Epoxidöffnungskaskaden zur Synthese polycyclischer Polyether-Naturstoffe

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.

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.

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.

Experimental and Computational Studies on Regiodivergent Chiral Phosphoric Acid Catalyzed Cycloisomerization of Mupirocin Methyl Ester

This article presents a new strategy for achieving regiocontrol over the endo versus exo modes of cycloisomerizations of epoxide-containing alcohols, which leads to the formation of five- or six-membered cyclic ethers. Unlike traditional methods relying on achiral reagents or enzymes, this approach utilizes chiral phosphoric acids to catalyze the regiodivergent selective formations of either tetrahydrofuran- or tetrahydropyran-containing products. By using methyl ester of epoxide-containing antibiotic mupirocin as the substrate, it is demonstrated that catalytic chiral phosphoric acids (R)-TCYP and (S)-TIPSY could be used to achieve the selective formation of either the six-membered endo product (95:5 r.r.) or the five-membered exo product (77:23 r.r.), correspondingly. This cyclization was found to be unselective under the standard conditions involving various achiral acids, bases, or buffers. The subsequent mechanistic studies using state-of-the-art quantum chemical solutions provided the description of the potential energy surface, which is fully consistent with the experimental observations. Based on these results, highly detailed reaction paths are obtained and a concerted and highly synchronous mechanism is proposed for the formation of both exo and endo products.

Critical Switching of Cyclization Modes of Polyepoxides in Acidic Aqueous Media and Neutral Water: Synthesis and Revised Structure of a Nerolidol-Type Sesquiterpenoid

Biomimetic epoxide-opening cascades of polyepoxides enable the efficient and rapid construction of polyether frameworks. Herein, we show that the epoxide-opening cascade cyclization that affords tetrahydrofuran products in acidic aqueous media produces tetrahydropyran (THP) in neutral water. THP formation proceeded by simply heating polyepoxides in neutral water and followed a different cyclization mode from those observed so far. The novel cascade cyclization in H₂ O was applied to the synthesis of a new nerolidol-type sesquiterpenoid, resulting in revision of the proposed structure and determination of the absolute configuration.

Synthesis, characterization and catalytic activity of novel large network polystyrene-immobilized organic bases

In view of searching for efficient solid catalysts to be used in environmentally friendly reaction conditions, novel gel-type cross-linked polystyrenes functionalized with diethylamine and 1,5,7-triazabicyclo[4.4.0]dec-5-ene, have been prepared.

Asymmetric organocatalytic epoxidations: reactions, scope, mechanisms, and applications

Chiral epoxides serve as versatile building blocks in the synthesis of complex organic frameworks. The high strain imposed by the three-membered ring system makes epoxides prone to a variety of nucleophilic ring-opening reactions. Since the development of the Sharpless epoxidation, there have been many important contributions and advances in this area. With the rapid development of the field of asymmetric organocatalysis, a wide range of organocatalysts is now able to catalyze the epoxidation of broad class of unsaturated carbonyl compounds. In this Minireview, recent progress in the development of organocatalytic asymmetric epoxidation methods, the proposed mechanisms of these reactions and their applications as intermediates is reported.

Heterocycle synthesis based on allylic alcohol transposition using traceless trapping groups

Allylic alcohols undergo transposition reactions in the presence of Re2 O7 whereby the equilibrium can be dictated by trapping one isomer with a pendent electrophile. Additional ionization can occur when the trapping group is an aldehyde or ketone, thus leading to cyclic oxocarbenium ion formation. Terminating the process through bimolecular nucleophilic addition into the intermediate provides a versatile method for the synthesis of diverse oxygen-containing heterocycles. Understanding the relative rates of the steps in the sequence leads to the design of reactions which create multiple stereocenters with good to excellent levels of control.

A dioxane template for highly selective epoxy alcohol cyclizations

Ladder polyether natural products are a class of natural products denoted by their high functional-group density and large number of well-defined stereocenters. They comprise the toxic component of harmful algal blooms (HABs), having significant negative economic and environmental ramifications. However, their mode of action, namely blocking various cellular ion channels, also denotes their promise as potential anticancer agents. Understanding their potential mode of biosynthesis will not only help with developing ways to limit the damage of HABs, but would also facilitate the synthesis of a range of analogs with interesting biological activity. 1,3-Dioxan-5-ol substrates display remarkable 'enhanced template effects' in water-promoted epoxide cyclization processes en route to the synthesis of these ladder polyether natural products. In many cases, they provide near complete endo-to-exo selectivity in the cyclization of epoxy alcohols, thereby strongly favoring the formation of tetrahydropyran (THP) over tetrahydrofuran (THF) rings. The effects of various Brønsted and Lewis acidic and basic conditions are explored to demonstrate the superior selectivity of the template over the previously reported THP-based epoxy alcohols. In addition, the consideration of other synthetic routes are also considered with the goal of gaining rapid access to a plethora of potential starting materials applicable towards the synthesis of ladder polyethers. Finally, cascade sequences with polyepoxides are investigated, further demonstrating the versatility of this new reaction template.

Cascade approach to stereoselective polycyclic ether formation: epoxides as trapping agents in the transposition of allylic alcohols

Complexity from simplicity: polycyclic ethers are synthesized by cascade reactions involving the use of epoxides as electrophilic traps in the transposition of allylic alcohols. Stereogenic centers are created by functionalizing prochiral sites under thermodynamic control, and remote stereoinduction can be achieved through the use of ketones as conduits.