Intramolecular Hydrogen Transfer Reaction: Menthon from Isopulegol

The flavor menthon (isomeric mixture of (-)-menthon and (+)-isomenthon) was obtained in good yields and selectivities by a solventless ruthenium catalyzed isomerization of the homoallylic alcohol (-)-isopulegol. In contrast to most previous assumptions on such isomerization reactions, this reaction follows an intermolecular pathway, with menthol and pulegon being the central intermediates in this transformation.