UV-induced photochemistry of methyl coumalate (methyl 2-pyrone-5-carboxylate) isolated in low-temperature inert matrices

Photochemical transformations of methyl coumalate have been studied by matrix-isolation technique. Two photoreactions were induced by UV (λ > 295 nm) light: isomerisation to the Dewar form and α-bond cleavage leading to the open-ring aldehyde-ketene. The first photoprocess was found to be strongly dominating. Experimental evidence of photoreversibility of both photoisomerisation processes has been provided. Upon λ > 200 nm irradiation, the photochemically formed monomeric Dewar species underwent decarboxylation, with production of methoxycarbonyl-substituted cyclobutadiene. This antiaromatic photoproduct was experimentally observed for the first time. All the photoproduced species were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level.