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-ox adiazoles into the corresponding 2-amino- and 2-methylamino-5-phenyl-1,3,4- oxadiazloles has been reinvestigated by examining the effect of a base on t he photoreaction. On irradiating at lambda = 254 nm in methanol, yields of the ring-photoisomers were found to be significantly enhanced by the additi on of triethylamine (TEA) in the photoreaction medium. By contrast, irradia tion 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 p hotolysis of 3-amino-5-phenyloxadiazole in acetonitrile containing triethyl amine also gave benzoylguanidine but no traces of the ring photoisomer.