performed on bicyclo[3.2.0]hept-3-en-6-ones and on bicyclo[3.2.0]hept-2-en-6-one. 3,3a,4,6a-Tetrahydro-2H-cyclopenta[b]furan-2-ones, important starting materials in the synthesis of linear condensed triquinane sesquiterpenes, have been prepared in an efficient manner by the easy bicyclization of 3-hydroxy-6-heptenoic acids, followed by a Baeyer-Villigeroxidation of the bicyclo[3.2.0]hept-3-en-6-one