A facile oxidation and oxygen insertion of the cyclopentadiene ring by molecular oxygen in solution.

[reaction: see text] Oxidation of substituted cyclopentadienes by molecular oxygen afforded the corresponding pyrylium cations in acidic solution, whereby an oxygen atom was inserted into the cyclopentadiene ring. This novel one-step reaction proceeds in a different yield depending on substitution patterns of the cyclopentadiene. A possible reaction pathway and formation mechanism of the pyrylium cation are proposed and discussed.