We achieved the expeditious chemical synthesis of insulin using a one-pot/stepwise disulfide bond-forming reaction enabled by S-protected cysteine sulfoxides. The concept of using cysteine sulfoxides for disulfide formation has been witnessed over three decades^{ref. 1} , but their application to insulin synthesis has remained to be achieved. Recently, we found that the S-chlorocysteine resulting from an S-protected cysteine sulfoxide can participate in aromatic electrophilic substitution with the indole of tryptophan (Trp) to afford 2’-sulfenyl Trp^{ref. 2} . This research has uncovered two facts indispensable for insulin synthesis as follows: (1) The p-methoxybenzyl (MBzl)-protected sulfoxide is converted to S-chlorocysteine in TFA in the presence of ammonium chloride while S-acetamidomethyl (Acm) becomes the chloride under less acidic conditions using MgCl₂ in AcOH; (2) The resulting S-chlorocysteine can react with S-protected cysteines including Cys(Acm) and Cys(MBzl) to form disulfide bonds. These findings prompted us to achieve the insulin synthesis featuring one-pot/stepwise interchain disulfide bond formation using two pairs of protected cysteine and cysteine sulfoxide.   

   Finally, the stepwise disulfide formation for Trp-substituted insulin followed by the Trp-lipidation using the indole sulfenylation proceeded in a one-pot manner to yield the lipidated insulin analogue expected to severe as a long-acting insulin (Fig.1).

 Fig.1 One-pot/stepwise native insulin and lipidated insulin analogue synthesis