Bond Alternation in Azulenes

5,6-Azulyne, the first azulyne to be prepared, has been trapped with furan to give an adduct whose crystal structure reveals marked bond alternation in the azulene π system. The double bond in the furan moiety is responsible in part for the bond localization, as hydrogenation of that bond attenuates the effect. The regiochemistry of electrophilic attack on the adduct has been examined briefly, with results contrary to the prediction of frontier orbital theory. Quantum mechanical calculations have shed light on these phenomena.