N-Arylation of Amino Acid Esters to Expand Side Chain Diversity in Ketoxime Peptide Ligations

Palladium-catalyzed N-arylations of amino acid tent-butyl esters using 4-bromo-N,N-dimethylaniline as a coupling partner are reported. The resulting N-aryl amino acid esters are suitable building blocks for the synthesis of electron-rich N-aryl peptides, which undergo oxidative couplings to aminooxy groups to afford ketoxime peptides under mild conditions. N-aryl amino acid tert-butyl esters possessing unnatural side chains were also accessed via glycine Schiff base alkylation, further increasing the scope of C alpha-substitution in ketoxime peptides.