Abstract Treatment of methyl mono- or bisubstituted 3-hydroxy-6-alkenoic acids in acetic anhydride and potassium acetate give the corresponding bicyclo[3.2.0]hept-3-en-6-ones in fair to good yields. This reaction appears to be of general applicability to prapare the methyl derivatives in all the portions of the five membered ring. The synthesis of 4,7,7-trimethylbicyclo[3.2.0]hept-3-en-6-one (filifolone) by an efficient bicyclization of 3-hydroxy-3-methyl-6-heptenoic acid followed by the geminal dimethylation of the intermediate 4-methylbicyclo[3.2.0lhept-3-en-6-one is reported. The latter reaction proved to be a general method when 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-2 H-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-Villiger oxidation of the bicyclo[3.2.0]hept-3-en-6-one intermediates.