Isomerization of dihydroazulenes to substituted azulenes - Naked eye observation of a novel cope-type rearrangement

Article abstract of DOI:10.1016/S0040-4020(01)81580-5

Readily accessible 3a-bridgehead allylic or propargylic substituted dihydroazulenes 9 rearrange at room temperature to thermodynamic more stable substituted azulenes. This process is paralleled by a gradual change in color: the yellow solution of the starting pentaenes e.g. 9a smoothly turns to green and finally to the brilliant blue of 10a at the end of the reaction. Activation parameters for one example of this type of reaction are given. the synthesis of 9a and some derivatives which are subjected to this rearrangement process are described.