Oxidative Dimerization of Tocopherols in Saturated and Unsaturated Triglycerides. III

Abstract

d-δ-Tocopherol (δ-Toc), which was dissolved in saturated and unsaturated trigiycerides, was oxidized with air under the condition of thermal oxidation. Experimental conditions and analytical methods were the same as those described in Parts I and II of this series. From the results of TLC and HPLC of the nonsaponifiable material obtained from the reaction mixture, it was assumed that the dimers of δ-Toc may be formed (Figs.-1 and 2). In order to confirm oxidative dimerization, the reaction mixture, which was obtained by the oxidation in trilaurin at 180°C for 5 h, was separated by chromatography on a silicic acid column eluted with hexane-ethyl ether mixtures, and three main oxidation products were isolated (Fig.-4). Product [I] was identified as 5- (δ-tocopheroxy) -δ- tocopherol (δ-TED), and both products [II] and [III] were identified as 5- (δ-tocopherol-5-yl) -δ-tocopherol (δ-TBD) by means of UV absorption, IR absorption, NMR and MS spectrometry (Table-1). Product [III] seems to be the atropisomer of product [II]. The decrease of δ-Toc concentration and the formation of the dimers during the course of thermal oxidation in trilaurin (Tri-La), in triolein (Tri-Ol), and in trilinolein (Tri-Li) were followed by HPLC. The decomposition of δ-Toc in Tri-Li was more rapid than that in Tri-La (Fig.-5). δ-TED was formed more abundantly than δ-TBD. The quantities of the formed dimers were greatly affected by the species of triglyceride molecules, and large amounts of dimers were obtained by the oxidation in Tri-Li. The amount of δ-TBD obtained by oxidation in Tri-Li reached a maximum within 5 h, and the quantity of this dimer formed by the oxidation at 150°C is much larger than that at 180°C (Figs.-6 and 7).