title: On the isomerization of ent-kaurenic acid.
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.
authors.affiliation: Posgrado de Quimica de Medicamento, Facultad de Farmacia y Bioanálisis, Universidad de Los Andes, Mérida, Venezuela.
authors.forename: Julio
authors.initials: J
authors.lastname: Rojas
authors.forename: Rosa
authors.initials: R
authors.lastname: Aparicio
authors.forename: Thayded
authors.initials: T
authors.lastname: Villasmil
authors.forename: Alexis
authors.initials: A
authors.lastname: Peña
authors.forename: Alfredo
authors.initials: A
authors.lastname: Usubillaga
mesh_terms: D019659:Asteraceae / Q000737:chemistry; D004224:Diterpenes / Q000737:chemistry*; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D013055:Spectrophotometry, Infrared; D014269:Trifluoroacetic Acid / Q000737:chemistry
chemical_list: D004224:Diterpenes; C051436:kaurenoic acid; D014269:Trifluoroacetic Acid