Tocopherol trimer

In 2012, Liao et al. disclosed total synthesis of ( )-schefflone 97 and tocopherol trimers 100 and 101 by the hetero-Diels-Alder trimerization of o-quinone methides 96 and 99, generated from the silver oxide-mediated oxidation of espintanol 95 and tocopherol 98, respectively (Scheme 28.33) [46]. 

FIGURE 6.25 Spiro trimers of a-tocopherol (31, 32) formed by reaction of o-QM 3 with the two diastereomers of spiro dimer 9. 

In addition to their use in combination with other antioxidants as melt stabilisers, phosphites are also used to reduce discoloration of polymer melts [37, 39, 40, 41, 42]. Using the phosphite Ultranox U-626 with tocopherol at a 2 1 w/w ratio, respectively, has resulted in a considerable colour reduction in the polymer (Fig. 5a-inset) [33]. This has also led to a 30% reduction in the total oxidation products formed in the polymer with only three products formed trimers, tocoquinone and aldehydes. This significant reduction in product formation results in an excellent retention of tocopherol in the polymer of over 95%. The preservation of the initial tocopherol con-... 

The overall effect of the initial tocopherol concentration on the product distribution in PE was somewhat similar to that observed in PP, in that increasing the tocopherol concentration resulted in the lowering of the concentrations of the trimers, aldehydes, and in only little change in the concentration of tocoquinone. The main difference found in the case of PP was the absence of the spirodimer, under all extrusion conditions examined (see e.g. Fig. 9) [34]. This difference is attributed to the greater oxidisability of PP at the higher extrusion temperatures used for this polymer (compared to PE). This is supported by the observation that the SPD, when used as an authentic (synthesised) sample in extruded PP, was much less thermally stable under PP processing conditions than the other tocopherol products such as the TRI and DHD [33]. 

The trimers are one of the major oxidative coupling products formed in extruded tocopherol-containing PP and PE. Trimers may either form by direct oxidation of the SPD by alkylperoxyl radicals (Scheme 6, reaction e) or through a reaction of SPD with QM in a Diels-Alder type process (reaction f). Oxidation studies of tocopherol in model compounds provided a clear evidence [44] in support of the latter reaction. It appears reasonable to suggest, therefore, that in the case of PP the major route for the formation of trimers (having much higher stability than SPD at PP processing tempera-... 

Scheme 6 Mechanism of melt stabilising action of a-tocopherol in polyolefins [34], TRI trimer, SPD spirodimer, DHD dihydroxydimer, QM quinine methide, ALD aldehyde, TQ tocoquinone...

Tocopherols are oxidized slowly by atmospheric oxygen and rapidly by ferric and silver salts. Oxidation products include tocopheroxide, tocopherylquinone, and tocopherylhydroqui-none, as well as dimers and trimers. Tocopherol esters are more stable to oxidation than the free tocopherols but are in consequence less effective antioxidants. See also Section 17. 

DHD and quinonoid-type products, trimers, TRI, spirodimers, SPD, quinone methides, QM, together with some aldehydes, ALD, see Fig. 3 for structures of these products. All the oxidation products were shown to be more highly colored than tocopherol itself, with the aldehydes being the most colored, and the trimers the least colored. 

Scheme 6 Oxidation products from a-tocopherol quinonoids (I), aldehydes (II), dimers (III, IV) and trimers (V)...

FIGURE 7.4 Tocopherol radicals, dimer, and trimer chemical structures. 

Studies of the metabolism of vitamin E were triggered by the observations of Alaupovic and coworkers [126, 127]. When " [Cj-D-a-tocopherol-5-methyl was administered to rats or pigs and attempts were made to detect metabolic derivatives, two compounds were separated by chromatography. One of the compounds is " [C]-D-a-tocopherol quinone the other is either a dimer or a trimer of a-tocopherol. The dimer and trimer are terminal oxidation products of a-tocopherol and are excreted in the bile. a-Tocopherol quinone can be converted to a-hydroquinone. a-Tocopherol hydroquinone may be esterified in liver and eliminated in the feces after concentration in the bile and excretion in the intestine, or it may be oxidized in the kidney to a-tocopheronic acid, which may be converted into an a-tocopheronolactone conjugate, which is excreted in the urine. In conclusion, vitamin E is excreted as such in the urine or the bile after conversion to a dimer or a trimer, in the form of a conjugated hydroxy-quinone or tocopheronic acid (see Fig. 4-43). 

HPO lyase was purified to a homogeneous state from immature fruits of green bell pepper (Shibata et al 1995a). It was deduced to be a homo-trimer of 55-kDa subunits. The activity was considerably inhibited by lipophilic antioxidants such as nordihydroguaiaretic acid and a-tocopherol. Furthermore, HPO lyase was rapidly and irreversibly inactivated... 

The free radical reaction may be accelerated and propagated via chain branching or homolytical fission of hydroperoxides formed to generate more free radicals (equations (11.4), (11.5)). Free radicals formed can initiate or promote fatty acid oxidation at a faster rate. Thus, once initiated, the free radical reaction is self-sustaining and capable of oxidizing large amounts of lipids. On the other hand, the free radical chain reaction may be terminated by antioxidants (AH) such as vitamin E (tocopherols) that competitively react with a peroxy radical and remove a free radical from the system (equation (11.6)). Also, the chain reaction may be terminated by self-quenching or pol)rmerization of free radicals to form non-radical dimers, trimers and polymers (equation (11.7)). 

Both reactions occur in lipids containing low concentrations of tocopherols and also in the presence of sufficient amounts of oxygen. Alternatively, other reactions take place. In the presence of large quantities of tocopherols, some tocopheroxyl radicals react with each other to form a dimer or trimer ... 

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