aReagents and conditions: (a) 1.5 equiv of 4 and 1.0 equiv of 5, 6 M Gnd·HCl, 300 mM Na2HPO4, 20 mM TCEP, pH 7.2, isolated 6 (38%); (b) 1 equiv of 7 and 1.1 equiv of 8, 6 M Gnd·HCl, 300 mM Na2HPO4, 20 mM TCEP, 200 mM MPAA, pH 7.6, isolated 9a (14%); (c) 9a, VA-044, t-BuSH, TCEP, solvent (see the Supporting Information); TCEP = tris(2-carboxyethyl)phosphine, MPAA = 4-mercaptophenylacetic acid, VA-044 = 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride.