R

HO"6

Figure 28.11 • Tocopherols and tocotrienols.

parenterally.88 The ethers of the tocopherols are inactive, because unlike esters that are readily hydrolyzed to the active antioxidant chromanol form by esterases, ethers are relatively metabolically more stable and as such do not possess antioxidant property.

Oxidation of the tocopherols to their corresponding quinones also leads to inactive compounds. Replacement of the methyl groups on the chromanol ring by ethyl groups decreases activity. The introduction of a double bond in the 3,4-position of «-tocopherol reduces its activity by about two thirds. Reduction of the size of the long alkyl side chain or the introduction of double bonds in this side chain markedly reduces activity.

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