Endoperoxide decomposition

A commonly used technique to follow lipid oxidation is to monitor the formation of malondialdehyde (MDA), a water-soluble compound produced from lipid hydroperoxide and endoperoxide decomposition. The formation of the MDA can be followed by measuring it directly via HPLC - , however most often it is reacted with thiobarbituric acid (TBA) with a 1 2 stoichiometry (equation 26), and formation of the product is followed spectrophotometrically at 535 nm or followed by fluorescence . ... 

Endoperoxide decomposition causes occmrence of oxygen-containing structiu es in the phenylene structure. Deeper oxidation causes the ring break, CQ2 and CQ release, where carbon from the aromatic ring combusts (e.g. oxidizes). 

Of the substrates that have worked well, let us first illustrate the 7-alkylidene-2,3-dioxabicyclo[2.2.1]heptane system 10. It was known that fulvenes react with singlet oxygen at low temperatures to afford the corresponding endoperoxides however, attempts to isolate these labile compounds led to decomposition, although NMR identification was possible at —70 °C 19>. When reduction of the singlet oxygenates with diimide was performed at —50 °C, the bicyclic peroxides 10 were obtained in high yield (Eq. 7) 20). 

Luminol semiquinone is further oxidized to luminol endoperoxide, which elicited CL at decomposition. It should be added that in our experiments peroxynitrite-stimulated luminol CL in cells was enhanced in the presence of the NO synthase substrate L-arginine and sharply diminished in the presence of NO synthase inhibitors. Thus, the application of substrates and inhibitors of NO synthases may discriminate luminol-amplified CL stimulated by superoxide and peroxynitrite. 

Pentaarylcyclopentadienols (375) are reported to yield endoperoxides (376) which when heated or treated with acids decompose to tetraarylfurans and the corresponding aryl acids.251 Tetraarylsubstituted cyclopentadienones such as tetracyclone (378) lead to m-diaroylstil-benes and carbon monoxide.252-255 Probably, as with other cyclopentadienes, endoperoxides are the primary products. However, no attempts seem to have been made to elucidate the mechanisms of rearrangements and decompositions involved in these reactions. 

CIEEL mechanism was originally proposed by Schuster for intense luminescence (or bioluminescence) in the decomposition of endoperoxides and dioxetanes. The relevance and importance of ET in the latter has recently been discussed. While the mechanism of reduction of peroxides in these early studies was described essentially as a concerted dissociative process, little insight into the fine detail of the mechanism could be provided. 

The emission spectrum is similar to fluorescence of 9,10-diphenyl-l,4-tiinethoxy anthracene although it is not excited directly. The decomposition of the endoperoxide to (1) and (3) is the excitation producing step in wtcatalyzed cleavage and (2) is activated by energy transfer. The formation of 102 by elimination reaction is a very minor reaction step. 

However, the formation of dysidiolide (1) requires the regiospecific removal of the hydrogen at C-l on the endoperoxide 31. This is achieved by treatment with a hindered base such as diisopropylethylamine (Hunig s base) at low temperature in order to favor base-catalyzed decomposition rather than thermal decomposition. 

The use of cobalt(II) tetraphenylporphyrin (CoTPP) as catalyst in the decomposition of unsaturated bicyclic endoperoxides represents the... 

The CoTPP-catalyzed decomposition of endoperoxides as 89 leads to divinylepoxides as 90 which can be converted through a 3,3-sigmatropic rearrangement to dihydrooxepine derivatives as 91 (Sch. 54) [33a], The result appears interesting due to the discovery of the oxepine nucleus in a number of biologically active natural products. 

The most prevalent base-catalyzed reaction of an endoperoxide is the Kornblum-DeLaMare decomposition [97a] which leads to a hydroxy ketone by removal of a proton from the carbon adjacent to the peroxy linkage [9b,90b,97], The formation, in a basic solvent as acetone, of hydroxyfuranones 69 (Sch. 38) in the photo-oxygenation of a,a -unsubstituted furans might occur via a similar rearrangement [60e]. Attempts to induce asymmetry in endoperoxides with enantiotopic hydrogens or with chiral bases have led to moderate success [98]. The Et3N-catalyzed rearrangement of substituted cycloheptatriene endoperoxides 92 leads to... 

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