“…[17] In order to investigate the conditions avoiding the aspartimide derivative 2 in detail, the reaction rate under several coupling conditions was extensively determined. In addition to 2-(1H-9-azobenzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphonate (HATU), [18] benzotriazole-1-yloxytrispyrrolidinophosphonium hexafluorophosphonate www.chemeurj.org (PyBOP) [19] and 3-(diethoxyphosphoryloxy)-1,2,3-benzotriazin-4(3H)-one (DEPBT), [20] which were used previously, [17] 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU), [21] 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphonate (HBTU) [22] and N,N'-diisopropyl carbodiimide (DIC)/1-hydroxy benzotriazole (HOBt) were chosen as additional coupling reagents. These coupling reagents are commonly used in solid-phase peptide synthesis.…”