readily undergo ringexpansion to yield the corresponding 8-membered carbocyclic dienes. Thus, flashvacuumpyrolysis of 4a, 4d, 4f, 6, and 14 at 500 degrees C affords dienes 13a-d and 15 in 70-96% yields. This interesting ringexpansion may be viewed as the insertion of an alkyne moiety into the carbon-carbon double bond of a cyclic olefin resulting in the enlargement of the ring by two carbons. Compound