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.

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.

Rh(II)-Catalyzed Ring Expansion of Pyrazoles with Diazocarbonyl Compounds as a Method for the Preparation of 1,2-Dihydropyrimidines.

A high yield synthesis of 1,2-dihydropyrimidines by the Rh(II)-catalyzed reaction of diazocarbonyl compounds with 1,4-di- and 1,4,5-trisubstituted pyrazoles is reported. This reaction represents the first example of a carbenoid insertion into a N-N bond and provides a novel approach to 4-unsubstituted 1,2-dihydropyrimidines with a broad range of functional group tolerance. According to DFT calculations, the pyrazole ring expansion proceeds via the sequential formation of the metal-bound pyrazolium ylide, metal-free pyrazolium ylide, and 1,5-diazahexatriene followed by 1,6-cyclization.

Synthesis of 1,2-Dihydropyrimidine-2-carboxylates via Regioselective Addition of Rhodium(II) Carbenoids to 2H-Azirine-2-carbaldimines.

An efficient two-step procedure "imine formation/azirine-carbenoid coupling" has been developed for the preparation of 1,2-dihydropyrimidines from azirine-2-carbaldehydes, primary amines, and diazo carbonyl compounds under Rh(II) catalysis. The formation of 1,2-dihydropyrimidines involves 100% regioselective addition of the rhodium carbenoid to endocyclic nitrogen atom of the 2H-azirine-2-carbaldimine. According to the DFT calculations the reaction proceeds via dissociation of the metal-bound complex of the azirinium ylide to metal-free azirinium ylide, ring-opening of the latter to give a 1,5-diazahexa-1,3,5-triene, followed by 1,6-cyclization. The 1,2-dihydropyrimidines with two different electron-withdrawing substituents at the C2 position can undergo in solution inversion of configuration of the stereogenic center at C2 via "the N1-C2 bond cleavage/rotation around the N-C single bond/1,6-cyclization" sequence.