Switchable Synthesis of Pyrroles and Pyrazines via Rh(II)-Catalyzed Reaction of 1,2,3-Triazoles with Isoxazoles: Experimental and DFT Evidence for the 1,4-Diazahexatriene Intermediate

Acridine-Isoxazole and Acridine-Azirine Hybrids: Synthesis, Photochemical Transformations in the UV/Visible Radiation Boundary Region, and Anticancer Activity

Easy-to-handle N-hydroxyacridinecarbimidoyl chloride hydrochlorides were synthesized as convenient nitrile oxide precursors in the preparation of 3-(acridin-9/2-yl)isoxazole derivatives via 1,3-dipolar cycloaddition with terminal alkynes, 1,1-dichloroethene, and acrylonitrile. Azirines with an acridin-9/2-yl substituent attached directly or via the 1,2,3-triazole linker to the azirine C2 were also synthesized. The three-membered rings of the acridine-azirine hybrids were found to be resistant to irradiation in the UV/visible boundary region, despite their long-wave absorption at 320-420 nm, indicating that the acridine moiety cannot be used as an antenna to transfer light energy to generate nitrile ylides from azirines for photoclick cycloaddition. The acridine-isoxazole hybrids linked at the C9-C3 or C2-C3 atoms under blue light irradiation underwent the addition of such hydrogen donor solvents, such as, toluene, o-xylene, mesitylene, 4-chlorotoluene, THF, 1,4-dioxane, or methyl tert-butyl ether (MTBE), to the acridine system to give the corresponding 9-substituted acridanes in good yields. The synthesized acridine-azirine, acridine-isoxazole, and acridane-isoxazole hybrids exhibited cytotoxicity toward both all tested cancer cell lines (HCT 116, MCF7, and A704) and normal cells (WI-26 VA4).

Substituent-Controllable Cascade Regioselective Annulation of β-Enaminones with N-Sulfonyl Triazoles for Modular Access to Imidazoles and Pyrroles

A controllable synthesis of trisubstituted imidazoles and pyrroles has been developed through rhodium(II)-catalyzed regioselective annulation of N-sulfonyl-1,2,3-trizaoles with β-enaminones. The imidazole ring was formed through a 1,1-insertion of the N-H bond to α-imino rhodium carbene, followed by a subsequent intramolecular 1,4-conjugate addition. This occurred when the α-carbon atom of the amino group was bearing a methyl group. Additionally, the pyrrole ring was constructed by utilizing a phenyl substituent and undergoing intramolecular nucleophilic addition. The mild conditions, good tolerance towards functional groups, gram-scale synthesis capability, and ability to undergo valuable transformations of the products qualify this unique protocol as an efficient tool for the synthesis of N-heterocycles.

Modular Synthesis of Highly Substituted 3-Azapyrroles by Rh(II)-Catalyzed N-H Bond Insertion and Cyclodehydration

A modular synthesis of highly substituted 3-azapyrroles has been developed using a three-step sequence comprising copper-catalyzed alkyne-azide cycloaddition (CuAAC), N-H bond insertion, and cyclodehydration. 1-Sulfonyl-1,2,3-triazoles (1-STs) can be accessed from common alkyne and sulfonyl azide building blocks by CuAAC using CuTC. Rhodium(II)-acetate-promoted 1-ST denitrogenation results in highly electrophilic rhodium azavinyl carbenes that, here, underwent insertion into the N-H bond of secondary α-aminoketones to form 1,2-aminoalkenes. These products were cyclized and dehydrated using BF₃·OEt₂ into highly substituted 3-azapyrroles. The three steps (CuAAC, N-H bond insertion, and cyclodehydration) could be telescoped into a one-pot process. The method proved to be highly efficient and tolerated a wide range of substituents.

Electrocyclizations of Conjugated Azapolyenes Produced in Reactions of Azaheterocycles with Metal Carbenes

Conjugated azapolyenes (azabuta-1,3-dienes, aza-/diaza-/oxaza-/oxadiazahexa-1,3,5-trienes) are highly reactive in electrocyclization reactions, which makes them convenient precursors for the synthesis of a wide range of four-, five-, and six-membered nitrogen heterocycles that are of relevance for medicinal chemistry. Ring opening reactions of 2H-azirines and azoles containing an N–N or N–O bond, initiated by a transition metal carbene, have become increasingly important in recent years, since they easily allow the generation of azapolyenes with different numbers of double bonds and heteroatoms in various positions. This review summarizes the literature, published mainly in the last decade, on the synthetic and mechanistic aspects of electrocyclizations of azapolyenes generated by the carbene method.

Rhodium-Catalyzed Synthesis of 2-Aroylpyrimidines via Cascade Heteropolyene Rearrangement

A one-step synthesis of cytotoxic 2-aroylpyrimidines by the denitrogenative reaction of 1-tosyl-1,2,3-triazoles with isoxazoles under rhodium catalysis has been developed. According to the density functional theory calculations and control experiments, the disclosed reaction proceeds via the rearrangement of an oxadiazatetraene intermediate involving a cascade of intramolecular aza-Diels-Alder and retro-aza-Diels-Alder reactions. The presence of a substituent at C4 of the isoxazole is a prerequisite for the formation of the pyrimidines.

Rhodium-Catalyzed Denitrogenative Diazole-Triazole Coupling toward Aza-Bridged Structures and Imidazole-Based Chelating Ligands

1,4,8-Triazaocta-1,3,5,7-tetraenes, generated in situ by Rh₂(Piv)₄-catalyzed denitrogenative coupling of pyrazoles with 1-sulfonyl-1,2,3-triazoles, smoothly form 2,6,8-triazabicyclo[3.2.1]octa-3,6-dienes via intramolecular aza-Diels-Alder cycloaddition. This domino reaction, combined with the subsequent thermal or acid-catalyzed rearrangement of the cycloadducts, provides direct and flexible access to N-sulfonylated (Z)-2-(2-aminovinyl)imidazoles.

Ru(II)-Catalyzed, Cu(II)-mediated carbene migratory insertion in the synthesis of trisubstituted pyrroles from isoxazoles

A convenient, "one-pot" synthesis of trisubstituted pyrroles via a Ru(ii)-catalyzed, Cu(ii)-mediated reaction of substituted isoxazoles with sulfonylhydrazones has been developed. A series of highly functionalized pyrroles are obtained via a synergistic formation of new C-C and C-N bonds. Mechanistic investigations were carried out to propose the plausible pathway. This protocol provides a facile and expeditious approach for the synthesis of various heterocyclic compounds bearing the pyrrole skeleton.

Light-induced one-pot synthesis of pyrimidine derivatives from vinyl azides

A one-pot procedure for the synthesis of tetrasubstituted dihydropyrimidine and pyrimidine derivatives from α-azidocinnamates was developed. The synthesis is based on the finding that the outcome of LED photolysis of α-azidocinnamates depends on the light wavelength employed. Blue light (455 nm) leads to the formation of 2H-azirines only, but violet light (395 nm), UV-A light (365 nm), or sunlight result in the transformation of the in situ formed 2H-azirines to 1,3-diazabicyclo[3.1.0]hex-3-enes. Under basic catalysis (DBU), the latter were isomerized to 1,6-dihydropyrimidines which were oxidized to pyrimidines using DDQ. A successful use of Cs2CO3 as a base and air as an oxidant was also demonstrated.

Recent Advancements in Pyrrole Synthesis

This review article features selected examples on the synthesis of functionalized pyrroles that were reported between 2014 and 2019. Pyrrole is an important nitrogen-containing aromatic heterocycle that can be found in numerous compounds of biological and material significance. Given its vast importance, pyrrole continues to be an attractive target for the development of new synthetic reactions. The contents of this article are organized by the starting materials, which can be broadly classified into four different types: substrates bearing π-systems, substrates bearing carbonyl and other polar groups, and substrates bearing heterocyclic motifs. Brief discussions on plausible reaction mechanisms for most transformations are also presented.

An isoxazole strategy for the synthesis of alkyl 5-amino-4-cyano-1H-pyrrole-2-carboxylates - versatile building blocks for assembling pyrrolo-fused heterocycles

A full atom-economical domino method has been developed for the preparation of alkyl 5-amino-4-cyano-1H-pyrrole-2-carboxylates by transannulation of 5-alkoxyisoxazoles with malononitrile under Fe(ii) catalysis. Alkyl 5-amino-4-cyano-1H-pyrrole-2-carboxylates are excellent building blocks for various annulation reactions, leading to new derivatives of 1H-pyrrolo[1,2-a]imidazole and pyrrolo[2,3-d]pyrimidine. The DFT calculations of mechanistic details of alkyl 5-amino-4-cyano-1H-pyrrole-2-carboxylate formation are presented.

Rh(ii)-Catalyzed denitrogenative 1-sulfonyl-1,2,3-triazole-1-alkyl-1,2,3-triazole cross-coupling as a route to 3-sulfonamido-1H-pyrroles and 1,2,3-triazol-3-ium ylides

The reaction of 1-alkyl-1H-1,2,3-triazoles with rhodium(ii) azavinyl carbenes, generated from 1-sulfonyl-1H-1,2,3-triazoles, was utilized to prepare 3-sulfonamido-1H-pyrroles and 1,2,3-triazol-3-ium ylides in good yields.

Direct synthesis of 2,3,5-trisubstituted pyrroles via copper-mediated one-pot multicomponent reaction

We have developed a copper-mediated one-pot synthesis of 2,3,5-trisubstituted pyrroles from 1,3-dicarbonyl compounds and acrylates using ammonium acetate as a nitrogen source. The reaction achieves C-C and C-N bond formation and provides an efficient approach to access highly functionalized pyrroles without further raw material preparation. This method is operationally simple, compatible with a wide range of functional groups, and provides the target products in moderate to good yields.

Regiodivergent synthesis of pyrazino-indolines vs. triazocines via α-imino carbenes addition to imidazolidines

Hexahydropyrazinoindoles were prepared in a single step from N-sulfonyl triazoles and imidazolidines. Under dirhodium catalysis, α-imino carbenes were generated and formed nitrogen ylide intermediates that, after subsequent aminal opening, afforded the pyrazinoindoles predominantly via formal [1,2]-Stevens and tandem Friedel–Crafts cyclizations. Of mechanistic importance, a regiodivergent reactivity was engineered through the use of a specific unsymmetrically substituted imidazolidine that promoted the exclusive formation of 8-membered ring 1,3,6-triazocines. Based on DFT calculations, an original Curtin–Hammett-like situation was demonstrated for the mechanism. Further derivatizations led to functionalized tetrahydropyrazinoindoles in high yields.

Hexahydropyrazinoindoles are prepared in a single step from N-sulfonyl triazoles and imidazolidines. Of mechanistic importance, a regiodivergent reactivity can be engineered towards the exclusive formation of 8-membered ring 1,3,6-triazocines.

Distinct Roles of Ag(I) and Cu(II) as Cocatalysts in Achieving Positional-Selective C-H Alkenylation of Isoxazoles: A Theoretical Investigation

For C-H alkenylation of aryl-substituted diarylisoxazoles, one mode is N-directed C-H alkenylation and the other is C-H alkenylation in the isoxazole ring. In this study, selective C-H alkenylations of 3,5-diarylisoxazoles have been investigated theoretically with the aid of density functional theory (DFT) calculations. With Cp*Rh^III as the catalyst, the N-directed C-H alkenylation is preferred as a result of the stronger interaction energy caused by the nitrogen-directing effect. With Pd(OAc)₂ as the catalyst and Ag₂CO₃ as the cocatalyst, their combination switches the regioselectivity to the C-H alkenylation in the isoxazole ring. The strong structural distortion involved in the competing N-directed olefin insertion transition state was found to suppress N-directed C-H alkenylation. With Pd(OAc)₂ as the catalyst and Cu(OTf)₂ as the cocatalyst, the N-directed C-H alkenylation becomes preferred due to the strong coordination of the nitrogen atom to the copper center. In particular, the structural and mechanistic information involved in the above two heterodimetallic Pd/Ag and Pd/Cu catalytic systems will help toward understanding and designing novel relevant heterodimetallic-catalyzed reactions.

Stereoselective assembly of 3,4-epoxypyrrolines via nucleophilic addition induced domino cyclization of 6-halo-1-oxa-4-azahexatrienes

A highly convergent assembly of 3,4-epoxypyrroline derivatives from azirines, diazo compounds and amines is developed based on the domino cyclization of 6-halo-1-oxa-4-azahexatrienes.

InCl3-catalyzed 5-exo-dig cyclization/1,6-conjugate addition of N-propargylamides with p-QMs to construct oxazole derivatives

An InCl₃-catalyzed tandem intramolecular 5-exo-dig cyclization/1,6-conjugate addition/aromatization of N-propargylamides with p-QMs to produce oxazoles tethering diarylmethane has been successfully developed.

[2 + 1 + 1] Assembly of spiro β-lactams by Rh(ii)-catalyzed reaction of diazocarbonyl compounds with azirines/isoxazoles

A domino synthesis of dispiro-fused N-vinyl β-lactams from diazo-Meldrum's acid and 2H-azirines or 5-alkoxyisoxazoles via the "2-azabuta-1,3-diene formation/Staudinger ketene-imine cycloaddition" sequence is described. The Rh2(Piv)4-catalyzed formation of the 2-azabuta-1,3-diene intermediates in the first stage of the reaction sequence proceeds via addition of the rhodium carbenoid to the azirines/isoxazoles and provides the first example of the generation of iminium-type ylides from diazo-Meldrum's acid. This methodology was extended to monospiro-β-lactams, which were synthesized in two steps using acyclic α-diazocarbonyl compounds in the stage of the formation of 2-azabuta-1,3-dienes. The method allows for the rapid assemblage of the carbon part of the azetidin-2-one system from diazo compounds only and affords spiro- and dispiro-β-lactams in moderate yields. A bromine atom in the 2-bromoalkenyl moiety of the synthesized β-lactams can be easily substituted by hydrogen under catalytic hydrogenation conditions or by 2-pyridyl-substituent via the Stille cross-coupling reaction.

Annulation-Induced Cascade Transformation of 5-Iodo-1,2,3-triazoles to 2-(1-Aminoalkyl)benzoxazoles

Base-mediated cyclization of (5-iodo-1,2,3-triazolyl)phenols was proposed as a new synthetic strategy for the in situ generation of diazoimines via electrocyclic ring opening of the fused heterocycle. Cu-catalyzed amination of the intermediate diazoalkanes was employed to develop an efficient cascade approach to functionalized benzoxazoles.

Zinc-catalyzed reaction of isoxazoles with thioynol ethers involving an unprecedented 1,2-sulfur migration

A novel zinc-catalyzed reaction of isoxazoles with thioynol ethers involving an unprecedented 1,2-sulfur migration has been developed, which represents the first example of a non-noble metal-catalyzed reaction between isoxazoles and alkynes. This method allows the facile and atom-economical synthesis of a range of valuable β-keto enamides. Moreover, the computational study provides further evidence for the feasibility of the proposed reaction mechanism.

Polycyclic Indoline-Benzodiazepines through Electrophilic Additions of α-Imino Carbenes to Tröger Bases

Polycyclic indoline-benzodiazepines can be accessed through the intermolecular reaction of Tröger bases with N-sulfonyl-1,2,3-triazoles. Under RhII catalysis, α-imino carbenes are generated and a subsequent cascade of [1,2]-Stevens, Friedel-Crafts, Grob, and aminal formation reactions yield the polycyclic heterocycles as single isomers (d.r.>49:1, four stereocenters including two bridgehead N atoms). Further ring expansion by insertion of a second α-imino carbene leads to elaborated polycyclic 9-membered-ring triazonanes.

Recent advances in transition-metal-catalyzed reactions of alkynes with isoxazoles

Isoxazoles, as masked 1,3-dicarbonyl equivalents, have proven to be versatile building blocks and pivotal intermediates for the construction of a variety of useful azacycles with molecular complexity. As a result, a range of new reactions have been discovered based on isoxazoles in the past decade. However, the relevant reactions of isoxazoles with alkynes have seldom been explored. In this review, we will focus on the recent progress in the transition-metal-catalyzed formal annulations for the efficient synthesis of N-heterocycles between alkynes and isoxazoles by highlighting their specificity and applicability, and the mechanistic rationale is presented where possible.

Brønsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water: access to oxygen-bridged tetrahydro-1,4-oxazepines

The unique property of the Brønsted acid, as well as introducing a small amount of water, shows distinct chemoselectivity from the corresponding gold-catalyzed cycloadditions.

A novel approach to 5H-pyrazino[2,3-b]indolesviaannulation of 3-diazoindolin-2-imines with 2H-azirines or 5-alkoxyisoxazoles under Rh(ii) catalysis

The first example of rhodium carbenoid-mediated intermolecular annulation of cyclic diazo compounds with azirines and alkoxyisoxazoles is presented.

Understanding the chemoselectivities between carbonyl and hydroxyl groups in the Rh(ii)–azavinyl carbene involved reactions

Computational studies were carried out to understand the chemoselectivities between carbonyl and hydroxyl groups in the Rh(ii)–AVC involved reactions.

Synthesis of substituted pyrazines from N-allyl malonamides

Synthesis of highly substituted pyrazines from geminal diazides via thermal or copper-mediated cyclization and modification of the obtained pyrazines.