Tocopherols vitamin 6-chromanol

More recently, the cerebral antioxidant activity of the 21-aminosteroid U-74006F has been enhanced by replacing the steroid functionality, which possesses only weak antioxidant activity without the complex amino substitution, with a more potent and effective antioxidant. A series of compounds has been synthesized in which the steroid of U-74006F has been replaced by the antioxidant ring structure (i.e. chromanol) of a-tocopherol (vitamin E). One of these compounds, U-78517F (Fig. 2) has been demonstrated to have predictably more potent effects with regard to inhibition of lipid peroxidation in vitro and enhancement of early neurological recovery of head-injured mice [62]. 

Synonyms 6-Chromanol, 2,5,7,8-tetramethyl-2-(4,8,12-trimethyl tridecyl)- 3,4-Dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H-benzopyran-6-ol all-rac-a-Tocopherol Vitamin E... 

The occurrence of a 5a-C-centered tocopherol-derived radical 10, often called chromanol methide radical or chromanol methyl radical, had been postulated in literature dating back to the early days of vitamin E research,12 19 which have been cited or supposedly reconfirmed later (Fig. 6.5).8,20-22 In some accounts, radical structure 10 has been described in the literature as being a resonance form (canonic structure) of the tocopheroxyl radical, which of course is inaccurate. If indeed existing, radical 10 represents a tautomer of tocopheroxyl radical 2, being formed by achemical reaction, namely, a 1,4-shift of one 5a-proton to the 6-oxygen, but not just by a shift of electrons as in the case of resonance structures (Fig. 6.5). In all accounts mentioning... 

Rosenau, T. Kloser, E. Gille, L. Mazzini, F. Netscher, T. Vitamin E. Chemistry studies into initial oxidation intermediates of a-tocopherol disproving the involvement of 5a-C-centered Chromanol Methide radicals.. /. Org. Chem. 2007, 72(9), 3268-3281. 

The pioneering work of Evans, Emerson and Emerson led to the isolation of a-tocopherol, a phenol exhibiting vitamin-E-like activity in biological systems. Now, eight compounds, usually oils, have been identified from plant sources, with similar vitamin-E-like actions. The tocopherols differ only in the number of methyl groups in the aromatic ring and therefore can be regarded as derivatives of 2-methyl-6-chromanol, onto which a 16-carbon isoprenoid chain is attached at C-2 and which is methylated at positions C-5, C-7 and C-8, respectively. 

Hydrophobic protective systems include vitamin E, i.e. a-tocopherol which, as all chromanol compounds, is a free radical scavenger which yields a long-lived radical upon hydrogen abstraction, thereby interrupting the chain reaction [160]. This property is optimized in a-tocopherol (a-TH) which is a remarkable scavenger of peroxyl radicals in phospholipid membrane bilayers [161,162] ... 

The term vitamin E refers to two groups of compounds, the tocophenols and the tocotrienols. The structures of these compounds appear in Figure 9.90. All forms of the vitamin contain two parts, a "head" and a "tail." The head consists of an aromatic ring structure, called chroman or chromanol, and is the site of antioxidant action. The tail of tocopherols is a phytyl group, while the tail of tocotrienols is a polyisoprenoid group. The tail of vitamin K setv es to anchor the vitamin in lipid membranes, in the lipids of adipose tissue, and in the lipid surface and core of the lipoproteins. 

Vitamin E is the nutritional term for the group of naturally occurring tocopherols and tocotrienols that have biological activity similar to RRR-a-tocopherol (formerly d-a-tocopherol). Both groups have a common 6-chromanol nucleus substituted with methyl groups at positions 2 and 8 and with a phytyl tail of isoprenoid units at position 2. The isoprenoid chain is saturated in the tocopherols, but unsaturated at positions 3", and iV for tocotrienols (Figure 30-... 

The term vitamin E describes a family of eight antioxidants, a-, P-, y-, and 5-tocopherol and a-, P-, y-, and 5-tocotrienol, which are synthesized only by plants. Tocopherols and tocotiienols are characterized by a substituted hydroxylated ring system (chromanol ring). However, tocotrienols have an unsaturated side chain, while tocopherols have a saturated side chain with three asymmetric carbon atoms (Figure 21.2). Out of eight possible tocopherol stereoisomers, only the AAA-stereoisomer occurs naturally. ... 

Tocopherols and tocotrienols comprise the group of eight chromanol homologs that possess vitamin E activity in the diet. They are natural monophenolic compounds with varying antioxidant activities. The a-, P-, y-, and 5-tocopherols are characterized by a saturated side chain consisting of three isoprenoid units, whereas their corresponding tocotrienols have double bonds at the 3, 7, and 11 position of the isoprenoid side chain (Figure 5.3). 

Tocopherols. Name for 3,4-dihydro-2//-l-benzopy-ran-6-ols (6-chromanols) substituted in the 2-position by a saturated or unsaturated 4,8,12-trimethyltridecyl residue. Oxidation of T. furnishes the tocoquinones. The latter, like the plastoquinones, ubiquinones, " boviquinones, and K vitamins (see vitamin K ) belong to the polyprenylated p-benzo- or p-naphthoqui-nones, which llfil important biological functions as redox pairs with the corresponding hydroquinones (so-called bioquinones). 

The well-known series of natural tocopherols consists of methylated tocols derived from phytol. Recently Green ct al. (1959a, 1960) showed the existence of a second, closely related series of natural tocopherols containing an unsaturated (trimethyltrideca-3,7,ll-trienyl) side chain, as represented by e- and fi-tocopherol (Table II). Further chromanols, chromenols, and benzoquinones which may play a role together with or similar to that of vitamin E in the living cell are also listed in Table II. 

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