In addition to small organic molecules, we were interested in testing the efficiency of cyclization with peptide substrates (Figure ). In the attempt to engraft a small azoline ring onto the macrocycle through direct cyclodehydration, the overall conformation of the macrocycle and arrangement of substituents around the amide of interest need to be considered. Furthermore, some amide bonds may be engaged in intramolecular H-bonding contributing to an even more challenging transformation. With these considerations in mind, we selected curacycline B (29), a cyclic peptide with immunomodulary activities, as a model system. The individual serine group in 29 could be directly converted into β-chloroalanine 30, β-azidoalanine 31 and mutated into S-phenylcysteine 32 through a common azoline intermediate formed by Mo-catalyzed cyclodehydration and opened with HCl, TMSN3, and PhSH, respectively. We note that in all transformations presented in Figure no epimerization at any potentially susceptible positions was observed.