Epihalohydrins in Organic Synthesis

Membrane-Free Electrosynthesis of Epichlorohydrins Mediated by Bromine Radicals over Nanotips

The industrial manufacture of epichlorohydrin (ECH) often suffers from excessive corrosive chlorine and multistep processes. Here, we report a one-pot membrane-free Br radical-mediated ECH electrosynthesis. Bromine radicals electro-oxidized from Br- ions initiate the reaction and then eliminate HBr from bromohydrin to give ECH and release Br- ions for reuse. A high energy barrier for *OH oxidation and isolated Br adsorption sites enables NiCo2O4 to suppress the competitive oxygen and bromine evolution reactions. The high-curvature nanotips with an increased electric field concentrate Br- and OH- ions to accelerate ECH electrosynthesis. This strategy delivers ECH with a Faradaic efficiency of 47% and a reaction rate of 1.4 mol h-1 gcat-1 at a high current density of 100 mA cm-2, exceeding the profitable target from the techno-economic analysis. Economically profitable electrosynthesis, methodological universality, and the extended synthesis of epoxide-drug blocks highlight their promising potential.

Bioactive Steroids Bearing Oxirane Ring

This review explores the biological activity and structural diversity of steroids and related isoprenoid lipids, with a particular focus on compounds containing an oxirane ring. These natural compounds are derived from fungi, fungal endophytes, as well as extracts of plants, algae, and marine invertebrates. To evaluate their biological activity, an extensive examination of refereed literature sources was conducted, including in vivo and in vitro studies and the utilization of the QSAR method. Notable properties observed among these compounds include strong anti-inflammatory, antineoplastic, antiproliferative, anti-hypercholesterolemic, antiparkinsonian, diuretic, anti-eczematic, anti-psoriatic, and various other activities. Throughout this review, 3D graphs illustrating the activity of individual steroids are presented, accompanied by images of selected terrestrial or marine organisms. Furthermore, this review provides explanations for specific types of biological activity associated with these compounds. The data presented in this review are of scientific interest to the academic community and carry practical implications in the fields of pharmacology and medicine. By analyzing the biological activity and structural diversity of steroids and related isoprenoid lipids, this review offers valuable insights that contribute to both theoretical understanding and applied research. This review draws upon data from various authors to compile information on the biological activity of natural steroids containing an oxirane ring.

Base-Mediated, Chemo- and Regioselective (4+2) Annulation of Indole-2-carboxamides with 2,3-Epoxy Tosylates toward 1,2-Fused Indoles

Base-mediated [4+2] annulation of indole-2-carboxamides with 2,3-epoxy tosylates has been explored. The protocol delivers 3-substituted pyrazino[1,2-a]indol-1-ones in high yields in diastereoselective fashion, and neither 4-substituted pyrazino[1,2-a]indol-1-ones nor tetrahydro-1H-[1,4]diazepino[1,2-a]indol-1-ones are generated, irrespective of whether the distal epoxide C3 substituent is alkyl or aryl, or the epoxide is cis- or trans-configured. This reaction proceeds in one pot via N-alkylation of the indole scaffold with 2,3-epoxy tosylates, concomitantly followed by 6-exo-selective epoxide-opening cyclization. Notably, the process is chemo- and regioselective with respect to both the starting materials. To our knowledge, the process represents the first successful example of one-pot annulation of indole-based diheteronucleophiles with epoxide-based dielectrophiles.

Asymmetric Synthesis of Methoxylated Ether Lipids: A Glyceryl Glycidyl Ether Key Building Block Design, Preparation, and Synthetic Application

The report describes the preparation and use of a double-C3 building block intended as a head group synthon in the synthesis of saturated, mono-, and polyunsaturated 1-O-alkyl-sn-glycerol type methoxylated ether lipids (MELs). The resulting head piece, an enantiopure isopropylidene-protected glyceryl glycidyl ether diastereomer, was accomplished in 49% yield (max 50%) from a 1:1 diastereomeric mixture obtained from R-solketal and racemic epichlorohydrin after treatment with the Jacobsen (S,S)-Co(III)salen catalyst for the hydrolytic kinetic resolution of terminal epoxides. The diol hydrolytic product obtained in 47% yield from the unwanted diastereomer was reconverted into epoxide with an inversion of configuration in a three-step operation involving a highly regioselective lipase. This enabled the recovery of a substantial amount of diastereopure material after a subsequent treatment with the Jacobsen catalyst to furnish the oxirane head piece in altogether 72% yield of higher than 99% diastereomeric purity. A modified synthesis of a monounsaturated 16:1 MEL confirmed the correct stereochemistry and excellent enantiopurity of the head piece and resulted in a dramatic improvement in yields, efficiency, and economy of the synthesis.

Development of a novel chemoenzymatic route to enantiomerically enriched β-adrenolytic agents. A case study toward propranolol, alprenolol, pindolol, carazolol, moprolol, and metoprolol

Efficient chemoenzymatic routes toward the synthesis of both enantiomers of adrenergic β-blockers were accomplished by identifying a central chiral building block, which was first prepared using lipase-catalyzed kinetic resolution (KR, Amano PS-IM) as the asymmetric step at a five gram-scale (209 mM conc.). The enantiopure (R)-chlorohydrin (>99% ee) subsequently obtained was used for the synthesis of a series of model (R)-(+)-β-blockers (i.e., propranolol, alprenolol, pindolol, carazolol, moprolol, and metoprolol), which were produced with enantiomeric excess in the range of 96–99.9%. The pharmaceutically relevant (S)-counterpart, taking propranolol as a model, was synthesized in excellent enantiomeric purity (99% ee) via acetolysis of the respective enantiomerically pure (R)-mesylate by using cesium acetate and a catalytic amount of 18-Crown-6, followed by acidic hydrolysis of the formed (S)-acetate. Alternatively, asymmetric reduction of a prochiral ketone, namely 2-(3-chloro-2-oxopropyl)-1H-isoindole-1,3(2H)-dione, was performed using lyophilized E. coli cells harboring overexpressed recombinant alcohol dehydrogenase from Lactobacillus kefir (E. coli/Lk-ADH-Lica) giving the corresponding chlorohydrin with >99% ee. Setting the stereocenter early in the synthesis and performing a 4-step reaction sequence in a ‘one-pot two-step’ procedure allowed the design of a ‘step-economic’ route with a potential dramatic improvement in process efficiency. The synthetic method can serve for the preparation of a broad scope of enantiomerically enriched β-blockers, the chemical structures of which rely on the common α-hydroxy-N-isopropylamine moiety, and in this sense, might be industrially attractive.

Efficient chemoenzymatic routes toward both enantiomers of β-blockers were accomplished by identifying a central chiral building block prepared using either lipase-catalyzed kinetic resolution methodology or ADH-catalyzed biotranshydrogenation.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Highly Diastereoselective Construction of Carbon- Heteroatom Quaternary Stereogenic Centers in the Synthesis of Analogs of Bioactive Compounds: From Monofluorinated Epoxyalkylphosphonates to α-Fluoro-, β-, or γ-Amino Alcohol Derivatives of Alkylphosphonates

The synthesis of the stable surrogates of an important amino acid (R)-4-amino-3-hydroxybutyric acid (GABOB) such as substituted hydroxy aminophosphonic acids bearing a quaternary stereogenic center is presented. Highly diastereoselective formations of fluorinated spiroepoxy alkylphosphonate or related tertiary carbon-containing oxiranes from β-keto phosphonates possessing methyl, phenyl, or cyclohexenyl substituents, are reported. Stereoselective acid-promoted epoxide opening by bromide or azide followed by reduction/protection afforded tertiary bromides or N-Boc derivatives of β-amino-γ-hydroxy alkylphosphonates in most cases, while the reactions of oxiranes with different amines yielded their β-hydroxy-γ-amino regioisomers. Surprisingly, during the synthesis of amino phosphonic acids, we observe that the acid-induced rearrangement proceeded in a high diastereospecific manner, leading finally to substituted β-hydroxy-γ-aminoalkylphosphonic acids.

The Role of Total Synthesis in Structure Revision and Elucidation of Decanolides (Nonanolides)

Ten-membered lactones are commonly observed structures of natural products. They are mostly fungal metabolites, which often act as plant pathogens, but recently ten-membered lactones were identified as pheromones of frogs and termites. Although modern spectroscopic methods are nowadays routinely used to elucidate the structure of natural products, structural assignments of ten-membered lactones often remain incomplete or are surprisingly often erroneous. Most errors concern the absolute configuration. The examples discussed in this chapter demonstrate that enantioselective total synthesis is not only an efficient tool for corroborating or revising a proposed structure, but that the synthesis of different stereoisomers as references for gas chromatographic investigations can be a vital part of the structure elucidation process if only minute amounts of material are available. As a method of outstanding importance for the synthesis of ten-membered lactones olefin metathesis has emerged. Most of the examples discussed herein use one or more olefin metathesis reactions as key steps.

Carbenoid-Mediated Homologation Tactics for Assembling (Fluorinated) Epoxides and Aziridines

Homologation strategies provide highly versatile tools in organic synthesis for the introduction of a CH2 group into a given carbon skeleton. The operation can result in diverse structural motifs by tuning of the reaction conditions and the nature of the homologating agent. In this Account, concisely contextualizing our work with lithium carbenoids (LiCH2X, LiCHXY etc) for homologating carbon-centered electrophiles, we focus on the assembly of three-membered cycles featuring fluorinated substituents. Two illustrative case studies are considered: (1) the development and employment of fluorinated carbenoids en route to rare α-fluoroepoxides and aziridines, and (2) the installation of up to halomethylenic groups on trifluoroimidoylacetyl chlorides (TFAICs) for preparing CF3-containing halo- and halomethylaziridines. Collectively, we demonstrate that the initial homologation event generated by the installation of the carbenoid, upon modulation of the conditions, serves as a tool for creating fluorinated building blocks in a single operation.

Highly Enantioselective, Hydrogen-Bond-Donor Catalyzed Additions to Oxetanes

A precisely designed chiral squaramide derivative is shown to promote the highly enantioselective addition of trimethylsilyl bromide (TMSBr) to a broad variety of 3-substituted and 3,3-disubstituted oxetanes. The reaction provides direct and general access to synthetically valuable 1,3-bromohydrin building blocks from easily accessed achiral precursors. The products are readily elaborated both by nucleophilic substitution and through transition-metal-catalyzed cross-coupling reactions. The enantioselective catalytic oxetane ring opening was employed as part of a three-step, gram-scale synthesis of pretomanid, a recently approved medication for the treatment of multidrug-resistant tuberculosis. Heavy-atom kinetic isotope effect (KIE) studies are consistent with enantiodetermining delivery of bromide from the H-bond-donor (HBD) catalyst to the activated oxetane. While the nucleophilicity of the bromide ion is expected to be attenuated by association to the HBD, overall rate acceleration is achieved by enhancement of Lewis acidity of the TMSBr reagent through anion abstraction.

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.

A Co(ii)-catalyzed asymmetric ring opening reaction of spiro-epoxyoxindoles with allylboron

A Co(ii)/BOX-catalyzed asymmetric allylation of spiroepoxyoxindoles was developed, which is the first stereoconvergent allylation of epoxides, yielding chiral oxindoles bearing quaternary stereocenters.

Does Oxygen Feature Chalcogen Bonding?

Using the second-order Møller-Plesset perturbation theory (MP2), together with Dunning's all-electron correlation consistent basis set aug-cc-pVTZ, we show that the covalently bound oxygen atom present in a series of 21 prototypical monomer molecules examined does conceive a positive (or a negative) σ-hole. A σ-hole, in general, is an electron density-deficient region on a bound atom M along the outer extension of the R-M covalent bond, where R is the reminder part of the molecule, and M is the main group atom covalently bonded to R. We have also examined some exemplar 1:1 binary complexes that are formed between five randomly chosen monomers of the above series and the nitrogen- and oxygen-containing Lewis bases in N2, PN, NH3, and OH2. We show that the O-centered positive σ-hole in the selected monomers has the ability to form the chalcogen bonding interaction, and this is when the σ-hole on O is placed in the close proximity of the negative site in the partner molecule. Although the interaction energy and the various other 12 characteristics revealed from this study indicate the presence of any weakly bound interaction between the monomers in the six complexes, our result is strongly inconsistent with the general view that oxygen does not form a chalcogen-bonded interaction.

Chiroptical Protocol for the Absolute Configurational Assignment of Alkyl-Substituted Epoxides Using Bis(zinc porphyrin) as a CD-Sensitive Bidentate Host

The absolute configurations of simple alkyl-substituted chiral epoxides not bearing other ligating groups are readily determined via the exciton-coupled circular dichroism (ECCD) protocol using bidentate bis(zinc porphyrin) host system BP1 as a CD-sensitive chirality probe. In this situation, chiral epoxides can successfully be incorporated into the cleft of V-shaped host BP1 by double coordination of both oxygen lone pairs of the guest to the two central zinc ions of the host. We also propose a working model based on an MM2 optimized structure of the substrates that enables nonempirical prediction of the chirality of the bound epoxide.

Behaviour Modelling of Organic Bases in the Oxyalkylation Reaction of Proton-Containing Nucleophiles

The mechanism of action of nucleophilic and non-nucleophilic bases in the system “acetic acid (HOAc) – epichlorohydrin (ECH) – R3X (R = alkyl, X = N, P)” has been studied. The observed orders of the reaction with respect to the components were found. It has been proved that the catalytic activity of the starting bases is determined by their nucleophilicity, rather than basicity. It is shown that the crucial stage of the reaction is the quaternisation of the starting base with the formation of a reactive intermediate, the acetate anion. Monitoring of possible paths of oxirane ring-opening by the initial base and the acetate anion was carried out. The behaviour of the model base, triphenylphosphine, in the quaternisation reaction was studied by UV spectroscopy. The rate constants of quaternisation in the reaction of tribenzylamine and triphenylphosphine with ECH have been estimated. The activation parameters of the stages of oxirane ring-opening by trimethylamine and the acetate anion were calculated using a quantum chemical method. It has been shown that the starting base attacks only the activated oxirane, while the acetate anion is capable of nucleophilic opening of both activated and non-activated ECH rings. A scheme for the catalytic opening of the oxirane ring by the bases in the system “ECH – HOAc – R3X” is proposed, which includes a series of both consecutive and parallel stages leading to the formation of the reaction product.

Porous Aromatic Frameworks for Size-Selective Halogenation of Aryl Compounds

Organic halides are vitally important chemical precursors or intermediates in the fields of agrochemical synthesis, molecular recognition, and material science. However, it is difficult to selectively synthesize these compounds due to the multiple reactive sites in aryl fragments. In this work, we prepared the first fully fluorinated porous aromatic framework (PAF). Its -C-F bond and hierarchical porosity have great benefits for PAF functionalization. After being decorated with different cyclodextrins (CDs), CD-PAF materials can incorporate diverse aryl compounds to protect their ortho sites from being attacked to produce para-substituted molecules. This selectivity obviously increased with a decrease in the substrate size (from 0.97 to 0.41 nm). In addition, the CD-PAFs can undergo long-term use in both chlorination and bromination.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

A palladium-catalyzed tandem reaction of 2-alkynylbenzenesulfonamides with 2-(2-bromoarylidene)cyclobutanones

The core of benzo[f][1,2]thiazonin-5(4H)-one 3,3-dioxide is efficiently assembled through a palladium-catalyzed tandem reaction of 2-alkynylbenzenesulfonamides with 2-(2-bromobenzylidene)cyclobutanones via double carbometallation. A range of polycycles with a nine-membered sultam ring are generated in moderate to good yields.

A metal catalyst-free and one-pot synthesis of (3,4-dihydro-2H-benzo[b][1,4]oxazin-2-yl)methanol derivatives in water

Regioselective synthesis of (3,4-dihydro-2H-benzo[b][1,4]oxazin-2-yl)methanols have been accomplished via a greener method.

Catalytic enantioselective synthesis of α-nitroepoxides via aminolytic kinetic resolution

The first enantioselective synthesis of β-aryl-substituted α-nitroepoxides, exploiting an organocatalyzed aminolytic kinetic resolution (AKR), has been developed.

Conformationally Restricted GABA with Bicyclo[3.1.0]hexane Backbone as the First Highly Selective BGT-1 Inhibitor

On the basis of the three-dimensional diversity-oriented conformational restriction strategy using key chiral cyclopropane units, we previously identified 3 ((2S,3R)-4-amino-3,4-methanobutyric acid) with a chiral trans-cyclopropane structure as a γ-aminobutyric acid (GABA) transporter inhibitor selective for GABA transporter (GAT) subtypes GAT-3 and BGT-1 (betaine/GABA transporter-1). Further conformational restriction of 3 with the rigid bicyclo[3.1.0]hexane backbone led to the successful development of the first highly potent and selective BGT-1 inhibitor 4 (IC50 = 0.59 μM). The bioactive conformation of 3 for BGT-1 was also identified.