Rh-Catalyzed Conversion of 3-Diazoindolin-2-imines to 5H-Pyrazino[2,3-b]indoles with Photoluminescent Properties

DBU-Promoted Cyclizations of Cyclopentyl-Substituted Oxazapolyenes to Cyclopentapyridones and Hydroxypyrroles: Experimental and DFT Study

1,1-Di(alkoxycarbonyl)-4-cyclopentyl-2-azabuta-1,3-dienes react with DBU to form two types of heterocyclic products: 1H-cyclopenta[c]pyrid-1-ones and 3-hydroxy-1H-pyrroles. These previously unobserved transformations proceed through the formation of 1-azapentadienyl anion which undergoes 1,6-shift of the alkoxycarbonyl group to the cyclopentyl moiety followed by 1,6-cyclization to form the cyclopentapyridone (path a) and 1,5-cyclization accompanied by 1,3-shift of the methoxy group followed by dialkyl carbonate elimination to afford the hydroxypyrrole (path b). The mechanisms of the reactions were studied using DFT calculations. Pyridones can be synthesized in one pot in three steps via Rh(II)-catalyzed isomerization of 4-cyclopentylisoxazoles to 2H-azirines and their subsequent reaction with diazomalonic esters, followed by heating of the resulting 2-azabuta-1,3-dienes with DBU.

Solvent- and Catalyst-Controlled Regioselective O- and N-Alkylation of 2-Pyridones by 2H-Azirines

The synthesis of O-substituted 2-hydroxypyridines and N-substituted 2-pyridones, crucial for many bioactive compounds and drugs, faces challenges due to the tautomeric nature of 2-pyridones, which complicates selective alkylation. Here we developed an efficient method for regioselective O- and N-alkylation of 2-pyridones using Bro̷nsted acid-catalyzed ring opening of 2H-azirines. The process involves triflic acid for O-alkylation and p-toluenesulfonic acid for N-alkylation, achieving high yields under optimized conditions. For O-alkylation, a variety of 2-pyridones and 2H-azirines were used, resulting in significant yields of the desired products. Similarly, for N-alkylation, the optimized conditions produced excellent yields, highlighting the method's versatility. This methodology was further demonstrated through scaled-up syntheses and subsequent transformations, showcasing its practicality for complex molecular architectures. The proposed mechanism involves the protonation of 2H-azirine, followed by a regioselective SN2-type attack and acid-catalyzed hydrolysis, leading to the desired alkylated products. This innovative approach, emphasizing Bro̷nsted acid catalysis and careful control of reaction conditions, represents a significant advancement in the selective alkylation of 2-pyridones, with broad implications for medicinal chemistry.

Brønsted acid-catalyzed regioselective ring opening of 2H-azirines by 2-mercaptopyridines and related heterocycles; one pot access to imidazo[1,2-a]pyridines and imidazo[2,1-b]thiazoles

A catalytic and versatile synthetic method for the synthesis of imidazo[1,2-a]pyridines has been developed. Brønsted acid-catalysis plays a major role in the regioselective ring opening of 2H-azirines. Nucleophilic attack via the N-centre of mercaptopyridines and their analogues, followed by cyclisation by cleaving the C-S bond, allowed a library of imidazo[1,2-a]pyridines and related heterocycles to be built. The reaction protocol has been applied to various 2H-azirines, 2-mercaptopyridines, and thiazole-2-thiols, illustrating the generality of reaction conditions. The practical applications include the synthesis of pharmaceuticals, such as anti-tumor agents. This study introduces a novel approach to the synthesis of functional molecules with extensive potential.

Metal-free construction of dihydropyrazino[2,3-b]indoles from 2-aminoacetophenones, isocyanates and 1,2-diamines

A Brønsted acid/iodine co-mediated approach to construct dihydropyrazino[2,3-b]indoles in 2-MeTHF as a green solvent at room temperature was established. A wide range of diversely substituted dihydropyrazino[2,3-b]indoles (42 examples, 61-85% yields) were synthesized under mild conditions, even on a gram scale. This protocol features organocatalysts, an eco-friendly solvent, mild conditions, readily accessible substrates, broad substrate scope and simple work-up.

An Accesss to 4,5,6-Trisubstituted Pyrimidines from 2H-Azirines and α-Isocyanoacetates or α-Isocyanoacetamides Enabled by 1,3-Dipolar [3 + 2] Cycloaddition/Ring-Expanding/Oxidative Aromatization Process

The products containing pyrimidine scaffolds exhibit various important physiological and biological activities. To date, the strategies to generate 4,5,6-trisubstituted pyrimidines were not reported. Here, a copper-catalyzed reaction of 2H-azirines with α-isocyanoacetates or α-isocyanoacetamides has been developed, rapidly preparing 4,5,6-trisubstituted pyrimidines. The mechanistic results reveal that this strategy underwent a formal 1, 3-dipolar [3 + 2] cycloaddition/ring-expanding/oxidative aromatization procedure to construct the desired pyrimidines.

Regioselective Transition Metal-Free Catalytic Ring Opening of 2H-Azirines by Phenols and Naphthols; One-Pot Access to Benzo- and Naphthofurans

Benzofuran and naphthofuran derivatives are synthesized from readily available phenols and naphthols. Regioselective ring openings of 2H-azirine followed by in situ aromatization using a catalytic amount of Brønsted acid have established the novelty of the methodology. The involvement of a series of 2H-azirines with a variety of phenols, 1-naphthols, and 2-naphthols showed the generality of the protocol. In-depth density functional theory calculations revealed the reaction mechanism with the energies of the intermediates and transition states of a model reaction. An alternate pathway of the mechanism has also been proposed with computer modeling.

Rhodium Catalyzed [4 + 1]-Annulation of o-Acylanilines with 3-Diazoindoline-2-imines

An efficient rhodium catalyzed [4 + 1]-annulation of o-acylanilines with 3-diazoindoline-2-imines has been successfully accomplished for the synthesis of spiroindolines in good to excellent yield. The reaction occurs through formation of N-ylide followed by cyclization and showed good tolerance to various functional groups. Gram-scale synthesis, diastereoselective construction of tetrasubstituted indoline, synthesis of spirooxindole, and isolation of potential intermediates have also been demonstrated.

Brønsted acid-promoted ring-opening and annulation of thioamides and 2H-azirines to synthesize 2,4,5-trisubstituted thiazoles

In this study, a metal-free synthesis of 2,4,5-trisubstituted thiazoles using 2H-azirines and thioamides is disclosed. Under the catalysis of HClO₄, the protocol was realized through a novel chemical bond breaking of 2H-azirine, which is usually achieved using a metal catalyst. It provides an efficient and green route for the synthesis of substituted thiazoles with a broad substrate scope. Preliminary mechanistic studies show that such a reaction may involve a ring-opening reaction, annulation, and a hydrogen atom rearrangement process.

Copper(II)-Catalyzed (3+2) Cycloaddition of 2H-Azirines to Six-Membered Cyclic Enols as a Route to Pyrrolo[3,2-c]quinolone, Chromeno[3,4-b]pyrrole, and Naphtho[1,8-ef]indole Scaffolds

A method for the [2+3] pyrroline annulation to the six-membered non-aromatic enols using 3-aryl-2H-azirines as annulation agents is developed in the current study. The reaction proceeds as a formal (3+2) cycloaddition via the N1-C2 azirine bond cleavage and is catalyzed by both Cu(II) and Cu(I) compounds. The new annulation method can be applied to prepare pyrrolo[3,2-c]quinoline, chromeno[3,4-b]pyrrole, and naphtho[1,8-ef]indole derivatives in good to excellent yields from enols of the quinolin-2-one, 2H-chromen-2-one, and 1H-phenalen-1-one series.

Synthesis of Imidazo[1,2-a]pyridines via Near UV Light-Induced Cyclization of Azirinylpyridinium Salts

An efficient one-pot synthesis of imidazo[1,2-a]pyridines from 2-bromoazirines and pyridines has been developed. The construction of the bicyclic framework of imidazo[1,2-a]pyridines occurs in two steps through the formation of (2H-azirin-2-yl)pyridinium bromides followed by dehydrobrominative UV light-induced cyclization. The method can also be applied for the synthesis of imidazo[2,1-a]isoquinolines. Unstable in solution, (2H-azirin-2-yl)pyridinium/isoquinolinium bromides were quantitatively converted to stable tetrafluoroborates, which can be cyclized to imidazo[1,2-a]pyridines under UV irradiation in the presence of bromide ions.

Copper-Catalyzed [4 + 1] Annulation of Enaminothiones with Indoline-Based Diazo Compounds

A concise synthetic route to spiroindoline-fused S-heterocycles was developed through copper-catalyzed [4 + 1] annulation using enaminothiones as donor-acceptor synthons. Both 3-diazoindolin-2-imines and 3-diazooxindoles were amenable to work as effective C1 building blocks. The reaction proceeds via a copper-catalyzed cascade process involving the in situ generation of copper(I) carbene and C-S/C-C bond formation. This synthetic protocol features the use of readily available substrates, diverse substituent tolerance, and good to excellent yields.

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.

Photocatalytic Approach for Construction of 5,6-Dihydroimidazo[2,1-a]isoquinolines and Their Luminescent Properties

A visible-light-driven photoredox reaction of tetrahydroisoquinoline with 2H-azirines is described. 4,7-Bis(4-methoxyphenyl)benzo[c][1,2,5]thiadiazole, a benzothiadiazole (BTD) derived fluorophore, is used as an organic photoredox catalyst, and the reaction offers an efficient access to 5,6-dihydroimidazo[2,1-a]isoquinolines with a broad range of functional groups. The resulting 5,6-dihydroimidazo[2,1-a]isoquinolines present strong photoluminecence in solutions and powders and could be applied in the fabrication of blue OLED devices.

Iron-Catalyzed Cycloaddition of Amides and 2,3-Diaryl-2H-azirines To Access Oxazoles via C-N Bond Cleavage

A novel and efficient iron-catalyzed cycloaddition reaction using readily available 2,3-diaryl-2H-azirines and primary amides is reported. A wide range of trisubstituted oxazoles could be achieved in good yields with good functional group compatibility. In this transformation, two C-N bonds were cleaed and new C-N and C-O bonds were formed.

Recent advances in the synthesis of indole embedded heterocycles with 3-diazoindolin-2-imines

The preparations, reactivity and synthetic applications of 3-diazoindolin-2-imines, a valuable class of α-diazo amidines, are reviewed.

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.

Regiodivergent Synthesis of Butenolide-Based α- and β-Amino Acid Derivatives via Base-Controlled Azirine Ring Expansion

A method for the preparation of 5-aminobutenolides from 2-bromo-2H-azirine-2-carboxylic esters/amides with arylacetic acids has been developed. The reaction regioselectivity can be switched by a change of the basic catalyst, making it possible to prepare both butenolide-based α- and β-amino acid derivatives. The change in the regioselectivity is interpreted in terms of the stability and reactivity of the enolates formed during the S_N2' substitution of the bromine in the azirine by the carboxylate ion.

Synthesis of 8-Alkoxy-5H-isochromeno[3,4-c]isoquinolines and 1-Alkoxy-4-arylisoquinolin-3-ols through Rh(III)-Catalyzed C-H Functionalization of Benzimidates with 4-Diazoisochroman-3-imines and 4-Diazoisoquinolin-3-ones

Rh(III)-catalyzed C-H activation/annulation of benzimidates with 4-diazoisochroman-3-imines furnished 8-alkoxy-5H-isochromeno[3,4-c]isoquinolines in moderate to excellent yields with a broad range of substrate scope. The reaction was carried out under mild reaction conditions and could be scaled up with practical usage. Similar reaction between benzimidates and 4-diazoisoquinolin-3-ones provided 1-alkoxy-4-arylisoquinolin-3-ols in excellent yields. Moreover, the synthesized products could be conveniently transformed to the corresponding heterocycles with a 1,8-naphthyridinone or isochromenopyridinone core, which are privileged structures in medicinal chemistry.

Emerging unconventional organic solvents for C-H bond and related functionalization reactions

Solvent engineering is an increasingly essential topic in the chemical sciences. In this context, some recently appeared unconventional solvents have shown their large potential in the field of C-H bond functionalization reactions. This review aims not only at recognizing and classifying a short selection of these emerging solvents, in particular halogenated ones, but also at providing a medium term perspective of the possibilities they will offer for synthetic method development.

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.

Preparation of spiro[imidazolidine-4,3′-indolin]-2′-imines via copper(i)-catalyzed formal [2 + 2 + 1] cycloaddition of 3-diazoindolin-2-imines and triazines

We report a facile and efficient synthesis of spiro[imidazolidine-4,3′-indolin]-2′-imines via a copper(i)-catalyzed cascade reaction of 3-diazoindolin-2-imines with 1,3,5-triazines.

Synthesis of 2 H-Azirines via Iodine-Mediated Oxidative Cyclization of Enamines

A facile and practical oxidative cyclization reaction of enamines to 2 H-azirines has been developed, employing molecular iodine. The features of the present synthetic approach include no use of transition metals, mild reaction conditions, and simplicity of operation. Under the optimal reaction conditions, a variety of 2 H-azirine derivatives were synthesized from simple and readily accessible enamine precursors in an efficient and scalable fashion.

Modular 2,3-diaryl-2H-azirine synthesis from ketoxime acetates via Cs2CO3-mediated cyclization

A practical and effective Cs₂CO₃-mediated cyclization of ketoxime acetates for the synthesis of 2H-azirines has been developed.

Facile access to 2-acyloxy-, aryloxy- and alkenyloxy-2H-azirinesviaan SN2′–SN2′ cascade in 2-halo-2H-azirines

A convenient method of synthesis of 2-oxygen-substituted 2H-azirinesviathe unusual S_N2′–S_N2′ cascade halogen substitution in 2-halo-2H-azirines is developed.