Fe(II)-Catalyzed Isomerization of 4‑Vinylisoxazoles into Pyrroles

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 FeCl2·4H2O 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.