The primary products formed from the autoxidation of lipids can be understood based upon a mechanism that involves five different reaction types. These reactions are: reaction of a carbon radical and molecular oxygen, atom transfer of a hydrogen from substrate to the chain carrying peroxyl, fragmentation of the chain carrying peroxyl to give oxygen and a carbon radical, rearrangement of the peroxyl, and cyclization of the peroxyl. The mechanism of these primary reaction steps has been the focus of extensive research over the past fifty years, and the current level of understanding of these transformations is the subject of this review.
Reaction of O-tertiary alkyl arylhydroxamate esters with nitrosyl chloride gives O-tertiary alkyl aryl peresters that can be hydrolyzed to the hydroperoxide. If the hydroxamate ester carbonyl oxygen or nitrosyl chloride is labeled with 18 O, the label appears in the product hydroperoxide's terminal oxygen. This strategy, which derives from the pioneering work of Koenig, permits the preparation of a variety of (2-18 O) hydroperoxides, including tertiary alkyl and cumyl hydroperoxides. tert-Butyl R-hydroperoxyisobutyrate, 27, prepared in this way with an 18 O label in the terminal oxygen, serves as a useful precursor for the preparation of other unsymmetrically labeled peroxides and hydroperoxides. Hydroperoxide 27 is protected as a perketal with 2-methoxypropene, and a six-step sequence involving Dibal-H reduction, oxidation (PDC), and Wittig chemistry provides the allylic hydroperoxide 12 that is used in mechanistic studies. Hydroxamate esters that give peresters prone to undergo the Criegee rearrangement upon reaction with NOCl do not give useful yields of product hydroperoxides in the transformation and one hydroxamate ester 17 that reacts via an allylalkoxyl radical gives epoxides 20a and 20b as the major isolable products of reaction.
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