Mass spectra of azetidines and 2-azetidinones

The mass spectra of twenty three azetidines and seventeen 2-azetidinones are reported and discussed. The majority of spectra contain molecular ions. Fragmentation modes are largely dependent upon the substitution pattern and involve specific cleavage of the four-membered ring. Several hydrogen rearrangement reactions occur, and in some cases, cis and trans isomers may be distinguished by large differences in the relative abundances of certain fragment ions.