Fatty acids free radical addition reactions

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

Fig. 3. Autoxidation of polyunsaturated fatty acids in phospholipid membranes. Addition of oxygen to lipid free radicals is extremely fast. It yields peroxyl radicals ROO which will tend to capture labile hydrogen atoms of neighbouring polyunsaturated lipids. Accidentally produced free radicals will therefore initiate a chain reaction of lipid peroxidation which will propagate along membranes. This process can result in several dozen propagation steps before it is stopped by a termination reaction. Examples of such termination reactions are the recombination of peroxyl radicals and the formation of a stable free radical from a free radical scavenger (scavH). Termination through recombination of low steady-state concentration of alkyl radicals is unlikely in aerobic medium.

The formation and reactions of guanine radicals in DNA and their reactions in the presence of carbon-centered radicals derived from lipid molecules provide instructive examples of free radical reactions in solutions involving these biologically relevant species. The reactivities of free radicals derived from biomolecules depend on their structures. The carbon-centered radicals produced by either by hydrogen atom abstraction or the addition of oxyl radicals to double bonds of polyunsaturated fatty acids (PUFAs) are primary intermediates of lipid peroxidation... 

A 2,5-disubstituted C g tetrahydrofuran fatty ester [13] was obtained from methyl ricinoleate by addition of bromine to the isomerized substrate, followed by hydrogenation over palladium on charcoal (68). Free radical and Lewis acid-induced reactions involving flie double bonds of unsaturated fetty esters have been conducted by Metzger et al. (69-74) these have resulted in the production of a large number of functionalized, cyclic, and branched fetty ester derivatives (e.g., [14], [15]). The synthesis of methyl rac-2-dodecyl-cyclopentane carboxylate from methyl 2-iodo-18 1(6Z) is presented in Scheme 5. 

Unsaturated fatty compounds such as oleic acid [la], 10-undecenoic acid [2a], pet-roselinic acid [3a], erucic acid [4a], and the respective esters, alcohols, and native oils (Fig. 1) are alkenes and contain an electron-rich double bond that can be functionalized in many different ways by reactions with electrophilic reagents. It is therefore remarkable that >90% of oleochemical reactions have been focused on the carboxylic acid functionality and < 10% have been reactions of the alkyl chain and the C,C-dou-ble bond (1). A review on radical additions to unsaturated fatty compounds that appeared in 1989 (2) quoted only very few C,C-bond-forming reactions giving branched and chain-elongated fatty compounds. Since then, modem preparative radical chemistry has been developing and has been applied also to fat chemistry (3-5). We report here on radical additions of activated haloalkanes such as alkyl 2-haloalka-noates and 2-haloalkanenitriles to unsaturated fatty compounds [l]-[4] initiated by electron transfer from copper in solvent-free systems. These additions were also car-... 

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