Phosphotungstic acid as a versatile catalyst for the synthesis of fragrance compounds by alpha-pinene oxide isomerization: Solvent-induced chemoselectivity

The remarkable effect of the solvent on the catalytic performance of H3PW12O40, the strongest heteropoly acid in the Keggin series, allows direction of the transformations of alpha-pinene oxide (1) to either campholenic aldehyde (2), trans-carveol (3), trans-sobrerol (4a), or pinol (5). Each of these expensive fragrance compounds was obtained in good to excellent yields by using an appropriate solvent. Solvent polarity and basicity strongly affect the reaction pathways: nonpolar nonbasic solvents favor the formation of aldehyde 2; polar basic solvents favor the formation of alcohol 3; whereas in polar weakly basic solvents, the major products are compounds 4a and 5. On the other hand, in 1,4-dioxane, which is a nonpolar basic solvent, both aldehyde 2 and alcohol 3 are formed in comparable amounts. The use of very low catalyst loading (0.005-1 mol%) and the possibility of catalyst recovery and recycling without neutralization are significant advantages of this simple, environmentally benign, and low-cost method. This method represents the first example of the synthesis of isomers from cc-pinene oxide, other than campholenic aldehyde, with a selectivity that is sufficient for practical usage.