“…1,2,[32][33][34][35][36][37][38] The mildness of the disulfide ligation and its established chemoselectivity for the cysteine thiol in the presence of all the proteinogenic amino acids stands in stark contrast to the various other methods for cysteine functionalization, most of which involve the capture of the cysteine thiol by electrophilic species, and which consequently have obvious potential chemoselectivity issues. 1,2,39 The practicality of the disulfide ligation, with its direct applicability to cysteine-containing peptides, also contrasts with the various ingenious indirect methods that have been developed for the preparation of S-functionalized cysteine derivatives, 17 including, for example, the Michael addition of thiols to dehydroalanine units, 40 the alkylation of thiolates with peptide-based β-halo-alanine units, [41][42][43] and other electrophiles, 44,45 the opening of peptide-based aziridines by thiolates, 46,47 and the synthesis of peptides with previously functionalized cysteine building blocks, [48][49][50] each of which requires the synthesis of modified peptides. The many advantages of the disulfide ligation are offset, however, by its impermanence, which results from the lability of the disulfide bond in the presence of thiols and other reducing agents.…”