Competing Ring-Photoisomerization Pathways in the 1,2,4-Oxadiazole Series. An Unprecedented Ring-Degenerate Photoisomerization

The irradiation of some 5-alkyl-3-amino-1,2,4-oxadiazoles at lambda = 254 nm in methanol in the presence of triethylamine (TEA) gave ring-photoisomerization both into 2-alkyl-5-amino-1,3,4-oxadiazoles and, unprecedently, into the ring-degenerate 3-alkyl-5-amino-1,2,4-oxadiazoles. The competing ring contraction-ring expansion route and the internal cyclization-isomerization mechanism explain the results.