Isomerization of a-pinene oxide over cerium and tin catalysts : selective synthesis of trans-carveol and trans-sobrerol.

Abstract

A remarkable effect of the solvent nature on the acid catalyzed transformation of _-pinene oxide allowed direction of the reaction to either trans-carveol or trans-sobrerol. Each of these highly valuable compounds was obtained in nearly 70% yield using an appropriate polar solvent, whose basicity affected strongly the product distribution. In acetone, a weakly basic solvent, the reaction over heterogeneous sol–gel Sn/SiO2 or Ce/SiO2 catalysts gave mainly trans-sobrerol. No leaching of active components occurs under the reaction conditions and the catalysts can be recovered and reused. On the other hand, in more basic solvent, i.e., dimethylacetamide, the reaction was essentially directed to trans-carveol. Due to the leaching problems with Sn/SiO2 and Ce/SiO2 materials, the synthesis of trans-carveol was performed under homogeneous conditions using CeCl3 or SnCl2 as catalysts with a catalyst turnover number up to ca. 1200. The method represents one of the few examples of the synthesis of isomers from _-pinene oxide, other than campholenic aldehyde, with a sufficient for practical usage selectivity.