Photochemical Dimerization of Resin Acids

Abstract

Ultraviolet irradiation of levopimaric acid with a sensitizer gave dimer (II), 8, 12-bridged valence tautomer (III) and dehydroabietic acid (VI). The yield of the dimer (III) increased with increasing amount of the sensitizer. Ultraviolet irradiation of abietic acid (IV) either with or without the sensitizer gave dimer (V), although the reaction with the sensitizer was more rapid. The dimer (II) of levopimaric acid was thermally stable and had a high melting point (above 300℃), while the dimer (V) of abietic acid was thermally unstable and regenerated abietic acid when heated at 70℃.