Studies on heterocyclic chemistry. Part VIII. Comparison of the thermally induced reaction of 4-substituted 5-aminoisoxazoles with the decomposition of isoxazoles by arylamine, with reference to a 2H-azirine intermediate

Thermal isomerisation of 5-amino-4-methyl-3-phenylisoxazole, which has previously been reported to yield 4-methyl-5-phenyl-4-imidazolin-2-one via an isocyanate (3), has been reinvestigated. It has been shown that conversion of the isoxazole into the imidazolin-2-one competes with intramolecular rearrangement to 2-methyl-3-phenyl-2H-azirine-2-carboxamide, and that the latter is not a precursor of the imidazolinone. This reaction is compared with that of isoxazoles with arylamines to give NN′-diarylureas by reaction of the amine with an intermediate 2H-azirine.