Abstract
A new method was used for the preparation of a pH-sen- sitive spin probe involving an intramolecular 1,3-dipolar cycloaddi- tion - 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-me- thylbutan-2-one, undergo intramolecular 1,3-dipolar cycloaddition upon heating in toluene. Treatment of the cycloadducts with low- valence titanium (LVT) reagent resulted in reductive cleavage of the N-O bond of the isoxazolidine ring to give 3¢-hydroxymethyl- 5,5-dimethyl-4-R-2,5-dihydrospiro(imidazol-2,4¢-chromane)s. The amine (R = NMe2) was then oxidized with MCPBA to the target ni- troxide - 3¢-hydroxymethyl-5,5-dimethyl-4-dimethylamino-2,5-di- hydrospiro(imidazole-2,4¢-chroman)-1-oxyl.
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