The synthesis of the starting material was performed following: https://www.chemotion-repository.net/mydb/collection/all/reaction/4202[Dimethylamino(triazolo[4,5-b]pyridin-3-yloxy)methylidene]-dimethylazanium hexafluorophosphate (HATU) (0.75 equiv) was dissolved in 30 mL of dimethylformamide. In parallel, the linear precursor N-(((2S)-2-amino-3-methylpentanoyl)-D-prolyl)-N-butylglycyl-L-phenylalanine (48.9 mg, 100 μmol, 1.00 equiv) was dissolved in 20 mL of dimethylformamide and N-ethyl-N-propan-2-ylpropan-2-amine (103 mg, 139 μL, 800 μmol, 8.00 equiv) was added. 10 mL of this mixture were added to the HATU solution over a period of 6 h at 21 °C. Afterwards, additional 0.75 equiv. of HATU were added to the reaction mixture in one portion. The remaining 10 mL of the linear precursor mixture were added over another period of 6 h. The reaction was stirred for 12 h and the solvent was removed under reduced pressure.
