And autoxidation

The fimction of an antioxidant is to divert the peroxy radicals and thus prevent a chain process. Other antioxidants fimction by reacting with potential initiators and thus retard oxidative degradation by preventing the initiation of autoxidation chains. The hydroperoxides generated by autoxidation are themselves potential chain initiators, and autoxidations therefore have the potential of being autocatalytic. Certain antioxidants fimction by reducing such hydroperoxides and thereby preventing their accumulation. 

Pyrrole itself is very easily converted by acid into intractable and readily autoxidized polymers. In this case and in the case of the alkyl pyrroles, it is important to distinguish between anaerobic acid-catalyzed reactions and autoxidative polymerizations the decomposition of pyrrole and its alkyl derivatives on standing in air belongs to the latter type, this review is concerned only with the former. 

Hunt, J.V., Dean, R.T. and Wolff, S. (1988). Hydroxyl radical production and autoxidative glycosylation. Glucose oxidation as the cause of protein damage in the experimental glycation model of diabetes mellitus and ageing. Biochem. J. 256, 205-212. 

We developed a convenient synthesis of 3-cyclopentenyl hydroperoxide via hydro-boration and autoxidation of cyclopentadiene, and bromination proceeded smoothly to afford 32 40). Ring closure with silver trifluoroacetate (Eq. 26) afforded a 5-bromo-2,3-dioxabicyclo[2.2.1]heptane 34 (6%) and a 5-trifluoroacetoxy-2,3-dioxabicyclo-[2.2.1]heptane 35 (14%), and it was shown independently that 34 is rapidly converted into 35 by reaction with Ag02CCF3. To avoid the trifluoroacetate bromide substitution that accompanies and competes with the dioxabicyclization, 32 was treated with silver oxide and this slowly yielded an isomeric 5-bromo-peroxide 33 (42 %) (Eq. 26). 

In the presence of a suitable heterocyclic base, the Fe(n) complex of this system is also a reversible carrier of 02 (Almog, Baldwin Huff, 1975). The stability of the dioxygen adduct depends largely upon the nature and concentration of the base present (in the absence of a base, rapid dimerization and autoxidation occurs). 

VM Goldberg, LK Obukhova. Progress in Chemistry of Organic Peroxides and Autoxidation. Moscow Khimiya, 1969, p.160 [in Russian]. 

Catalysis by radicals will usually be due to a radical addition or displacement reaction, hydrogen and halogen being the atoms on which the displacement most often occurs. It is usually a chain reaction once the substrate is converted into a radical it carries the reaction to many molecules of substrate. Examples are polymerization and autoxidation. 

The synthesis12 of indochrome A involved cyclization, catalyzed by boron tribromide, of the acyclic intermediate 242, treatment of the resulting deazauridine 243 with benzenediazonium chloride, hydrogenation of the product, and autoxidation of the resulting 5-ben-... 

A semiquinone can be readily oxidized to the parent compormd by molecular oxygen and can then re-enter the reductase-catalyzed reaction. The enzymatic reduction and autoxidation of quinones rmder aerobic conditions generates superoxide anion radicals, and this process is known as redox cycling (Figure 2). Flydroquinones are less prone to transfer electrons to oxygen, because the second-electron potential is often too high. 

Water-soluble diorganyl tellurides with thiolperoxidase and autoxidant activity have been prepared as depicted in the following scheme. ... 

It should be stated here that the suggestion of Cu(III) as the oxidation state in different oxidation and autoxidation reactions does not exclude the participation of Cu(II)-Cu(I) couple in other reactions, where Cu(II) is easily reduced by the substrate-e.g., iodide or sulfite ions (4). Likewise, the Co(II)-Co(III) couple was shown to operate in the oxidation of cystein (69), which was shown to proceed according to... 

The volatiles produced by the LOX pathway and autoxidation are typically volatile aldehydes and alcohols responsible for fresh and green sensorial notes. In the LOX pathway these volatile compounds are produced in response to stress, during ripening or after damage of the plant tissue. The pathway is illustrated in Scheme 7.2. Precursors of the LOX (EC 1.13.11.12) catalysed reactions are Cis-polyunsaturated fatty acids with a (Z,Z)-l,4-pentadiene moiety such as linoleic and a-linolenic acids that are typically oxidised into 9-, 10- or 13-hydro-peroxides depending on the specificity of the LOX catalyst. These compounds are then cleaved by hydroperoxide lyase (HPL) into mainly C, C9 and Cio aldehydes, which can then be reduced into the corresponding alcohols by alcohol dehydrogenase (ADH EC 1.1.1.1) (Scheme 7.2) [21, 22]. The production of volatile compounds by the LOX pathway depends, however, on the plants as they have different sets of enzymes, pH in the cells, fatty acid composition of cell walls, etc. 

Hikino, H., Y. Hikino, and I. Yosioka. 1964. Studies on the constituents of Atracty-lodes. IX. Structure and autoxidation of Atractylon. Chem. Pharm. Bull. 12 755-760. 

Values (fimol I 1) of Flavonoids for Iron-Induced Linoleate Peroxidation (I) and Autoxidation of Rat Cerebral Membranes (II) and Log Capacity factor log k ... 

Food flavor is governed by many factors, including lipid oxidation and protein degradation. Enzyme-catalyzed oxidation ( ) and autoxidation (2) can substantially alter the flavor q ality of foods. In "addition, protein degradation, whether caused by enzymes, heat, or interactions with other compounds, can also affect flavor characteristics of certain foods (3, 4, ... 

Sh2 substitutions frequently occur in chain reaction sequences, some of which we have already encountered in the earlier discussion. Equations 9.66-9.69 illustrate some reactions the simple stoichiometric equations serve only to emphasize the overall substitution nature of the process and do not reveal the complexities, often considerable, of the actual pathways followed. We shall not attempt to analyze these processes in detail, but in order to give a better idea of the mechanisms will describe more fully two of them, halogenation and autoxidation. 

Application of this synthesis to tetraarylporphyrins was less favorable due to the inferior stability of, e.g., OsCO(TTP)L, at high temperatures. Thermal decarbonyl-ation and autoxidation always lead to contamination of the product with Os02(TTP) (see Sect. 3.2.2) [47]. 

Gooch (1997, 2002) emulsified fatty acids and oils and autoxidized them under greater than atmospheric pressure of oxygen or air to produce a network polymer or gel that did not flow after applying the emulsion to a surface and allowing the aqueous phase to evaporate. By this method, the viscosity and crosslink density was adjusted to control rheological properties, including viscosity and flow-out. 

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