Reactions of Unsaturated Fatty Acids

A number of reactions which are known for olefinic hydrocarbons play an important role in the analysis and processing of lipids containing unsaturated fatty acids. 

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In the area of moleculady designed hot-melt adhesives, the most widely used resins are the polyamides (qv), formed upon reaction of a diamine and a dimer acid. Dimer acids (qv) are obtained from the Diels-Alder reaction of unsaturated fatty acids. Linoleic acid is an example. Judicious selection of diamine and diacid leads to a wide range of adhesive properties. Typical shear characteristics are in the range of thousands of kilopascals and are dependent upon temperature. Although hot-melt adhesives normally become quite brittle below the glass-transition temperature, these materials can often attain physical properties that approach those of a stmctural adhesive. These properties severely degrade as the material becomes Hquid above the melt temperature. 

Raw potato possesses little aroma. Approximately 50 compounds have been reported to contribute to raw potato aroma. Raw potatoes have a high content of LOX, which catalyses the oxidation of unsaturated fatty acids into volatile degradation products (Scheme 7.2) [187]. These reactions occur as the cells are disrupted, e.g. during peeling or cutting. Freshly cut, raw potatoes contain ( ,Z)-2,4-decadienal, ( ,Z)-2,6-nonadienal, ( )-2-octenal and hexanal, which are all products of LOX-initiated reactions of unsaturated fatty acids [188,189]. It is reported that two compounds represent typical potato aroma in raw potato methional and ( ,Z)-2,6-nonadienal [189]. Other important volatiles in raw potatoes produced via the LOX pathway are l-penten-3-one, heptanal, 2-pen-tyl furan, 1-pentanol and ( , )-2,4-heptadienal [189]. Pyrazines such as 3-iso-propyl-2-methoxypyrazine could be responsible for the earthy aroma of potato [35]. Some of the most important character-impact compounds of raw potatoes are summarised in Table 7.8. Aroma compounds from cooked, fried and baked potatoes have previously been reviewed [35]. 

Not surprisingly, heat treatment, such as commercial and household frying, accelerates autoxidation. In addition to undergoing autoxidation, when fats are heated in the presence of moisture, as often is the case in food applications, fatty acids are released via hydrolysis of the ester linkages (233). The free fatty acids can accelerate oxidation of the oil. During heat treatment, the formation of dimeric and cyclic compounds seems to be the predominant thermolytic reaction of unsaturated fatty acids. In the presence of oxygen during heat treatment, however, oxidative... 

In reactions where rearrangement is likely, a rule of thumb is that the most stable available cartenium ion will lead to the product. Such thermodynamic control may be used deliberately to carry out complex rearrangements in high yields for suitable cases. When the potential cartenium ions are of similar energy, for example in Ritter reactions of unsaturated fatty acids or esters, then mixtures of products are likely. In such cases, mass spectrometry has proved to be a particularly useful analytical technique. ... 

Hamberg, M. and B. Gotthammar. 1973. A new reaction of unsaturated fatty acid hydroperoxides formation of 11-hydroxy-12,13-epoxy-9-octadecenoic acid from... 

A thermal ene-reaction of unsaturated fatty acids with maleic anhydride produces branched triacids. As an example, the Diels-Alder reaction of conjugated triene fatty acids, that is calendic acid and maleic anhydride, yields a branched triacid product with high regioselectivity and stereoselectivity (Fig. 3.13). 

Ene-reaction of unsaturated fatty acids with maleic anhydride to produce branched triacids. 

Figge, K. Dimeric fatty acid[l-14C]methyl esters. I. Mechanisms and products of thermal-oxidative reactions of unsaturated fatty acid esters - literature review. Chem. Phys. Upids. 6,159-177 (1971). 

LCAT catalyzes the transfer reaction of unsaturated fatty acids in position II of lecithin (PC) to free cholesterol (FC), giving rise to the formation of CE and lysolecithin. LCAT does not act on artificial Upid emulsions containing PC and FC without any apoUpoprotein, but there exist some cofactors which catalyze the above reaction [1]. 

A photometric procedure which is based on the reaction of unsaturated fatty acids with concentrated sulfuric acid and a mixture of phosphoric acid and vanil-line with development of a pink color was introduced by Zollner and Kirsch (1962). Since in this reaction the various lipids show different extinction coefficients calibration is necessary according to gravimetric values. This method is also applicable directly to plasma samples without previous extraction. 

Chlorine-containing fatty acids arise as contaminants in reactions of unsaturated fatty acids with chlorine or chlorine dioxide (see Section 11.4.2.2.2). 

Cross-metathesis reaction of unsaturated fatty acid esters... 

Chemical peroxidation is an important reaction of unsaturated fatty acids... 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Clay-catalyzed dimerization of unsaturated fatty acids appears to be a carbonium ion reaction, based on the observed double bond isomerization, acid catalysis, chain branching, and hydrogen transfer (8,9,11). 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

Figure 22-4. Sequence of reactions in the oxidation of unsaturated fatty acids, eg, linoleic acid. A -c/s-fatty acids or fatty acids forming A -c/s-enoyl-CoA enter the pathway at the position shown. NADPH for the dienoyl-CoA reductase step is supplied by intramitochondrial sources such as glutamate dehydrogenase, isocitrate dehydrogenase,and NAD(P)H transhydrogenase.

The specific behaviour of unsaturated fatty acids under oxidation is determined by the position and the number of double bonds in the fatty acid molecule. The stepwise oxidation of an unsaturated acid to the position of a double bond in it proceeds in a manner similar to that of saturated acid oxidation. If the double bond retains the same configuration (trans-configuration) and position (A2,3) as those of the enoyl-CoA, which is produced during the oxidation of saturated fatty acids, the subsequent oxidation proceeds via conventional route. Otherwise, the oxidation reaction proceeds with the involvement of an accessory enzyme, A3,4-CiS-A2,3jrans-enoyl-CoA isomerase this facilitates the translocation of the double bond to an appropriate position and alters the double-bond configuration from cis to trans. 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

As mentioned earlier, oxidation of LDL is initiated by free radical attack at the diallylic positions of unsaturated fatty acids. For example, copper- or endothelial cell-initiated LDL oxidation resulted in a large formation of monohydroxy derivatives of linoleic and arachi-donic acids at the early stage of the reaction [175], During the reaction, the amount of these products is diminished, and monohydroxy derivatives of oleic acid appeared. Thus, monohydroxy derivatives of unsaturated acids are the major products of the oxidation of human LDL. Breuer et al. [176] measured cholesterol oxidation products (oxysterols) formed during copper- or soybean lipoxygenase-initiated LDL oxidation. They identified chlolcst-5-cnc-3(3, 4a-diol, cholest-5-ene-3(3, 4(3-diol, and cholestane-3 3, 5a, 6a-triol, which are present in human atherosclerotic plaques. 

UV-induced ROS are extremely toxic to cells by causing oxidative damage to all biomolecules (Sies 1991). For instance, lipids, which are major compounds of all biological membranes, may be destroyed by ROS. After a first initiation reaction an unsaturated fatty acid is converted to a peroxyl radical, which in turn attacks another unsaturated fatty acid finally leading to free radical cascades. This photochemical peroxidation of unsaturated fatty acids may be particularly damaging for membrane structure and function (Bischof et al 2006a). 

Polymers Catalytic reactions involving C=C bonds are widely used for the conversion of unsaturated fatty compounds to prepare useful monomers for polymer synthesis. Catalytic C-C coupling reactions of unsaturated fatty compounds have been reviewed by Biermann and Metzger [51]. Metathesis reactions involving unsaturated fatty compounds to prepare co-unsaturated fatty acid esters have been applied by Warwel et al. [52], Ethenolysis of methyl oleate catalyzed by ruthenium carbenes developed by Grubb yields 1-decene and methyl 9-decenoate (Scheme 3.6), which can be very useful to prepare monomers for polyolefins, polyesters, polyethers and polyamide such as Nylon 11. 

Metabolism of unsaturated fatty acids is similar to that of the saturated compounds just described, but additional enzymic reactions are necessary. 

The reactions studied were the catalytic formation of methane from carbon monoxide and hydrogen (according to Sabatier (34), normal pressure), the catalytic hydrogenation of unsaturated hydrocarbons and also of unsaturated fatty acids ( fat hardening according to Normann (35)). Here again, a certain analogy was established between... 

Sec. 9-2a). The crosslinking process is referred to as drying and is directly dependent on the content of unsaturated fatty acid. The crosslinking reaction involves chemical reactions different from those involved in prepolymer synthesis. 

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