Study of the fragmentation pathway of α-aminophosphonates by chemical ionization and fast atom bombardment mass spectrometry

The diastereoisomers of alpha-aminophosphonates are key intermediates in the synthesis of enantiomerically pure alpha-aminophosphonic acids, which are analogs of alpha-amino acids. Although several methods have been reported for the diastereoselective synthesis of alpha-aminophosphonates, their mass spectrometry (MS) fragmentation patterns have not yet been fully investigated. The work described here involved a detailed study of the fragmentation of enriched alpha-aminophosphonate diastereoisomers by chemical ionization (CI-MS) and fast atom bombardment (FAB)-MS. The complete characterization of the different conventional MS fragmentation pathways is represented and this intriguing exercise required the use of tandem mass spectrometry (MS/MS) experiments and high-resolution accurate mass measurements. All alpha-aminophosphonates gave prominent pseudomolecular ions, protonated molecules [MH](+), and their fragmentations mainly showed a loss of dimethyl phosphite to give the corresponding iminium ions as base peaks for alpha-aminophosphonates bearing methylbenzyl and 2,2-dimethylbutyl fragments. The loss of the chiral fragment from the iminium ions bearing the (S)-1-(1-naphthyl)ethyl group gave rise to a base peak due to aryl cations. The nature of all fragment ions were confirmed by high-resolution mass spectrometry (HRMS). Copyright (C) 2011 John Wiley & Sons, Ltd.