A stereospecific Favorskii rearrangement of pulegone dibromide, from D-pulegone, gives the trans-pulegenic acid which is converted into the corresponding bicyclic ?-lactone (V), a useful intermediate for the synthesis of the cyclopentanoid monoterpenes. A stereospecific rearrangement of pulegone epoxide (VIIa) yields the lactone (V), together with the trans,trans-hydroxyacid (VIIIb). A nonstereo-specific Favorskii rearrangement of the dibromide gives a mixture of the trans- and cis-pulegenic acids, but a comparable rearrangement of the epoxide gives the hydroxyacids of type (X). Mechanisms for these transformations are discussed.