4-Aminopyrrole-3-carboxylates and pyrazine-2-carboxylates were synthesized from 5-alkoxyisoxazoles and 1-sulfonyl-1,2,3-triazoles by tuning the Rh(II) catalyst and the reaction conditions. The reaction in chloroform at 100 °C under Rh(OAc) catalysis provides 4-aminopyrrole-3-carboxylates in good yields. The use of Rh(Piv) in refluxing toluene results in the formation of 1,2-dihydropyrazine-2-carboxylates as the main products, which can be converted by a one-pot procedure to pyrazine-2-carboxylates by heating with catalytic amounts of TsOH. According to the NMR and DFT investigations of the reaction mechanism, pyrroles and dihydropyrazines are formed, respectively, via 1,5- and 1,6-cyclization of common (5Z)-1,4-diazahexa-1,3,5-triene intermediates. The influence of the nature of the catalyst on the product distribution is rationalized in terms of the Rh-catalyzed isomerization of a pyrrolin-2-ylium-3-aminide zwitterion, the primary product of 1,4-diazahexatriene 1,5-cyclization.

Various 2-oxygen-substituted 2H-azirine-2-carboxylic acid derivatives were synthesized in high yields under mild conditions from readily available precursors, 2-halo-2H-azirines and OH-reagents having pKa values in the range of 3-10. This reaction is the first example of substitution at the azirine carbon atom for which an unusual SN2'-SN2' cascade mechanism was revealed.

A novel one-step method for the preparation of 5H-pyrazino[2,3-b]indoles with different substitution patterns in all rings of the tricyclic system via the Rh(OAc)-catalyzed reaction of 2H-azirines with 3-diazoindolin-2-imines is reported. Alkyl 5H-pyrazino[2,3-b]indole-3-carboxylates were also prepared by a one-pot procedure from synthetic equivalents of alkyl 2H-azirine-2-carboxylates, 5-alkoxyisoxazoles. The reactions provide the first examples of the use of Rh(ii) catalysis for intermolecular annulations with 2H-azirines and isoxazoles.

A convenient gram-scale method for the preparation of 2-halo-2H-azirine-2-carboxylic acid esters, thioesters and amides via metal-catalyzed isomerization of 5-heteroatom-substituted 4-haloisoxazoles is developed. The formation of the esters and amides is efficiently catalyzed by Rh2(Piv)4, while FeCl2·4H2O is the catalyst of choice for the synthesis of the thioesters. In addition, rhodium catalysis is successfully applied in the synthesis of azirine-2-carboxylates from non-halogenated 5-alkoxyisoxazoles.