Triethylamine-Promoted Oxidative Cyclodimerization of 2H-Azirine-2-carboxylates to Pyrimidine-4,6-dicarboxylates: Experimental and DFT Study.

An unprecedented oxidative cyclodimerization reaction of 2H-azirine-2-carboxylates to pyrimidine-4,6-dicarboxylates under heating with triethylamine in air is described. In this reaction, one azirine molecule undergoes formal cleavage across the C-C bond and another across the C=N bond. According to the experimental study and DFT calculations, the key steps of the reaction mechanism include nucleophilic addition of N,N-diethylhydroxylamine to an azirine to form an (aminooxy)aziridine, generation of an azomethine ylide, and its 1,3-dipolar cycloaddition to the second azirine molecule. The crucial condition for the synthesis of pyrimidines is generation of N,N-diethylhydroxylamine in the reaction mixture in a very low concentration, which is ensured by the slow oxidation of triethylamine with air oxygen. Addition of a radical initiator accelerated the reaction and resulted in higher yields of the pyrimidines. Under these conditions, the scope of the pyrimidine formation was elucidated, and a series of pyrimidines was synthesized.

Gold vs Light: Chemodivergent Reactivity of Diazoesters toward 2H-Azirine-2-carboxylic Acids.

An orthogonal reactivity of diazo compounds toward azirine-2-carboxylic acids, switching with the reaction conditions, is demonstrated. A gold-catalyzed reaction is N-selective and produces 1,3-oxazin-6-ones, whereas a blue light activation leads to O-H insertion products, azirine-2-carboxylic esters. The observed chemodivergence is explained by the metal-bound and metal-free carbenes exhibiting different electronic properties in these reactions. In addition, a high antibacterial potential of the 1,3-oxazin-6-ones synthesized is shown.

Selective and Reversible 1,3-Dipolar Cycloaddition of 2-(2-Oxoindoline-3-ylidene)acetates with Nitrones in the Synthesis of Functionalized Spiroisoxazolidines.

The 1,3-dipolar cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates with functionalized aldo- and ketonitrones proceeds with good selectivity to provide new highly functionalized 5-spiroisoxazolidines. A characteristic feature of these reactions is reversibility that allows for the control of the diastereoselectivity of cycloaddition. The reduction of obtained adducts using zinc powder in acetic acid leads to 1,3-aminoalcohols or spirolactones. For a number of the spiro compounds obtained, anticancer activity was found.

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.

One-Pot Synthesis of Multifunctionalized 1-Pyrrolines from 2-Alkyl-2H-azirines and Diazocarbonyl Compounds.

A novel strategy for the synthesis of 1-pyrrolines based on formal [4 + 1] annulation of 2-alkyl-2H-azirines with diazocarbonyl compounds has been developed. This one-pot approach includes the Rh(II)-catalyzed formation of 4-alkyl-2-azabuta-1,3-dienes, followed by the DBU-promoted cyclization, and features a good substrate tolerance. The 1-pyrrolines containing an ester group at the C3 were prepared in a three-step one-pot procedure starting from 5-alkoxyisoxazoles. The cyclization of 2-azabutadienes to 1-pyrrolines most likely proceeds via the 6π electrocyclization of a conjugated NH-azomethine ylide.

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.

Blue Light-Promoted Cross-Coupling of α-Diazo Esters with Isocyanides: Synthesis of Ester-Functionalized Ketenimines.

A metal-free scalable synthesis of functionalized ketenimines from alkyl α-(aryl/heteroaryl)-α-diazoacetates and alkyl isocyanides induced by blue light irradiation has been developed. The reaction proceeds at room temperature without any photocatalyst and provides ketenimines in moderate to good yields. Density functional theory (DFT) calculations and the experimental study showed that aryl(alkoxycarbonyl)carbenes in both singlet and triplet states can react with isocyanides but only the reaction of the former leads to the smooth formation of ketenimines. The obtained ketenimines were used for the synthesis of functionalized amidines under mild metal-free conditions.

Synthesis of 2-(2-Pyridyl)-2H-azirines via Metal-Free C-C Cross-Coupling of Bromoazirines with 2-Stannylpyridines.

A high-yield method for the introduction of a 2-pyridyl substituent in the C2 position of the three-membered ring of 2-bromo-2H-azirine-2-carboxylic acid derivatives by the direct cross-coupling with 2-(trialkylstannyl)pyridines has been described. The reaction works well with 3-, 4-, or 5-substituted 2-stannylpyridines and can be also employed for the synthesis of 2-(thiazol-2-yl)-2H-azirines. According to DFT calculations, the reaction proceeds through the sequence of SN2'-substitution of bromine, 1,4-stannyl shift, and [2,3]-sigmatropic shift of the pyridine ring.

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.

A Hydroxypyrrole Approach to 2,2'-Bi(4-pyrrolin-3-ones) and Pyrrolone-Based α-Amino Esters.

A high yield synthesis of 2-amino-4-pyrrolin-3-ones including 4-pyrrolin-3-one-based amino esters by the NBS- or TfCl-mediated reaction of 3-hydroxypyrroles with amines has been developed. The reaction of 3-hydroxypyrroles with triflyl chloride in the absence of amine affords 2,2'-bi(4-pyrrolin-3-ones) in excellent yields and diastereoselectivity. The relative configuration of stereocenters in these compounds can be changed from (2RS,2'RS) to (2RS,2'SR) by the action of NaH in THF, while the reverse stereoisomerization occurs in the presence of 2,5-lutidine in MeCN. Bi(4-pyrrolin-3-ones) also can be converted to 2-amino-4-pyrrolin-3-ones in excellent yields. Bi(4-pyrrolin-3-ones) and 2-amino-4-pyrrolin-3-ones were evaluated for their antiproliferative activity toward four human tumor cell lines.

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.

Azirine-containing dipeptides and depsipeptides: synthesis, transformations and antibacterial activity.

Azirine-containing dipeptides and depsipeptides with a wide range of substituents have been synthesized in high yields via the Passerini and Ugi multicomponent reactions (MCRs) using 2H-azirine-2-carboxylic acids as the acid component. The obtained MCR adducts have been transformed to lactam-fused aziridines, as well as pyrrole, imidazole, aziridine, and other derivatives, containing the dipeptide or depsipeptide moiety. The azirine-containing depsipeptides exhibit antibacterial activity against the ESKAPE pathogens, especially Gram-positive bacterial strains (E. faecium - MIC 16 µg mL-1, S. aureus - MIC 9 µg mL-1).

Synthesis of 3-Alkoxy-4-Pyrrolin-2-ones via Rhodium(II)-Catalyzed Denitrogenative Transannulation of 1H-1,2,3-Triazoles with Diazo Esters.

A method for the synthesis of densely substituted 4-pyrrolin-2-ones by Rh(II)-catalyzed denitrogenative transannulation of 1-alkyl-4-aryl-1H-1,2,3-triazoles with diazo esters has been developed. The reaction proceeds via an attack of the rhodium-bound carbene at the N2 atom of the triazole and the formation of unstable 3,4-dihydro-1,2,4-triazine, which further undergoes ring contraction to a 4-pyrrolin-2-one under rhodium catalysis. The method is inapplicable to 1,2,3-triazoles with primary alkyl substituent at C4, which afford stable 1,2,3-triazol-3-ium ylides as the main products.

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.

Regiodivergent Synthesis of Butenolide-Based α- and ß-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 ß-amino acid derivatives. The change in the regioselectivity is interpreted in terms of the stability and reactivity of the enolates formed during the SN2' substitution of the bromine in the azirine by the carboxylate ion.

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

A domino synthesis of dispiro-fused N-vinyl ß-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-ß-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-ß-lactams in moderate yields. A bromine atom in the 2-bromoalkenyl moiety of the synthesized ß-lactams can be easily substituted by hydrogen under catalytic hydrogenation conditions or by 2-pyridyl-substituent via the Stille cross-coupling reaction.

2 H-Azirines as C-C Annulation Reagents in Cu-Catalyzed Synthesis of Furo[3,2- c]quinolone Derivatives.

A method of furo-annulation of 4-hydroxy-2-oxoquinoline-3-carboxylates with 3-arylazirines under Cu(II) catalysis was developed to synthesize a variety of 2,3-dihydrofuro[3,2- c]quinolones bearing a carbamate group at the C2 position. The reaction involves an azirine ring opening across the N-C2 bond and formation of a dihydrofuran ring with the inclusion of two azirine carbon atoms, accompanied by a shift of the ester group to the nitrogen. The discovered reaction is the first example of the use of 2 H-azirines for furo-annulation.

Synthesis of 1-(2-Aminovinyl)indoles and 1,3'-Biindoles by Reaction of 2,2-Diaryl-Substituted 2 H-Azirines with α-Imino Rh(II) Carbenoids.

An effective and operationally simple method for the preparation of 1-(2-(sulfonamido)vinyl)indoles (SAV-indoles) by the Rh(II)-catalyzed reaction of 2,2-diaryl-2 H-azirines with 1-sulfonyl-1,2,3-triazoles has been developed. This method enables the stereoselective synthesis of a variety of 1,2,3-trisubstituted indoles having a Z configuration of the (1-aryl-2-(sulfonamido)vinyl) substituent. The reaction mechanism, supported by DFT calculations, involves the formation of 1,4-diazahexa-1,3,5-trienes, which rapidly cyclize to 2,2-diaryl-1-sulfonyl-1,2-dihydropyrazines. These compounds can be isolated at an early stage of the reaction, but under prolonged heating they isomerize into 7a H-indolium ylides, followed by a barrierless 1,5-prototropic shift to SAV-indoles. The developed methodology was also applied to the preparation of 1,3'-biindoles from 2,2-diaryl-2 H-azirines using 3-diazoindolin-2-imines instead of 1-sulfonyl-1,2,3-triazoles.

Non-natural 2H-azirine-2-carboxylic acids: an expedient synthesis and antimicrobial activity.

Non-natural 2H-azirine-2-carboxylic acids were obtained in high yields by FeCl2-catalyzed isomerization of 5-chloroisoxazoles to azirine-2-carbonyl chlorides followed by their hydrolysis. The 3-aryl- and 3-heteroaryl-substituted acids are stable during prolonged storage, exhibit antibacterial activity against ESKAPE pathogens and show a low level of cytotoxicity.

Synthesis of 2-(Di/tri/tetraazolyl)-2 H-azirine-2-carboxylates by Halogen Substitution: Evidence for an SN2'-SN2' Cascade Mechanism.

An effective and operationally simple method for the preparation of methyl 2-(di/tri/tetraazol-1-yl)-2 H-azirine-2-carboxylates from accessible methyl 2-halo-2 H-azirine-2-carboxylates and NH-azoles has been developed. The azoles having enhanced NH acidity, react with 2-halo-2 H-azirines in the presence of Et3N in a deprotonated form, while the azoles having highly nucleophilic sp2 nitrogen react in neutral form. According to the DFT calculations, the substitution of the halogen by the azole proceeds via an SN2'-SN2' cascade, with the initial conjugate substitution of the halogen being the rate-determining step. The formation of 2,3-dipyrazolylaziridines in the reaction of pyrazole provides experimental evidence for the cascade mechanism of the reaction.