Epoxide-opening cascades in the synthesis of polycyclic polyether natural products

Studies on the Biomimetic Synthesis of Marine Ladder Polyethers via Endo-Selective Epoxide-to-Epoxonium Ring-Opening Cascades

Marine ladder polyethers have attracted the attention of chemists and biologists because of their potent biological activities. Synthetic chemists have attempted to construct their polyether frameworks by epoxide ring-opening cascades, as Nakanishi hypothesis describes. However, Baldwin's rules of ring closure state that exo-selective intramolecular cyclization of epoxy alcohols is preferred over endo-selective cyclization. Herein, we investigated epoxide ring-opening cascades of polyepoxy alcohols in [EMIM]BF4/PFTB (1-ethyl-3-methylimidazolium tetrafluoroborate /perfluoro-tert-butyl alcohol) and found that all-endo products were formed via epoxide-to-epoxonium ring-opening cyclizations (not restricted by Baldwin's rules, which only apply to intramolecular hydroxyl-to-epoxide cyclizations). We determined that the key factor enabling polyepoxy alcohols to undergo a high proportion of all-endo-selective cyclization was inhibition of exo-selective hydroxyl-to-epoxide cyclization starting from the terminal hydroxyl group of a polyepoxy alcohol. By introducing a slow-release protecting group to the terminal hydroxyl group, we could markedly increase the cyclization yields of polyether fragments with hydrogen atoms at the ring junctions. For the first time, we constructed consecutively fused six-membered-ring and fused seven-, eight-, and nine-membered-ring polyether fragments by epoxide-to-epoxonium ring-opening cyclizations through the addition of a suitable Lewis acid. We also suggest that the biosynthesis of marine ladder polyethers may proceed via epoxide-to-epoxonium ring-opening cyclization of polyepoxide.

Effective In Situ Alkoxide Trapping by TMSX in the Presence of TPP Catalyst in Glycidol Acetal Rearrangements

This study demonstrates a nucleophile-catalyzed, trimethylsilyl halide-promoted rearrangement reaction of glycidol acetals to form halogenated cyclic acetals. The acetal group has been activated selectively in the presence of trimethylsilyl cation, which is used as in situ-generated alkoxide trapping reagent. Nucleophilic chloride and bromide ions participate in addition reactions with epoxides predominantly via SN1-type epoxide opening, while non-nucleophilic iodide and triflate ions induce a positive charge at the epoxide carbon. A systematic investigation of acetal-initiated polyene cyclization of epoxy polyenes has been conducted using bicyclic epoxonium ions as transient intermediates. Unfavorable orbital orientation and other stereoelectronic factors hinder the much-anticipated polyene cyclizations. The potential of this method has been showcased through its application in the total synthesis of parvistone A, a chlorinated styryllactone.

Exploring the synthetic potential of epoxide ring opening reactions toward the synthesis of alkaloids and terpenoids: a review

Epoxides are oxygen containing heterocycles which are significantly employed as crucial intermediates in various organic transformations. They are considered highly reactive three-membered heterocycles due to ring strain and they undergo epoxide ring opening reactions with diverse range of nucleophiles. Epoxide ring-opening reactions have gained prominence as flexible and effective means to obtain various functionalized molecules. These reactions have garnered substantial attention in organic synthesis, driven by the need to comprehend the synthesis of biologically and structurally important organic compounds. They have also found applications in the synthesis of complex natural products. In this review article, we have summarized the implementation of epoxide ring opening reactions in the synthesis of alkaloids and terpenoids based natural products reported within the last decade (2014-2023).

Gold-catalyzed endo-selective Ring-opening of Epoxides and its Application in Construction of Poly-ethers

Tetrahydropyran and tetrahydropyran-fused poly-ethers scaffolds are found in many classes of natural products and medicinally relevant small molecules. Here we describe a catalytic system for 6-endo selective ring-opening of epoxides by Au(I) or Au(III) catalyst that provides rapid access to various tetrahydropyran-derived motifs. It also could efficiently construct the subunits of marine ladder-like poly-ethers through emulating the Nakanishi's hypothesis on the biosynthesis of these toxins. The synthetic utility of this method is also demonstrated in the preparation of the tricyclic core of tetrahydropyran-containing macrolide natural products lituarines A-C.

Establishment of "Ring-Size-Divergent" Synthetic Strategy: Divergent Synthesis, Stereochemical Assignments, and Biological Activity Studies of Nerolidol-Type Sesquiterpenoids and Feroniellins

Biomimetic epoxide-opening cascade cyclizations of polyepoxides enable the efficient and rapid construction of polyether skeletons. In this study, we discovered a method for switching the cyclization mode from tetrahydrofuran to tetrahydropyran (THP) formation in epoxide-opening cascades of polyepoxides. The THP formation proceeded via an epoxonium-ion intermediate by simple heating in neutral water. Next, by expanding the switching reaction, we successfully established a "ring-size-divergent" synthetic strategy that enabled the synthesis of the five-, six-, and seven-membered ether rings from identical diepoxide cyclization precursors under simple acidic or neutral conditions. The "ring-size-divergent" synthetic strategy was applied to the short divergent synthesis of nerolidol-type sesquiterpenoids and feroniellins, resulting in the revision of the proposed stereochemistry of certain natural products and the determination of all of the absolute configurations. Additionally, the anti-inflammatory activities of the synthetic samples were evaluated.

Anion-(π)n -π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)n -π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)n -π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)n -π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.

The Origin of Anion-π Autocatalysis

The autocatalysis of epoxide-opening ether cyclizations on the aromatic surface of anion-π catalysts stands out as a leading example of emergent properties expected from the integration of unorthodox interactions into catalysis. A working hypothesis was proposed early on, but the mechanism of anion-π autocatalysis has never been elucidated. Here, we show that anion-π autocatalysis is almost independent of peripheral crowding in substrate and product. Inaccessible asymmetric anion-π autocatalysis and sometimes erratic reproducibility further support that the origin of anion-π autocatalysis is more complex than originally assumed. The apparent long-distance communication without physical contact calls for the inclusion of water between substrate and product on the catalytic aromatic surface. Efficient anion-π autocatalysis around equimolar amounts but poor activity in dry solvents and with excess water indicate that this inclusion of water requires high precision. Computational models suggest that two water molecules transmit dual substrate activation by the product and serve as proton shuttles along antiparallel but decoupled hydrogen-bonded chains to delocalize and stabilize evolving charge density in the transition state by "anion-π double bonds". This new transition-state model of anion-π autocatalysis provides a plausible mechanism that explains experimental results and brings anion-π catalysis to an unprecedented level of sophistication.

Theoretical Study on the Origin of Abnormal Regioselectivity in Ring-Opening Reaction of Hexafluoropropylene Oxide

That nucleophiles preferentially attack at the less sterically hindered carbon of epoxides under neutral and basic conditions has been generally accepted as a fundamental rule for predicting the regioselectivity of this type of reaction. However, this rule does not hold for perfluorinated epoxides, such as hexafluoropropylene oxide (HFPO), in which nucleophiles were found to attack at the more hindered CF₃ substituted β-C rather than the fluorine substituted α-C. In this contribution, we aim to shed light on the nature of this intriguing regioselectivity by density functional theory methods. Our calculations well reproduced the observed abnormal regioselectivities and revealed that the unusual regiochemical preference for the sterically hindered β-C of HFPO mainly arises from the lower destabilizing distortion energy needed to reach the corresponding ring-opening transition state. The higher distortion energy required for the attack of the less sterically hindered α-C results from a significant strengthening of the C(α)-O bond by the negative hyperconjugation between the lone pair of epoxide O atom and the antibonding C-F orbital.

Rational Tuning of the Reactivity of Three-Membered Heterocycle Ring Openings via SN 2 Reactions

The development of small-molecule covalent inhibitors and probes continuously pushes the rapidly evolving field of chemical biology forward. A key element in these molecular tool compounds is the "electrophilic trap" that allows a covalent linkage with the target enzyme. The reactivity of this entity needs to be well balanced to effectively trap the desired enzyme, while not being attacked by off-target nucleophiles. Here we investigate the intrinsic reactivity of substrates containing a class of widely used electrophilic traps, the three-membered heterocycles with a nitrogen (aziridine), phosphorus (phosphirane), oxygen (epoxide) or sulfur atom (thiirane) as heteroatom. Using quantum chemical approaches, we studied the conformational flexibility and nucleophilic ring opening of a series of model substrates, in which these electrophilic traps are mounted on a cyclohexene scaffold (C₆ H₁₀ Y with Y=NH, PH, O, S). It was revealed that the activation energy of the ring opening does not necessarily follow the trend that is expected from C-Y leaving-group bond strength, but steeply decreases from Y=NH, to PH, to O, to S. We illustrate that the HOMO_Nu -LUMO_Substrate interaction is an all-important factor for the observed reactivity. In addition, we show that the activation energy of aziridines and phosphiranes can be tuned far below that of the corresponding epoxides and thiiranes by the addition of proper electron-withdrawing ring substituents. Our results provide mechanistic insights to rationally tune the reactivity of this class of popular electrophilic traps and can guide the experimental design of covalent inhibitors and probes for enzymatic activity.

Precyclization Conformer Profiles of -SiR3+- and -Bcat+-Activated Linear Si-Protected Hexitols Explain Condensative Cyclization Selectivities

The condensative cyclization of sp³ C-O bonds in per-silylated hexitols is investigated by computation. Conformer searches using the Monte Carlo algorithm, followed by successively higher levels of theory (MMFF, PM3, and B3LYP), of -SiR₃⁺- and -Bcat⁺-activated substrates lead to structures primed for intramolecular chemistry. Silane activation features O4 to C1 attack, while borane activation suggests boronium ions that activate O5 to C2 reactivity. This, in conjunction with Boltzmann population analysis, parallels reported reactivity for sorbitol, mannitol, and galactitol. Calculations using the meta-hybrid M06-2X functional additionally provide free-energy profiles for each cyclization event. In most of the cases presented, precyclization conformers that position a nucleophilic oxygen less than 3.0 Å from the C-O leaving group correlate to efficient experimental reactivities. Two examples of galactitol containing bridging silyl groups are analyzed computationally, and the experimental outcomes match predictions. The computational regime presented is a step closer to providing predictive power for the reduction of per-functionalized molecules.

Highly Regio- and Stereoselective Intramolecular Rearrangement of Glycidol Acetal to Alkoxy Cyclic Acetals

A new class of highly regio- and stereoselective intramolecular rearrangements of glycidol acetals to produce alkoxylated 1,3-dioxolane/1,3-dioxane is demonstrated. Selective Lewis acid activation of acetal generates an oxocarbenium ion that initiates the epoxide ring-opening event, giving the bicyclic [3,1,0]epoxonium ion intermediate that undergoes exo/endo-selective opening by a tethered alkoxide. Mechanistic insights into preferential acetal activation over the epoxide were obtained by crossover experiments. The method was applied in the total synthesis of parvistone B and its 8-ethoxy analogue.

Gambierdiscus and Its Associated Toxins: A Minireview

Gambierdiscus is a dinoflagellate genus widely distributed throughout tropical and subtropical regions. Some members of this genus can produce a group of potent polycyclic polyether neurotoxins responsible for ciguatera fish poisoning (CFP), one of the most significant food-borne illnesses associated with fish consumption. Ciguatoxins and maitotoxins, the two major toxins produced by Gambierdiscus, act on voltage-gated channels and TRPA1 receptors, consequently leading to poisoning and even death in both humans and animals. Over the past few decades, the occurrence and geographic distribution of CFP have undergone a significant expansion due to intensive anthropogenic activities and global climate change, which results in more human illness, a greater public health impact, and larger economic losses. The global spread of CFP has led to Gambierdiscus and its toxins being considered an environmental and human health concern worldwide. In this review, we seek to provide an overview of recent advances in the field of Gambierdiscus and its associated toxins based on the existing literature combined with re-analyses of current data. The taxonomy, phylogenetics, geographic distribution, environmental regulation, toxin detection method, toxin biosynthesis, and pharmacology and toxicology of Gambierdiscus are summarized and discussed. We also highlight future perspectives on Gambierdiscus and its associated toxins.

Natural Polyether Ionophores and Their Pharmacological Profile

This review is devoted to the study of the biological activity of polyether ionophores produced by bacteria, unicellular marine algae, red seaweeds, marine sponges, and coelenterates. Biological activities have been studied experimentally in various laboratories, as well as data obtained using QSAR (Quantitative Structure-Activity Relationships) algorithms. According to the data obtained, it was shown that polyether toxins exhibit strong antibacterial, antimicrobial, antifungal, antitumor, and other activities. Along with this, it was found that natural polyether ionophores exhibit such properties as antiparasitic, antiprotozoal, cytostatic, anti-mycoplasmal, and antieczema activities. In addition, polyethers have been found to be potential regulators of lipid metabolism or inhibitors of DNA synthesis. Further study of the mechanisms of action and the search for new polyether ionophores and their derivatives may provide more effective therapeutic natural polyether ionophores for the treatment of cancer and other diseases. For some polyether ionophores, 3D graphs are presented, which demonstrate the predicted and calculated activities. The data presented in this review will be of interest to pharmacologists, chemists, practical medicine, and the pharmaceutical industry.

Synthetic and biosynthetic methods for selective cyclisations of 4,5-epoxy alcohols to tetrahydropyrans

Tetrahydropyrans (THPs) are common structural motifs found in natural products and synthetic therapeutic molecules. In Nature these 6-membered oxygen heterocycles are often assembled via intramolecular reactions involving either oxy-Michael additions or ring opening of epoxy-alcohols. Indeed, the polyether natural products have been particularly widely studied due to their fascinating structures and important biological properties; these are commonly formed via endo-selective epoxide-opening cascades. In this review we outline synthetic approaches for endo-selective intramolecular epoxide ring opening (IERO) of 4,5-epoxy-alcohols and their applications in natural product synthesis. In addition, the biosynthesis of THP-containing natural products which utilise IERO reactions are reviewed.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

Decoded fingerprints of hyperresponsive, expanding product space: polyether cascade cyclizations as tools to elucidate supramolecular catalysis

Simple enough to be understood and complex enough to be revealing, cascade cyclizations of diepoxides are introduced as new tools to characterize supramolecular catalysis. Decoded product fingerprints are provided for a consistent set of substrate stereoisomers, and shown to report on chemo-, diastereo- and enantioselectivity, mechanism and even autocatalysis. Application of the new tool to representative supramolecular systems reveals, for instance, that pnictogen-bonding catalysis is not only best in breaking the Baldwin rules but also converts substrate diastereomers into completely different products. Within supramolecular capsules, new cyclic hemiacetals from House–Meinwald rearrangements are identified, and autocatalysis on anion–π catalysts is found to be independent of substrate stereochemistry. Decoded product fingerprints further support that the involved epoxide-opening polyether cascade cyclizations are directional, racemization-free, and interconnected, at least partially. The discovery of unique characteristics for all catalysts tested would not have been possible without decoded cascade cyclization fingerprints, thus validating the existence and significance of privileged platforms to elucidate supramolecular catalysis. Once decoded, cascade cyclization fingerprints are easily and broadly applicable, ready for use in the community.

Hyperresponsive XL product space identifies polyether cascade fingerprinting as an attractive tool to elucidate supramolecular catalysis, including pnictogen-bonding, capsule and anion–π catalysts.

Pnictogen-Bonding Catalysis and Transport Combined: Polyether Transporters Made In Situ

The combination of catalysis and transport across lipid bilayer membranes promises directional access to a solvent-free and structured nanospace that could accelerate, modulate, and, at best, enable new chemical reactions. To elaborate on these expectations, anion transport and catalysis with pnictogen and tetrel bonds are combined with polyether cascade cyclizations into bioinspired cation transporters. Characterized separately, synergistic anion and cation transporters of very high activity are identified. Combined for catalysis in membranes, cascade cyclizations are found to occur with a formal rate enhancement beyond one million compared to bulk solution and product formation is detected in situ as an increase in transport activity. With this operational system in place, intriguing perspectives open up to exploit all aspects of this unique nanospace for important chemical transformations.

Enantioselective Synthesis of Spiro-oxiranes: An Asymmetric Addition/Aldol/Spirocyclization Domino Cascade

Enantioenriched spiro-oxiranes bearing three contiguous stereocenters were synthesized using a rhodium-catalyzed asymmetric addition/aldol/spirocyclization sequence. Starting from a linear substrate, the cascade enabled the formation of a spirocyclic framework in a single step. sp² - and sp-hybridized carbon nucleophiles were found to be competent initiators for this cascade, giving arylated or alkynylated products, respectively. Derivatization studies demonstrated the synthetic versatility of both the epoxide and the alkyne moieties of the products. DFT calculations were used to reconcile spectroscopic discrepancies observed between the solution- and solid-state structures of the products.

Bioinspired Ether Cyclizations within a π-Basic Capsule Compared to Autocatalysis on π-Acidic Surfaces and Pnictogen-Bonding Catalysts

While the integration of supramolecular principles in catalysis attracts increasing attention, a direct comparative assessment of the resulting systems catalysts to work out distinct characteristics is often difficult. Herein is reported how the broad responsiveness of ether cyclizations to diverse inputs promises to fill this gap. Cyclizations in the confined, π-basic and Brønsted acidic interior of supramolecular capsules, for instance, are found to excel with speed (exceeding general Brønsted acid and hydrogen-bonding catalysts by far) and selective violations of the Baldwin rules (as extreme as the so far unique pnictogen-bonding catalysts). The complementary cyclization on π-acidic aromatic surfaces remains unique with regard to autocatalysis, which is shown to be chemo- and diastereoselective with regard to product-like co-catalysts but, so far, not enantioselective.

Establishing a "Ring-Size-Divergent" Synthetic Strategy: Synthesis, Structural Revision, and Absolute Configuration of Feroniellins

Feroniellin analogs isolated from Feroniella lucida possess a furanocoumarin skeleton connected to monoterpenic five- to seven-membered ethereal rings by an ether linkage and exhibit a broad spectrum of biological activities. In this contribution, we intended to establish a "ring-size-divergent" synthetic strategy for the monoterpenic five- to seven-membered ethereal rings through the chemical sythesis of feroniellins. The short and comprehensive synthesis of feroniellins was achieved in only two steps from easily available bergamottin based on the "ring-size-divergent" strategy. In addition, these syntheses resulted in revision of the proposed structures for feroniellins A and B and the determination of all the absolute configurations of feroniellins; their preliminary anti-inflammatory activities were investigated as well.

Ambruticins: tetrahydropyran ring formation and total synthesis

The ambruticins are a family of polyketide natural products which exhibit potent antifungal activity. Gene knockout experiments are in accord with the proposal that the tetrahydropyran ring of the ambruticins is formed via the AmbJ catalysed epoxidation of the unsaturated 3,5-dihydroxy acid, ambruticin J, followed by regioselective cyclisation to ambruticin F. Herein, a convergent approach to the total synthesis of ambruticin J is described as well as model studies involving epoxidation and cyclisations of unsaturated hydroxy esters to give tetrahydropyrans and tetrahydrofurans. The total synthesis involves preparation of three key fragments which were united via a Suzuki-Miyaura cross-coupling and Julia-Kocienski olefination to generate the required carbon framework. Global deprotection to a triol and selective oxidation of the primary alcohol gave, after hydrolysis of the lactone, ambruticin J.

Chemical Synthesis of Multifunctional Air Pollutants: Terpene-Derived Nitrooxy Organosulfates

Nitrooxy organosulfates derived from terpenes (NOS_TP) represent an important class of products formed between anthropogenic pollution (e.g., SO₂ and NO_x) and natural emissions. NOS_TP compounds have been consistently detected in atmospheric environments under varying urban influences. Their chemical linkages to both anthroposphere and biosphere make them valuable markers for tracking anthroposphere-biosphere interactions. However, their quantification, formation, and transformation kinetics in atmospheric aerosols are hindered due to the lack of NOS_TP standards. In this work, we developed two routes for the first concise chemical synthesis of eight NOS_TP from terpenes including α-pinene, β-pinene, limonene, limonaketone, and β-caryophyllene. Subsequently, six of the synthesized NOS_TP were for the first time positively identified in ambient aerosol samples, clarifying certain misidentifications in previous studies. More significantly, the availability of authentic standards allows irrefutable observation of three carbon skeleton-rearranged NOS_TP, two derived from α-pinene, and one derived from β-caryophyllene, revealing the occurrence of previously unrecognized transformation pathways in the formation of NOS_TP. Two synthesized NOS_TP from β-pinene and limonene could not be detected, likely due to rapid hydrolysis of their immediate hydroxynitrate precursors outcompeting sulfation. Such mechanistic evidence is valuable in understanding the atmospheric chemistry of NOS_TP and related compounds. This work demonstrates the usefulness of authentic standards in probing the NOS_TP formation mechanisms in the atmosphere. Comparison of NOS_TP ambient samples collected from four Chinese cities in two winter months indicates that anthropogenic chemical factors could outcompete terpene emissions in the formation of NOS_TP.

Prins cyclization-mediated stereoselective synthesis of tetrahydropyrans and dihydropyrans: an inspection of twenty years

Functionalized tetrahydropyran (THP) rings are important building blocks and ubiquitous scaffolds in many natural products and active pharmaceutical ingredients (API). Among various established methods, the Prins reaction has emerged as a powerful technique in the stereoselective synthesis of the tetrahydropyran skeleton with various substituents, and the strategy has further been successfully applied in the total synthesis of bioactive macrocycles and related natural products. In this context, hundreds of valuable contributions have already been made in this area, and the present review is intended to provide the systematic assortment of diverse Prins cyclization strategies, covering the literature reports of the last twenty years (from 2000 to 2019), with an aim to give an overview on exciting advancements in this area and designing new strategies for the total synthesis of related natural products.

Enantiospecific Total Synthesis of (-)-Japonicol C

An efficient and convergent first total syntheses of (±)-japonicol B and (-)-japonicol C have been completed. The notable points of the synthetic route are Lewis-acid-catalyzed Friedel-Crafts reaction for one pot C-C and C-O bond formations resulting in construction of the tricyclic meroterpenoid skeleton, one pot Pd(OH)₂/C-catalyzed isomerization/hydrogenation, and site selective sp³ C-H oxidation.

How Lewis Acids Catalyze Ring-Openings of Cyclohexene Oxide

We have quantum chemically studied the Lewis acid-catalyzed epoxide ring-opening reaction of cyclohexene epoxide by MeZH (Z = O, S, and NH) using relativistic dispersion-corrected density functional theory. We found that the reaction barrier of the Lewis acid-catalyzed epoxide ring-opening reactions decreases upon ascending in group 1 along the series Cs+ > Rb+ > K+ > Na+ > Li+ > H+. Our activation strain and Kohn-Sham molecular orbital analyses reveal that the enhanced reactivity of the Lewis acid-catalyzed ring-opening reaction is caused by the reduced steric (Pauli) repulsion between the filled orbitals of the epoxide and the nucleophile, as the Lewis acid polarizes the filled orbitals of the epoxide more efficiently away from the incoming nucleophile. Furthermore, we established that the regioselectivity of these ring-opening reactions is, aside from the "classical" strain control, also dictated by a hitherto unknown mechanism, namely, the steric (Pauli) repulsion between the nucleophile and the substrate, which could be traced back to the asymmetric orbital density on the epoxide. In all, this work again demonstrates that the concept of Pauli-lowering catalysis is a general phenomenon.

Progress, challenges and future directions of heterocycles as building blocks in iterative methodologies towards sequence-defined oligomers and polymers

Heterocyclic building blocks for iterative methodologies leading to sequence-defined oligomers and polymers are reviewed. Solid- as well as solution-phase methods, challenges surrounding these systems and potential future directions are presented.

An Enolate-Structure-Enabled Anionic Cascade Cyclization Reaction: Easy Access to Complex Scaffolds with Contiguous Six-, Five-, and Four-Membered Rings

Catalyst-free addition of ketone enolate to non-activated multiple C-C bonds involves non-complementary reaction partners and typically requires super-basic conditions. On the other hand, highly aggregated or solvated enolates are not reactive enough to undergo direct addition to alkenes or alkynes. Herein, we report a new anionic cascade reaction for one-step assembly of intriguing molecular scaffolds possessing contiguous six-, five-, and four-membered rings, representing a formal [2+2] enol-allene cycloaddition. Reaction proceeds under very mild conditions and with excellent diastereoselectivity. Deeper mechanistic and computational studies revealed unusually slow proton transfer phenomenon in cyclic ketone intermediate and explained peculiar stereochemical outcome.

An Endo-Selective Epoxide-Opening Cascade for the Fast Assembly of the Polycyclic Core Structure of Marine Ladder Polyethers

The rapid synthesis of marine ladder polyethers from polyepoxide precursors (in analogy with the biosynthetic pathway hypothesized by Nakanishi) is hampered by the fact that the exo-selective epoxide-opening cyclization cascade that gives THF-type polyethers is preferred over the endo-selective cascade that gives the desired products. We found that perfluoro-tert-butanol (PFTB) cooperating with 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF₄ ) can promote endo-selective epoxide-opening cyclization reactions of trisubstituted epoxy alcohols. Starting from readily accessible homochiral polyepoxy alcohols with a methyl group at all the endo-cyclization sites, we were able to construct polyethers up to five consecutive fused 6-, 7-, and/or 8-membered rings in one step. Notably, molecules with the 7/7/6/6 and 7/7/6/7/6 polyether frameworks of hemibrevetoxin B and brevenal, respectively, could be synthesized in 40 % and 17 % chemical yields.

Polyether Natural Product Inspired Cascade Cyclizations: Autocatalysis on π-Acidic Aromatic Surfaces

Anion-π catalysis functions by stabilizing anionic transition states on aromatic π surfaces, thus providing a new approach to molecular transformation. The delocalized nature of anion-π interactions suggests that they serve best in stabilizing long-distance charge displacements. Aiming therefore for an anionic cascade reaction that is as charismatic as the steroid cyclization is for conventional cation-π biocatalysis, reported here is the anion-π-catalyzed epoxide-opening ether cyclizations of oligomers. Only on π-acidic aromatic surfaces having a positive quadrupole moment, such as hexafluorobenzene to naphthalenediimides, do these polyether cascade cyclizations proceed with exceptionally high autocatalysis (rate enhancements k_auto /k_cat >10⁴  m^-1 ). This distinctive characteristic adds complexity to reaction mechanisms (Goldilocks-type substrate concentration dependence, entropy-centered substrate destabilization) and opens intriguing perspectives for future developments.

Palladium-catalyzed synthesis of β-hydroxy compounds via a strained 6,4-palladacycle from directed C-H activation of anilines and C-O insertion of epoxides

A palladium-catalyzed C–H activation of acetylated anilines (acetanilides, 1,1-dimethyl-3-phenylurea, 1-phenylpyrrolidin-2-one, and 1-(indolin-1-yl)ethan-1-one) with epoxides using O-coordinating directing groups was accomplished. This C–H alkylation reaction proceeds via formation of a previously unknown 6,4-palladacycle intermediate and provides rapid access to regioselectively functionalized β-hydroxy products. Notably, this catalytic system is applicable for the gram scale mono-functionalization of acetanilide in good yields. The palladium-catalyzed coupling reaction of the ortho-C(sp²) atom of O-coordinating directing groups with a C(sp³) carbon of chiral epoxides offers diverse substrate scope in good to excellent yields. In addition, further transformations of the synthesized compound led to biologically important heterocycles. Density functional theory reveals that the 6,4-palladacycle leveraged in this work is significantly more strained (>10 kcal mol⁻¹) than the literature known 5,4 palladacycles.

The combined experimental and computational study on palladium-catalyzed regioselective C–H functionalization of O-coordinating directing groups with epoxides is described.

Studies of the Enantiocontrolled Synthesis of the C(10)-C(25) Subunit of Amphidinolide C

A stereocontrolled synthesis of 28, the C(10)-C(25) component of amphidinolide C (1), has been efficiently achieved. Reaction of the dithiane component 21 with nonracemic bis(epoxide) 19 directly affords functionalized 2,5-trans-disubstituted tetrahydrofuran 22. Propargylation is highly diastereoselective for the formation of the desired C(12)-C(13) anti stereochemistry, and the resulting terminal alkyne 25 is utilized for a regioselective syn-silylstannylation. A general strategy is illustrated for sequential replacement of stannyl and silyl substituents of the trisubstituted alkene to yield (E)-alkenyl iodide 28.