Natural product communications 2011-07-01

On the isomerization of ent-kaurenic acid.

Julio Rojas, Rosa Aparicio, Thayded Villasmil, Alexis Peña, Alfredo Usubillaga

Index: Nat. Prod. Commun. 6(7) , 935-8, (2011)

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Abstract

Kaurenic acid (1a) is a tetracyclic diterpene that has an exocyclic double bond at delta16. Isokaurenic acid (2a) has an endocyclic delta15double 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 la, 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% 16alpha-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 16alpha-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.