“…Compounds such as 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU, Figure 3, j), 2-(7-Aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU, Figure 3, k), and benzotriazole-1-yl-oxy-tripyrrolidinophosphonium hexafluorophosphate (PyBOP, Figure 3, l) are useful for the activation of sterically hindered carboxylic components. [28] Although HBTU and HATU were historically named uronium salts, the reagents were recently reported to crystallize in the guanidinium N-oxide form. [30,32] Furthermore, many variations of the discussed coupling reagents exist which differ slightly in terms of reactivity and designated use.…”