Photoinduced transformation of matrix-isolated methyl 2-pyrone-3-carboxylate into methyl 2-pyrone-5-carboxylate via intramolecular hydrogen shift in open-ring aldehyde–ketene

Photochemical transformations of methyl 2-pyrone-3-carboxylate (mp3c) were studied by matrix-isolation technique. The dominating primary photoreaction induced by UV (λ > 295 nm) light was α-bond cleavage leading to open-ring aldehyde–ketene. Another reaction characteristic of α-pyrones, isomerisation to the Dewar form, did not occur for mp3c. The ring-opening photoreaction was followed by intramolecular hydrogen shift and subsequent ring-closure reaction converting the aldehyde–ketene into methyl 2-pyrone-5-carboxylate (mp5c). Upon prolonged UV irradiation, mp3c completely transformed into mp5c and its Dewar isomer Dmp5c. Unequivocal identification of mp5c and Dmp5c photoproducts was enabled by exact knowledge of their experimental IR spectra, recorded in separate experiments.