The promising functional group tolerance and high chemoselectivity observed on tetramers drove us to apply the methodology to more complex peptides (Scheme 2b). The chemoselectivity of the peptides labelling was confirmed by MS/MS analysis (see Supporting Information for details). The neurohypophysial hormones vasopressin (1o) and oxytocin (1p), both containing a disulfide bond, were successfully functionalized at the tyrosine position (3o and 3p) with high conversion and no damage to the macrocyclic structure. High labelling conversion was also obtained at the N-terminal tyrosine of HIV-1-tat (1q) and β-casomorphin human (1r)