“…The side chain protectedA -chain, by tert-butyl-based groups (P) includingB oc protection of the depsidipeptide unit (A8-A9), was selectivelyd eprotected at Cys(StBu) A6 with 25 % b-mercaptoethanol/ DMF, RT,2 1.5 h, and then treated with 10 equiv 2,2'-dithiobis(5nitropyridine, DTNP) [50] to produce Cys(Npys) A6 .C ys(Mmt) A11 was deprotected with 1.1 %TFA, 5% TIS (TIS = triisopropylsilane) in dichloromethane (6 2min) to form the intrachain Cys A6 -Cys A11 disulfide by thiolysis. [58] Although Cys(tBu) A7 is deprotected by 10 %t rifluoromethanesulfonic acid (TFMSA) in TFAa nd concomitantly activated by an excess of dipyridyl disulfide to yield Cys(Pyr) A7 , [59] the photolysis of Cys(oNv) B7 occurs quantitatively in 45 min to generate insulin in 36 % yield over these last two steps. Oxidation with I 2 in AcOH/H 2 O, purificationb yHPLC, and lyophilizationf rom NH 4 HCO 3 produced human insulin in 82 %y ield.…”