Scheme 2. Synthesis of BBPs by ClAc ligation at the example of 27c. One letter codes are used for amino acids, upper case letters = l-amino acids, lower case letters = d-amino acids, italics = branching amino acids, z = branching d-γ-thia-homoglutamate, arrow bonds indicate peptide bonds in C → N direction. Conditions: (a) Fmoc SPPS; (b) Pd(PPh₃)₄, PhSiH₃, anh. CH₂Cl₂; (c) HATU, iPr₂EtN, NMP–DMSO 4 : 1; (d) piperidine–DMF 1 : 4; (e) (ClCH₂CO)₂O, CH₂Cl₂; (f) CF₃CO₂H–iPr₃SiH–H₂O 94 : 5 : 1; (g) KI, iPr₂EtN, CH₃CN–H₂O 1 : 1.