First characterization at atomic resolution of the C‐activating groups in a peptide synthesis acid chloride, acid azide and carboxylic‐carboxylic mixed anhydride*

The x-ray diffraction structures of three N α -protected, C α -activated MeAib derivatives are reported. They are Tos-MeAib-Cl, Tos-MeAib-N 3 , and Tos-MeAib-O-Piv. The geometry and conformation of these classical carboxyl activating groups, which have been characterized at atomic resolution for the first time, are discussed