UNUSUAL OXIDATION BEHAVIOR OF SESQUITERPENES

Oxygenated derivatives of sesquiterpenes like α-humulene (αH) or β-caryofyllene (βC) are important contributors to the natural flavouring of beverages. However, the relations between the molecular structure of oxidized sesquiterpenes and their aroma are unclear. This incited us to study the heterolytic as well as the homolytic oxidation of αH, βC and 1,5,9-trimethyl-1,5,9-cyclododecatriene (TMCDT). In the presence of hydrogen peroxide chemoselective epoxidation of these sesquiterpenes is observed, even without the addition of a catalyst. Activation of hydrogen peroxide is achieved through hydrogen bonding with the solvent; the oxidation rate correlates well with the hydrogen donor properties of the solvent. While the activation of hydrogen peroxide by fluorinated alcohols or phenols is well-known, αH, βC and TMCDT are reactive in common alcoholic solvents. The epoxidation is not inhibited by TEMPO or 4,6-di-tert-butyl-m-cresol, demonstrating the non-radical nature of the epoxidation. The high oxidation rates of these sesquiterpenes indicate that the electron density on their trisubstituted double bonds is unusually high. The reactivity of αH, βC and TMCDT is of the same magnitude as that of 2,3-dimethyl-2-butene, while other trisubstituted cyclic olefins such as 1-methylcyclohexene, limonene and α–pinene are inert under the same experimental conditions. αH, βC and TMCDT demonstrate unusual behavior in the aerobic autoxidation as well. Analysis of the mass balance of the autoxidation of these sesquiterpenes shows that both volatile and non-volatile products are formed. The volatile fraction contains mainly epoxides; for αH the chemoselectivity for epoxides within this volatile fraction even exceeds 98%. The proposed mechanisms in current literature are insufficient to explain this high chemoselectivity. The main challenge here is to elucidate the fate of the generated alkoxy radicals and their involvement in the formation of the non-volatile product fraction.