Total synthesis and evaluation of a key series of C5-substituted vinblastine derivatives

Aryl Annulation: A Powerful Simplifying Retrosynthetic Disconnection

Retrosynthetic deconstruction of a core aromatic ring is an especially simplifying retrosynthetic step, reducing the complexity of the precursor synthetic target. Moreover, when implemented to provide a penultimate intermediate, it enables late-stage divergent aryl introductions, permitting deep-seated core aryl modifications ordinarily accessible only by independent synthesis. Herein, we highlight the use of a ketone carbonyl group as the functionality to direct such late-stage divergent aryl introductions onto a penultimate intermediate with a projected application in the total synthesis of vinblastine and its presently inaccessible analogs containing indole replacements. Although the studies highlight this presently unconventional strategy with an especially challenging target in mind, the increase in molecular complexity (intricacy) established by the synthetic implementation of the powerful retrosynthetic disconnection, the use of a ketone as the precursor enabling functionality, and with adoption of either conventional or new wave (hetero)aromatic annulations combine to define a general and powerful strategy suited for wide-spread implementation with near limitless scope in target diversification.

Tris(4-bromophenyl)aminium Hexachloroantimonate-Mediated Intermolecular C(sp2)-C(sp3) Free Radical Coupling of Vindoline with β-Ketoesters and Related Compounds

A powerful tris(4-bromophenyl)aminium hexachloroantimonate (BAHA) mediated regioselective intermolecular coupling reaction of vindoline with a wide range of substrates that include β-ketoesters, β-diketones, β-ketoaldehydes, β-ketonitriles, β-ketolactones, β-ketolactams, β-cyanoesters, and malononitriles is detailed. The BAHA-promoted intermolecular sp3/sp2 coupling, representing a special class of selective C-H functionalization reactions with direct carbon-carbon bond formation, proceeds with generation of a quaternary center bound to the aryl C15 center of vindoline capable of accommodating of the vinblastine C16' methyl ester and functionalized for subsequent divergent heterocycle introduction. A comprehensive examination of the reaction scope, optimization of subtle reaction parameters, and key insights into the reaction mechanism are described. Contrary to what might be prevailing expectations, studies suggest the plausible mechanism entails initial single-electron oxidation of the substrate enolate, not vindoline, and subsequent regiospecific addition of the resulting electrophilic radical to vindoline. As such and beyond the new arylation reaction with vindoline, the studies define a host of new, previously unrecognized, applications of BAHA and related triarylaminium radical cations that arises from their ability to generate stabilized electrophilic radicals from β-ketoesters and related substrates under nonreducing and metal-free conditions. Those exemplified herein include mediating stabilized enolate free radical arylation, dimerization, allylation, alkene addition, and α-oxidation reactions.

Total synthesis of (-)-4-desacetoxy-1-oxovindoline: Single atom exchange of an embedded core heteroatom in vindoline

A concise total synthesis of (-)-4-desacetoxy-1-oxovindoline is disclosed, bearing a single heteroatom exchange in the core structure of the natural product 4-desacetoxyvindoline. Central to the synthesis is powerful oxadiazole intramolecular [4+2]/[3+2] cycloaddition cascade that formed four C-C bonds, created three new rings, and established five contiguous stereocenters about the new formed central 6-membered ring.

Synthetic, Mechanistic, and Biological Interrogation of Ginkgo biloba Chemical Space En Route to (-)-Bilobalide

Here we interrogate the structurally dense (1.64 mcbits/Å3) GABAA receptor antagonist bilobalide, intermediates en route to its synthesis, and related mechanistic questions. 13C isotope labeling identifies an unexpected bromine migration en route to an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computation converge on the driving forces behind two surprising observations. First, an oxetane acetal persists in concentrated mineral acid (1.5 M DCl in THF-d8/D2O); its longevity is correlated to destabilizing steric clash between substituents upon ring-opening. Second, a regioselective oxidation of des-hydroxybilobalide is found to rely on lactone acidification through lone-pair delocalization, which leads to extremely rapid intermolecular enolate equilibration. We also establish equivalent effects of (-)-bilobalide and the nonconvulsive sesquiterpene (-)-jiadifenolide on action potential-independent inhibitory currents at GABAergic synapses, using (+)-bilobalide as a negative control. The high information density of bilobalide distinguishes it from other scaffolds and may characterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (ca. 132 000 molecules/min) calculate information content (Böttcher scores), which may prove helpful to identify important features of NP space.

Mitotic Poisons in Research and Medicine

Cancer is one of the greatest challenges of the modern medicine. Although much effort has been made in the development of novel cancer therapeutics, it still remains one of the most common causes of human death in the world, mainly in low and middle-income countries. According to the World Health Organization (WHO), cancer treatment services are not available in more then 70% of low-income countries (90% of high-income countries have them available), and also approximately 70% of cancer deaths are reported in low-income countries. Various approaches on how to combat cancer diseases have since been described, targeting cell division being among them. The so-called mitotic poisons are one of the cornerstones in cancer therapies. The idea that cancer cells usually divide almost uncontrolled and far more rapidly than normal cells have led us to think about such compounds that would take advantage of this difference and target the division of such cells. Many groups of such compounds with different modes of action have been reported so far. In this review article, the main approaches on how to target cancer cell mitosis are described, involving microtubule inhibition, targeting aurora and polo-like kinases and kinesins inhibition. The main representatives of all groups of compounds are discussed and attention has also been paid to the presence and future of the clinical use of these compounds as well as their novel derivatives, reviewing the finished and ongoing clinical trials.

Metabolites of Vinca Alkaloid Vinblastine: Tubulin Binding and Activation of Nausea-Associated Receptors

Vinblastine (VLB) is an antimitotic drug that binds to the vinca site of tubulin. The molecule possesses a high molecular weight and a complex chemical structure with many possibilities of metabolization. Despite advances in drug discovery research in reducing drug toxicity, the cause and mechanism of VLB-induced adverse drug reactions (ADRs) remains poorly understood. VLB is metabolized to at least 35 known metabolites, which have been identified and collected in this present work. This study also explores how VLB metabolites affect nausea-associated receptors such as muscarinic, dopaminergic, and histaminic. The metabolites have stronger binding interactions than acetylcholine (ACh) for muscarinic M₁, M₄, and M₅ receptors and demonstrate similar binding profiles to that of the natural substrate, ACh. The affinities of VLB metabolites to dopaminergic and histaminic receptors, their absorption, distribution, metabolism, excretion, toxicity properties, and the superiority of VLB to ACh for binding to M₅R, indicate their potential to trigger activation of nausea-associated receptors during chemotherapy with VLB. It has been shown that metabolite 20-hydroxy-VLB (metabolite 10) demonstrates a stronger binding affinity to the vinca site of tubulin than VLB; however, they have similar modes of action. VLB and metabolite 10 have similar gastric solubility (FaSSGF), intestinal solubility (FeSSIF), and log P values. Metabolite 10 has a more acceptable pharmacokinetic profile than VLB, a better gastric and intestinal solubility. Furthermore, metabolite 10 was found to be less bound to plasma proteins than VLB. These are desired and essential features for effective drug bioavailability. Metabolite 10 is not a substrate of CYP2D6 and thus is less likely to cause drug-drug interactions and ADRs compared to its parent drug. The hydroxyl group added upon metabolism of VLB suggests that it can also be a reasonable starting compound for designing the next generation of antimitotic drugs to overcome P-glycoprotein-mediated multidrug resistance, which is often observed with vinca alkaloids.

Inverse Electron Demand Diels-Alder Reactions of Heterocyclic Azadienes, 1-Aza-1,3-Butadienes, Cyclopropenone Ketals, and Related Systems. A Retrospective

A summary of the investigation and applications of the inverse electron demand Diels-Alder reaction is provided that have been conducted in our laboratory over a period that now spans more than 35 years. The work, which continues to provide solutions to complex synthetic challenges, is presented in the context of more than 70 natural product total syntheses in which the reactions served as a key strategic step in the approach. The studies include the development and use of the cycloaddition reactions of heterocyclic azadienes (1,2,4,5-tetrazines; 1,2,4-, 1,3,5-, and 1,2,3-triazines; 1,2-diazines; and 1,3,4-oxadiazoles), 1-aza-1,3-butadienes, α-pyrones, and cyclopropenone ketals. Their applications illustrate the power of the methodology, often provided concise and nonobvious total syntheses of the targeted natural products, typically were extended to the synthesis of analogues that contain deep-seated structural changes in more comprehensive studies to explore or optimize their biological properties, and highlight a wealth of opportunities not yet tapped.

Ultra-potent vinblastine analogues improve on-target activity of the parent microtubulin-targeting compound

In recent efforts, several C20' urea vinblastine analogues were discovered that displayed remarkable potency against vinblastine-sensitive tumor cell lines (IC₅₀ 50-75 pM), being roughly 100-fold more potent than vinblastine, and that exhibited decreased susceptibility to Pgp efflux-derived resistance in a vinblastine-resistant cell line. Their extraordinary activity indicate that it is not likely or even possible that their cellular functional activity is derived from stoichiometric occupancy of the intracellular tubulin binding sites. Rather, their potency indicates sub-stoichiometric or even catalytic occupancy of candidate binding sites may be sufficient to disrupt tubulin dynamics or microtubule assembly during mitosis. We detail efforts to delineate the underlying behavior responsible for the increased potency and show that the ultra-potent extended C20' ureas retain the mechanistic behavior of vinblastine, display enhanced affinity for tubulin and on-target activity approximately 100-fold both in vitro and in HeLa cells, but do not show evidence of catalytic disassembly of microtubulin. We also use the analogues to show that, in live interphase cells, the effects of the vinblastine class of drugs do not display a catastrophic effect on the microtubule skeleton, but rather a subtler insult to its dynamicity, acting as sub-stoichiometric drugs that inhibit normal microtubulin maturation and dynamics.

Copper-catalyzed [3 + 2] annulation of ethynyl epoxides with malononitrile to access highly substituted dihydrofurans with an all-carbon quaternary stereocenter

A copper-catalyzed [3 + 2] annulation of ethynyl epoxides with malononitrile to afford dihydrofurans with an all-carbon quaternary stereocenter has been described.

Vinca alkaloids and analogues as anti-cancer agents: Looking back, peering ahead

Cancer still represents a "nightmare" worldwide, causing annually millions of victims. Several antiproliferative molecules are currently used as drugs market and offer a pharmaceutical opportunity for attenuating and treating tumor manifestations. In this context, natural sources have a relevant role, since they provide the 60% of currently-used anticancer agents. Among the numerous natural products, acting via different mechanisms of action, microtubule-targeting agents (MTAs) have a high therapeutic potential, since they disrupt the abnormal cancer cell growth, interfering with the continuous mitotic division. Vinca alkaloids (VAs) are the earliest developed MTAs and approved for clinical use (Vincristine, Vinblastine, Vinorelbine, Vindesine, and Vinflunine) as agents in the treatment of hematological and lymphatic neoplasms. Here, we review the state-of-art of VAs, discussing their mechanism of action and pharmacokinetic properties and highlighting their therapeutic relevance and toxicological profile. Additionally, we briefly disclosed the technological approaches faced so far to ameliorate the pharmacological properties, as well as to avoid the drug resistance. Lastly, we introduced the recent advances in the discovery of new derivatives.

High expression of class III β-tubulin has no impact on functional cancer cell growth inhibition of a series of key vinblastine analogs

Clinical association studies have implicated high expression of class III β-tubulin as a predictive factor for lower response rates and reduced overall survival in patients receiving tubulin binding drugs, most notably the taxanes. Because of the implications, we examined a series of key vinblastine analogs that emerged from our studies in functional cell growth inhibition assays for their sensitivity to high expression of class III β-tubulin (human non-small cell lung cancer cell line A549 vs taxol-resistant A549-T24). Unlike taxol, vinblastine and a set of key analogs 3-10 did not exhibit any loss in sensitivity toward A549-T24. The results suggest that vinblastine and related analogs are not likely prone to resistance derived from high expression of class III β-tubulin unlike the taxanes. Most significant are the results with 4-6, a subset of 20' amide vinblastine analogs. They match or exceed the potency of vinblastine and they display more potent activity against taxol-resistant A549-T24 than even wild type A549 cells (1.2-2-fold), complementing our prior observations that they also display no sensitivity to overexpression of Pgp (HCT116/VM46 vs HCT116) and are not subject to resistance derived from Pgp efflux.

The Difference a Single Atom Can Make: Synthesis and Design at the Chemistry-Biology Interface

A Perspective of work in our laboratory on the examination of biologically active compounds, especially natural products, is presented. In the context of individual programs and along with a summary of our work, selected cases are presented that illustrate the impact single atom changes can have on the biological properties of the compounds. The examples were chosen to highlight single heavy atom changes that improve activity, rather than those that involve informative alterations that reduce or abolish activity. The examples were also chosen to illustrate that the impact of such single-atom changes can originate from steric, electronic, conformational, or H-bonding effects, from changes in functional reactivity, from fundamental intermolecular interactions with a biological target, from introduction of a new or altered functionalization site, or from features as simple as improvements in stability or physical properties. Nearly all the examples highlighted represent not only unusual instances of productive deep-seated natural product modifications and were introduced through total synthesis but are also remarkable in that they are derived from only a single heavy atom change in the structure.

Vinblastine 20' Amides: Synthetic Analogues That Maintain or Improve Potency and Simultaneously Overcome Pgp-Derived Efflux and Resistance

A series of 180 vinblastine 20' amides were prepared in three steps from commercially available starting materials, systematically exploring a typically inaccessible site in the molecule enlisting a powerful functionalization strategy. Clear structure-activity relationships and a structural model were developed in the studies which provided many such 20' amides that exhibit substantial and some even remarkable enhancements in potency, many that exhibit further improvements in activity against a Pgp overexpressing resistant cancer cell line, and an important subset of the vinblastine analogues that display little or no differential in activity against a matched pair of vinblastine sensitive and resistant (Pgp overexpressing) cell lines. The improvements in potency directly correlated with target tubulin binding affinity, and the reduction in differential functional activity against the sensitive and Pgp overexpressing resistant cell lines was found to correlate directly with an impact on Pgp-derived efflux.

Key analogs of a uniquely potent synthetic vinblastine that contain modifications of the C20' ethyl substituent

A key series of vinblastine analogs 7-13, which contain modifications to the C20' ethyl group, was prepared with use of two distinct synthetic approaches that provide modifications of the C20' side chain containing linear and cyclized alkyl groups or added functionalized substituents. Their examination revealed the unique nature of the improved properties of the synthetic vinblastine 6, offers insights into the origins of its increased tubulin binding affinity and 10-fold improved cell growth inhibition potency, and served to probe a small hydrophobic pocket anchoring the binding of vinblastine with tubulin. Especially noteworthy were the trends observed with substitution of the terminal carbon of the ethyl group that, with the exception of 9 (R=F vs H, equipotent), led to remarkably substantial reductions in activity (>10-fold): R=F (equipotent with H)>N3, CN (10-fold)>Me (50-fold)>Et (100-fold)>OH (inactive). This is in sharp contrast to the maintained (7) or enhanced activity (6) observed with its incorporation into a cyclic C20'/C15'-fused six-membered ring.

Synthetic route to chiral indolines via Cu(OAc)2-catalyzed ring-opening/C(sp2)–H activation of activated aziridines

A novel synthetic route to functionalized indolinesviaLewis acid catalyzed ring-opening of activated aziridines followed by Cu(OAc)₂-mediated intramolecular C–H amination in one-pot with excellent enantio- and diastereospecificity (ee 99%; de >99%).

Synthesis of Bisindole Alkaloids from the Apocynaceae Which Contain a Macroline or Sarpagine Unit: A Review

Bisindole natural products consist of two monomeric indole alkaloid units as their obligate constituents. Bisindoles are more potent with respect to their biological activity than their corresponding monomeric units. In addition, the synthesis of bisindoles are far more challenging than the synthesis of monomeric indole alkaloids. Herein is reviewed the enantiospecific total and partial synthesis of bisindole alkaloids isolated primarily from the Alstonia genus of the Apocynaceae family. The monomeric units belong to the sarpagine, ajmaline, macroline, vobasine, and pleiocarpamine series. An up-to-date discussion of their isolation, characterization, biological activity as well as approaches to their partial and total synthesis by means of both synthetic and biosynthetic strategies are presented.

Total synthesis of a key series of vinblastines modified at C4 that define the importance and surprising trends in activity

An expanded scope of a powerful oxadiazole cycloaddition cascade was used for the total synthesis of 17 synthetic vinblastines systematically modified at C4. Their evaluation defined a surprisingly significant impact and provided an unrecognized role of the C4 substituent on activity.

The total synthesis and evaluation of a key systematic series of vinblastines that incorporate the first deep-seated changes to the substituent at C4 are detailed. The synthetic approach features an expanded and redefined scope of a 1,3,4-oxadiazole [4 + 2]/[3 + 2] cycloaddition cascade in which electronically mismatched electron-deficient trisubstituted alkenes and unactivated trisubstituted alkenes were found to productively initiate the cycloaddition cascade with tethered electron-deficient 1,3,4-oxadiazoles. Such cycloaddition cascades were used to directly introduce altered C4 substituents, providing the basis for concise total syntheses of a series of C4 modified vindolines and their subsequent single-step incorporation into the corresponding synthetic vinblastines in routes as short as 8–12 steps. Evaluation of the synthetic vinblastines revealed a surprisingly large impact and role of the C4 substituent on activity even though it was previously not thought to intimately interact with the biological target tubulin. Only the introduction of a C4 methyl ester, a constitutional isomer of vinblastine in which the carbonyl carbon and ester oxygen of the C4 acetate are transposed, provided a synthetic vinblastine that matched the potency of the natural product. In contrast, even introduction of a C4 acetamide or N-methyl carboxamide, which incorporate single heavy atom exchanges (amide NH for ester oxygen) in vinblastine or the C4 methyl ester, provided compounds that were ≥10-fold less active than vinblastine. Other C4 acetate replacements, including a C4 amine, carboxylic acid, hydroxymethyl or acetoxymethyl group, led to even greater reductions in potency. Even replacement of the C4 acetoxy group or its equally active C4 methyl ester with an ethyl or isopropyl ester led to 10-fold or more reductions in activity. These remarkable trends in activity, which correlate with relative tubulin binding affinities, retrospectively may be ascribed to the role the substituent serves as a H-bond acceptor for α-tubulin Lys336 and Asn329 side chains at a site less tolerant of a H-bond donor, placing the methyl group of the C4 acetate or C4 methyl ester in a spatially restricted and well-defined hydrophobic half pocket created by a surrounding well-ordered loop. This remarkable impact of the C4 substituent, its stringency, and even the magnitude of its effect are extraordinary, and indicate that its presence was selected in Nature to enhance the effects of vinblastine and related natural products.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Synthesis of a Potent Vinblastine: Rationally Designed Added Benign Complexity

Many natural products, including vinblastine, have not been easily subjected to simplifications in their structures by synthetic means or modifications by late-stage semisynthetic derivatization in ways that enhance their biological potency. Herein, we detail a synthetic vinblastine that incorporates added benign complexity (ABC), which improves activity 10-fold, and is now accessible as a result of advances in the total synthesis of the natural product. The compound incorporates designed added molecular complexity but no new functional groups and maintains all existing structural and conformational features of the natural product. It constitutes a member of an analogue class presently inaccessible by semisynthetic derivatization of the natural product, by its late-stage functionalization, or by biosynthetic means. Rather, it was accessed by synthetic means, using an appropriately modified powerful penultimate single-step vindoline-catharanthine coupling strategy that proceeds with a higher diastereoselectivity than found for the natural product itself.

Tandem Intramolecular Diels-Alder/1,3-Dipolar Cycloaddition Cascade of 1,3,4-Oxadiazoles: Initial Scope and Applications

A summary of the development and initial studies on the scope of a powerful tandem intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of 1,3,4-oxadiazoles is detailed and provides the foundation for its subsequent use in organic synthesis. Implemented with substrates in which both the initiating dienophile and subsequent dipolarophile are tethered to the 1,3,4-oxadiazoles, the studies expanded the scope of oxadiazoles that participate in the reaction cascade, permitted the use of differentiated dienophiles and dipolarophiles, extended their use to unsymmetrical dienophiles and dipolarophiles, provided exclusive control of the cycloaddition regioselectivities, and imposed exquisite control on the cycloaddition stereochemistry. As key reactivity and stereochemical features of the reactions were being defined, the cascade cycloaddition reaction was implemented in the total synthesis of a series of alkaloids including (-)-vindoline, (-)-vindorosine, the closely related natural products (+)-4-desacetoxyvindoline and (+)-4-desacetoxyvindorosine, natural minovine, (+)-N-methylaspidospermidine, (+)-spegazzinine, (-)-aspidospermine, and a number of key analogues. Most recently, it was used in the divergent total syntheses of (+)-fendleridine, (-)-kopsinine, (-)-kopsifoline D, and (-)-deoxoapodine, in which four different strategic bonds in four different classes of the hexacyclic alkaloids were formed from a common cascade cycloaddition intermediate. A large number of vindoline analogues were prepared by variations on the cascade cycloaddition reaction for single step incorporation into analogues of vinblastine. These structural changes to vindoline permitted both systematic alterations to the peripheral substituents as well as deep-seated changes to the core structure and embedded functionality of vinblastine not previously accessible. Although explored initially for accessing vindoline and vinblastine, the use of the cycloaddition cascade in the total synthesis of an impressive range of additional natural products illustrate the power of the methodology. Alternative tethering strategies for the cascade cycloaddition reaction, combined intramolecular and intermolecular variants of either the initiating Diels-Alder reaction or the subsequent carbonyl ylide 1,3-dipolar cycloaddition, an expanded examination of the tethered dipolarophile scope, and applications to additional natural product classes represent attractive areas for future work.

Total Synthesis of (-)-Vindoline and (+)-4-epi-Vindoline Based on a 1,3,4-Oxadiazole Tandem Intramolecular [4 + 2]/[3 + 2] Cycloaddition Cascade Initiated by an Allene Dienophile

It is reported that an allene dienophile can initiate a tandem intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of 1,3,4-oxadiazoles, that the intermediate cross-conjugated 1,3-dipole (a carbonyl ylide) can participate in an ensuing [3 + 2] dipolar cycloaddition in a remarkably effective manner, and that the reaction can be implemented to provide the core pentacyclic ring system of vindoline. Its discovery improves a previous total synthesis of (-)-vindoline and was used in a total synthesis of (+)-4-epi-vindoline and (+)-4-epi-vinblastine that additionally enlists an alternative series of late-stage transformations.

Total synthesis of (-)-kopsinine and ent-(+)-kopsinine

The total synthesis of (-)-kopsinine and its unnatural enantiomer is detailed, enlisting a late-stage SmI₂-mediated transannular free radical conjugate addition reaction for construction of the core bicyclo[2.2.2]octane ring system with strategic C21-C2 bond formation. Key to the approach is assemblage of the underlying skeleton by an intramolecular [4+2]/[3+2] cycloaddition cascade of a 1,3,4-oxadiazole that provided the precursor C21 functionalized pentacyclic ring system 1 in a single step in which the C3 methyl ester found in the natural product served as a key 1,3,4-oxadiazole substituent, activating it for participation in the initiating Diels-Alder reaction and stabilizing the intermediate 1,3-dipole.

Total synthesis of vinblastine, related natural products, and key analogues and development of inspired methodology suitable for the systematic study of their structure-function properties

Biologically active natural products composed of fascinatingly complex structures are often regarded as not amenable to traditional systematic structure-function studies enlisted in medicinal chemistry for the optimization of their properties beyond what might be accomplished by semisynthetic modification. Herein, we summarize our recent studies on the Vinca alkaloids vinblastine and vincristine, often considered as prototypical members of such natural products, that not only inspired the development of powerful new synthetic methodology designed to expedite their total synthesis but have subsequently led to the discovery of several distinct classes of new, more potent, and previously inaccessible analogues. With use of the newly developed methodology and in addition to ongoing efforts to systematically define the importance of each embedded structural feature of vinblastine, two classes of analogues already have been discovered that enhance the potency of the natural products >10-fold. In one instance, remarkable progress has also been made on the refractory problem of reducing Pgp transport responsible for clinical resistance with a series of derivatives made accessible only using the newly developed synthetic methodology. Unlike the removal of vinblastine structural features or substituents, which typically has a detrimental impact, the additions of new structural features have been found that can enhance target tubulin binding affinity and functional activity while simultaneously disrupting Pgp binding, transport, and functional resistance. Already analogues are in hand that are deserving of full preclinical development, and it is a tribute to the advances in organic synthesis that they are readily accessible even on a natural product of a complexity once thought refractory to such an approach.

Total syntheses of (-)-kopsifoline D and (-)-deoxoapodine: divergent total synthesis via late-stage key strategic bond formation

Divergent total syntheses of (-)-kopsifoline D and (-)-deoxoapodine are detailed from a common pentacyclic intermediate 15, enlisting the late-stage formation of two different key strategic bonds (C21-C3 and C21-O-C6) unique to their hexacyclic ring systems that are complementary to its prior use in the total syntheses of kopsinine (C21-C2 bond formation) and (+)-fendleridine (C21-O-C19 bond formation). The combined efforts represent the total syntheses of members of four classes of natural products from a common intermediate functionalized for late-stage formation of four different key strategic bonds uniquely embedded in each natural product core structure. Key to the first reported total synthesis of a kopsifoline that is detailed herein was the development of a transannular enamide alkylation for late-stage formation of the C21-C3 bond with direct introduction of the reactive indolenine C2 oxidation state from a penultimate C21 functionalized Aspidosperma-like pentacyclic intermediate. Central to the assemblage of the underlying Apidosperma skeleton is a powerful intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of a 1,3,4-oxadiazole that provided the functionalized pentacyclic ring system 15 in a single step in which the C3 methyl ester found in the natural products served as a key 1,3,4-oxadiazole substituent, activating it for participation in the initiating Diels-Alder reaction and stabilizing the intermediate 1,3-dipole.

Transition metal-promoted biomimetic steps in total syntheses

Important biomimetic steps in natural product synthesis have been promoted by transition metals, as exemplified by this beautiful ruthenium-catalyzed rearrangement of an endoperoxide into elysiapyrone A. Such reactions are supposed to occur during the biosynthesis, yet under different catalysis conditions.

Transannular Diels-Alder/1,3-dipolar cycloaddition cascade of 1,3,4-oxadiazoles: total synthesis of a unique set of vinblastine analogues

A powerful tandem [4 + 2]/[3 + 2] cycloaddition cascade of 1,3,4-oxadiazoles initiated by a transannular [4 + 2] cycloaddition is detailed. An impressive four rings, four carbon-carbon bonds, and six stereocenters are set on each site of the newly formed central six-membered ring in a cascade thermal reaction that proceeds at temperatures as low as 80 °C. The resulting cycloadducts provide the basis for the synthesis of unique analogues of vinblastine containing metabolically benign deep-seated cyclic modifications at the C3/C4 centers of the vindoline-derived subunit of the natural product.

Potent Vinblastine C20' Ureas Displaying Additionally Improved Activity Against a Vinblastine-Resistant Cancer Cell Line

A series of disubstituted C20'-urea derivatives of vinblastine were prepared from 20'-aminovinblastine that was made accessible through a unique Fe(III)/NaBH₄- mediated alkene functionalization reaction of anhydrovinblastine. Three analogs were examined across a panel of 15 human tumor cell lines, displaying remarkably potent cell growth inhibition activity (avg. IC₅₀ = 200-300 pM), being 10-200-fold more potent than vinblastine (avg. IC₅₀ = 6.1 nM). Significantly, the analogs also display further improved activity against the vinblastine-resistant HCT116/VM46 cell line that bears the clinically relevant overexpression of Pgp, exhibiting IC₅₀ values on par with that of vinblastine against the sensitive HCT116 cell line, 100-200-fold greater than the activity of vinblastine against the resistant HCT116/VM46 cell line, and display a reduced 10-20-fold activity differential between the matched sensitive and resistant cell lines (vs 100-fold for vinblastine).

Synthetic route to chiral indolines via ring-opening/C-N cyclization of activated 2-haloarylaziridines

A practical approach for the synthesis of 3-substituted indolines via regio- and stereoselective SN2-type ring-opening of 2-(2-halophenyl)-N-tosylaziridines with heteroatomic nucleophiles (O, N, and S) followed by palladium-catalyzed intramolecular C-N cyclization is reported in excellent yields (up to >99%) and enantiomeric excess (ee 99%).

Hypervalent iodine(III)-promoted intermolecular C-C coupling of vindoline with β-ketoesters and related substrates

The regioselective intermolecular coupling reaction of vindoline with a wide range of substrates including β-ketoesters, β-diketones, β-ketoaldehydes, β-ketonitriles, malononitriles, and β-cyanoesters provides an opportunity for the synthesis of vinblastine analogues containing deep-seated changes in the upper velbanamine subunit. The transition-metal-free hypervalent iodine(III)-promoted intermolecular sp(3)/sp(2) coupling, representing a special class of selective C-H activation with direct carbon-carbon bond formation, proceeds with generation of a quaternary center capable of incorporation of the vinblastine C16' methyl ester and functionalized for subsequent divergent heterocycle introduction.

Total synthesis of kopsinine

The use of a powerful intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of an 1,3,4-oxadiazole in the divergent total synthesis of kopsinine (1), featuring an additional unique SmI(2)-promoted transannular cyclization reaction for formation of the bicyclo[2.2.2]octane central to its hexacyclic ring system, is detailed.

Diversity oriented synthesis of a vinblastine-templated library of 7-aryl-octahydroazonino[5,4-b]indoles via a three-component reaction

A vinblastine-templated library of 7-aryl-octahydroazonino[5,4-b]indoles was prepared by a three-component reaction from indolizino[8,7-b]indoles, chloroformates, and activated arenes via a chloroformate mediated fragmentation of the indolizinoindole nucleus followed by insertion of an activated arene. In addition to N3-carbamoyl-7-aryl-octahydroazonino[5,4-b]indoles prepared in one step, a wide range of N3-substituted substrates were synthesized in one pot via the derivatization of a versatile N3-H-azonino[5,4-b]indole intermediate generated in situ by application of the same strategy. A subset of 308 compounds out of a virtual library of 3216, representing 13 different chemotypes, was prepared by high throughput solution-phase synthesis and subsequently purified by mass-triggered high performance liquid chromatography (HPLC). A total of 188 compounds with a minimum purity of 80% by UV214 nm and 85% by evaporative light scattering detection (ELSD) was isolated for primary screening.

Total synthesis and evaluation of vinblastine analogues containing systematic deep-seated modifications in the vindoline subunit ring system: core redesign

The total synthesis of a systematic series of vinblastine analogues that contain deep-seated structural modifications to the core ring system of the lower vindoline subunit is described. Complementary to the vindoline 6,5 DE ring system, compounds with 5,5, 6,6, and the reversed 5,6 membered DE ring systems were prepared. Both the natural cis and unnatural trans 6,6-membered ring systems proved accessible, with the latter representing a surprisingly effective class for analogue design. Following Fe(III)-promoted coupling with catharanthine and in situ oxidation to provide the corresponding vinblastine analogues, their evaluation provided unanticipated insights into how the structure of the vindoline subunit contributes to activity. Two potent analogues (81 and 44) possessing two different unprecedented modifications to the vindoline subunit core architecture were discovered that matched the potency of the comparison natural products and both lack the 6,7-double bond whose removal in vinblastine leads to a 100-fold drop in activity.

A remarkable series of vinblastine analogues displaying enhanced activity and an unprecedented tubulin binding steric tolerance: C20' urea derivatives

A systematic series of previously inaccessible key C20' urea and thiourea derivatives of vinblastine were prepared from 20'-aminovinblastine that was made accessible through a unique Fe(III)/NaBH(4)-mediated alkene functionalization reaction of anhydrovinblastine. Their examination defined key structural features of the urea-based analogues that contribute to their properties and provided derivatives that match or exceed the potency of vinblastine by as much as 10-fold in cell-based functional assays, which is directly related to their relative tubulin binding affinity. In contrast to expectations based on apparent steric constraints of the tubulin binding site surrounding the vinblastine C20' center depicted in an X-ray cocrystal structure, remarkably large C20' urea derivatives are accommodated.

New insights into the mechanism and an expanded scope of the Fe(III)-mediated vinblastine coupling reaction

A definition of the scope of aromatic substrates that participate with catharanthine in an Fe(III)-mediated coupling reaction, an examination of the key structural features of catharanthine required for participation in the reaction, and the development of a generalized indole functionalization reaction that bears little structural relationship to catharanthine itself are detailed. In addition to providing insights into the mechanism of the Fe(III)-mediated coupling reaction of catharanthine with vindoline suggesting the reaction conducted in acidic aqueous buffer may be radical mediated, the studies provide new opportunities for the preparation of previously inaccessible vinblastine analogs and define powerful new methodology for the synthesis of indole-containing natural and unnatural products.

Domino constructions of pentacyclic indeno[2,1-c]quinolines and pyrano[4,3-b]oxepines by [4+1]/[3+2+1]/[5+1] and [4+3] multiple cyclizations

The novel three-component domino reactions have been discovered. The reactions are easy to perform simply by mixing three common reactants in HOAc under microwave heating. The reaction proceeds at fast rates and can be finished within 20–36 min, which makes work-up convenient. Most of multiple stereocenters and geometry have been controlled well. The stereochemistry has been unequivocally determined by X-ray structural analysis.

Modifications on the basic skeletons of vinblastine and vincristine

The synthetic investigation of biologically active natural compounds serves two main purposes: (i) the total synthesis of alkaloids and their analogues; (ii) modification of the structures for producing more selective, more effective, or less toxic derivatives. In the chemistry of dimeric Vinca alkaloids enormous efforts have been directed towards synthesizing new derivatives of the antitumor agents vinblastine and vincristine so as to obtain novel compounds with improved therapeutic properties.

Divergent total syntheses of (-)-aspidospermine and (+)-spegazzinine

Divergent total syntheses of (+)-spegazzinine (1) and (-)-aspidospermine (2) and their extensions to the synthesis of C19-epi-aspidospermine and C3-epi-spegazzinine are detailed, confirming the relative stereochemistry and establishing the absolute configuration of (+)-spegazzinine. A powerful intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of a 1,3,4-oxadiazole provided the pentacyclic skeleton and all the requisite stereochemistry of the natural products in a single reaction that forms three rings, four C-C bonds, and five stereocenters.

Iron(III)/NaBH4-mediated additions to unactivated alkenes: synthesis of novel 20'-vinblastine analogues

An Fe(III)/NaBH(4)-mediated reaction for the functionalization of unactivated alkenes is described defining the alkene substrate scope, establishing the exclusive Markovnikov addition, exploring a range of free radical traps, examining the Fe(III) salt and initiating hydride source, introducing H(2)O-cosolvent mixtures, and exploring catalytic variants. Its use led to the preparation of a novel, potent, and previously inaccessible C20'-vinblastine analogue.

10'-Fluorovinblastine and 10'-Fluorovincristine: Synthesis of a Key Series of Modified Vinca Alkaloids

A study on the impact of catharanthine C10 and C12 indole substituents on the biomimetic Fe(III)-mediated coupling with vindoline led to the discovery and characterization of two new and substantially more potent derivatives, 10'-fluorovinblastine and 10'-fluorovincristine. In addition to defining a pronounced and unanticipated substituent effect on the biomimetic coupling, fluorine substitution at C10', which minimally alters the natural products, was found to uniquely enhance the activity 8-fold against both sensitive (IC(50) = 800 pM, HCT116) and vinblastine-resistant tumor cell lines (IC(50) = 80 nM, HCT166/VM46). As depicted in the X-ray structure of vinblastine bound to tubulin, this site resides at one end of the upper portion of the T-shaped conformation of the tubulin-bound molecule, suggesting the 10'-fluorine substituent makes critical contacts with the protein at a hydrophobic site uniquely sensitive to steric interactions.