Oxidation products of abietic acid and its methyl ester

Abstract

Abietic acid (1) and its methyl ester (2) were investigated under various storage conditions to provide an indication of their preferred oxidation mechanisms and to investigate the most susceptible positions for modification in the abietane skeleton. Six known compounds, methyl 7α,13β-dihydroxyabiet-8(14)-enoate (4a), methyl 7α,13α-dihydroxyabiet-8(14)-enoate (4b), methyl 12-oxoabietate (6), methyl 7-oxodehydroabietate (7), methyl 7α-hydroxydehydroabietate (8), and 13,14-seco-13,14-dioxoabiet-7(8)-enoic acid (11), were identified. Compounds 7 and 8 are regarded as potent allergens. In addition, six new oxidation products were isolated, methyl 13β-ethoxy-7α-hydroxyabiet-8(14)-enoate (3a), methyl 13α-ethoxy-7αhydroxyabiet-8(14)-enoate (3b), methyl 7α-hydroperoxy-13α-hydroxyabiet-8(14)-enoate or methyl 13αhydroperoxy-7α-hydroxyabiet-8(14)-enoate (5), 7α,13β-dihydroxyabiet-8(14)-enoic acid (9a), 7α,13αdihydroxyabiet-8(14)-enoic acid (9b), and 7α,15-dihydroxydeydroabietic acid (10). Their structures were characterized on the basis of spectroscopic data interpretation. The cytotoxicity of several compounds against KB cells was evaluated, and weak activity was observed for 6, 7, and 8 with IC50 values of 12.5, 4.5, and 5.8 μg/mL, respectively.