Synthesis of Chiral Nitroxides and an Unusual Racemization Reaction

Lai's protocol for the synthesis of nitroxides has been extended to the synthesis of several new chiral piperazine and morpholine nitroxides. This strategy utilizes the Bargellini reaction as the key bond-forming step. Several optically pure nitroxides incorporating alpha-aromatic and alpha-spiro centers were prepared by this route. These chiral nitroxides will be of interest as enantioselective oxidants, as traps for prochiral radicals, and in the preparation of new materials. One of these nitroxides, compound 43, was found to racemize under mild oxidizing conditions. The mechanism for this unusual racemization reaction was investigated.