PREPARATION OF UNSYMMETRICALLY LABELED HYDROPEROXIDES - A HYDROXAMATEESTER-NITROSATION APPROACH

Reaction of O-tertiary alkyl arylhydroxamate esters with nitrosyl chlo ride gives O-tertiary alkyl aryl peresters that can be hydrolyzed to t he hydroperoxide. If the hydroxamate ester carbonyl oxygen or nitrosyl chloride is labeled with O-18, the label appears in the product hydro peroxide's terminal oxygen. This strategy, which derives from the pion eering work of Koenig, permits the preparation of a variety of(2-O-18) hydroperoxides, including tertiary alkyl and cumyl hydroperoxides. te rt-Butyl alpha-hydroperoxyisobutyrate, 27, prepared in this way with a n O-18 label in the terminal oxygen, serves as a useful precursor for the preparation of other unsymmetrically labeled peroxides and hydrope roxides. Hydroperoxide 27 is protected as a perketal with 2-methoxypro pene, 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 transformatio n and one hydroxamate ester 17 that reacts via an allylalkoxyl radical gives epoxides 20a and 20b as the major isolable products of reaction .