Selective Reduction of Doxyl and Proxyl Nitroxide Carboxylic Acids to the Corresponding Alcohols with Borane Methyl Sulfide

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Abstract
Doxyl2 (4,4-dimethyloxazolidine-N-oxy1) nitroxides and the more recently introduced proxyl3 (for example, 8) nitroxides are important stable free radicals for the study of biological systems using the spin labeling technique.4 Chemical transformations on molecules already bearing a nitroxide grouping lead to new nitroxide spin labels, providing of course, that the synthetic steps do not destroy the somewhat sensitive nitroxide grouping.4 We wish to report that nitroxide carboxylic acids can be smoothly reduced to the corresponding nitroxide alcohols in high yield using borane methyl sulfide5 in ether solution at 25°C. Heretofore, LiAlH4 has been employed for this reduction.6 Since borane methyl sulfide and LiAlH4 differ considerably in their reactivity toward other functional groups5, the borane methyl sulfide reagent constitutes a valuable alternative to LiAlH4 for chemically modifying nitroxide carboxylic acids which also bear other functional groups susceptible toward reduction by LiAlH4.