4.3.3. Synthesis of Cyclic Peptides Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt) and Leu-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt-Leu)
The cyclic peptides were synthesized using the imidazole-promoted cyclization methods as we previously reported [
36]. Under N
2 protection, the Boc-Tyr(tBu)-D-Lys(Alloc)-Dap(Ant)-Thr-Gly-S-POSS (1.0 mmol) or the Boc-Leu-D-Lys(Alloc)-Dap(Ant)-Thr-Gly-S-POSS (1.0 mmol) was dissolved in 100 mL of DCM to a concentration of 10 mM. A portion of PhSiH
3 (24 mmol) and Pd(PPh
3)
4 (0.07 mmol) were then added to facilitate the removal of the Alloc protective group. After reacting overnight, imidazole (1.36 g, 20 mmol) was directly added to the reaction mixture to a concentration of 0.2 M and the reaction was carried out for 24 hrs. After removal of the solvent
in vacuo, 75% acetonitrile (aq.) was added to dissolve the yielded cyclic peptide while precipitating the POSS-SH released from peptide cyclization. The supernatant was collected and then lyophilized to dry. The resulting powder was washed with water to remove imidazole. The precipitation was collected and re-dissolved in 65% acetonitrile. After centrifugation at 4000 rpm for 10 min, the supernatant was collected and freeze-dried to yield the protected cyclic peptide Boc-Tyr(tBu)-[D-Lys-Dap(Ant)-Thr-Gly] or Boc-Leu-[D-Lys-Dap(Ant)-Thr-Gly]. Following the treatment with 55% TFA for 30 min, the fully unprotected cyclic peptide Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt) or Leu-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt) was yielded. The crude compound was purified by reverse-phase HPLC. The cyclic peptide was obtained as a TFA salt after HPLC purification.
Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt). 1H NMR (400 MHz, DMSO-d6) δ 0.99-1.22 (m, 2H), 1.03 (d, 3H, J = 6 Hz), 1.08-1.18 (m, 1H), 1.29-1.39 (m, 3H), 1.49-1.56 (m, 1H), 2.85-2.87 (m, 1H), 2.90 (m, 1H), 2.96-2.98 (m, 1H), 3.08-3.12 (m, 1H), 3.50 (td, 1H, J = 14, 6 Hz), 3.88 (d, 1H, J = 6.8 Hz), 3.90-3.94 (m, 1H), 3.99-4.02 (m, 2H), 4.08 (t, 1H, J = 6 Hz), 4.2-4.24 (m, 1H), 5.07 (d, 1H, J = 4.8 Hz), 6.34 (br.s, 2H), 6.49-6.52 (dd, 1H, J = 7.2, 1 Hz), 6.54 (d, 1H, J = 2 Hz), 6,72 (d, 3H, J = 8 Hz), 7.03 (d, 2 H, J = 8 Hz), 7.16 (td, 1H , J = 7.6, 1.6 Hz ), 7.33 (t, 1H, J = 5.2 Hz), 7.46 (dd, 1H, J = 8, 1.2 Hz), 7.64 (d, 1H, J = 5.2 Hz), 7.72 (br.s, 1H), 8.13 (br.s, 2H), 8.29 (t, 1H, J = 5.6 Hz), 8.38 (t, 1H, J = 5.6 Hz), 8.43 (d, 1H, J = 7.2 Hz), 8.47 (d, 1H, J = 6.4 Hz), 9.39 (s, 1H). 13C NMR (100 MHz, DMSO-d6) δ 20.1, 21.2, 28.1, 31.5, 36.8, 38.2, 40.7, 43.8, 53.6, 54.2, 56.4, 60.2, 66.3, 114.9, 115.3, 115.7, 115.8, 116.9, 118.7, 125.2, 128.8, 130.9, 132.5, 149.7, 157.1, 158.4, 158.7, 168.2, 169.2, 170.5(2), 170.6, 172.1. 19F NMR (376 MHz) δ -37.7 (s).
Leu-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt-Leu). 1H NMR (400 MHz, DMSO-d6) δ 0.92 (d, 6H, J = 8 Hz), 1.03 (d, 3H, J = 4 Hz), 1.29-1.32 (m, 2H), 1.38-1.39 (m, 2H), 1.57-1.65 (m, 5H), 2.96-2.99 (m, 1H), 3.12-3.16 (m, 1H), 3.36 (dd, 1H, J = 16, 4 Hz), 3.53 (m, 1H), 3.88 (dd, 1H, J = 16, 8 Hz), 3.93-4.00 (m, 1H), 4.02-4.04 (m, 2H), 4.12 (t, 1H, J = 8 Hz), 4.19 (t, 1H, J= 8 Hz), 6.51 (td, 1H, J = 8, 1 Hz), 6.73 (d, 1H, J = 8 Hz), 7.16 (td, 1 H, J = 8, 1 Hz), 7.41 (t, 1H, J = 8 Hz), 7.48 (dd, 1H , J = 7.6, 1.6 Hz ), 7.66 (d, 1H, J = 8 Hz), 8.15 (br.s, 3H), 8.26 (t, 1H, J = 6 Hz), 8.38 (t, 1H, J = 5.6 Hz), 8.57 (d, 1H, J = 8 Hz), 8.83 (d, 1H, J = 6.8 Hz). 13C NMR (100 MHz, DMSO-d6) δ 20.1, 21.7, 22.1, 23.2, 24.1, 28.2, 31.6, 38.2, 40.4, 40.5, 43.8, 51.4, 54.4, 56.4, 60.0, 66.3, 114.7, 115.2, 115.8, 116.9, 118.8, 128.8, 132.5, 149.8, 158.4, 158.8, 169.2, 169.5, 170.5(2), 170.7, 172.4.