Cyclization of citronellal to p-menthane-3,8-diols in water and carbon dioxide

A clean process has been developed for the synthesis of p-menthane-3,8-diols from cyclization of citronellal in CO(2)-H(2)O medium without any additives. With the addition of CO(2), the reaction rate could be enhanced about 6 times for the cyclization of citronellal in H(2)O, because CO(2) dissolved into water and formed carbonic acid inducing an increase of the acidity. Although, the reaction conversion in CO(2)-H(2)O is slightly lower compared to that obtained with sulfuric acid as catalyst, CO(2)-H(2)O could replace the sulfuric acid at a relative higher reaction temperature. The reaction kinetics studies showed that the hydration of isopulegols to p-menthane-3,8-diols is a reversible reaction. The equilibrium constant and the maximum equilibrium yield obtained in CO(2)-H(2)O at a range of CO(2) pressures are similar to that with sulfuric acid catalyst.