Environmentally benign peptide synthesis using liquid-assisted ball-milling: application to the synthesis of Leu-enkephalin

This paper describes an original methodology for peptide bond synthesis avoiding toxic solvents and reactants. Ball-milling stoichiometric amounts of Boc-protected α-amino acid N-carboxyanhydrides (Boc-AA-NCA) or Boc-protected α-amino acid N-hydroxysuccinimide esters (Boc-AA-OSu) with α-amino acid alkyl ester salts in the presence of NaHCO3 and a minimal quantity of EtOAc led to the production of di- to pentapeptides in an efficient and environmentally benign manner. This approach was successfully applied to the synthesis of Leu-enkephalin.