Transformation of α-Tocopherol (Vitamin E) and Related Chromanol Model Compounds into Their Phenoxonium Ions by Chemical Oxidation with the Nitrosonium Cation

Abstract

[reaction: see text] Alpha-tocopherol (alpha-TOH), the main oil component making up vitamin E, and its nonnatural solid 6-hydroxy-2,2,5,7,8-pentamethylchroman and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid structurally related analogues were oxidized quantitatively with 2 mol equiv of NO+ SbF6(-) in CH3CN at 233 K to form phenoxonium cations (alpha-TO+ SbF6(-)) in a chemically reversible two-electron/one-proton process. Solution-phase infrared spectroscopy, 1H and 13C NMR spectroscopy, and corresponding theoretical calculations of the spectroscopic data using density-based and wave-function-based models support the identity of the remarkably stable phenoxonium cations. The presence of an oxygen atom in the para position to the hydroxyl group and the chromanol ring structure appear to be important factors in stabilization of the phenoxonium ions, which raises the interesting possibility that the cations play a crucial role in the mode of action of vitamin E in biological systems. Although the phenoxonium cations are reactive toward nucleophiles such as water, they may be moderately stable in the hydrophobic (lipophilic) environment where vitamin E is known to occur naturally.

Voltammetric Studies on Vitamins D2 and D3 in Organic Solvents

“…A fast and straightforward method for oxidizing biological compounds was previously achieved with the use of NOSbF 6 as a one-electron chemical oxidant, which was found to closely mimic the results of electrochemical oxidation experiments [27]. There are many advantages of using chemical oxidation over electrochemical oxidation.…”

Voltammetric Behavior of Trolox in Methanol and Ethanol Solutions

“…To date, the oxidation of trolox in two wellseparated one-electron steps was observed by Webster et al [3,4] in CH 3 CN and CH 2 Cl 2 in the presence of strong acid (1 -3% CF 3 SO 3 H) that resulted in the formation of phenoxyl cation radical as a stable intermediate. To characterize this process more completely, cyclic voltammograms of trolox were recorded.…”

“…Despite the great practical importance of trolox, its electrochemical behavior was not thoroughly investigated, particular in nonaqueous media. Electrochemical studies about the mechanism of trolox oxidation were carried out only in acetonitrile [3,4], dichloromethane [3,4] and acetic acid [5] and water [6]. To date, such investigations have not been performed in methanol and ethanol.…”

“…The dissociation constant of CCl 3 COOH in methanol is known from the literature [10] as pK a ¼ 4.9. Consequently, one can assume that the concentration of protons in 0.1 M methanolic solution of CCl 3 COOH is approximately 1 Â 10 À3 M. The voltammetric responses in methanol and ethanol are depicted in Figure 2 (curves a and b).…”

Electrochemical oxidation of trolox and α-tocopherol in acetic acid: A comparative study

“…In our work [14] we have investigated the anodic oxidation of TrOH on GC in aqueous solutions of various pHs. A systematic study of TrOH anodic oxidation on GC and Pt was very recently carried out by Webster et al [15] in acetonitrile and dichloromethane.…”