Six linear precursors of linaclotide containing different protected cysteine residues were synthesized by Fmoc solid-phase methods. Wang resin was used in the peptide syntheses. The protective groups of cysteine thiol were trityl (Trt) and acetamidomethyl (Acm) in the different positions. The six linear precursors of linaclotide include three[4 Trt+2 Acm] and three[2 Trt+4 Acm] ones. The linaclotide with three disulfide bonds was prepared from these linear precursors by semiregioselective strategy. Firstly, linear peptides were cleaved from Wang resins by TFA-TIS-H₂O. At the same time, the Trt groups were removed to give free thiol groups, whereas Acm groups were still remained in the peptides. Secondly, the free thiol groups were oxidized by 20% hemin/DIEA system to from disulfide bond(s). Finally, cysteines containing Acm groups were deprotected by CH₃SiCl₃/PhS(O)Ph/TFA coaktail and disulfide bond(s) were formed simultaneously. The precursors of peptides[4 Trt(2,5,10,13)+2 Acm(1,6)],[2 Trt(1,6)+4 Acm(2,5,10,13)] and[2 Trt(5,13)+2 Acm(1,2,6,10)] give linaclotide at the conversion ratios of 71.9%, 31.5%, and 81.4% respectively. Other three peptides failed in the conversion or were found to be less suitable to prepare linaclotide. Our results indicated that the order of disulfide bond formation is very important to prepare linaclotide by using semiregioselective or regioselective strategies. The Cys5-Cys13 disulfide bond is the most privileged and should be formed firstly among the three disulfide bonds in linaclotide.

Key words: linaclotide, disulfide, oxidation, semiregioselective