Isomerization and Autoxidation of Resin Acids

Abstract

Levopimaric acid (I), palustric acid (II), neoabietic acid (III), and abietic acid (IV) isomerized in chloroform, and rapidly approached to an equilibrium state under nitrogen atmosphere in the dark (92% abietic acid (IV), 5% palustric acid (II), 3% neoabietic acid (III), but the resin acids dissolved in a-pinene or octyl alcohol did not isomerize in the absence of air. Abietic acid (IV) gave dehydroabietic acid (V) and some complex oxidation products when kept in chloroform in the presence of air. The resin acids as well as apinene used as solvent were oxidized in the dark, but not oxidized in octyl alcohol even in the presence of air oxygen. Homoannular dienes, such as palustric acid (II) and levopimaric acid (I), yielded dehydroabietic acid (V) on oxidation, but heteroannular dienes, such as abietic acid (IV) and neoabietic acid (III), yielded only oxygen adducts, but not dehydroabietic acid (V). Levopimaric acid (I) was presumed to be the precursor in the biosynthesis of palustric acid (II), neoabietic acid (III), abietic acid (IV) and dehydroabietic acid (V).