Abstract: Cycloheptatrienylidene (1) adds to the terminal double bond of cis-and trans-l, 3- pentadiene. The adduct from the cis diene, l-cis-propenylspiro [2.6] nona-4, 6, 8-triene, rearranges above 50" to yield the geometric isomers of 8-methylbicyclo [5.4. 0] undeca-l, 3, 5, 9-tetraene (3a and 3b). l-trans-Propenylspiro [2.6] nona-4, 6, 8-triene is unstable at room temperature and also rearranges to 3a and 3b, although giving different relative amounts. ...