Nitrone protecting groups for enantiopure N-hydroxyamino acids: synthesis of N-terminal peptide hydroxylamines for chemoselective ligations.

The synthesis of enantiopure N-benzylidene nitrones of N-hydroxy-alpha-amino acids and their incorporation using standard Fmoc-based peptide chemistry into solid-supported peptide chains is described. Deprotection and resin cleavage affords N-terminal peptide hydroxylamines, which are the key substrates for chemoselective ligations with C-terminal peptide alpha-ketoacids. This general route is applicable to a variety of different N-terminal residues and provides a general approach to the solid phase synthesis of peptide hydroxylamines.