Several tocopherols have been isolated. Some have the 4',8',12'-trimethyltridecyl-saturated side chain; others have unsaturation in the side chain. It has been suggested that these polyunsaturated tocols be named tocotrienols. The best-known tocopherol is a-tocopherol, which has the greatest biological activity. The base structure, represented in Figure 28.11, shows that the tocopherols are methyl-substituted tocol derivatives: a-tocopherol is 5,7,8-trimethyltocol; ¡-tocopherol is 5,8-dimethyltocol; the y-compound is 7,8-dimethyltocol; and S-tocopherol is 8-methyltocol. The tocotrienols have similar substituents. Vitamin E is a generic descriptor for all tocol and tocotrienol derivatives that have biological activity similar to a-tocopherol.86 Currently, eight of these tocols, four tocopherols and four tocotrienols, are considered the vitamin E family. These tocols possess two important structural components, the chromanol ring and the side chain, and differ by the methylation patterns of the chromanol ring (a-, 3-, y-, and S-) and presence or absence of double bonds in the side chain (no double bond in tocopherols whereas three double bonds in tocotrienols).
Natural (+)-a-tocopherol has the configuration 2R,4'R,8'R. The natural tocotrienol has a 2R,3'E,7'E configuration. The tocopherols are diterpenoid natural products biosynthesized from a combination of four isoprenoid units; geranylgeranyl pyrophosphate is the key intermediate that leads to these compounds.
(+)-a-Tocopherol is about 1.36 times as effective as (±)-a-tocopherol in rat antisterility bioassays. As mentioned previously, (+)-a-tocopherol is the naturally occurring form of a-tocopherol with 2R,4'R,8'R configuration. (± )-a-Tocopherol is synthetic, and is called all-rac-a-toco-pherol. The configuration at C2 is critical and the Nutrition Board of the National Academy of Sciences, Institute of Medicine recognized in 2000 that 2R-forms of a-tocopherol meet the human requirements. ¡-Tocopherol is about half as active as a-tocopherol, and the y- and S-tocopherols are only 0.01 times as active as a-tocopherol.
The esters of tocopherol (e.g., acetate, propionate, and butyrate) are more active than the parent compound because of better bioavailability.87 This is also true of the phosphoric acid ester of (±)-S-tocopherol when it is administered
R2 el3 A PHs CH3
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