The first synthesis of pyrroles by Fe(II)-catalyzed isomerization of 4-vinylisoxazoles is reported. 5-Alkoxy, amino, and N,N-dialkylamino-3-aryl/alkyl-4-(2-R-vinyl)isoxazoles afford 2-aryl/alkyl-5-aryl/alkyl/methoxycarbonyl-1H-pyrrol-3-carboxylic acid derivatives typically under mild conditions with cheap and available FeCl·4HO as a catalyst. The isomerization of 5-alkoxy/amino-3-arylisoxazoles, bearing unsaturated carbo and heterocyclic substituents at the position 4, gives the corresponding fused pyrrolecarboxylic acid derivatives in high yields. DFT calculations were used to elucidate a probable mechanism of the isomerization and explain the influence of steric congestion of the vinyl moiety on the isomerization pathway.

An effective strategy was developed for the synthesis of new 2,2′-bipyridine ligands, symmetrical and unsymmetrical 6,6′-binicotinates, and 2,2′-bipyridine-5-carboxylates, from 4-propargylisoxazoles. The synthesis of symmetrical 2,2′-disubstituted 6,6′-binicotinates was achieved using the Eglinton reaction of 5-methoxy-4-(prop-2-yn-1-yl)­isoxazoles with Cu­(OAc)2, followed by Fe­(NTf2)2/Au­(NTf2)tBuXPhos-catalyzed isomerization of the so-formed mixture of isoxazole/azirine-substituted biacetylenic intermediates. Unsymmetrical 2,2′-disubstituted 6,6′-binicotinates were prepared via a copper-free Sonogashira coupling of 5-methoxy-4-(prop-2-yn-1-yl)­isoxazoles with 6-bromonicotinates to give methyl 6-(3-(5-methoxyisoxazol-4-yl)­prop-1-ynyl)­pyridine-3-carboxylates, followed by a transformation of the propargylisoxazole moiety of the adduct into the pyridine fragment under Fe­(II)/Au­(I) relay catalysis conditions. 6-(Pyrid-2-yl)­nicotinates were synthesized by a Stille-type coupling of 2-(tributylstannyl)­pyridine with 6-bromonicotinates. Several cyclopalladated complexes of a new series of 6,6′-binicotinates and 2,2′-bipyridine-5-carboxylates and the homoleptic Cu­(I) complex were synthesized in high yields.

The Co­(III)-catalyzed reaction of 1,3-dicarbonyl compounds with 2-(diazoacetyl)-2H-azirines, prepared by a simplified procedure from 2H-azirin-2-carbonyl chlorides, led in high yields to the formation of 2-(diazoacetyl)­pyrroles, while leaving the diazoacetyl function intact. The intramolecular aromatic substitution reaction of 2-(diazoacetyl)­pyrroles, catalyzed by Cu­(OTf)2, provided selectively previously unknown benzo­[e]- and hetero­[e]-fused indol-7-oles in good yields. Formylation of benzo­[e]­indol-4-ol led selectively to the 5-formyl derivative, which is a good precursor for an unusual salen ligand and its Ni-complex. Triflates prepared from benzo­[e]­indol-4-oles gave various 4-substituted benzo­[e]­indoles carrying aryl, 2-thienyl, 2-pyridyl, and alkynyl groups, in excellent yields using cross-coupling reactions. 4-(2-Pyridyl)­benzo­[e]­indoles, upon treatment with BF3·Et2O/Et3N, afforded a new type of fluorescent boron complexes with large Stokes shifts.

The reaction conditions for the Buchner reaction of 2-(diazoacetyl)-2H-azirines with benzene leading to 2-(cyclohepta-2,4,6-trien-1-yl)-2H-azirines have been found. The azirine fragment of these compounds was subsequently used to prepare 5-(cyclohepta-2,4,6-trien-1-yl)­isoxazoles and α-(cyclohepta-2,4,6-triene-1-ylcarbonyl)-1H-pyrroles. NMR and DFT calculations showed that the cycloheptatriene-norcaradiene valence isomerization in the corresponding moieties of synthesized azirines, isoxazoles, and pyrroles is very fast at room temperature. Cycloheptatriene valence isomer predominates in azirines and pyrroles, while 5-(cycloheptatrienyl)­isoxazoles exist in a pure cycloheptatriene form at 25 °C in chloroform solution. In accordance with the X-ray analysis, two of the synthesized pyrroles crystallized as norcaradiene isomers from a solution of equilibrium mixture of valence isomers. Reductive isoxazole ring opening using the Mo­(CO)6/H2O reductive system afforded (Z)-3-amino-3-aryl-1-(cyclohepta-2,4,6-trien-1-yl)­prop-2-en-1-ones, which are pure cycloheptatriene isomers. α-(Cycloheptatrieneylcarbonyl)-1H-pyrroles were transformed into the α-(benzylcarbonyl)-1H-pyrroles under cooperative TFA/Rh2(TFA)4 catalysis.

A simple strategy was developed for the synthesis of pyrrole-containing polyheterocycles by sequential annulation of 2H-azirine-2-carbonylbenzotriazoles derived from 5-chloroisoxazoles.