[3 + 2]-Cycloaddition of 2H-Azirines with Nitrosoarenes: Visible-Light-Promoted Synthesis of 2,5-Dihydro-1,2,4-oxadiazoles

Multiple-cycle photochemical cascade reactions

Cascade reactions represent an efficient and economical synthetic approach, enabling the rapid synthesis of a wide array of structurally complex organic compounds. These compounds, previously inaccessible, can now be synthesized in a remarkably limited number of steps. Concurrently, the photochemical reactions of organic molecules have gained prominence as a potent strategy for accessing a diverse range of radical species and intermediates. This is achieved in a controlled manner under mild conditions. Owing to the relentless endeavors of chemists, significant strides have been made in the realm of photochemical cascade reactions. These advancements have facilitated the synthesis of novel molecular structures with high complexity, structures that are typically challenging to generate under thermal conditions. In this review, we comprehensively summarize and underscore the recent pivotal advancements in visible-light-induced cascade reactions. Our focus is on the elucidation of multiple photochemical catalytic cycles, emphasizing the catalytic activation modes and the types of reactions involved.

Design, synthesis, anti-mycobacterial activity, molecular docking and ADME analysis of spiroquinoxaline-1,2,4-oxadiazoles via [3 + 2] cycloaddition reaction under ultrasound irradiation

The development of anti-tuberculosis (anti-TB) drugs has become a challenging task in medicinal chemistry. This is because Mycobacterium tuberculosis (TB), the pathogen that causes tuberculosis, has an increasing number of drug-resistant strains, and existing medication therapies are not very effective. This resistance significantly demands new anti-TB drug profiles. Here, we present the design and synthesis of a number of hybrid compounds with previously known anti-mycobacterial moieties attached to quinoxaline, quinoline, tetrazole, and 1,2,4-oxadiazole scaffolds. A convenient ultrasound methodology was employed to attain spiroquinoxaline-1,2,4-oxadiazoles via [3 + 2] cycloaddition of quinoxaline Schiff bases and aryl nitrile oxides at room temperature. This approach avoids standard heating and column chromatography while producing high yields and shorter reaction times. The target compounds 3a-p were well-characterized, and their in vitro anti-mycobacterial activity (anti-TB) was evaluated. Among the screened compounds, 3i displayed promising activity against the Mycobacterium tuberculosis cell line H37Rv, with an MIC99 value of 0.78 µg/mL. However, three compounds (3f, 3h, and 3o) exhibited potent activity with MIC99 values of 6.25 µg/mL. To further understand the binding interactions, the synthesized compounds were docked against the tuberculosis protein 5OEQ using in silico molecular docking. Moreover, the most active compounds were additionally tested for their cytotoxicity against the RAW 264.7 cell line, and the cytotoxicity of compounds 3f, 3h, 3i, and 3o was 27.3, 28.9, 26.4, and 30.2 µg/mL, respectively. These results revealed that the compounds 3f, 3h, 3i, and 3o were less harmful to humans. Furthermore, the synthesized compounds were tested for ADME qualities, and the results suggest that this series is useful for producing innovative and potent anti-tubercular medicines in the future.

Rhodium-Catalyzed [3+2]-Cycloaddition of in-situ Generated Nitrile Ylides with Nitrosoarenes

Herein we report the rhodium-catalyzed one-pot three-component reaction of diazo compounds, nitriles, and nitrosoarenes to construct 2,5-dihydro-1,2,4-oxadiazole derivatives. Mechanistic studies indicate that the transformation may proceed through the formation of nitrile ylides intermediates, which then undergo [3+2]-cycloaddition with nitrosoarenes. The strategy exhibits several synthetic advantages, including operational simplicity, good functional group tolerance, and scalability.

Silver-catalyzed nitrosation and nitration of aromatic amides using NOBF4

Divergent aromatic ring nitrosation and nitration of aromatic amides are reported using NOBF4 as the electrophile under silver-catalyzed conditions. The reactions proceed efficiently with a wide range of compatible functionalities providing ortho-position nitrosation products, deacylation nitrosation products, and nitration products from different tertiary and secondary aromatic amides.

Three-Component Synthesis of Di-Keto Aziridines and Highly Functionalized Alkenes from Sulfoxonium Ylides, Nitrosoarenes, and Alkynes

A catalyst-free one-pot three-component method of sulfoxonium ylides, nitrosoarenes, and alkynes for the synthesis of highly functionalized di-keto aziridines and alkenes is described. This strategy features the catalyst-free and additive-free approach, the employment of safe, more stable, and readily accessible sulfoxonium ylides, which bear a much wider substrate scope as starting materials. In terms of terminal alkynes, a cascade reaction of nitrone formation/1,3-diploar cycloaddition/Baldwin rearrangement is involved to afford a wide variety of di-keto aziridines. However, highly functionalized alkenes could be obtained instead of di-keto aziridines through the same nitrone formation/1,3-diploar cycloaddition and another different rearrangement reaction when internal alkynes are employed as starting materials.

Alkylation/Ipso-cyclization of Active Alkynes Leading to 3-Alkylated Aza- and Oxa-spiro[4,5]-trienones

A method for the preparation of 3-alkylated spiro[4.5]trienones via alkylation/ipso-cyclization of activated alkynes with 4-alkyl-DHPs under transition-metal-free conditions is proposed. This alkylation successively undergoes the generation of alkyl radicals, addition of alkyl radicals to the alkynes, and intramolecular ipso-cyclization. The mechanism studies suggest that the alkylation/ipso-cyclization involves a radical process. This ipso-cyclization procedure shows a series of advantages, such as accessibility, mild conditions, high efficiency, greater safety, and an environmentally friendly method.

Visible light-driven [3 + 3] annulation reaction of 2H-azirines with Huisgen zwitterions and synthesis of 1,2,4-triazines

A visible light-driven [3 + 3] annulation reaction of 2H-azirines with Huisgen zwitterions is developed for the first time.

Stannyl radical-mediated synthesis of 6H-1,3-oxazin-6-ones from 2-acyloxyazirines or whether free radicals can open the azirine ring?

A fundamentally new radical cascade reaction of 2-acyloxyazirines underlays an effective one-step method for the preparation of 5-hydroxy-6H-1,3-oxazin-6-ones from methyl 2-acyloxy-2H-azirine-2-carboxylates using Bu3SnH/ACHN system as a source of stannyl radicals....

Switchable assembly of substituted pyrimidines and 2H-imidazoles via Cu(i)-catalysed ring expansion of 2 methoxyl-2H-azirines

The unprecedented switchable synthesis of substituted pyrimidines and 2H-imidazoles via the Cu(i)-catalyzed ring expansion of 2-methoxyl-2H-azirines is described.

SET activation of nitroarenes by 2-azaallyl anions as a straightforward access to 2,5-dihydro-1,2,4-oxadiazoles

The use of nitroarenes as amino sources in synthesis is challenging. Herein is reported an unusual, straightforward, and transition metal-free method for the net [3 + 2]-cycloaddition reaction of 2-azaallyl anions with nitroarenes. The products of this reaction are diverse 2,5-dihydro-1,2,4-oxadiazoles (>40 examples, up to 95% yield). This method does not require an external reductant to reduce nitroarenes, nor does it employ nitrosoarenes, which are often used in N-O cycloadditions. Instead, it is proposed that the 2-azaallyl anions, which behave as super electron donors (SEDs), deliver an electron to the nitroarene to generate a nitroarene radical anion. A downstream 2-azaallyl radical coupling with a newly formed nitrosoarene is followed by ring closure to afford the observed products. This proposed reaction pathway is supported by computational studies and experimental evidence. Overall, this method uses readily available materials, is green, and exhibits a broad scope.

Visible light organic photoredox catalytic cascade reactions

Over the past years, impressive progress has been made in the development of organic photoredox catalytic cascade reactions without the participation of expensive and toxic transition metals under visible light irradiation. These transformations highly depend on the in situ generation of various radical species in the photoredox catalytic cycles. Numerous chemically and biomedically valuable building blocks have been synthesized through this efficient and sustainable protocol. In this review, we highlight the recent progress in this blooming area by presenting a series of new catalytic cascade reactions mediated by organic photoredox catalysts and describe their mechanisms and applications which have appeared in the recent literature.

The visible-light-induced acylation/cyclization of alkynoates with acyl oximes for the construction of 3-acylcoumarins

A nitrogen-centered radical-mediated carbon-carbon bond cleavage strategy is described to synthesize functionalized 3-acylcoumarins. The strategy is enabled by the visible-light-induced acylation/cyclization of alkynoates with various acyl oxime compounds in acetonitrile. The difunctionalization of carbon-carbon triple bonds precedes the generation of iminyl radicals, which is followed by the formation of acyl radicals. The acyl radicals then attack the carbon-carbon triple bonds, followed by 5-exo-trig cyclization and 1,2-ester migration. This strategy has wide substrate adaptability and good substituent tolerance.

Visible-light-promoted nitrone synthesis from nitrosoarenes under catalyst- and additive-free conditions

A green and sustainable nitrone formation reaction via visible-light-promoted reaction of aryl diazoacetates with nitrosoarenes is described. This protocol exhibits good functional group tolerance and broad substrate scope for both aryl diazoacetates with nitrosoarenes. Comparing the reported methods for the synthesis of nitrones from nitrosoarenes, the reaction described herein occurs under sole visible-light irradiation without the need of any catalysts and additives.

Divergent Synthesis of Aziridine and Imidazolidine Frameworks under Blue LED Irradiation

We develop a visible light-promoted divergent cycloaddition of α-diazo esters with hexahydro-1,3,5-triazines, leading to a series of aziridine and imidazolidine frameworks in average good yield, by simply changing the reaction media used. It is noteworthy that the reaction occurs under sole visible light irradiation without the need for exogenous photoredox catalysts. More significantly, a reasonable reaction mechanism was proposed on the basis of the control experiments and density functional theory calculation results.

Dual Functional Pd-Catalyzed Multicomponent Reaction by Umpolung Chemistry of the Oxygen Atom in Electrophiles

A Pd-catalyzed multicomponent reaction was developed by trapping oxomium ylide with nitrosobenzene via Pd-promoted umpolung chemistry. The Pd catalyst plays two important roles: diazo compound decomposed catalyst and Lewis acid for the activation of nitrosobenzene. This strategy provides some insight into a new way for discovery of multicomponent methodology to construct complex molecules. The developed method also provides rapid access to a series of O-(2-oxy) hydroxylamine derivatives, which exhibit good anticancer activity in osteosarcoma cells.

Redox-Neutral 1,3-Dipolar Cycloaddition of 2H-Azirines with 2,4,6-Triarylpyrylium Salts under Visible Light Irradiation

A novel visible light mediated redox-neutral 1,3-dipolar cycloaddition of 2H-azirines with 2,4,6-triarylpyrylium tetrafluoroborate salts providing tetrasubstituted pyrroles has been developed. The 2,4,6-triarylpyrylium salt acts as dipolarophile as well as photosensitizer in the reaction, under blue light irradiation. The control experiments indicated single electron oxidation of 2H-azirines by photoexcited pyrylium salts, followed by coupling between an azaallenyl radical cation and triarylpyranyl radical as the key mechanistic feature. The mild conditions, wide substrate scope, and complete regioselectivity are the noticeable attributes of the reaction.

Strain Release Chemistry of Photogenerated Small-Ring Intermediates

Photochemical processes, such as isomerizations and cycloadditions, have proven to be very useful in the construction of highly strained molecular frameworks. Photoinduced ring strain enables subsequent exergonic reactions which do not require the input of additional chemical energy and provides a variety of attractive synthetic options leading to complex structures. This review covers the progress achieved in the application of sequences combining excitation by ultraviolet light to form strained intermediates, which are further transformed to lower energy products in strain-release reactions. As ring strain is considerable in small ring systems, photogenerated three- and four-membered rings will be covered, mainly focusing on examples from 2000 to May 2020.

Synthesis of trisubstituted hydroxylamines by a visible light-promoted multicomponent reaction

A green and efficient route for the synthesis of trisubstituted hydroxylamines from β-keto ester, 2-nitrosopyridine and aryldiazoacetates has been reported. This multicomponent reaction occurred under mild conditions without catalysts or additives.

Visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates leading to coumarins via SO2 insertion

A visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates with cycloketone oxime compounds for the preparation of 3-cyanoalkylsulfonylcoumarins via SO₂ insertion is reported.

Visible-Light-Promoted Polysubstituted Olefins Synthesis Involving Sulfur Ylides as Carbene Trapping Reagents

A blue-light-emitting diode (LED) promoted coupling of aryl diazoacetates with sulfur ylides is described. This protocol features mild conditions, good functional group tolerance, and broad substrate scope for both aryl diazoacetates with sulfur ylides. Under optimal reaction conditions, a wide range of trisubstituted olefins is obtained in moderate to good yield, which can be further transferred to other biologically important heterocycles after a two-step simple operation.

General Synthesis of Tri-Carbo-Substituted N2-Aryl-1,2,3-triazoles via Cu-Catalyzed Annulation of Azirines with Aryldiazonium Salts

The general synthesis of fully substituted N²-aryl-1,2,3-triazoles is hitherto challenging compared with that of the N¹-aryl counterparts. Herein, we describe a Cu-catalyzed annulation reaction of azirines and aryldiazonium salts. This regiospecific method allows access to a broad spectrum of tri-carbo N²-aryl-1,2,3-triazoles substituted with diverse aryl and alkyl moieties. Its utility is highlighted by the synthesis of several triazole precursors applicable in drug discovery, as well as novel chiral binaphthyl ligands bearing triazole moieties.

BI-OAc-Accelerated C3-H Alkylation of Quinoxalin-2(1H)-ones under Visible-Light Irradiation

An efficient, photoredox-catalyst-free radical alkylation of quinoxalin-2(1H)-ones has been described. This reaction utilizes 4-alkyl-1,4-dihydropyridines (R-DHPs) as alkyl radical precursors and acetoxybenziodoxole (BI-OAc) as an electron acceptor to undergo single-electron transfer with photoexcited R-DHPs. The benign conditions allow for good compatibility in the scope of both quinoxalin-2(1H)-ones and R-DHPs. The synthetic value of the protocol was also demonstrated by the successful functionalization of natural products and drug-based complex molecules.

Novel 1,2,4-Oxadiazole Derivatives in Drug Discovery

Five-membered 1,2,4-oxadiazole heterocyclic ring has received considerable attentionbecause of its unique bioisosteric properties and an unusually wide spectrum of biological activities.Thus, it is a perfect framework for the novel drug development. After a century since the1,2,4-oxadiazole have been discovered, the uncommon potential attracted medicinal chemists'attention, leading to the discovery of a few presently accessible drugs containing 1,2,4-oxadiazoleunit. It is worth noting that the interest in a 1,2,4-oxadiazoles' biological application has been doubledin the last fifteen years. Herein, after a concise historical introduction, we present a comprehensiveoverview of the recent achievements in the synthesis of 1,2,4-oxadiazole-based compounds and themajor advances in their biological applications in the period of the last five years as well as briefremarks on prospects for further development.

Visible-light photoredox-catalyzed dual C–C bond cleavage: synthesis of 2-cyanoalkylsulfonylated 3,4-dihydronaphthalenes through the insertion of sulfur dioxide

A novel visible-light photoredox-catalyzed dual C–C bond cleavage of methylenecyclopropanes and cycloketone oximes for accessing 2-cyanoalkylsulfonated 3,4-dihydronaphthalenes is established.

Visible light-promoted ring-opening functionalization of three-membered carbo- and heterocycles

In this tutorial review, synthetic and mechanistic aspects of visible light-promoted ring-opening functionalization of three-membered carbo- and heterocycles are highlighted.

A copper-catalyzed insertion of sulfur dioxide via radical coupling

A copper-catalyzed reaction of O-acyl oximes, DABCO·(SO₂)₂, and 2H-azirines is developed under mild conditions, leading to diverse tetrasubstituted β-sulfonyl N-unprotected enamines with excellent stereoselectivity and regioselectivity.

Radical chemistry of nitrosoarenes: concepts, synthetic applications and directions

The radical chemistry of nitrosoarenes has recently emerged as an attractive and important topic in organic chemistry.

Visible-Light-Enabled Oxidative Coupling of Alkenes with Dialkylformamides To Access Unsaturated Amides

A practical and direct method for oxidative cross-coupling of alkenes with dialkylformamides is established employing visible-light-enabled photoredox catalysis. This strategy allows efficient access to diverse unsaturated amides under mild reaction conditions. The application of an appropriate diaryliodonium salt was demonstrated to be critical to the success of this process. This catalyst system is well tolerant of a variety of useful functional groups.