efficiently reacted with the azide functional group of the O-VBX 3a, leading to 11a in 51% isolated yield. With this result in hand, we then extended the click reaction to more complex peptides using a one-pot two-step procedure. β-Casomorphin human (1r) was successfully labelled with 10a to give 12a in 86% HPLC-yield. Both β-casomorphin human (1r) and vasopressin (1o) peptides were successfully labelled with N3-EBX (2a), followed by cycloaddition with BCN 10b-d functionalized with biotin, a fluorophore or a carbohydrate derivative (bioconjugates 12b-d and 13b-d).