Electron ionization induced fragmentation of some oxadiazole and thiadiazole derivatives.

The mass spectrometric behaviour under electron ionization of several 3,4-(alkyl/aryl)-disubstituted 1,2,4-oxadiazole-5(4H)-ones (1–13) and 1,2,4-thiadiazole-5(4H)-thiones (14–17), and that of 3-aryl-5-alkyl- or arylthio-1,2,4-thiadiazoles (18–24), was studied. These five-membered rings split similarly to the corresponding 1,2,4-thiadiazole-5(4H)-ones, although substitution has also a clear effect on the routes of fragmentation and the magnitude of secondary processes. In particular, the fragmentation of 1,2,4-oxadiazole-5(4H)-ones (1–6), which do not bear aromatic substituents, was, in addition to the ring ruptures, fairly complicated. The other compounds fragmented more systematically and relatively few unpredictable fragmentations occurred. Copyright © 2004 John Wiley & Sons, Ltd.