Synthetic studies on tricyclic diterpenoids: convenient synthesis of 16-arylisopimaranes

A series of 16-arylisopimaranes has been synthesized by palladium-catalyzed arylation of the corresponding tricyclic diterpenoid isopimaric acid or its methyl ester in the presence of silver carbonate. No ligand was essential for the formation of new type optically active styrene derivatives. Good-to-excellent yields were obtained with good chemical tolerance. Silver carbonate enhanced the rate of the reaction. The use of cesium carbonate as the base requires an increase in reaction time and need for increasing amounts (2 mol%) of the palladium salt. The cross-coupling reaction proceeds with the formation of 16-arylisopimaranes with the (E)-type geometry of the double bond. Several transformations on the aryl substituent were carried out. The structure of target compounds was confirmed by X-ray diffraction study. Graphic abstract: