Synthesis of the first stable phosphonamide transition state analogue.

Three methods were selected for the one-pot synthesis of the fully protected beta-fluoroaminophosphonic acids, using the readily accessible N-protected beta-fluoroaminals. These were activated by acylation leading, by beta-elimination, to a transient N-acylimine immediately trapped by reactive forms of dialkyl phosphites. Avoiding basic conditions, the complete or partial deprotection of these N-protected beta-fluoroaminophosphonic esters allowed the synthesis of the free amino acids, their esters, and a racemic beta-trifluorophosphonamidic acid. The latter, which represents a transition state analogue formed by the bacterial transpeptidase, is perfectly stable at pH 4.7, contrary to the nonfluorinated compounds.