The preferred mode of cycloreversion of some 1,3,4-substituted azetidin-2-ones upon electron impact

The mass spectra of some 1,3,4-substituted azetidin-2-ones are measured. Fragmentation pathways are confirmed by use of both high-resolution and mass-analysed ion kinetic energy. They are consistent with the possible 2 + 2]- cycloadditions through which the azetidin-2-ones may be synthesized. More attention has to be paid to further fragmentations of the main fragments in assessing the preferred fragmentation mode. Influence of side-chain substituents is discussed.