1-(2H-Azirine-2-carbonyl)benzotriazoles: building blocks for the synthesis of pyrrole-containing heterocycles

Catalysts-Controlled Roles Exchange between 2,3-Diaryl-2H-azirines and Acetone: Chemodivergent Synthesis of Pyrroles and 3-Oxazolines

We report two practical and step-economical methodologies for the chemodivergent synthesis of tri-substituted pyrroles and 3-oxazolines from the domino reactions of 2H-azirines and acetone. For instance, acetone served as a nucleophile to react with 2H-azirines under the basic conditions to furnish pyrroles. Upon changing the catalyst to TfOH, acetone served as an electrophile to synthesize 3-oxazolines. Moreover, the products could be synthesized on a gram scale, and the possible catalytic cycles were proposed.

Divergent Diazo Approach toward Alkyl 5/4-Hydroxy-3H-benzo[e]indole-4/5-carboxylates

A divergent diazo approach toward alkyl 5/4-hydroxy-3H-benzo[e]indole-4/5-carboxylates has been developed. The reaction of 1,3-diketones with alkyl 2-diazo-3-oxo-3-(2H-azirin-2-yl)propanoates catalyzed by Co(acac)3 or Ni(acac)2 gives various alkyl 3-(1H-pyrrol-2-yl)-2-diazo-3-oxopropanoate in good yields. The latter undergo Wolff rearrangement followed by the 6π-cyclization of transient ketene to form alkyl 5-hydroxy-3H-benzo[e]indole-4-carboxylates bearing various substituents in positions 1, 2, 7, and 8, as well as derivatives of methyl 4-hydroxy-6H-thieno[2,3-e]indole-5-carboxylates and methyl 5-hydroxy-7H-benzo[c]carbazole-6-carboxylate under thermolysis or Rh2(OAc)4 catalysis. Isomeric benzoindoles, alkyl 4-hydroxy-3H-benzo[e]indole-5-carboxylates, have been prepared by Boc-protection of the pyrrole nitrogen of alkyl 3-(1H-pyrrol-2-yl)-2-diazo-3-oxopropanoates followed by an intramolecular formal carbene insertion into the aromatic C-H bond catalyzed by Cu(OTf)2. The hydroxyl group of alkyl 5/4-hydroxy-3H-benzo[e]indole-4/5-carboxylates, through the formation of the corresponding triflates, allows the introduction of various substituents into the 5/4 position of benzo[e]indoles using the cross-coupling reaction and even form a new heterocyclic backbone, benzo[k]pyrrolo[2,3-i]phenanthridine, via a tandem Suzuki reaction/nucleophilic acyl substitution.

5-Chloroisoxazoles: A Versatile Starting Material for the Preparation of Amides, Anhydrides, Esters, and Thioesters of 2H-Azirine-2-carboxylic Acids

Amides, anhydrides, esters, and thioesters of 2H-azirine-2-carboxylic acids were prepared by a rapid procedure at room temperature involving FeCl₂-catalyzed isomerization of 5-chloroisoxazoles to 2H-azirine-2-carbonyl chlorides, followed by reaction with N-, O-, or S-nucleophiles mediated by an ortho-substituted pyridine. With readily available chloroisoxazoles and a nucleophile, 2-picoline can be used as an inexpensive base. When a high yield of the acylation product is important, the reagent 2-(trimethylsilyl)pyridine/ethyl chloroformate is more suitable for the acylation with 2H-azirine-2-carbonyl chlorides.

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.

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.

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).

The Fe(iii)-catalyzed decarboxylative cycloaddition of β-ketoacids and 2H-azirines for the synthesis of pyrrole derivatives

An Fe(iii)-catalyzed Mannich/decarboxylative ring expansion reaction to generate pyrrole derivatives has been achieved using Fe(OTf)₃ as the catalyst. A series of pyrrole derivatives was obtained in moderate to excellent yields.