A One-Pot Approach to Δ 2 -Isoxazolines from Ketones and Arylacetylenes

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.

Access to Functionalized Cyclohex-2-enones from a Multicomponent Cascade Reaction of Readily Available Alkynes, Ketones, and Ethyl Acetoacetate

The synthesis of cyclohex-2-enone derivatives is a topic of current interest in organic chemistry. A novel three-component cascade reaction of alkynes with ketones and ethyl acetoacetate has been uncovered. This process provides di- and trisubstituted cyclohex-2-enones in good yields with excellent functional group tolerance. A variety of terminal alkynes and a wide range of aryl, alkyl, and cyclic ketones are viable in this transformation. Successful scale-up preparation and synthetic transformations have demonstrated the potential of this simple operating protocol.

KOtBu-catalysed α-homoallylic alkylation of acyclic amides with 1-aryl-1,3-dienes

Herein, we report a KOtBu-catalysed α-homoallylic alkylation of acyclic amides with 1-aryl-1,3-dienes. With this transition metal-free and atom-economic protocol, a series of α-homoallylic alkylated acyclic amides were synthesized in good to excellent yields. This transformation is proposed to proceed through a cation-π interaction-based C-C bond formation from the in situ-generated potassium enolate with the diene unit.

A Study on Synthesis of Chalcone Derived -5- Membered Isoxazoline and Isoxazole Scaffolds

Chalcone-derived Isoxazole scaffolds remain the central focus because of their greater biological, clinical, and pharmacological properties. The present study reviews the synthesis of various chalcone derived - 5- Membered Isoxazoline and Isoxazole Scaffolds with the available literature until 2021. Keywords: Chalcones, Isoxazoles, Isoxazolines, Biological and Pharmacological properties.

Nickel(II)-Catalyzed [5 + 1] Annulation of 2-Carbonyl-1-propargylindoles with Hydroxylamine To Synthesize Pyrazino[1,2-a]indole-2-oxides in Water

An atom-economical and practical method for the efficient synthesis of various pyrazino[1,2-a]indole-2-oxides was developed through a nickel(II)-catalyzed [5 + 1] annulation of 2-carbonyl-1-propargylindoles with hydroxylamine in water without using an organic solvent. The reaction involved an initial condensation of 2-carbonyl-1-propargylindoles with hydroxylamine to afford oxime intermediates, which then underwent a nickel(II)-catalyzed 6-exo-dig cyclization. Preliminary studies showed that (n-Bu)₄NI served as a phase transfer catalyst and promoted the formation of active nickel(II) species. More importantly, the nickel(II) salt and phase transfer catalyst-in-water could be recycled seven times, and a gram scalable product was easily obtained in good yields through a filtration and washing protocol.

Transition-Metal-Free α-Vinylation of Enolizable Ketones with β-Bromostyrenes

An α-vinylation of enolizable ketones has been developed by using β-bromostyrenes and a KO tBu/NMP system. β,γ-Unsaturated ketones of E configuration were obtained in excellent yield and selectivity. Further synthetic possibilities are highlighted by one-pot functionalization via trapping of intermediate dienolates with alkyl, allyl, benzyl, and propargyl halides to generate quaternary centers. The reported transformation is believed to involve phenylacetylene and propargylic alcohol derivatives.

Copper nitrate-mediated synthesis of 3-aryl isoxazolines and isoxazoles from olefinic azlactones

A copper nitrate-mediated [2 + 2 + 1] cycloaddition reaction was developed for the expedient synthesis of isoxazolines and isoxazoles from olefinic azlactones.

Acetylenes in the Superbase-Promoted Assembly of Carbocycles and Heterocycles

In this Account, we briefly discuss the recently discovered and rapidly developing superbase-promoted self-organization reactions of several equivalents of acetylenes and ketones to afford complex compounds that represent promising synthetic building blocks common in natural products. Notably, acetylenes play a special role in these reactions because of their dual (acting as an electrophile and a nucleophile) and flexible reactivity. These unique properties of acetylenes are elegantly expressed in superbasic media, where acetylenes are more deprotonated and their electrophilicity increases as a result of complexation with alkali metal cations, with simultaneous enhancement of the nucleophilic reactants due to desolvation. Under these conditions, acetylenes behave as a driving and organizing force toward other reactants. Various combinations of nucleophilic addition to the triple bond and acetylene deprotonation in the presence of other reactants with dual reactivity (e.g., ketones) enables the self-organization of complex molecular architectures that are inaccessible by conventional reactions. Herein we analyze recent achievements in this area concerning the reactions of acetylenes with ketones in superbasic KOH/DMSO-type systems that selectively afford synthetically and pharmaceutically valuable carbo- and heterocycles. Most of the reactions are triggered by the nucleophilic addition of deprotonated ketones (enolate anions) to acetylenes (superbase-catalyzed C-vinylation of ketones with acetylenes, which was recently introduced by our group into a toolkit of organic chemistry). The β,γ-ethylenic ketones thus formed can then take part in cascade processes with ketones and acetylenes to afford either carbocycles (e.g., hexahydroazulenones, acyl terphenyls, functionalized and cyclopentenols) or heterocycles (e.g., furans, benzoxepines, dioxabicyclo[3.2.1]octanes, and dioxadispiro[5.1.5.2]pentadecanes), depending on the structure of the reactants and the reaction conditions. Most of these compounds are selectively built from several equivalents of ketones and acetylenes in different combinations, and despite the presence of two or more asymmetric carbons in the products, they are generated as single diastereomers. When other nucleophiles (hydroxylamine, hydrazines, guanidine, and oximes) and ketones are involved in these self-organization processes, the intermediate β,γ-ethylenic ketones allow the formation of diverse heterocyclic systems (pyrroles, isoxazolines, pyrazolines, aminopyrimidines, and azabicyclo[3.1.0]hexanes). The discovered unique chemical transformations do not require transition metal catalysts and proceed under mild and operationally simple conditions. Most of these syntheses involve cascade addition reactions and therefore represent pot-, atom-, step-, and energy-saving processes that meet the requirements of green chemistry. The significance of the approach discussed herein is that it represents a viable alternative to existing classic and modern transition-metal-based catalytic syntheses of some fundamental carbo- and heterocycles. This is demonstrated by its employment of readily available, inexpensive starting materials like acetylenes and ketones and simple, widely accessible superbasic systems such as KOH/DMSO, which serves as a highly active universal catalyst and auxiliary. As shown in this Account, as this approach has developed, the number of preparatively attractive methods for the synthesis of diverse and potentially useful compounds has rapidly ballooned. The impressive experimental results presented in this Account will hopefully draw the attention of large circles of organic chemists involved in the design of rational and ecologically sound synthetic procedures and thus increase the application of these techniques in medicinal chemistry and materials science.

Catalytic enantioselective synthesis of carboxy-substituted 2-isoxazolines by cascade oxa-Michael-cyclization

An efficient quinidine-based phase-transfer-catalyzed enantioselective cascade oxa-Michael-cyclization reaction of hydroxylamine with various β-carboxy-substituted α,β-unsaturated ketones has been achieved for the preparation of chiral carboxy-substituted 2-isoxazolines. This cascade reaction provided the desired products in good yields (up to 98%) with excellent enantioselectivities (91-96% ee). In addition, the cascade reaction was effectively applied to the first catalytic asymmetric synthesis of the herbicide (S)-methiozolin.

Base-Mediated Hydroamination of Alkynes

Inter- or intramolecular hydroamination reactions are a paradigmatic example of modern sustainable organic chemistry, as they are a catalytic, 100% atom-economical, and waste-free process of fundamental simplicity in which an amine is added to an alkyne substrate. Many enamines are found in many natural and synthetic compounds possessing interesting physiological and biological activities. The development of synthetic protocols for such molecules and their transformation is a persistent research topic in pharmaceutical and organic chemistry. Hydroamination is conspicuously superior to the other accessible methods, such as the imination of ketones or the aminomercuration/demercuration of alkynes, that involve the stoichiometric use of toxic reagents. Additionally, the hydroamination of alkyne substrates has been successfully employed as a key step in synthesizing target molecules through total syntheses containing substituted indoles, pyrroles, imidazoles, and other heterocycles as core moieties. Many research groups have explored inter- or intramolecular hydroamination of alkynes for the synthesis of diversely substituted nitrogen heterocycles using expensive metal catalysts. However, in contrast to metal-catalyzed hydroamination, the base-mediated hydroamination of alkynes has not been extensively studied. Various inorganic (such as hydroxides, phosphates, and carbonates) and organic bases have been proven to be valuable reagents for achieving the hydroamination process. This method represents an attractive strategy for the construction of a broad range of nitrogen-containing compounds that prevents the formation of byproducts in the creation of a C-N linkage. The presence of a base is thought to facilitate the attack of nitrogen nucleophiles, such as indoles, pyrroles, and imidazoles, on unsaturated carbon substrates through the activation of the triple bond and thus transforming the electron-rich alkyne into an electrophile. In the past few years, we have been involved in the development of methods for the nucleophilic addition of N-heterocycles onto terminal and internal alkynes using alkali base catalysts to achieve new carbon-nitrogen bond-forming reactions. During our study, we discovered the regioselective preferential nucleophilic addition of N-heterocycles onto the haloarylalkyne over N-arylation of the aryl halide. In this Account, we summarize our latest achievements in regio-, stereo-, and chemoselective hydroamination chemistry of N-nucleophiles with alkynes using a superbasic medium to produce a broad range of highly functionalized vinyl and styryl enamines, which are valuable and versatile synthetic intermediates for the synthesis of bioactive compounds. Interestingly, the stereoselectivity of the addition products (kinetically stable Z and thermodynamically stable E isomers) was found to be dependent upon time. It is worthwhile to note that hydroaminated products formed by the addition reaction can further be utilized for the synthesis of indolo-/pyrrolo[2,1-a]isoquinolines, naphthyridines, and bisindolo/pyrrolo[2,1-a]isoquinolinesvia tandem cyclization. This chemistry is expected to find application in organic synthesis for constructing a diverse variety of fused π-conjugated compounds, enaminones, and C-C coupled products.

Copper Nitrate Mediated Regioselective [2+2+1] Cyclization of Alkynes with Alkenes: A Cascade Approach to Δ(2)-Isoxazolines

An efficient method for the regioselective synthesis of pharmacologically relevant polysubstituted Δ(2)-isoxazolines is based on the copper-mediated direct transformation of simple terminal alkynes and alkenes. The overall process involves the formation of four chemical bonds with inexpensive and readily available copper nitrate trihydrate as a novel precursor of nitrile oxides. The reaction can be easily handled and proceeds under mild conditions.

Synthesis and diverse general oxidative cyclization catalysis of high-valent MoVIO2(HL) to ubiquitous heterocycles and their chiral analogues with high selectivity

The first synthesis and diverse oxidative cyclization catalysis properties of high-valent Mo^VI–triazole are demonstrated towards highly selective construction of benzimidazoles, benzothiazoles, isoxazolines, isoxazoles and their chiral analogues.

Transition metal free synthesis of 2,4,6-trisubstituted pyrimidines via Cope-type hydroamination of 1,4-diarylbuta-1,3-diynes

We have developed an efficient and transition metal free methodology for the synthesis of 2,4,6-trisubstituted pyrimidines by the Cope-type hydroamination reaction of 1,4-diarylbuta-1,3-diynes with amidines in DMSO solvent.