Oxidation products of abietic acid and its methyl ester

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 7alpha,13beta-dihydroxyabiet-8(14)-enoate (4a), methyl 7alpha,13alpha-dihydroxyabiet-8(14)-enoate (4b), methyl 12-oxoabietate (6), methyl 7-oxodehydroabietate (7), methyl 7alpha-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 13beta-ethoxy-7alpha-hydroxyabiet-8(14)-enoate (3a), methyl 13alpha-ethoxy-7alpha-hydroxyabiet-8(14)-enoate (3b), methyl 7alpha-hydroperoxy-13alpha-hydroxyabiet-8(14)-enoate or methyl 13alpha-hydroperoxy-7alpha-hydroxyabiet-8(14)-enoate (5), 7alpha,13beta-dihydroxyabiet-8(14)-enoic acid (9a), 7alpha,13alpha-dihydroxyabiet-8(14)-enoic acid (9b), and 7alpha,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 IC(50) values of 12.5, 4.5, and 5.8 microg/mL, respectively.