Epoxycyclopentenone intermediates

Formation and Rearrangement of Epoxycyclopentenone Intermediates Evidence for Oxyallyl Zwitterion Intermediates... 

An epoxycyclopentenone (26) is also thought to be an intermediate in the photochemical conversion of 2,6-dimethylpyrone into 4,5-dimethylfurfuraldehyde 25 [Eq. (6)], but the reasons for the formation of the furan rather than a 2-pyrone are not clear. 

At first sight, the photochemistry of 4-hydroxypyryliums, i.e. of 4-pyrones in acid solution, seems extraordinary, in that they are converted into 2-pyrones, however a rationalisation involving first, a bicyclic hydroxyallyl cation, secondly a bicyclic epoxycyclopentenone, and then a second photo-excitation, makes the transformation clear, the sequence is shown below. Irradiation at higher pH leads to a trapping of first-formed photo-intermediate by solvent and thus the isolation of dihydroxy-cyclopentenones. ... 

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