Unsaturated acids oxidation

The kinetics of the heterogeneously catalysed vapor-phase oxidation of a,p-unsaturated aldehydes to the a,p-unsaturated acid has been investigated for a Mo-V-Cu-oxid catalyst. The reaction rates of aldehyde and oxygen consumption as function of the aldehyd, oxygen, acid and water partial pressure are described by a kinetic model based on a modified Mars-van Krevelen mechanism Also the rate of the a, p-unsaturated acid oxidation has been measured and described for varied partial pressures of acid, oxygen and water. 

Reactions in which a product remains in the him (as above) are complicated by the fact that the areas of reactant and product are not additive, that is, a nonideal mixed him is formed. Thus Gilby and Alexander [310], in some further studies of the oxidation of unsaturated acids on permanganate substrates, found that mixed hlms of unsaturated acid and dihydroxy acid (the immediate oxidation product) were indeed far from ideal. They were, however, able to ht their data for oleic and erucic acids fairly well by taking into account the separately determined departures from ideality in the mixed hlms. 

The prostaglandins (qv) constitute another class of fatty acids with aUcycHc structures. These are of great biological importance and are formed by i vivo oxidation of 20-carbon polyunsaturated fatty acids, particularly arachidonic acid [27400-91-5]. Several prostaglandins, eg, PGE [745-65-3] have different degrees of unsaturation and oxidation when compared to the parent compound, prostanoic acid [25151 -18-9]. 

The two oxidoreductase systems most frequentiy used for preparation of chiral synthons include baker s yeast and horse hver alcohol dehydrogenase (HLAD). The use of baker s yeast has been recendy reviewed in great detail (6,163) and therefore will not be coveted here. The emphasis here is on dehydrogenase-catalyzed oxidation and reduction of alcohols, ketones, and keto acid, oxidations at unsaturated carbon, and Bayer-Vidiger oxidations. 

The reaction capability of PS is weak, but the reaction capability can be improved by anchoring the functional group to the aliphatic chain or aromatic ring of PS using chemical or conversion reactions. Aliphatic chain reactions are halogenation reactions, oxidation reactions, or unsaturated acids to bonded aliphatic chain of PS (in the presence of a radical catalysis). 

The mechanism suggested by these kinetics depends on the simultaneous oxidation of two ions in substrate-metal ion complexes so that free radicals are not produced. A few data on Cr(II) reduction of these unsaturated acids indicate simple second-order kinetics ... 

Heteropoly acids can be synergistically combined with phase-transfer catalysis in the so-called Ishii-Venturello chemistry for oxidation reactions such as oxidation of alcohols, allyl alcohols, alkenes, alkynes, P-unsaturated acids, vic-diols, phenol, and amines with hydrogen peroxide (Mizuno et al., 1994). Recent examples include the epoxidations of alkyl undecylenates (Yadav and Satoskar, 1997) and. styrene (Yadav and Pujari, 2000). 

Chemical oxidation technology is primarily used for the detoxification of cyanide and other oxi-dizable organics such as aldehydes, mercaptans, phenols, unsaturated acids, and certain pesticides.40... 

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. 

The unsaturated fatty acid oxidation proceeds at a rate higher over that for saturated acids. For example, if the oxidation rate for saturated stearic acid is taken as a reference value, the oxidation rate for oleic acid is i 1 times, linolic acid, 114 times, linolenic acid, 170 times, and arachidonic acid, nearly 200 times as high as that for stearic acid. 

The attack of peroxyl radicals on 0-CH2 groups produces the same functional groups (hydroperoxyl, hydroxy, oxo) as in the case of subsequent hydrocarbon oxidation. The oxidation of unsaturated acids proceeds similarly to the oxidation of olefins [4,7]. 

Low-density lipoproteins in plasma and arterial wall are susceptible to oxidation to form oxidized LDL, which are thought to promote the development of atherosclerosis. LDL particles have a density of about 1.05, a molecular weight of about 2.5 x 106, and a diameter of about 20 nm [119]. LDL composition from different donors varies widely an average LDL particle contains about 1200 molecules of unsaturated acids and antioxidants about six molecules of a-tocopherol, about 0.53 molecule of 7-tocopherol, about 0.33 molecule of (3-carotene, and about 0.18 molecule of lycopene [120], Rapid oxidation of LDL is started only after the depletion of tocopherols and carotenoids [121]. 

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. 

An important characteristic of mammalian 15-LOX is its capacity to oxidize the esters of unsaturated acid in biological membranes and plasma lipoproteins without their hydrolysis to free acids. Jung et al. [19] found that human leukocyte 15-LOX oxidized phosphatidylcholine at carbon-15 of the AA moiety. Soybean and rabbit reticulocyte 15-LOXs were also active while human leukocyte 5-LOX, rat basophilic leukemia cell 5-LOX, and rabbit platelet 12-LOX were inactive. It was suggested that the oxygenation of phospholipid is a unique property of 15-LOX. However, Murray and Brash [20] showed that rabbit reticulocyte... 

LOX catalyzed the oxidation of arachidonoylphosphatidylcholine at both carbon-12 and carbon-15. Later on, it has been found [21] that reticulocyte lipoxygenase oxidized rat liver mitochondrial membranes, beef heart submitochondrial particles, rat liver endoplasmic membranes, and erythrocyte plasma membranes without preliminary release of unsaturated acids by phospholipases. 

Schnurr et al. [22] showed that rabbit 15-LOX oxidized beef heart submitochondrial particles to form phospholipid-bound hydroperoxy- and keto-polyenoic fatty acids and induced the oxidative modification of membrane proteins. It was also found that the total oxygen uptake significantly exceeded the formation of oxygenated polyenoic acids supposedly due to the formation of hydroxyl radicals by the reaction of ubiquinone with lipid 15-LOX-derived hydroperoxides. However, it is impossible to agree with this proposal because it is known for a long time [23] that quinones cannot catalyze the formation of hydroxyl radicals by the Fenton reaction. Oxidation of intracellular unsaturated acids (for example, linoleic and arachidonic acids) by lipoxygenases can be suppressed by fatty acid binding proteins [24]. 

Besides the oxidation of unsaturated acids such as arachidonic and linoleic acids, LOXs are able to oxidize other substrates. One of the most important oxidative processes catalyzed by LOXs is the oxidation of low-density lipoproteins (LDL). (Nonenzymatic LDL oxidation has been discussed in detail in Chapter 25.) As already mentioned, the oxidation of LDL in the arterial intimal space is an important step in the development of atherogenesis. Now, we will consider the involvement of LOXs in this process. In 1989, Parthasarathy et al. [25] found that... 

The mechanism of LOX-catalyzed LDL oxidation is still not clearly understood [31]. On one hand, it has been proposed that LDL oxidation may be initiated by oxygen radicals, which are released from the active site of the enzyme. On the other hand, the formation of lipid peroxide by direct oxygenation of unsaturated acids without the participation of free... 

LOX-dependent superoxide production was also registered under ex vivo conditions [55]. It has been shown that the intravenous administration of lipopolysaccharide to rats stimulated superoxide production by alveolar and peritoneal macrophages. O Donnell and Azzi [56] proposed that a relatively high rate of superoxide production by cultured human fibroblasts in the presence of NADH was relevant to 15-LOX-catalyzed oxidation of unsaturated acids and was independent of NADPH oxidase, prostaglandin H synthase, xanthine oxidase, and cytochrome P-450 activation or mitochondrial respiration. LOX might also be involved in the superoxide production by epidermal growth factor-stimulated pheochromo-cytoma cells [57]. 

Starting from quinic acid, a highly substituted, cis-afi unsaturated nitrile oxide has been synthesized and used in a 1,3-dipolar cycloaddition, to afford a precursor of the cis -decalin system of branimycin (468). 

Passi, S., Picardo, M., Deluca, A., et al. (1993). Saturated dicarboxylic-acids as products of unsaturated fatty-acid oxidation. Biochimica et Biophysica Acta 1168 190-198. 

Unsaturated acids may be split chemically at their double bonds. Permanganate-periodate oxidation has been used to produce the corresponding carboxylic acids, while an alternative technique of ozonolysis results in the formation of aldehydes and aldehyde esters. All these reaction products may be identified by GLC and the information used to determine the position of the double bond in the original fatty acid. 

---