A new method was used for the preparation of a pH-sensitive spin probe involving an intramolecular 1,3-dipolarcycloaddition- reductive isoxazolidine ring-opening - oxidation reaction sequence. 2-(2-Allyloxyphenyl)-5-R-4,4-dimethyl-4H-imidazole-3-oxides (R = Me, NMe2), prepared from 3-hydroxylamino-3-methylbutan-2-one, undergo intramolecular 1,3-dipolarcycloaddition upon heating in toluene. Treatment