Hydroperoxy cyclic peroxides, from

Figure 3. The formation of hydroperoxy cyclic peroxides and prostaglandin-like endoperoxides from the 13S-hydroperoxide of linolenic acid by peroxy radical rearrangement. Structures are...

Figure 3.6 Oxidative breakdown of hydroperoxy cyclic peroxides of linoleate leading to pentane and hexanal. Reproduced from Min, D. B., Lee, S. H. and Lee, E. C., Singlet oxygen oxidation of vegetable oils, in Flavor Chemistry of Lipid Foods (eds D. B. Min and J. H.

Porter et al, in two significant papers, have offered a unified mechanism of polyene autoxidation. This seeks to account for the various products obtained particularly from linoleic acid and from arachidonic acid (six hydroperoxides with one E double bond, six hydroperoxides with two E double bonds, and four hydroperoxy cyclic peroxides). The serial cyclisation of peroxy radicals observed with (51) is interesting in that the 1,5-diene units give rise to six-membered cyclic peroxides whilst the 1,4-diene units in natural... 

Neff, W.E., Frankel, E.N. and Weisleder, D. High-pressure liquid chromatography of autoxidized lipids II. Hydroperoxy-cyclic peroxides and other secondary products from methyl linolenate. Lipids 16, 439-448 (1981). 

Thus, reactions of carbonyl compounds with hydrogen peroxide and acids lead to products similar to those obtained by the ozonization of olefins (Section III), which also yields 1,2,4-trioxolans and 1,2,4,5-tetroxans. The similarity of the two reactions is understandable, since the intermediates )C+—OO- in the ozonization27a and )C+—OOH in the hydrogen peroxide reactions are related as a conjugate base-acid pair. A number of cyclic peroxides of structure 7 are prepared from bis(hydroperoxy)dialkyl peroxides (5) by reaction with lead(IV) acetate, as described by Criegee et al.la This reaction is also thought to involve a carbonium ion intermediate,31 which reacts with the second OOH group. 

More recently, Nojima utilized the DBPO-TBHP procedure for making 1,2,4-trioxanes and trioxepanes from unsaturated hydroperoxy acetals (cf. Scheme 46) [82], The preparation of 1,2,4-trioxepanes is especially noteworthy since seven-membered cyclic peroxides have not been previously obtained by such pathways. 

As an example, the formation of 1,2-dioxanes from 12-hydroperoxyoctadeca-9,13,15-trienoic acid derived from hnolenic acid is shown in Figure 3.35.1,2-Dioxanes are unstable compounds and decompose to low molecular weight flavour-active products. For example, 10-hydroperoxy-9,12-peroxyoctadeca-13,15-dienoic acid is decomposed into octa-3,5-dien-2-one. The mechanism of formation of 1,2-dioxolanes, cyclic peroxides and cycloendoperox-ides from 13-hydroperoxyoctadeca-9,ll,15-trienoic acid is shown in Figure 3.36. Peroxohydroperoxides of the 1,2-dioxolane type are considered to be the main precursors of toxic malondialdehyde (see Section 3.8.1.12.1). 

When active oxygen species are produced this may lead to a cycle of oxidation and reduction (redox cycle) with electrons being donated to oxygen to yield superoxide. This is the case with paraquat and also a number of cytotoxic quinones. Thus, there is a cyclic process which produces superoxide by adding electrons from paraquat (see Chapter 7) or a semiquinone to oxygen (figure 6.10). The superoxide produced may then be metabolized to hydrogen peroxide by superoxide dismutase, which is further metabolized to water (see below) or protonated to the hydroperoxy radical, HOO ... 

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