**1. Introduction**

246 Nitroxides – Theory, Experiment and Applications

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Nitroxides are stable free radicals which have the >N-O moiety. In most cases, nitroxides have a ring structure. For example, imidazoline, isoindoline, piperidine and pyrrolidine ring nitroxides (Fig. 1) have been used as agents for spin labeling, imaging, and as antioxidants. These nitroxides have four substituents at the α-position; two substituents on each α-carbon. All four substituents are needed for avoiding the disproportionation reaction of nitroxides except for the case of a bridgehead at the α-position. Methyl groups have been chosen as simple and inert substituents. However, it has been reported that other types of substituents, especially ethyl groups, showed unique characteristics that were unlike those of the conventional methyl group.

In this chapter, we will introduce the conventional as well as the latest synthetic methods used to introduce the various substituents to the α-position. Also, we will describe the structure–reactivity relationships of α-substituted nitroxides.
