Radical Cyclization-Initiated Difunctionalization Reactions of Alkenes and Alkynes

Radical reactions are powerful in the synthesis of diverse molecular scaffolds bearing functional groups. In previous review articles, we have presented 1,2-difunctionalizations, remote 1,3-, 1,4-, 1,5-, 1,6- and 1,7-difunctionalizations, and addition followed by cyclization reactions. Presented in this paper is radical cyclization followed by the second functionalization reaction. The second functionalization could be realized by atom transfer reactions, radical or transition metal-assisted coupling reactions, and reactions with neutral molecules, cationic and anionic species.

Manganese-Catalyzed 5-Endo-trig Oxygenative Cyclization of α,β-Unsaturated Oximes under Air and Ambient Conditions for the Synthesis of 4,5-Dihydroisoxazoles

The stereoselective 5-endo-trig oxygenative cyclization of α,β-unsaturated oximes was achieved using molecular oxygen (O2) and a manganese catalyst. Several 4-hydroxy-4,5-dihydroisoxazoles were obtained in high yields by directly incorporating O2 from the atmosphere (eliminating the necessity for a pure oxygen environment) and using an unprecedentedly low loading of Mn(acac)3 (as little as 0.020 mol %) without additional additives. Because of its desirable features, such as operational simplicity, inexpensive catalyst, mild reaction conditions (open flask conditions at room temperature), and broad substrate compatibility, this novel reaction provides an attractive synthetic approach to producing 4-hydroxy-4,5-dihydroisoxazoles.

Regioselective Cycloaddition of Alkenes with tert-butyl Nitrite: A Cascade Approach to the Formation of Δ2-Isoxazolines

A route for cycloaddition reaction of alkenes and tert-butyl nitrite to synthesize Δ2-isoxazolines has been developed. The overall process involves the formation of multiple chemical bonds without the use of a catalyst. This methodology features mild reaction conditions and good functional group tolerance, providing a direct approach for the preparation of isoxazolines.

Copper-catalyzed regioselective C2-H chlorination of indoles with para-toluenesulfonyl chloride

A copper-catalyzed, pyrimidine directed regioselective C-H chlorination of indoles with para-toluenesulfonyl chloride (TsCl) has been developed. The reactions proceeded smoothly in the presence of 20 mol% of Cu(OAc)₂ as the catalyst and TsCl as the chlorine source, delivering C2-chlorinated indoles with structural diversity in moderate to excellent yields. Mechanistic studies suggested that single electron transfer (SET) from Cu(II) to TsCl accompanied by the release of the p-toluenesulfonyl radical and the related Cu(III)Cl species might be involved in the reactions.

Anionic Chiral Co(III) Complexes Mediated Asymmetric Halocyclization─Synthesis of 5-Halomethyl Pyrazolines and Isoxazolines

An asymmetric synthesis of 5-halomethyl pyrazolines and isoxazolines which bear a tertiary stereocenter by catalytic halocyclization of β,γ-unsaturated hydrazones and ketoximes is described. By using Brønsted acids of anionic chiral Co(III) complexes as catalysts, a variety of chiral 5-halomethyl pyrazolines and isoxazolines were obtained in good yields with high enantioselectivities (up to 99% yield, 97:3 er). Preliminary bioassay results indicated that several isoxazoline derivatives exhibited significant antifungal activities.

Three-Component Radical Iodonitrosylative Cyclization of 1,6-Enynes under Metal-Free Conditions

A three-component, metal-free radical cascade iodonitrosylative cyclization reaction was described. The nitroso radical was generated from tert-butyl nitrite and triggered the radical addition/cyclization/iodination/oxidation sequences. A variety of 1,6-enynes were tested and proved to be compatible, delivering various highly functionalized hetero- and all-carbon cycles and nitro and vinyl C-I bonds containing pyrrolidines, tetrahydrofuran, and cyclopentane in moderate to excellent isolated yields.

External-oxidant-free amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines

A new copper-catalyzed two-component amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines as the benzoyloxy sources is developed. Chemoselectivity of this method toward amino-benzoyloxylation or oxy-benzoyloxylation of alkenyl ketoximes relies on the position of the tethered olefins, and provides an external-oxidant-free alkene difunctionalization route that directly utilizes O-benzoylhydroxylamines as the benzoyloxy radical precursors and internal oxidants for the divergent synthesis of cyclic nitrones and isoxazolines.

Recent advances in the oxime-participating synthesis of isoxazolines

Isoxazoline compounds are used as important intermediates for the synthesis of organic molecules, which are widely used in the chemical and life science industries. Oxime-participating cyclization has emerged as an efficient strategy for the construction of isoxazolines. This review is devoted to highlighting the main achievements (since 2010) in the development of methodologies for the synthesis of isoxazolines. According to the reaction mechanism, the oxime-participating synthesis of isoxazolines can be mainly classified into four reaction types: iminoxyl radical-initiated intramolecular cyclization, intermolecular radical addition-initiated cyclization, intramolecular nucleophilic cyclization, and [3 + 2] cycloaddition. Meanwhile, miscellaneous examples are also illustrated, such as [2 + 2 + 1] cycloaddition. Representative reactions will be discussed for each of the highlighted synthetic strategies. In addition, the enantioselective synthesis of isoxazolines is also illustrated in this review.

Oxime radicals: generation, properties and application in organic synthesis

N-Oxyl radicals (compounds with an N-O• fragment) represent one of the richest families of stable and persistent organic radicals with applications ranging from catalysis of selective oxidation processes and mechanistic studies to production of polymers, energy storage, magnetic materials design and spectroscopic studies of biological objects. Compared to other N-oxyl radicals, oxime radicals (or iminoxyl radicals) have been underestimated for a long time as useful intermediates for organic synthesis, despite the fact that their precursors, oximes, are extremely widespread and easily available organic compounds. Furthermore, oxime radicals are structurally exceptional. In these radicals, the N-O• fragment is connected to an organic moiety by a double bond, whereas all other classes of N-oxyl radicals contain an R2N-O• fragment with two single C-N bonds. Although oxime radicals have been known since 1964, their broad synthetic potential was not recognized until the last decade, when numerous selective reactions of oxidative cyclization, functionalization, and coupling mediated by iminoxyl radicals were discovered. This review is focused on the synthetic methods based on iminoxyl radicals developed in the last ten years and also contains some selected data on previous works regarding generation, structure, stability, and spectral properties of these N-oxyl radicals. The reactions of oxime radicals are classified into intermolecular (oxidation by oxime radicals, oxidative C-O coupling) and intramolecular. The majority of works are devoted to intramolecular reactions of oxime radicals. These reactions are classified into cyclizations involving C-H bond cleavage and cyclizations involving a double C=C bond cleavage.

Room temperature iron(ii)-catalyzed radical cyclization of unsaturated oximes with hypervalent iodine reagents

An iron(ii)-catalyzed radical cyclization of oximes with hypervalent iodine reagents was developed, which enabled the construction of the isoxazoline backbone.

General 5-Halomethyl Isoxazoline Synthesis Enabled by Copper-Catalyzed Oxyhalogenation of Alkenes

A general and efficient oxyhalogenation of unsaturated ketoximes has been achieved through copper catalysis with diethyl bromomalonate, N-chlorosuccinimide, and N-iodosuccinimide, yielding 5-chloromethyl, 5-bromomethyl, and 5-iodomethyl isoxazolines in good to excellent yields.

Synthesis of Isoxazolines by the Electrophilic Chalcogenation of β,γ-Unsaturated Oximes: Fishing Novel Anti-Inflammatory Agents

Herein, we describe a new strategy to prepare chalcogen-functionalized isoxazolines. The strategy involves the reaction of β,γ-unsaturated oximes with electrophilic selenium and tellurium species, affording 19 new selenium- and tellurium-containing isoxazolines in good yields after 1 h at room temperature. The method was efficiently extended to the synthesis of 5 new (bis)isoxazoline ditellurides. One of the prepared compounds, 3-phenyl-5-((phenylselanyl)methyl)-isoxazoline, demonstrated better anti-inflammatory and antiedematogenic effects than the reference drug Celecoxib.

Metal-free oxysulfonylation and aminosulfonylation of alkenyl oximes: synthesis of sulfonylated isoxazolines and cyclic nitrones

Intramolecular oxysulfonylation of alkenyl oximes was reported. Using iodine as the catalyst, TBHP as the oxidant, and sulfonyl hydrazides as the sulfonyl radical source, a variety of sulfonylated isoxazolines were obtained in moderate to excellent yields. Cyclic nitrones could also be readily obtained under the same conditions.

Solvent-controlled chemoselective N-dealkylation-N-nitrosation or C-nitration of N-alkyl anilines with tert-butyl nitrite

A metal-free, acid-free and chemoselective N-dealkylation-N-nitrosation or C-nitration of N-alkyl anilines has been developed.

Metal-Involving Synthesis and Reactions of Oximes

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

Copper-mediated oxysulfonylation of alkenyl oximes with sodium sulfinates: a facile synthesis of isoxazolines featuring a sulfone substituent

A novel and efficient Cu(OAc)₂-mediated oxysulfonylation of alkenyl oximes with sodium sulfonates was developed. The reactions are easy to conduct, occur under mild conditions, and form a broad range of sulfone-substituted isoxazolines in good yields.