On the Isomerization of ent -Kaurenic Acid

Abstract

Kaurenic acid (1a) is a tetracyclic diterpene that has an exocyclic double bond at Δ16. Isokaurenic acid (2a) has an endocyclic Δ15double bond. This compound has been isolated from Espeletia tenore (Espeletinae), a resinous plant from the Venezuelan Andes, but its occurrence is rare. In order to obtain a larger amount of 2a, the isomerization of 1a, which is easily obtained from other Espeletinae, was tried. Kaurenic acid methyl ester (1b) was treated with dil. HCl in CH₃Cl/EtOH, after 6 h under reflux a yield of 41.5% isokaurenic acid methyl ester (2b) was obtained but 35.7% 16α-ethoxy-kauran-19-oic acid methyl ester (3b) had formed as a byproduct. Treating 1b with CF₃COOH in refluxing CH₂Cl₂ permitted to obtain a yield of 66.6 % of 2b in 4 h and only traces of 16α-hydroxy-kauran-19-oic acid methyl ester (3a) as a byproduct. Both isomers were separated on a silica gel column impregnated with 20% AgNO₃. Treating 2b with KOH in refluxing DMSO yielded pure isokaurenic acid, no back isomerization was observed.