As shown in Figure , various protected amino acids were compatible with this reaction using the standard conditions with various P2CAs (for polar amino acids, the protecting groups were removed after cyclization). Cyclization proceeded efficiently (see Supplementary Figures ) with substrates containing Tyr (MP12), Trp (MP13), Arg (MP13), Lys (MP14), Cys (MP15), His (MP16), Glu (MP17). Methionine residues are oxidized quantitatively to the sulfoxide (see compound MP14) by NaIO4, as expected. The structures of these macrocycles were confirmed by LCMS and NMR spectroscopy (see Supplementary Figures ).