Intramolecular ring rearrangements in the mass spectrometric fragmentation of 1-(N-ethoxycarbonylamino)arylmethylphosphonic monoesters under electrospray ionization conditions

The mass spectrometric fragmentation of 1-(N-ethoxycarbonylamino)arylmethylphosphonic monoesters has been investigated under positive ion electrospray ionization conditions. All protonated title compounds predominantly eliminate a phosphite monoester via a four-membered ring hydrogen rearrangement to yield protonated N-ethoxycarbonyl arylmethylimines, which could further undergo four-, six-, or eight-membered ring rearrangements to produce mainly N-substituted/unsubstituted arylmethylimine ions by loss of carbon dioxide, ethene, and cyanic acid alone or together. It is interesting to note that the protonation of 1-(N-ethoxycarbonylamino)arylmethylphosphonic monoesters should be occurred in their arene rings.