Diprotonation of 5-Acyl-1,2,3,4,5-pentamethylcyclopentadienes

5-Acyl-1,2,3,4,5-pentamethylcyclopentadienes could potentially be protonated on either or both of two sites, the diene moiety in the five-membered ring or the carbonyl oxygen. The protonation of a series of 5-acylpentamethylcyclopentadienes has been examined and evidence is presented which shows that the former site is essentially completely protonated in FSO 3 H and that the carbonyl oxygen is also partially protonated. The extent of diprotonation is shown to be dependent both on the nature of the acyl groups and also the acidity of the medium. At temperatures above 40 °C these protonated species undergo a clean fragmentation to give protonated pentamethylcyclopentadiene from which pentamethylcyclopentadiene can be recovered in good yield on quenching the acid solution.