Preparation, mass spectra, and acid-catalysed rearrangement of aryl-substituted azetidin-2-ones

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Abstract
Several acid-catalysed reactions of aryl-substituted azetidin-2-ones are described. Thus 1,3,4-triphenylazetidin-2-one underwent rearrangement in concentrated sulphuric acid to yield the isomeric 3,4-dihydro-3,4-diphenylquinolin-2(1H)-one; however, reaction in toluene with boron trifluoride as catalyst yielded N,2,3-triphenyl-3(4-tolyl)propionamide, a toluene adduct. 1,4-Diphenylazetidin-2-one underwent similar reactions, but when the rearrangement was performed in chlorobenzene the isomeric cinnamanilide was obtained. In contrast, treatment of 1,3,3,4,4-pentaphenylazetidin-2-one with boron trifluoride in toluene caused a fission reaction and α-phenylbenzylideneaniline and diphenylacetic acid were isolated, whereas 1,3,4,4-tetraphenylazetidin-2-one yielded 2,3-diphenylindene by extrusion of a molecule of aniline. The mass spectra of several aryl-substituted azetidin-2-ones are discussed in terms of the principal decomposition modes which yield ions due to keten, anil, olefin, and isocyanate fragments.
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