Oxidations of the α‐tocopherol model compound 2,2,5,7,8‐pentamethyl‐6‐chromanol. Formation of 2,2,7,8‐tetramethylchroman‐5,6‐dione

Abstract

AbstractOveroxidation of α‐tocopherol (1a) by silver nitrate produces tocored (9a) as a major product. The aim of the present work was to elucidate the pathway of formation of tocored using the α‐tocopherol model compound, 2,2,5,7,8‐pentamethyl‐6‐chromanol (1b). Oxidation of 1b by silver nitrate in ethanol produces 2‐(3‐hydroxy‐3‐methylbutyl)‐3,5,6‐trimethyl‐1,4‐benzoquinone (6b) and 2,2,7,8‐tetramethylchroman‐5,6‐dione (9b, the model compound of tocored) as major products. Formation of 6b is rapid and is accompanied by an equally rapid fall in pH. Formation of 9b only occurs after 6b has reached maximum concentration and has begun to decline. It appears that acid promotes the dehydration and recyclization of 6b into a quinone methide (2b), which is then rehydrated into 5‐hydroxymethyl‐2,2,7,8‐tetramethyl‐6‐chromanol (5b), the phenolic isomer of the quinone 6b. Oxidative deformylation of 5b leads to 9b. It is also demonstrated that 6b, heated in ethanol in the presence of acid and in the absence of any oxidizing agent, is converted into 9b, 1b, 5‐ethoxymethyl‐2,2,7,8‐tetramethyl‐6‐chromanol (4b) and 2‐(3‐hydroxy‐3‐methylbutyl)‐3‐ethoxymethyl‐5,6‐dimethyl‐1,4‐benzoquinone (7b). It seems that dehydration and recyclization of 6b into 5b occurs as above and that 6b then oxidizes 5b into 9b, while being reduced into the hydroquinone of 6b (6bH2). Compound 6bH2 then cyclizes in acid to 1b. A possible alternative pathway from 6b to 9b that does not involve 5b is also discussed. These results suggest that 6b and, by implication, α‐tocopheryl quinone (6a), is not a stable compound and, in the presence of acid, is readily oxidized to 9b.