Intramolecular four-membered ring rearrangements of 1-(N-benzyloxycarbonylamino)arylmethylphosphonate monoesters under electrospray ionization conditions

The mass spectrometric behavior of 1-[N-benzyloxycarbonyl(Cbz)amino]arylmethylphosphonate monoesters was studied under positive ion electrospray ionization (ESI) conditions. The most interesting feature is that all title compounds predominantly undergo four-membered ring rearrangements to yield mainly nitrogen-containing fragment ions by loss of a carbon dioxide, phosphite, carbon dioxide plus phosphite, or benzyloxycarbonylphosphonate monoester. The fragmentation is obviously different from the corresponding 1-(N-Cbz-amino)arylmethylphosphonate diesters, which show a tendency to undergo an intramolecular six-membered ring benzyl rearrangement.