Mass spectrometry of heterocyclic compounds. VI —retro‐1,3‐dipolar cycloaddition and competitive fragmentation reaction of 2,5‐diaryl‐1,3,4‐dioxazoles

The process of retro‐1,3‐dipolar cycloaddition of 2,5‐diaryl‐1,3,4‐dioxazoles induced by electron‐impact is discussed. For this purpose, 2‐phenyl‐5‐ p ‐chlorophenyl‐1,3,4‐dioxazole was prepared, labelled with 18 O in position 1; the spectrum of this compound showed clear evidence of retro‐cycloaddition, the dipole fragment retaining the positive charge. This labelled compound also provides a reliable interpretation of other cleavage modes of the heterocyclic ring. The elemental compositions of the ions considered were determined by exact mass measurements and the metastable transitions were detected through the defocusing technique of the electrostatic sector.