Photoinduced molecular rearrangements. Some comments on the ring‐photoisomerization of 1,2,4‐oxadiazoles into 1,3,4‐oxadiazoles

The ring‐photoisomerization of 3‐amino‐ and 3‐methylamino‐5‐phenyl‐1,2,4‐oxadiazoles into the corresponding 2‐amino‐ and 2‐methylamino‐5‐phenyl‐1,3,4‐oxadiazoles has been reinvestigated by examining the effect of a base on the photoreaction. On irradiating at λ = 254 nm in methanol, yields of the ring‐photoisomers were found to be significantly enhanced by the addition of triethylamine (TEA) in the photoreaction medium. By contrast, irradiation of the 3‐amino‐5‐phenyloxadiazole in acetonitrile containing TEA gave an almost complete photoreduction into benzoylguanidine, while few percent of the ring photoisomer were detected. Furthermore, the pyrene‐sensitized photolysis of 3‐amino‐5‐phenyloxadiazole in acetonitrile containing triethylamine also gave benzoylguanidine but no traces of the ring photoisomer.