamics to alter various steps of angiogenesis, including endothelial sprout formation and maintenance.

Stereoselective Synthesis of Five Biologically Active, Naturally Occurring Medium and Large Ring Lactones

Stereoselective syntheses of five naturally occurring, pharmacologically active medium and large ring lactones are described. Key synthetic methods used were, depending on the cases, olefin metatheses, asymmetric allylations and C-glycosidations.

The laulimalide family: total synthesis and biological evaluation of neolaulimalide, isolaulimalide, laulimalide and a nonnatural analogue

We herein describe in full detail the first total synthesis of the antitumor agents neolaulimalide and isolaulimalide as well as a highly efficient route to laulimalide. A Kulinkovich reaction followed by a cyclopropyl-allyl rearrangement is used to install the exo-methylene group. The C(2)-C(16) aldehyde fragment is coupled with the C(17)-C(28) sulfone fragments by a highly (E)-selective Julia-Lythgoe-Kocienski olefination to deliver the key intermediates of all three syntheses. Various conditions for the Yamaguchi macrolactonization are applied to close the individual macrocycles. Finally a carefully elaborated endgame was developed to solve the problem of acyl migration in the case of neolaulimalide. All compounds were tested against several cell lines. The cytotoxicity of neolaulimalide could be confirmed for the first time since its original isolation and it could be shown that it induces tubulin polymerization as efficiently as laulimalide.

Biomimetic transannular oxa-conjugate addition approach to the 2,6-disubstituted dihydropyran of laulimalide yields an unprecedented transannular oxetane

2,6-Disubstituted dihydropyrans are a common feature in many bioactive polyketides, including the anticancer marine polyketide laulimalide. While much of the uncharacterized biosynthetic pathway for laulimalide can be confidently postulated, the biosynthetic origins of the trans 2,6-disubstituted dihydropyran cannot. We hypothesize that a transannular oxa-conjugate addition in a macrocyclic laulimalide precursor could be the origin of the 2,6-dihydropyran. To test this hypothesis, we constructed a model containing the key functional groups for oxa-conjugate addition-mediated dihydropyran formation. Under acid-mediated conditions, the model under went regiospecific oxa-conjugate addition producing a stable trans oxetane as the only regioisomer. The desired, more stable dihydropyran was not detected. This unprecedented regiospecificity is unexpected due to the ring strain of the oxetane and the anticipated facile ring opening retro-oxa-conjugate addition. The oxetane is stable to acid and basic conditions, as are a number of literature acyclic oxetanes that could undergo similar retro-oxa-conjugate addition. While the source of the oxetane kinetic stability is yet to be characterized, it may enable general oxetane construction via oxa-conjugate addition. The more stable dihydropyran regioisomer could not be generated due to poor geometrical orbital alignment and hard-soft incompatibility between the hard oxygen nucleophile and the soft activated polyenoate electrophile. These factors disfavor the breaking of conjugation by oxa-conjugate addition. Based on these results we propose that dihydropyran formation does not occur on completed polyketide macrocycles as we had proposed but rather during polyketide biosynthesis on the growing polyketide chain.

A selective account of effective paradigms and significant outcomes in the discovery of inspirational marine natural products

Marine natural products continue to be a source of significant molecular structures that serve as a stimulus to seed further significant research. This account reviews some of the major advances in the study of marine biomolecules made at UC Santa Cruz over more than three decades. The continuing challenge of discovery and characterization of what we term "inspirational molecular structures" will be presented in a comprehensive fashion. Examples of privileged molecular structures and their impact on biomedicinal research will be an important theme. The three major groups of organisms explored include seaweeds, sponges, and marine-derived fungi, and the study of their active principles has greatly benefited from synergistic collaborations with both academic and biopharmaceutical groups. The concluding sections of this chronicle will touch on prospects for future outcomes involving new sources and strategies.

Total synthesis of neolaulimalide and isolaulimalide

The first total syntheses of the potential antitumoral leads neolaulimalide (2) and isolaulimalide (3) have been achieved. Key steps in our convergent, fully stereocontrolled route are a Yamaguchi macrolactonization, a Julia-Lythgoe-Kocienski olefination, a Kulinkovich reaction, and a cyclopropyl-allyl rearrangement to install the exo-methylene group. Overall, we synthesized 2 in 21 linear steps (3% yield) and 3 in 24 steps (2% yield).

Alcohol-assisted phosphine catalysis: one-step syntheses of dihydropyrones from aldehydes and allenoates

This paper describes the phosphine-catalyzed annulation of methyl allenoate with various aromatic aldehydes to form 6-aryl-4-methoxy-5,6-dihydro-2-pyrones. In this reaction, the addition of an alcohol was necessary to induce dihydropyrone formation, with the optimal agent being methanol. Moreover, the addition of n-butyllithium suppressed the formation of the noncyclized product, leading to the exclusive isolation of the dihydropyrone. This method provides an efficient, one-step route toward disubstituted dihydropyrones from simple, stable starting materials.

Function-oriented synthesis, step economy, and drug design

This Account provides an overview and examples of function-oriented synthesis (FOS) and its increasingly important role in producing therapeutic leads that can be made in a step-economical fashion. Biologically active natural product leads often suffer from several deficiencies. Many are scarce or difficult to obtain from natural sources. Often, they are highly complex molecules and thus not amenable to a practical synthesis that would impact supply. Most are not optimally suitable for human therapeutic use. The central principle of FOS is that the function of a biologically active lead structure can be recapitulated, tuned, or greatly enhanced with simpler scaffolds designed for ease of synthesis and also synthetic innovation. This approach can provide practical access to new (designed) structures with novel activities while at the same time allowing for synthetic innovation by target design. This FOS approach has been applied to a number of therapeutically important natural product leads. For example, bryostatin is a unique natural product anticancer lead that restores apoptosis in cancer cells, reverses multidrug resistance, and bolsters the immune system. Remarkably, it also improves cognition and memory in animals. We have designed and synthesized simplified analogs of bryostatin that can be made in a practical fashion (pilot scale) and are superior to bryostatin in numerous assays including growth inhibition in a variety of human cancer cell lines and in animal models. Laulimalide is another exciting anticancer lead that stabilizes microtubules, like paclitaxel, but unlike paclitaxel, it is effective against multidrug-resistant cell lines. Laulimalide suffers from availability and stability problems, issues that have been addressed using FOS through the design and synthesis of stable and efficacious laulimalide analogs. Another FOS program has been directed at the design and synthesis of drug delivery systems for enabling or enhancing the uptake of drugs or drug candidates into cells and tissue. We have generated improved transporters that can deliver agents in a superior fashion compared with naturally occurring cell-penetrating peptides and that can be synthesized in a practical and step-economical fashion. The use of FOS has allowed for the translation of exciting, biologically active natural product leads into simplified analogs with superior function. This approach enables the development of synthetically innovative strategies while targeting therapeutically novel structures.

Sponge-derived fijianolide polyketide class: further evaluation of their structural and cytotoxicity properties

The sponge-derived polyketide macrolides fijianolides A (1) and B (2), isolaulimalide and laulimalide, have taxol-like microtubule-stabilizing activity, and the latter exhibits potent cytotoxicity. Insight on the biogeographical and phenotypic variations of Cacospongia mycofijiensis is presented that will enable a future study of the biosynthetic pathway that produces the fijianolides. In addition to fijianolides A and B, six new fijianolides, D-I (7-12), were isolated, each with modifications to the C-20 side chain of the macrolide ring. Compounds 7-12 exhibited a range of in vitro activities against HCT-116 and MDA-MB-435 cell lines. Fijianolides 8 and 10 were shown to disrupt interphase and mitotic division, but were less potent than 2. An in vivo evaluation of 2 using tumor-bearing severe combined immuno-deficiency mice demonstrated significant inhibition of growth in HCT-116 tumors over 28 days.

Anticancer drugs from nature--natural products as a unique source of new microtubule-stabilizing agents

This review article provides an overview on the current state of research in the area of microtubule-stabilizing agents from natural sources, with a primary focus on the biochemistry, biology, and pharmacology associated with these compounds. A variety of natural products have been discovered over the last decade to inhibit human cancer cell proliferation through a taxol-like mechanism. These compounds represent a whole new range of structurally diverse lead structures for anticancer drug discovery.

Highly Enantio- and Diastereoselective Brassard Type Hetero-Diels−Alder Approach to 5-Methyl-Containing α,β-Unsaturated δ-Lactones

Two efficient new chiral copper (II) Schiff base complexes were developed for the highly enantio- and diastereoselective HDA reaction of Brassard type diene 1b with aldehydes, to afford the corresponding 5-methyl-containing alpha,beta-unsaturated delta-lactone derivatives in moderate yields, high enantioselectivities (up to 99% ee) and excellent diastereoselectivities (up to 99:1 anti/syn). On the basis of the absolute configuration of 4a-4j disclosed by X-ray diffraction and CD analysis, a possible transition-state model for the enantio- and diastereoselective catalytic reaction has been proposed.

Apoptolidin: Induction of Apoptosis by a Natural Product

Apoptolidin is a natural product that selectively induces apoptosis in several cancer cell lines. Apoptosis, programmed cell death, is a biological key pathway for regulating homeostasis and morphogenesis. Apoptotic misregulations are connected with several diseases, in particular cancer. The extrinsic way to apoptosis leads through death ligands and death receptors to the activiation of the caspase cascade, which results in proteolytic degradation of the cell architecture. The intrinsic pathway transmits signals of internal cellular damage to the mitochondrion, which loses its structural integrity, and forms an apoptosome that initiates the caspase cascade. Compounds which regulate apoptosis are of high medical significance. Many natural products regulate apoptotic pathways, and apoptolidin is one of them. The known synthetic routes to apoptolidin are described and compared in this Review. Selected further natural products which regulate apoptosis are introduced briefly.

Natural product-like chemical space: search for chemical dissectors of macromolecular interactions

Macromolecular interactions (i.e. protein-protein or DNA/RNA-protein interactions) play important cellular roles, including cellular communication and programmed cell death. Small-molecule chemical probes are crucial for dissecting these highly organized interactions, for mapping their function at the molecular level and developing new therapeutics. The lack of ideal chemical probes required to understand macromolecular interactions is the missing link in the next step of dissecting such interactions. Unfortunately, the classical combinatorial-chemistry community has not successfully provided the required probes (i.e. natural product inspired chemical probes that are rich in stereochemical and three-dimensional structural diversity) to achieve these goals. The emerging area of diversity-oriented synthesis (DOS) is beginning to provide natural product-like chemical probes that may be useful in this arena.