In general, cyclic compounds containing the cyclopropyl-vinylcyclopropane ring system, upon thermolysis, undergo an internal [Za + 2a + Za] cycloaddition reaction,* regardless of whether the second cyclopropane ring conjugates with the double bond or not. The pyrolysis of one of bis-homotropilidenes, pentacyclo[5.5.0.02"2.06y8.03,9 ]dodeca-4,10-diene (A, X = -C=C-)2b is unexceptional and undergoes the homo Cope rearrangement3 of the cyclopropyl-vinylcyclopropane system to afford the tetracyclic hydrocarbon, tetracyclo[5.5.0 2'4.03'10]dodeca-5,8,11-triene @, X = -C=C-) even though the retro-Oiels-Alder reaction would be expected to give the tricyclic hydrocarbon, tricyclo[5.5.0.0*"* ]dodeca-3,5,8,10-tetraene (C, X = -C=C-C=C-) which is of interest as a potential precursor of tricyclo[5.5.0.04"0]dodeca-2,5,8,11-teraene.4 This recent communication 2b prompts us to report the skeletally related bis-homotropilidenes our results on the investigation of pyrolyses of (1 and 2). We wish to report here a marked difference in thermal behavior between these two compounds that the C,,H,OO ketone (l_) containing the cyclopentanone ring undergoes the retro-Oiels-Alder reaction such as A-+ C, while the cyclohexanone homologue, the C,2H,20 ketone (2_) behaves differently and instead, undergoes the homo Cope rearrangement such as A+ h Both bis-homotropilidenes (1 and 2) were prepared starting from the readily accessible carboxylic acid (z)5 'in good yields. Chlorination with oxalyl chloride followed by diazomethylation afforded the diazoketone (4_)7 in 83% yield from (2). Copper-catalyzed decomposition of (4) in refluxing tetrahydrofurane yielded the novel C,,HIOO ketone, tetracyclo-[5 4 0 . . . 0*+06*8 ]undeca-3-en-g-one (l), mp 68"C, in 73% yield. The structure of (1) was (1977). a) The typical example is the retro-Diels-Alder reaction of bis-homobenzenes.