Oxidation and Mass Spectra of 4,4-Dimethyloxazolidine-N-oxyl (Doxyl) Derivatives of Ketones

4,4-Dimethyloxazolidine-N-oxyls (doxyls), the very useful nitroxide spin labels, can be reconverted rapidly and efficiently to their parent ketones with nitrogen dioxide (conveniently as contained in commercial nitric oxide) in ethanol at room temperature. This reaction is interpreted as involving initial oxidation to give an oxoammonium salt, e.g., 5 from 1, followed by fragmentation to an oxonium ion, e.g., 6, and cleavage to ketone. The mass spectra of several doxyls and their precursor oxazolidines have been studied. The latter show a fragmentation pattern like that of ethylene ketals. The more interesting mass spectra of the doxyls can be interpreted on the basis of formation of the same molecular ion, e.g., 5, which results from chemical oxidation. This leads in a major fragmentation mode to protonated parent ketone as the base peak. Deuterium-labeled substrates and high-resolution identification of ion formulas were used to gain evidence for this pathway as well as for several other modes of breakdown of the oxoammonium molecular ions.