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

The diastereoisomers of α‐aminophosphonates are key intermediates in the synthesis of enantiomerically pure α‐aminophosphonic acids, which are analogs of α‐amino acids. Although several methods have been reported for the diastereoselective synthesis of α‐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 α‐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 α‐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 α‐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 © 2011 John Wiley & Sons, Ltd.