4.3.5. d-Phe-cyclo[Cys-Tyr-d-Trp-Lys(ivDde)-Thr-Cys]-Thr (5)

d-Phe-Cys(Acm)-Tyr(tBu)-d-Trp(Boc)-Lys(ivDde)-Thr(tBu)-Cys(Acm)-Thr(tBu)-resin was synthesized by a SPPS strategy similar to the synthesis of 4. Fmoc deprotection and coupling with Fmoc-l-Thr(tBu)-OH, Fmoc-l-Cys(Acm)-OH, Fmoc-l-Thr(tBu)-OH, Fmoc-l-Lys(ivDde)-OH, Fmoc-D-Trp(Boc)-OH, Fmoc-l-Tyr(tBu)-OH, Fmoc-l-Cys(Acm)-OH, and Fmoc-d-Phe-OH were achieved following the same protocols described above. After the final Fmoc deprotection, the protected linear peptide was cyclized on resin by using Tl(TFA)₃ (2.0 equiv.) in DMF for 1 h at rt. The cyclization was monitored by analytical HPLC. After completion of the reaction, the resin was washed with dichloromethane (3 × 3 mL). A solution of TFA/H₂O/TIPS (v/v/v = 95:2.5:2.5) was added to the resin, and the mixture was stirred for 2 h at rt. The solution was removed from the resin by filtration, and ice-cold diethyl ether was added to the filtrate. The precipitated peptide was collected by centrifugation and purified by semi-preparative HPLC (Condition 3) to give 110 mg of peptide 5 as a white solid (22%). ESI-MS: m/z calc’ for C₆₂H₈₂N₁₀O₁₄S₂ 1254.55; found 1255.54 [M+H]⁺.