Although the Cope rearrangement has been recently observed in several naturally occurring tan-membered carbocyclic compounds (e.g. I + II) (1) or their derivatives with &~.6, .&zM~-cyclodeca-1,5-diene systems (2) , none has recorded formation of bicyclic products. Herein, we wish to report for the first time such results obtained during the thermolysis of dihydrocostunolide (I). Heating of dihydrocostunolide (I) in an aEmosphere of nitrogen at 230f10Β° for 6 hours afforded a mixture from which three bicyclic crystalline lactones (III, IV and V) were isolated in variable yields*. The a-isomer (III) [m.p. 139-1400, Cd; + 85.3Β°(c,0.73)] and the B-isomer (IV) [m-p. 139-141Β°, [a12z + 164~(c,0.70)1+ were identified by their rigorous comparison with authentic materials (mixed m.p., I.R., N.M.R. and mass spectra)(3). The third y-isomer (V) Cm-p. 88-89O, [a12z + 32.2Β°(c,l.ll); WZ 1777 a-1 (y-lactone); mass spectra m/,(%L18): 234 (3.06, M+), 219 (13.01, M-CH3, base peak)] analyzed for C15H2202, further confirmed by high resolution mass measurements (Found: 234.1622. Calcd,: 234.1620). Evidence for structure (V) was garnered frae its n.l.r. spectrum (CDCl,) which displayed a broad one-proton signal centered at 5.43r due to the hydrogen at C-6. This downfield shiftt is compatible with the location of &uble bead between C-4 and C-5, a fact further corroborated by the complete absence of any vinyl * The overall yields of the three crystalline isomers actually isolated from several experiments fluctuated from 12-21% (III), 7-13% (IV) and l-10% (V). Furthem=, the ratio of these isomers also varied considerably. t Unless otherwise stated, specific rotations were determined in CHC13 solution on a Perkin-Elmer model 141 polarimeter. ,N.M.R. spectra were obtained with a Varian Associates HA-60 spectrometer using Me4Si as internal standard.