The development of endo-selective epoxide-opening cascades in water

Beyond Strain Release: Delocalization-Enabled Organic Reactivity

The release of strain energy is a fundamental driving force for organic reactions. However, absolute strain energy alone is an insufficient predictor of reactivity, evidenced by the similar ring strain but disparate reactivity of cyclopropanes and cyclobutanes. In this work, we demonstrate that electronic delocalization is a key factor that operates alongside strain release to boost, or even dominate, reactivity. This delocalization principle extends across a wide range of molecules containing three-membered rings such as epoxides, aziridines, and propellanes and also applies to strain-driven cycloaddition reactions. Our findings lead to a "rule of thumb" for the accurate prediction of activation barriers in such systems, which can be easily applied to reactions involving many of the strained building blocks commonly encountered in organic synthesis, medicinal chemistry, polymer science, and bioconjugation. Given the significance of electronic delocalization in organic chemistry, for example in aromatic π-systems and hyperconjugation, we anticipate that this concept will serve as a versatile tool to understand and predict organic reactivity.

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.

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.

Rational enzyme design for enabling biocatalytic Baldwin cyclization and asymmetric synthesis of chiral heterocycles

Chiral heterocyclic compounds are needed for important medicinal applications. We report an in silico strategy for the biocatalytic synthesis of chiral N- and O-heterocycles via Baldwin cyclization modes of hydroxy- and amino-substituted epoxides and oxetanes using the limonene epoxide hydrolase from Rhodococcus erythropolis. This enzyme normally catalyzes hydrolysis with formation of vicinal diols. Firstly, the required shutdown of the undesired natural water-mediated ring-opening is achieved by rational mutagenesis of the active site. In silico enzyme design is then continued with generation of the improved mutants. These variants prove to be versatile catalysts for preparing chiral N- and O-heterocycles with up to 99% conversion, and enantiomeric ratios up to 99:1. Crystal structural data and computational modeling reveal that Baldwin-type cyclizations, catalyzed by the reprogrammed enzyme, are enabled by reshaping the active-site environment that directs the distal RHN and HO-substituents to be intramolecular nucleophiles.

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.

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.

Expanding Stereoelectronic Limits of endo-tet Cyclizations: Synthesis of Benz[b]azepines from Donor-Acceptor Cyclopropanes

The importance of intramolecular constraints in cyclic transition-state geometries is especially pronounced in n-endo-tet cyclizations, where the usual backside approach of a nucleophile to the breaking bond is impossible for the rings containing less than eight atoms. Herein, we expand the limits of endo-tet cyclizations and show that donor-acceptor cyclopropanes can provide a seven-membered ring via a genuine 6-endo-tet process. Substrates containing a N-alkyl-N-arylcarbamoyl moiety as an acceptor group undergo Lewis acid-induced cyclization to form tetrahydrobenz[b]azepin-2-ones in high yields. The reaction proceeds with the inversion of the configuration at the electrophilic carbon. In this process, a formally six-membered transition state yields a seven-membered ring as the pre-existing cycle is merged into the forming ring. The stereochemistry of the products can be controlled by the reaction time and by the nature of Lewis acid, opening access to both diastereomers by tuning of the reaction conditions.

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.

Architectural and catalytic aspects of designer materials built using metalloligands of pyridine-2,6-dicarboxamide based ligands

This perspective presents the design, synthesis and crystal structures of a large number of architectures constructed using assorted metalloligands of pyridine-2,6-dicarboxamide based ligands. The metalloligands offered various appended functional groups, whereas design strategies included varying both their position and number. A combination of these parameters resulted in the development of assorted architectures including discrete trinuclear and tetranuclear complexes as well as 1D/2D/3D coordination polymers. The metalloligand strategy not only assisted in the construction of ordered crystalline materials with varied dimensionalities but also judiciously allowed the incorporation of Lewis acidic and redox-active secondary metals in the resultant architectures. As a result, such designer architectures illustrated their noteworthy role both as homogenous and heterogeneous catalysts in different organic transformation reactions.

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.

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.

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.

Switching of regioselectivity in base-mediated diastereoselective annulation of 2,3-epoxy tosylates and their N-tosylaziridine analogs with 2-mercaptobenzimidazole

A base-mediated dinucleophilic cyclization of readily accessible 2,3-epoxy tosylates with 2-mercaptobenzimidazole has been developed for the one-pot diastereoselective synthesis of benzimidazole-based tricyclic compounds equipped with two stereogenic centres. With trans-substrates bearing an aryl or alkyl substituent at the C3 position, the reaction involves an initial S-C1 bond-forming intermolecular alkylation followed by an N-C3 bond-forming, endo-selective intramolecular epoxide ring-opening cyclization reaction. A spectacular regioselectivity switching (tandem S-C3 and N-C1 bond formation reactions) was observed with related trans-N-tosylaziridine substrates. Wide substrate scope, complete diastereoselectivity, high to complete regioselectivity and mild transition metal-free conditions render this protocol particularly efficient and practical.

Base-mediated intramolecular one-pot double-cyclization of epoxide-tethered 2-fluorobenzenesulfonamides: an avenue to 1,4-benzoxazine-fused benzothiaoxazepine-1,1-dioxides

Herein, we describe the synthesis of hitherto unknown 1,4-benzoxazine-fused benzothiaoxazepine-1,1-dioxides by a NaH-promoted intramolecular one-pot double-cyclization of epoxide-tethered 2-fluorobenzene sulfonamides. Mechanistically, the reactions proceed via an intramolecular epoxide ring-opening followed by an intramolecular nucleophilic aromatic substitution. The high yields, mild conditions, complete regio- and diastereoselectivity, and a wide substrate scope render this protocol well suited for drug discovery efforts.

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.

Selective recognition of Fe3+ and CrO42− ions using a Zn(ii) metallacycle and a Cd(ii) coordination polymer and their heterogeneous catalytic application

A Zn(ii) metallacycle [Zn₂{(L)₂(DMF)₂(NO₃)₂}]·(NO₃)₂ (1) along with a Cd(ii) coordination polymer [{Cd(L)₂(DMF)₂}·(NO₃)₂]_n (2) have been synthesized, structurally characterized and exploited as a multifunctional material.

Studies toward brevisulcenal F via convergent strategies for marine ladder polyether synthesis

Shortly after the initial isolation of marine ladder polyether natural products, biomimetic epoxide-opening cascade reactions were proposed as an efficient strategy for the synthesis of these compounds. However, difficulties in assembling the cascade precursors have limited the realization of these cascades. In this report, we describe strategies that provide convergent access to cascade precursors via regioselective allylation and efficient fragment coupling. We then investigate epoxide-opening cascades promoted by strong bases for the formation of fused tetrahydropyrans. These strategies are evaluated in the context of the synthesis of rings CDEFG of brevisulcenal F.

Interfacial kinetics in a model emulsion polymerisation system using microelectrochemical measurements at expanding droplets (MEMED) and time lapse microscopy

Physicochemical processes that take place at the oil-water interface of an epoxy-amine emulsion polymerisation system influence the properties and structural morphology of the polymeric microparticles formed. Investigating these processes, such as the transport of monomers across the liquid/liquid interface brings new understanding which can be used to tune polymeric morphology. Two different approaches are used to provide new insights on these processes. Microelectrochemical measurements at expanding droplets (MEMED) is used to measure the transfer of amine from an organic phase comprised of epoxide and amine into an aqueous receptor phase. The rate of amine transfer across the liquid/liquid interface is characterised using MEMED and finite element method modelling and kinetic values are reported. Time lapse microscopy of epoxide droplets held in deionised water or an aqueous amine solution heated to different temperatures is further used to characterise epoxide dissolution into the aqueous phase. Mass-transport of epoxide into the aqueous phase is shown to be temperature-dependent. Epoxide homopolymerisation at the droplet-water interface is found to influence the rate of epoxide droplet dissolution. The rate of the epoxy-amine cure reaction is shown to be faster than the rate of the epoxide homopolymerisation reaction. The combination of methods used here is not limited to emulsion polymerisation and should find application in a myriad of processes at liquid/liquid interfaces.

Ring opening of epoxides with C-nucleophiles

Ring opening of epoxides has been an area of interest for organic chemists, owing to their reactivity toward nucleophiles. Such reactions yield important products depending on the type of nucleophiles used. This review article covers the synthetic approaches (1991-2015) used for the ring opening of epoxides via carbon nucleophiles.

Cascade polycyclizations in natural product synthesis

Cascade (domino) reactions have an unparalleled ability to generate molecular complexity from relatively simple starting materials; these transformations are particularly appealing when multiple rings are forged during this process.

Rhodium-catalyzed endo-selective epoxide-opening cascades: formal synthesis of (-)-brevisin

[Rh(CO)2Cl]2 is as an effective catalyst for endo-selective cyclizations and cascades of epoxy-(E)-enoate alcohols, thus enabling the synthesis of oxepanes and oxepane-containing polyethers from di- and trisubstituted epoxides. Syntheses of the ABC and EF ring systems of (-)-brevisin via all endo-diepoxide-opening cascades using this method constitute a formal total synthesis and demonstrate the utility of this methodology in the context of the synthesis of marine ladder polyether natural products.

Intramolecular etherification and polyene cyclization of π-activated alcohols promoted by hot water

Hot water, acting as a mildly acidic catalyst, efficiently promoted intramolecular direct nucleophilic substitution reactions of unsaturated alcohols with heteroatom or carbon nucleophiles. In a mixed solvent of water and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), polyene cyclizations using allylic alcohols as initiators gave the desired cyclized products, and in neat HFIP, a tricyclization reaction gave a tetracyclic product in 51% chemical yield.

Hydroxyl-substituted ladder polyethers via selective tandem epoxidation/cyclization sequence

A new and highly selective method for the synthesis of hydroxyl-substituted tetrahydropyrans is described. This method utilizes titanium(IV) isopropoxide and diethyl tartrate to perform a diastereoselective epoxidation followed by in situ epoxide activation and highly selective endo-cyclization to form the desired tetrahydropyran ring. The HIJ ring fragment of the marine ladder polyether yessotoxin was synthesized using this two-stage tactic that proceeds with high efficiency and excellent regioselectivity.

Heterometallic coordination polymers: syntheses, structures and heterogeneous catalytic applications

Heterometallic coordination polymers function as heterogeneous catalysts for various organic transformation reactions via preferential interaction with the substrates.

Hot water-promoted cyclopropylcarbinyl rearrangement facilitates construction of homoallylic alcohols

Hot water can promote the rearrangements of various types of cyclopropyl carbinols to afford acyclic or cyclic homoallylic alcohols in high yields.

A catalytic approach to the metal-free reaction of epoxides with ketene silyl acetals for accessing γ-lactones

The first catalytic approach to the nucleophilic addition of silyl ketene acetals 2 to epoxides 1 is reported. The defined protocol is metal-free using tetrabutylammonioum fluoride as the catalyst. It works in a very efficient manner under solvent-free conditions (SolFC) allowing γ-lactones 3 to be directly obtained with high regioselectivities and yields.

Mononuclear complexes of amide-based ligands containing appended functional groups: role of secondary coordination spheres on catalysis

Coordination complexes of amide-based ligands with appended heterocyclic rings create a hydrogen bonding cavity that effectively binds the substrate(s). Such cavity-based complexes function as reusable and heterogeneous catalysts for various organic transformations.