Staudinger ligation of α-azido acids retains stereochemistry

The Staudinger ligation of peptides with a C-terminal phosphinothioester and N-terminal azide is an emerging method in protein chemistry. Here, the first Staudinger ligations of nonglycyl azides are reported and shown to proceed both in nearly quantitative yield and with no detectable effect on the stereochemistry at the a-carbon of the azide. These results demonstrate further the potential of the Staudinger ligation as a general method for the total synthesis of proteins from peptide fragments.