Protonated Cycloheptadienones

Stoichiometrically protonated cycloheptadienones appear to undergo electrocyclic ring-opening/ring-closure processes R. Noyori, Y. Ohnishi and M. Kato, J. Am, Chem. Soc., 94, 5105 (1972). 

The photolysis of protonated 2,6-cycloheptadienone (73) and its 5-methyl derivative (74) at — 78° in FHSOs yields as a single product in each case the corresponding protonated vinylcyclopentadienones 75 and 76, wliich subsequently isomerize thermally to the more stable cations 77 and 78 (Noyori et al., 1972). 

---