Polymeric Antioxidants. V. Ortho Methylene-bridged and Direct-linked Sesamol Oligomers

Two types of sesamol oligomers, ortho methylene-bridged and direct-linked oligomers, were prepared and their antioxidant effects on the autoxidation of lard and tetralin were evaluated. The methylene-bridged oligomers were prepared by the reaction of sesamol, a major antioxidative compound in sesame seed, with ethylmagnesium bromide and subsequent oligomerization with paraformaldehyde. During the reaction, methylidenedioxy moieties of the sesamol oligomers were partially cleaved and the addition of hydroxyl groups was observed. The methylene-bridged oligomers had much stronger antioxidant effects on the autoxidation of lard than the sesamol monomer. And the oligomers, having a larger average number of hydroxyl groups per sesamol unit, were a more efficient antioxidant for lard. Direct-linked oligomers were synthesized by an oxidative coupling of sesamol by hydrogen peroxide using horseradish peroxidase as a catalyst. Molecular structures of the direct-linked oligomers were influenced by the pH of the buffer solution in which the oxidations were carried out. Oligomers having a larger number of hydroxyl groups were prepared under more acidic conditions. The direct-linked oligomers prepared in acidic conditions were better antioxidants of lard than the sesamol, whereas oligomers prepared under neutral and alkaline conditions did not improve the antioxidant effect. Both methylene-bridged and direct-linked oligomers inhibited the autoxidation of tetralin; but, in contrast to the lard results, almost all of the oligomers were less effective antioxidants for tetralin than the sesamol monomer.