2,2-Dimethyl cyclopentanones by acid catalyzed ring expansion of isopropenylcyclobutanols. A short synthesis of ((plus-minus))-(alpha)-cuparenone and ((plus-minus))-herbertene

2,2-Dimethyl cyclopentanones are readily prepared by acid catalyzed ring expansion of isopropenylcyclobutanols; the method allows ready access to the family of sesquiterpenes cuparanes and herbertanes, as demonstrated by the synthesis of ((plus-minus))-(alpha)-cuparenone and the direct precursor of ((plus-minus))-herbertene.