Methyl esters of diterpenoids ent-pimara-9(11),15-dien-19-oic acid (1), ent-pimara-7,15-dien-19-oic acid (2), and ent-pimara-8,15-dien-19-oic acid (3) were submitted to photooxygenation reactions with sensitized singlet oxygen. While compounds 2 and 3 were converted to the products, compound 1 reacted only partially, suggesting an influence of the steric hindrance on the endocyclic double bond of 1. The oxidation products obtained, methyl-7alpha,11beta-dihydroxypimara-8,15-dien-19-oate (5), methyl-7alpha-hydroperoxypimara-8(14),15-dien-19 oate (6), methyl-7alpha-hydroxy-14-oxopimara-15-en-19-oate (8), methyl-7alpha,9alpha-dihydroxypimara-8(14),15-dien-19-oate (9) and methyl-7alpha,14alpha-dihydroxypimara-8,15-dien-19-oate (10), are new and their structures were elucidated by spectral data analysis.
diterpenes; ent-pimaradiene derivatives; singlet oxygen; photooxygenation
