Oxidation of the α-tocopherol model compound 2,2,5,7,8-pentamethyl-6-chromanol in the presence of alcohols

Oxidation of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol (1b) byt-butyl hydroperoxide in chloroform has been studied in the presence of ethanol, heptanol and cholesterol. In the absence of an alcohol, the major products were the spirodimer (13b) and spirotrimer (14b) of 1b, together with 1H,2,3-dihydro-3,3,5,6,9,10,11a(R)-heptamethyl-7a(S)-(3-hydroxy-3-methylbutyl)-pyrano[2,3-a] xanthene 8(7aH), 11(11aH) dione (6b). In the presence of ethanol, heptanol and cholesterol, the major products were 5-ethoxymethyl-2,2,7,8-tetramethyl-6-chromanol (16b), 5-heptoxymethyl-2,2,7,8-tetramethyl-6-chromanol (17) and 5-cholesteroxymethyl-2,2,7,8-tetramethyl-6-chromanol (18). However, when water was present in a homogeneous reaction, the most rapidly formed product was 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (5b). Compounds 13b, 14b, 16b, 17 and 18 are formedvia a quinone methide intermediate, and compound 5b is formedvia a phenoxylium ion. The phenoxylium species appears to be the preferred intermediate when water is present, whereas the quinone methide species is prefered in the absence of water.