3.2. Experimental Part
Boc-NMe-d-Phe-OMe (16). To a solution of d-Phe 9 (1.7 g, 10.3 mmol) and Boc2O (3.5 mL, 15.4 mmol) in a mixture THF and H2O (1:1, 50 mL) was added NaOH (0.6 g, 15.4 mmol) at room temperature. After stirring overnight, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. To a solution of above crude Boc-d-Phe-OH 14 (10.3 mmol) in dry THF (20 mL) was added NaH (60% dispersion in mineral oil, 2.1 g, 51.5 mmol) at room temperature. After stirring for 1 h, iodomethane (3.2 mL, 51.5 mmol) was added to the reaction mixture. The reaction mixture was stirred for 12 h, quenched with 1N HCl, and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was used in the next step without further purification. To a solution of crude acid 15 (10.3 mmol) in dry DMF (20 mL) were added iodomethane (1.3 mL, 20.6 mmol) and K2CO3 (2.8 g, 20.6 mmol) at room temperature. After stirring overnight, the reaction mixture was quenched with 1N HCl and extracted with Et2O. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:20) to give 2.5 g (84% for 3 steps) of ester 16 as a colorless oil. [α]D20 = +109.84 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3, 3:2 mixture of two rotamers). Major rotamer δ 7.28–7.20 (m, 2H), 7.19–7.07 (m, 3H), 4.50 (dd, J = 10.4, 3.8 Hz, 1H), 3.70 (s, 3H), 3.23 (dd, J = 14.2, 4.4 Hz, 1H), 3.01–2.94 (m, 1H), 2.68 (s, 3H), 1.29 (s, 9H), minor rotamer δ 7.28–7.20 (m, 2H), 7.19–7.07 (m, 3H), 4.89 (dd, J = 10.6, 5.2 Hz, 1H), 3.68 (s, 3H), 3.27 (dd, J = 14.4, 5.1 Hz, 1H), 3.01–2.94 (m, 1H), 2.66 (s, 3H), 1.33 (s, 9H); 13C-NMR (200 MHz, CDCl3, 3:2 mixture of two rotamers). Major rotamer δ 171.4, 154.8, 137.5, 128.9, 128.4, 126.5, 80.1, 61.5, 52.0, 35.4, 32.4, 28.0, minor rotamer δ 171.7, 155.6, 137.2, 128.8, 128.2, 126.3, 79.8, 59.4, 52.0, 34.9, 31.8, 28.1; IR (thin film, neat) νmax 2977, 1746, 1698, 1393, 1332, 1227, 1145, 751 cm−1; LR-MS (ESI+) m/z 316 (M + Na+); HR-MS (ESI+) calcd for C16H23NNaO4 (M + Na+) 316.1519; found 316.1523.
Boc-L-Ile-L-Ala-OH (21). To a solution of l-alanine methyl ester hydrochloride 17 (1.7 g, 12.2 mmol), Boc-l-Ile-OH (2.3 g, 9.9 mmol), DIPEA (5.2 mL, 29.8 mmol), and HOAt (1.4 g, 10.4 mmol) in CH2Cl2 (40 mL) was added EDC∙HCl (3.8 g, 19.9 mmol) at room temperature. After stirring for 4 h, the reaction mixture was quenched with 1N HCl, extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:2 to 1:1) to give 2.8 g (88%) of dipeptide 19 as white solid. [α]D20 = +16.54 (c 1.00, CHCl3); 1H NMR (800 MHz, CDCl3) δ 6.40 (s, 1H), 5.03 (d, J = 6.7 Hz, 1H), 4.56 (dq, J = 7.2, 7.1 Hz, 1H), 3.94 (t, J = 6.2 Hz, 1H), 3.72 (s, 3H), 1.88–1.81 (m, 1H), 1.53–1.44 (m, 1H), 1.42 (s, 9H), 1.39 (d, J = 7.2 Hz, 3H), 1.17–1.09 (m, 1H), 0.92 (d, J = 6.8 Hz, 3H), 0.89 (t, J = 7.4 Hz, 3H); 13C NMR (200 MHz, CDCl3) δ 173.1, 171.1, 155.7, 79.9, 59.1, 52.4, 48.0, 37.3, 28.3, 24.7, 18.3, 15.4, 11.4; IR (thin film, neat) νmax 3313, 2977, 1751, 1682, 1651, 1528, 1367, 1252, 870 cm−1; LR-MS (ESI+) m/z 317 (M + H+); HR-MS (ESI+) calcd for C15H29N2O5 (M + H+) 317.2071; found 317.2072. To a solution of Boc-l-Ile-l-Ala-OMe 19 (2.0 g, 6.3 mmol) in a mixture of THF, MeOH and H2O (1:1:1, 10 mL) was added LiOH∙H2O (0.6 g, 14.3 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was dried over MgSO4 and concentrated in vacuo to afford 1.9 g (99%) of Boc-l-Ile-l-Ala-OH 21 as a white solid. The free acid 21 was used in the next step without further purification.
Boc-L-Ile-Gly-OH (22). To a solution of glycine methyl ester hydrochloride 18 (1.5 g, 11.9 mmol), Boc-l-Ile-OH (2.3 g, 9.9 mmol), DIPEA (5.2 mL, 29.8 mmol), and HOAt (1.4 g, 10.4 mmol) in CH2Cl2 (40 mL) was added EDC∙HCl (3.8 g, 19.9 mmol) at room temperature. After stirring for 5 h, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:2 to 1:1) to give 2.7 g (91%) of dipeptide 20 as a white solid. [α]D20 = −5.85 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 6.81 (s, 1H), 5.17 (d, J = 7.5 Hz, 1H), 4.08–3.91 (m, 3H), 3.70 (s, 3H), 1.84 (s, 1H), 1.51–1.45 (m, 1H), 1.39 (s, 9H), 1.13–1.06 (m, 1H), 0.91 (d, J = 6.9 Hz, 3H), 0.86 (t, J = 7.4 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 172.1, 170.1, 155.8, 79.9, 59.0, 52.2, 41.0, 37.2, 28.2, 24.6, 15.4, 11.3; IR (thin film, neat) νmax 3324, 2968, 1759, 1687, 1661, 1527, 1367, 1173, 862 cm−1; LR-MS (ESI+) m/z 303 (M + H+); HR-MS (ESI+) calcd for C14H27N2O5 (M + H+) 303.1914; found 303.1923. To a solution of Boc-l-Ile-Gly-OMe 20 (2.0 g, 6.6 mmol) in a mixture of THF, MeOH, and H2O (1:1:1, 33 mL) was added LiOH∙H2O (0.6 g, 14.3 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was dried over MgSO4 and concentrated in vacuo to afford 1.9 g (99%) of Boc-l-Ile-Gly-OH 22 as white solid. The free acid 22 was used in the next step without further purification.
Boc-l-Ile-l-Ala-NMe-d-Phe-OMe (24). To a solution of Boc-NMe-d-Phe-OMe 16 (440 mg, 1.5 mmol) in CH2Cl2 (6.0 mL) was added TFA (1.5 mL) dropwise at room temperature. After stirring for 1 h, the reaction mixture was concentrated in vacuo. The residue was used in the next step without further purification. To a solution of above amine salt 23 (1.5 mmol), Boc-l-Ile-l-Ala-OH 21 (544 mg, 1.8 mmol), and DIPEA (0.8 mL, 4.6 mmol) in CH2Cl2 (7.5 mL) was added DEPBT (898 mg, 3.0 mmol) at 0 °C. After stirring for overnight at 0 °C, the reaction mixture was quenched with 1N HCl, extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:1) to give 407 mg (57% for 2 steps) of tripeptide 24 as white solid and 145 mg (20% for 2 steps) of its epimer as white solid. [α]D20 = +61.58 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 7.24 (t, J = 7.4 Hz, 2H), 7.18 (t, J = 7.4 Hz, 1H), 7.13 (d, J = 7.3 Hz, 2H), 6.81 (d, J = 7.1 Hz, 1H), 5.29 (d, J = 7.1 Hz, 1H), 4.98 (d, J = 8.3 Hz, 1H), 4.69 (dq, J = 6.9, 6.8 Hz, 1H ), 3.95 (dd, J = 7.3, 6.1 Hz, 1H), 3.73 (s, 3H), 3.39 (dd, J = 14.8, 5.0 Hz, 1H), 3.01 (dd, J = 14.7, 11.8 Hz, 1H), 2.83 (s, 3H), 1.87–1.79 (m, 1H), 1.74–1.64 (m, 1H), 1.40 (s, 9H), 1.12–1.02 (m, 1H), 0.88 (d, J = 6.8 Hz, 3H), 0.87 (t, J = 7.4 Hz, 3H), 0.84 (d, J = 6.8 Hz, 3H).; 13C-NMR (200 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 172.9, 170.7, 170.5, 155.6, 136.4, 128.7, 128.6, 126.9, 79.8, 59.2, 58.3, 52.4, 45.5, 37.6, 34.7, 32.5, 28.3, 24.6, 17.9, 15.6, 11.5; IR (thin film, neat) νmax 3319, 2970, 1743, 1710, 1637, 1498, 1366, 1173, 1017, 871 cm−1; LR-MS (ESI+) m/z 478 (M + H+); HR-MS (ESI+) calcd for C25H40N3O6 (M + H+) 478.2912; found 478.2915.
Boc-l-Ile-Gly-NMe-d-Phe-OMe (25). To a solution of Boc-NMe-d-Phe-OMe 16 (426 mg, 1.5 mmol) in CH2Cl2 (6.0 mL) was added TFA (1.5 mL) dropwise at room temperature. After stirring for 1 h, the reaction mixture was concentrated in vacuo. The residue was used in the next step without further purification. To a solution of above amine salt 23 (1.5 mmol), Boc-l-Ile-Gly-OH 22 (502 mg, 1.7 mmol) and DIPEA (0.8 mL, 4.4 mmol) in CH2Cl2 (7.3 mL) was added DEPBT (869 mg, 2.9 mmol) at 0 °C. After stirring overnight at 0 °C, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:1) to give 586 mg (87% for 2 steps) of tripeptide 25 as white solid. [α]D20 = +38.96 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 7.21 (t, J = 7.5 Hz, 2H), 7.15 (t, J = 7.4 Hz, 1H), 7.09 (d, J = 7.3 Hz, 2H), 6.84 (s, 1H), 5.19 (dd, J = 10.7, 5.3 Hz, 1H), 5.10 (d, J = 8.4 Hz, 1H), 4.04–4.01 (m, 1H), 3.95 (dd, J = 17.9, 3.7 Hz, 1H), 3.81 (dd, J = 17.7, 3.1 Hz, 1H), 3.67 (s, 3H), 3.32 (dd, J = 14.6, 5.4 Hz, 1H), 2.98 (dd, J = 14.6, 10.9 Hz, 1H), 2.74 (s, 3H), 1.81 (s, 1H), 1.44–1.36 (m, 1H), 1.37 (s, 9H), 1.07–1.02 (m, 1H), 0.85 (d, J = 6.9 Hz, 3H), 0.83 (t, J = 7.4 Hz, 3H); 13C-NMR (200 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 171.3, 170.6, 168.4, 155.5, 136.4, 128.5, 128.5, 126.8, 79.5, 58.9, 58.7, 52.3, 41.2, 37.5, 34.4, 31.7, 28.1, 24.5, 15.4, 11.4; IR (thin film, neat) νmax 3328, 2967, 1743, 1712, 1643, 1497, 1366, 1172, 1016, 867 cm−1; LR-MS (ESI+) m/z 464 (M + H+); HR-MS (ESI+) calcd for C24H38N3O6 (M + H+) 464.2755; found 464.2751.
Boc-l-Ile-l-Ala-NMe-d-Phe-l-Pro-OMe (4). To a solution of Boc-l-Ile-l-Ala-NMe-d-Phe-OMe 24 (491 mg, 1.0 mmol) in a mixture of THF, MeOH, and H2O (1:1:1, 10 mL) was added LiOH∙H2O (86 mg, 2.1 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was dried over MgSO4 and concentrated in vacuo. The free acid 26 was used in the next step without further purification. To a solution of above acid 26 (1.0 mmol), l-proline methyl ester hydrochloride (203 mg, 1.3 mmol), and DIPEA (0.6 mL, 3.4 mmol) in CH2Cl2 (10 mL) was added DEPBT (615 mg, 2.1 mmol) at 0 °C. After stirring for overnight at 0 °C, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:1 to 2:1) to give 496 mg (84% for 2 steps) of tetrapeptide 4 as white solid. [α]D20 = +45.90 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 7.23–7.13 (m, 5H), 6.64 (d, J = 7.2 Hz, 1H), 5.66 (dd, J = 9.5, 6.4 Hz, 1H), 4.98 (d, J = 8.2 Hz, 1H), 4.74 (dq, J = 7.1, 6.9 Hz, 1H), 4.42 (t, J = 7.5 Hz, 1H), 3.90 (t, J = 6.4 Hz, 1H), 3.70 (s, 3H), 3.47–3.42 (m, 1H), 3.22–3.15 (m, 1H), 2.98 (s, 3H), 2.94 (dd, J = 14.4, 9.6 Hz, 1H), 2.22–2.17 (m, 1H), 1.97–1.76 (m, 6H), 1.39 (s, 9H), 1.08–1.01 (m, 1H), 0.84 (t, J = 7.4 Hz, 3H), 0.83 (d, J = 6.8 Hz, 3H), 0.82 (d, J = 6.9 Hz, 3H); 13C-NMR (200 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 172.3, 172.2, 170.5, 168.1, 155.5, 136.7, 129.5, 128.2, 126.6, 79.7, 59.2, 59.0, 55.5, 52.2, 46.9, 45.1, 37.6, 34.6, 30.2, 28.8, 28.2, 25.3, 24.6, 17.7, 15.4, 11.5; IR (thin film, neat) νmax 3316, 2971, 1748, 1709, 1643, 1497, 1365, 1245, 1174, 1045, 870 cm−1; LR-MS (ESI+) m/z 575 (M + H+); HR-MS (ESI+) calcd for C30H47N4O7 (M + H+) 575.3439; found 575.3442.
Boc-l-Ile-Gly-NMe-d-Phe-l-Pro-OMe (5). To a solution of Boc-l-Ile-Gly-NMe-d-Phe-OMe 25 (511 mg, 1.0 mmol) in a mixture of THF, MeOH, and H2O (1:1:1, 11 mL) was added LiOH∙H2O (92 mg, 2.2 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was dried over MgSO4 and concentrated in vacuo. The free acid 27 was used in the next step without further purification. To a solution of above acid 27 (1.1 mmol), l-proline methyl ester hydrochloride (216 mg, 1.4 mmol) and DIPEA (0.6 mL, 3.4 mmol) in CH2Cl2 (11 mL) was added DEPBT (660 mg, 2.2 mmol) at 0 °C. After stirring overnight at 0 °C, the reaction mixture was quenched with 1N HCl, extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:1 to 2:1) to give 501 mg (81% for 2 steps) of tetrapeptide 5 as a white solid. [α]D20 = +69.20 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 7.23 (t, J = 7.4 Hz, 2H), 7.20 (d, J = 6.9 Hz, 2H), 7.17 (t, J = 7.2 Hz, 1H), 6.77 (s, 1H), 5.56 (t, J = 7.6 Hz, 1H), 5.00 (d, J = 8.0 Hz, 1H), 4.40 (dd, J = 8.5, 5.7 Hz, 1H), 4.09–4.01 (m, 1H), 3.92–3.85 (m, 1H), 3.70 (s, 3H), 3.38–3.34 (m, 1H), 3.33–3.29 (m, 1H), 3.25 (dd, J = 13.7, 8.0 Hz, 1H), 2.96 (s, J = 6.5 Hz, 3H), 2.81 (dd, J = 13.7, 7.1 Hz, 1H), 2.16–2.11 (m, 1H), 1.95–1.74 (m, 6H), 1.42 (s, 9H), 1.12–1.02 (m, 1H), 0.90 (d, J = 6.8 Hz, 3H), 0.88 (t, J = 7.4 Hz, 3H).; 13C-NMR (200 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 172.4, 171.5, 167.9, 167.9, 155.7, 136.9, 129.4, 128.4, 126.7, 79.9, 59.2, 58.9, 56.2, 52.2, 46.8, 41.1, 37.5, 34.9, 29.7, 28.8, 28.3, 25.0, 24.5, 15.6, 11.6; IR (thin film, neat) νmax 3328, 2968, 1748, 1710, 1640, 1497, 1365, 1246, 1174, 1044, 869 cm−1; LR-MS (ESI+) m/z 561 (M + H+); HR-MS (ESI+) calcd for C29H45N4O7 (M + H+) 561.3283; found 561.3290.
Allyl (2R,3S)-2-hydroxy-3-methylpentanoate (12). To a solution of d-allo-Ile 31 (1.4 g, 10.7 mmol) in dilute sulfuric acid (0.8N in H2O, 26.7 mL, 21.3 mmol) was added NaNO2 (2.2 g, 32.0 mmol) slowly at 0°C. After stirring for 6 h at the same temperature, the reaction mixture was diluted with Et2O and extracted with Et2O. The combined organic layer was dried over MgSO4 and concentrated in vacuo. The residue was used in the next step without further purification. To a solution of crude acid 32 (10.7 mmol) and TBAI (0.79 g, 2.13 mmol) in dry DMF (500 mL) were added allyl bromide (1.8 mL, 21.3 mmol) and K2CO3 (3.0 g, 21.3 mmol) at room temperature. After stirring for overnight, the reaction mixture was quenched with 1N HCl and extracted with Et2O. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:10) to give 1.2 g (65% for 2 steps) of ester 12 as a colorless oil. [α]D20 = +45.94 (c 0.50, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 5.89 (ddt, J = 17.1, 10.5, 5.9 Hz, 1H), 5.31 (ddd, J = 17.2, 2.9, 1.5 Hz, 1H), 5.24 (dd, J = 10.4, 2.3, 1.2 Hz, 1H), 4.67 (ddt, J = 13.0, 5.9, 1.2 Hz, 1H), 4.63 (ddt, J = 13.1, 5.9, 1.3 Hz, 1H), 4.17 (d, J = 3.0 Hz, 1H), 2.61 (s, 1H), 1.83–1.75 (m, 1H), 1.50 (ddq, J = 13.8, 7.4, 7.2 Hz, 1H), 1.28 (ddq, J = 13.7, 7.5, 7.4 Hz, 1H), 0.91 (t, J = 7.5 Hz, 3H), 0.78 (d, J = 7.0 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 175.0, 131.5, 119.1, 72.9, 66.0, 38.5, 25.9, 13.1, 11.8; IR (thin film, neat) νmax 3524, 2966, 1742, 1461, 1384, 1200, 1138, 934 cm−1; LR-MS (ESI+) m/z 173 (M + H+); HR-MS (ESI+) calcd for C29H45N4O7 (M + H+) 173.1172; found 173.1163.
Allyl (R)-2-hydroxy-3-methylbutanoate (13). To a solution of d-valic acid 33 (1.2 g, 10.2 mmol) and TBAI (0.75 g, 2.0 mmol) in dry DMF (25 mL) were added allyl bromide (1.8 mL, 20.3 mmol) and K2CO3 (2.8 g, 20.3 mmol) at room temperature. After stirring overnight, the reaction mixture was quenched with 1N HCl and extracted with Et2O. The combined organic layer was washed with brine, dried over MgSO4 and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:20) to give 1.5 g (93%) of ester 13 as a colorless oil. [α]D20 = +24.77 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 5.84 (ddt, J = 17.1, 10.6, 5.9 Hz, 1H), 5.26 (dd, J = 17.2, 1.3 Hz, 1H), 5.18 (dd, J = 10.4, 1.1 Hz, 1H), 4.61 (dd, J = 13.1, 5.9 Hz, 1H), 4.57 (dd, J = 13.2, 5.9 Hz, 1H), 3.98 (d, J = 3.9 Hz, 1H), 2.87 (s, 1H), 2.01 (dtd, J = 13.9, 6.9, 3.7 Hz, 1H), 0.94 (d, J = 7.3 Hz, 3H), 0.79 (d, J = 7.2 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 174.4, 131.4, 118.9, 74.9, 65.8, 32.0, 18.6, 15.8; IR (thin film, neat) νmax 3516, 2968, 1743, 1468, 1372, 1197, 1130, 933 cm−1; LR-MS (ESI+) m/z 181 (M + Na+); HR-MS (ESI+) calcd for C8H14NaO3 (M + Na+) 181.0835; found 181.0834.
(S)-2-((tert-Butyldiphenylsilyl)oxy)-3-methylbutanoic acid (35). To a solution of l-valic acid (1.2 g, 10.2 mmol) and DMAP (0.12 g, 1.0 mmol) in dry DMF (5.0 mL) were added imidazole (3.5 g, 50.8 mmol) and TBDPSCl (7.9 mL, 30.5 mmol) at room temperature. After stirring overnight, the reaction mixture was quenched with 1N HCl and extracted with Et2O. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was used in the next step without further purification. To a solution of crude silyl ether 34 (10.2 mmol) in a mixture of MeOH and H2O (3:1, 40 mL) was added K2CO3 (2.1 g, 15.2 mmol) at room temperature. After stirring for 1 h, the reaction mixture was quenched with 1N HCl and extracted with Et2O. The combined organic layer was washed with brine, dried over MgSO4 and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:10) to give 1.1 g (92% for 2 steps) of acid 35 as a colorless oil. [α]D20 = −12.98 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 7.64 (d, J = 6.9 Hz, 2H), 7.60 (d, J = 6.9 Hz, 2H), 7.42 (dt, J = 13.4, 7.4 Hz, 2H), 7.39–7.33 (m, 4H), 4.12 (d, J = 3.8 Hz, 1H), 1.95–1.85 (m, 1H), 1.11 (s, 9H), 0.88 (d, J = 7.0 Hz, 3H), 0.86 (d, J = 6.9 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 174.4, 135.9, 135.8, 132.7, 132.2, 130.2, 130.1, 127.8, 127.8, 77.4, 33.2, 27.0, 19.4, 17.6, 17.3; IR (thin film, neat) νmax 2962, 1720, 1471, 1428, 1183, 1113, 1071, 939 cm−1; LR-MS (ESI+) m/z 355 (M - H+); HR-MS (ESI+) calcd for C21H27OSi (M - H+) 355.1735; found 355.1742.
Allyl (2R,3S)-2-(((S)-2-((tert-Butyldiphenylsilyl)oxy)-3-methylbutanoyl)oxy)-3-methylpentanoate (36). To a solution of acid 35 (591 mg, 1.7 mmol), alcohol 12 (238 mg, 1.4 mmol), and DMAP (338 g, 2.8 mmol) in CH2Cl2 (14 mL) was added EDC∙HCl (530 g, 2.8 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:20) to give 649 mg (92%) of ester 36 as a colorless oil. [α]D20 = −23.21 (c 1.00, CHCl3); 1H NMR (800 MHz, CDCl3) δ 7.67–7.64 (m, 4H), 7.42–7.38 (m, 2H), 7.36–7.33 (m, 4H), 5.84 (ddt, J = 17.2, 10.4, 5.8 Hz, 1H), 5.29 (ddd, J = 17.2, 2.9, 1.4 Hz, 1H), 5.21 (ddd, J = 10.4, 2.3, 1.1 Hz, 1H), 4.82 (d, J = 3.5 Hz, 1H), 4.59 (ddt, J = 13.1, 5.7, 1.3 Hz, 1H), 4.52 (ddt, J = 13.1, 5.9, 1.3 Hz, 1H), 4.25 (d, J = 4.3 Hz, 1H), 2.06–1.96 (m, 1H), 1.90–1.85 (m, 1H), 1.36 (ddq, J = 13.8, 7.4, 7.3 Hz, 1H), 1.16 (ddq, J = 13.7, 7.6, 7.5 Hz, 1H), 1.14 (d, J = 7.5 Hz, 3H), 1.10 (s, 9H), 0.92 (d, J = 7.0 Hz, 3H), 0.89 (d, J = 6.9 Hz, 3H), 0.86 (t, J = 7.5 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 172.0, 169.2, 136.0, 135.9, 133.5, 133.3, 131.7, 129.7, 129.6, 127.5, 127.4, 118.7, 77.6, 75.1, 65.5, 36.7, 33.3, 26.9, 25.7, 19.6, 18.1, 17.1, 14.4, 11.7; IR (thin film, neat) νmax 2965, 1756, 1463, 1428, 1192, 1113, 1008, 935, 822 cm−1; LR-MS (ESI+) m/z 528 (M + NH4+); HR-MS (ESI+) calcd for C30H46NO5Si (M + NH4+) 528.3140; found 528.3149.
Allyl (R)-2-(((S)-2-((tert-Butyldiphenylsilyl)oxy)-3-methylbutanoyl)oxy)-3-methylbutanoate (37). To a solution of acid 35 (741 g, 2.1 mmol), alcohol 13 (274 mg, 1.7 mmol) and DMAP (423 mg, 3.5 mmol) in CH2Cl2 was added EDC∙HCl (664 mg, 3.5 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl, extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4 and concentrated in vacuo. The residue was purified by flash column chromatography (EtOAc/Hexane = 1:20) to give 809 mg (94%) of ester 37 as a colorless oil. [α]D20 = +2.49 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 7.64 (td, J = 8.4, 1.2 Hz, 4H), 7.40 (tdt, J = 7.7, 7.6, 1.3 Hz, 2H), 7.34 (dd, J = 7.2, 7.0 Hz, 4H), 5.83 (ddt, J = 17.2, 10.4, 5.8 Hz, 1H), 5.28 (dq, J = 17.2, 1.4 Hz, 1H), 5.20 (ddt, J = 10.4, 1.2, 1.0 Hz, 1H), 4.58 (dt, J = 13.1, 5.8, 1.3 Hz, 1H), 4.58 (d, J = 4.4 Hz, 1H), 4.51 (ddt, J = 13.2, 5.9, 1.2 Hz, 1H), 4.22 (d, J = 4.4 Hz, 1H), 2.12–2.06 (m, 1H), 2.05–1.98 (m, 1H), 1.09 (s, 9H), 0.92 (d, J = 6.9 Hz, 3H), 0.92 (d, J = 6.9 Hz, 3H), 0.90 (d, J = 6.9 Hz, 3H), 0.90 (d, J = 7.0 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 172.0, 168.9, 136.0, 135.9, 133.4, 133.3, 131.6, 129.7, 129.6, 127.5, 127.4, 118.8, 77.6, 77.0, 65.5, 33.3, 30.1, 26.9, 19.6, 18.5, 18.3, 17.4, 17.2; IR (thin film, neat) νmax 2965, 1754, 1471, 1428, 1185, 1113, 990, 934, 822 cm−1; LR-MS (ESI+) m/z 519 (M + Na+); HR-MS (ESI+) calcd for C29H40NaO5Si (M + Na+) 519.2537; found 519.2543.
(2R,3S)-2-(((S)-2-((tert-Butyldiphenylsilyl)oxy)-3-methylbutanoyl)oxy)-3-methylpentanoic acid (38). To a solution of ester 36 (345 mg, 0.7 mmol) and N-methylaniline (0.15 mL, 1.4 mmol) in dry THF (6.8 mL) was added Pd(PPh3)4 (78 mg, 0.1 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography through a short pad of silica gel (CH2Cl2/MeOH = 16:1) to afford 280 mg (88%) of the acid 38 as a yellow oil. [α]D20 = −15.06 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 7.64–7.62 (m, 4H), 7.42–7.36 (m, 2H), 7.35–7.29 (m, 4H), 4.84 (d, J = 3.3 Hz, 1H), 4.25 (d, J = 4.1 Hz, 1H), 2.03–1.97 (m, 1H), 1.89 (dtd, J = 7.5, 7.0, 3.3 Hz, 1H), 1.36 (ddq, J = 13.9, 7.3, 7.1 Hz, 1H), 1.18 (ddq, J = 13.8, 7.6, 7.4 Hz, 1H), 1.09 (s, 9H), 0.92 (d, J = 7.0 Hz, 3H), 0.91 (d, J = 6.9 Hz, 3H), 0.90 (d, J = 6.9 Hz, 3H), 0.86 (t, J = 7.4 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 174.8, 172.0, 136.0, 135.9, 133.5, 133.3, 129.7, 129.7, 127.5, 127.5, 77.4, 74.4, 36.6, 33.3, 26.9, 25.8, 19.6, 18.4, 17.1, 14.3, 11.7; IR (thin film, neat) νmax 2965, 1760, 1730, 1463, 1428, 1182, 1113, 1000, 822 cm−1; LR-MS (ESI+) m/z 488 (M + NH4+); HR-MS (ESI+) calcd for C27H42NO5Si (M + NH4+) 488.2827; found 488.2829.
(R)-2-(((S)-2-((tert-Butyldiphenylsilyl)oxy)-3-methylbutanoyl)oxy)-3-methylbutanoic acid (39). To a solution of ester 37 (362 mg, 0.7 mmol) and N-methylaniline (0.2 mL, 1.5 mmol) in dry THF (7.3 mL) was added Pd(PPh3)4 (84 mg, 0.1 mmol) at room temperature. After stirring for 2 h, the reaction mixture was quenched with 1N HCl and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography through a short pad of silica gel (CH2Cl2/MeOH = 16:1) to afford 306 mg (92%) of the acid 39 as a yellow oil. [α]D20 = −11.79 (c 1.00, CHCl3); 1H-NMR (800 MHz, CDCl3) δ 7.63 (td, J = 8.0, 1.1 Hz, 4H), 7.41–7.36 (m, 2H), 7.32 (dt, J = 9.0, 7.6 Hz, 4H), 4.61 (d, J = 4.3 Hz, 1H), 4.24 (d, J = 4.2 Hz, 1H), 2.14–2.09 (m, 1H), 2.06–1.98 (m, 1H), 1.08 (s, 9H), 0.93 (d, J = 7.2 Hz, 3H), 0.92 (d, J = 7.0 Hz, 6H), 0.92 (d, J = 6.9 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 173.6, 172.0, 136.0, 135.9, 133.4, 133.3, 129.7, 129.7, 127.5, 127.4, 77.4, 76.3, 33.3, 30.0, 26.9, 19.6, 18.6, 18.4, 17.2, 17.1; IR (thin film, neat) νmax 2965, 1760, 1726, 1470, 1428, 1181, 1113, 999, 822 cm−1; LR-MS (ESI+) m/z 474 (M + NH4+); HR-MS (ESI+) calcd for C26H40NO5Si (M + NH4+) 474.2670; found 474.2674.
Izenamide C (3). To a solution of Boc-l-Ile-Gly-NMe-d-Phe-l-Pro-OMe 5 (34 mg, 0.1 mmol) in CH2Cl2 (0.8 mL) was added TFA (0.2 mL) dropwise at room temperature. After stirring for 1 h, the reaction mixture was concentrated in vacuo. To a solution of above amine salt 40 (0.1 mmol), acid 38 (37 mg, 0.1 mmol), DIPEA (0.1 mL, 0.2 mmol), and HOAt (11 mg, 0.1 mmol) in CH2Cl2 (1.0 mL) was added EDC∙HCl (24 mg, 0.1 mmol) at room temperature. After stirring overnight, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was used in the next step without further purification. To a solution of crude hexapeptide (0.1 mmol) in THF (1.0 mL) was added TBAF (1M in THF, 0.2 mL, 0.2 mmol) at room temperature. After stirring for 4 h, the reaction mixture was quenched with H2O and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (Acetone/Hexane = 1:3) to give 33 mg (83% for 3 steps) of izenamide C (3) as white solid. [α]D20 = +21.50 (c 0.10, MeOH); 1H-NMR (800 MHz, CD3OD, a mixture of rotamers) Major rotamer δ 7.25–7.19 (m, 4H), 7.18–7.14 (m, 1H), 5.57 (t, J = 7.6 Hz, 1H), 5.06 (d, J = 4.5 Hz, 1H), 4.37 (dd, J = 8.3, 5.6 Hz, 1H), 4.33 (d, J = 7.0 Hz, 1H), 4.15 (d, J = 17.0 Hz, 1H), 4.11 (d, J = 4.4 Hz, 1H), 3.90 (d, J = 17.0 Hz, 1H), 3.70 (s, 3H), 3.44 (dt, J = 10.2, 6.9 Hz, 1H), 3.39 (dt, J = 10.2, 5.6 Hz, 1H), 3.19 (dd, J = 13.7, 7.8 Hz, 1H), 3.02 (s, 3H), 2.84 (dd, J = 13.7, 7.4 Hz, 1H), 2.21–2.17 (m, 1H), 2.11 (qqd, J = 6.9, 6.8, 4.5 Hz, 1H), 1.97–1.89 (m, 3H), 1.88–1.81 (m, 2H), 1.50–1.43 (m, 1H), 1.44 (ddq, J = 13.8, 7.4, 7.3 Hz, 1H), 1.28 (ddq, J = 13.7, 7.6, 7.5 Hz, 1H), 1.18–1.11 (m, 1H), 1.01 (d, J = 6.9 Hz, 3H), 0.95 (t, J = 6.7 Hz, 3H), 0.95 (d, J = 6.5 Hz, 3H), 0.93 (d, J = 6.9 Hz, 3H), 0.91 (d, J = 6.8 Hz, 3H), 0.89 (t, J = 7.4 Hz, 3H); 13C-NMR (200 MHz, CD3OD, a mixture of rotamers) Major rotamer δ 175.2, 174.0, 173.4, 172.2, 170.2, 170.1, 138.6, 130.5, 129.4, 127.6, 78.0, 76.5, 60.6, 58.9, 57.8, 52.7, 48.3, 41.9, 38.5, 37.9, 35.7, 33.3, 30.5, 29.9, 27.0, 26.0, 25.7, 19.4, 16.9, 16.1, 14.7, 11.8, 11.4; IR (thin film, neat) νmax 3317, 2960, 2877, 1744, 1637, 1519, 1447, 1199, 1176, 1137, 1030, 753 cm−1; LR-MS (ESI+) m/z 675 (M + H+); HR-MS (ESI+) calcd for C35H55N4O9 (M + H+) 675.3964; found 675.3949.
Pentapeptide (42). To a solution of Boc-l-Ile-l-Ala-NMe-d-Phe-l-Pro-OMe 4 (140 mg, 0.2 mmol) in CH2Cl2 (1.6 mL) was added TFA (0.4 mL) dropwise at room temperature. After stirring for 1 h, the reaction mixture was concentrated in vacuo. The residue was used in the next step without further purification. To a solution of above amine salt 41 (0.2 mmol), acid 6 (100 mg, 0.3 mmol), DIPEA (0.1 mL, 0.7 mmol), and HOAt (43 mg, 0.3 mmol) in CH2Cl2 (2.4 mL) was added EDC∙HCl (93 mg, 0.5 mmol) at room temperature. After stirring overnight, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (Acetone/Hexane = 1:3) to give 145 mg (77% for 2 steps) of pentapeptide 42 as white solid. [α]D20 = +67.88 (c 0.50, CHCl3); 1H-NMR (800 MHz, CDCl3, a mixture of rotamers) Major rotamer δ 7.23–7.18 (m, 4H), 7.16 (t, J = 7.2 Hz, 1H), 6.64 (s, 1H), 6.51 (d, J = 6.8 Hz, 1H), 5.66 (dd, J = 9.5, 6.5 Hz, 1H), 4.72 (dq, J = 7.2, 7.0 Hz, 1H), 4.44 (dd, J = 8.1, 6.7 Hz, 1H), 4.27–4.24 (m, 1H), 4.22–4.19 (m, 1H), 3.71 (s, 3H), 3.65–3.60 (m, 1H), 3.48–3.42 (m, 1H), 3.21 (dt, J = 10.6, 7.4 Hz, 1H), 3.19 (dd, J = 14.5, 6.4 Hz, 1H), 2.98 (s, 3H), 2.95 (dd, J = 14.4, 9.6 Hz, 1H), 2.58–2.51 (m, 1H), 2.48–2.40 (m, 1H), 2.23–2.18 (m, 1H), 1.93–1.87 (m, 1H), 1.87–1.75 (m, 4H), 1.61 (s, 3H), 1.52–1.46 (m, 5H), 1.46 (s, 9H), 1.46–1.42 (m, 1H), 1.12–1.02 (m, 1H), 0.90 (d, J = 6.2 Hz, 3H), 0.88 (d, J = 6.8 Hz, 3H), 0.85 (t, J = 7.4 Hz, 3H), 0.83 (d, J = 6.8 Hz, 3H), 0.82 (d, J = 6.9 Hz, 3H); 13C-NMR (200 MHz, CDCl3) δ 172.6, 172.4, 172.2, 169.8, 168.1, 156.3, 136.6, 129.5, 128.3, 126.7, 93.8, 79.3, 70.4, 59.3, 57.7, 55.6, 52.3, 52.2, 46.9, 45.2, 41.6, 40.1, 37.5, 34.7, 30.9, 30.3, 28.8, 28.4, 25.3, 24.9, 24.8, 24.8, 23.1, 22.1, 17.6, 15.3, 11.4; IR (thin film, neat) νmax 2961, 2875, 1751, 1700, 1632, 1532, 1455, 1366, 1257, 1175, 1047, 859 cm−1; LR-MS (ESI+) m/z 789 (M + NH4+); HR-MS (ESI+) calcd for C41H69N6O9 (M + NH4+) 789.5121; found 789.5134.
Izenamide A (1). To a solution of pentapeptide 42 (41 mg, 0.1 mmol) in CH2Cl2 (0.8 mL) was added TFA (0.2 mL) dropwise at room temperature. After stirring for 1 h, the reaction mixture was concentrated in vacuo. To a solution of above amine salt 43 (0.1 mmol), acid 39 (32 mg, 0.1 mmol), DIPEA (0.1 mL, 0.2 mmol), and HOAt (10 mg, 0.1 mmol) in CH2Cl2 (1.0 mL) was added EDC∙HCl (21 mg, 0.1 mmol) at room temperature. After stirring for 6 h, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4 and concentrated in vacuo. The residue was used in the next step without further purification. To a solution of crude heptapeptide (0.1 mmol) in THF (1.0 mL) was added TBAF (1M in THF, 0.2 mL, 0.2 mmol) at room temperature. After stirring for 6 h, the reaction mixture was quenched with H2O and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4 and concentrated in vacuo. The residue was purified by flash column chromatography (Acetone/Hexane = 1:3 to 1:1) to give 29 mg (66% for 3 steps) of izenamide A (1) as white solid. [α]D20 = −17.15 (c 0.20, MeOH); 1H-NMR (800 MHz, CD3OD, a mixture of rotamers) Major rotamer δ 7.26–7.19 (m, 4H), 7.18–7.13 (m, 1H), 5.69 (dd, J = 10.0, 5.9 Hz, 1H), 4.71 (d, J = 5.9 Hz, 1H), 4.68 (q, J = 7.1 Hz, 1H), 4.41 (dd, J = 7.7, 7.4 Hz, 1H), 4.19 (d, J = 7.6 Hz, 1H), 4.11 (d, J = 4.4 Hz, 1H), 4.00–3.95 (m, 2H), 3.71 (s, 3H), 3.47–3.45 (m, 1H), 3.42–3.37 (m, 1H), 3.11 (dd, J = 14.3, 5.9 Hz, 1H), 3.06 (s, 3H), 2.93 (dd, J = 14.3, 10.0 Hz, 1H), 2.41–2.34 (m, 2H), 2.29–2.23 (m, 1H), 2.17–2.12 (m, 1H), 2.11–2.07 (m, 1H), 1.97–1.90 (m, 1H), 1.87–1.82 (m, 2H), 1.80–1.73 (m, 1H), 1.60–1.52 (m, 2H), 1.48 (ddd, J = 13.4, 7.6, 3.4 Hz, 1H), 1.31 (ddd, J = 9.3, 7.5, 4.3 Hz, 1H), 1.13 (ddd, J = 13.6, 9.5, 7.3 Hz, 1H), 1.01 (d, J = 6.8 Hz, 3H), 1.00 (d, J = 6.8 Hz, 6H), 0.92 (d, J = 6.8 Hz, 3H), 0.92 (d, J = 6.5 Hz, 3H), 0.89 (d, J = 6.3 Hz, 3H), 0.87 (t, J = 7.4 Hz, 3H), 0.86 (d, J = 6.7 Hz, 3H), 0.82 (d, J = 7.0 Hz, 3H); 13C-NMR (200 MHz, CD3OD, a mixture of rotamers) Major rotamer δ 175.8, 174.6, 173.9, 173.8, 173.0, 171.8, 170.4, 138.3, 130.8, 129.3, 127.6, 80.6, 76.4, 71.3, 60.9, 58.7, 57.2, 52.6, 52.2, 48.5, 46.6, 41.4, 41.4, 38.3, 35.6, 33.3, 31.7, 31.1, 30.0, 26.2, 26.0, 25.7, 23.8, 22.2, 19.3, 19.2, 18.1, 17.2, 16.6, 15.8, 11.4; IR (thin film, neat) νmax 3318, 2960, 2878, 1742, 1637, 1544, 1450, 1366, 1265, 1198, 1038, 753 cm−1; LR-MS (ESI+) m/z 854 (M + Na+); HR-MS (ESI+) calcd for C43H69N5NaO11 (M + Na+) 854.4886; found 854.4888.
Izenamide B (2). To a solution of pentapeptide 42 (46 mg, 0.1 mmol) in CH2Cl2 (0.8 mL) was added TFA (0.2 mL) dropwise at room temperature. After stirring for 1 h, the reaction mixture was concentrated in vacuo. To a solution of above amine salt 43 (0.1 mmol), acid 38 (36 mg, 0.1 mmol), DIPEA (0.1 mL, 0.2 mmol), and HOAt (11 mg, 0.1 mmol) in CH2Cl2 (1.0 mL) was added EDC∙HCl (23 mg, 0.1 mmol) at room temperature. After stirring for 6 h, the reaction mixture was quenched with 1N HCl and extracted with CH2Cl2. The combined organic layer was washed with aqueous NaHCO3, dried over MgSO4, and concentrated in vacuo. The residue was used in the next step without further purification. To a solution of crude heptapeptide (0.1 mmol) in THF (1.0 mL) was added TBAF (1M in THF, 0.2 mL, 0.2 mmol) at room temperature. After stirring for 6 h, the reaction mixture was quenched with H2O and extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. The residue was purified by flash column chromatography (Acetone/Hexane = 1:3 to 1:1) to give 31 mg (61% from S5) of izenamide B (2) as white solid. [α]D20 = −11.62 (c 1.30, MeOH); 1H-NMR (800 MHz, CD3OD) δ 7.25–7.19 (m, 4H), 7.18–7.13 (m, 1H), 5.69 (dd, J = 10.0, 5.9 Hz, 1H), 4.93 (d, J = 4.4 Hz, 1H), 4.67 (q, J = 7.0 Hz, 1H), 4.41 (dd, J = 7.8, 7.3 Hz, 1H), 4.19 (d, J = 7.5 Hz, 1H), 4.13 (d, J = 4.3 Hz, 1H), 4.00–3.96 (m, 2H), 3.71 (s, 3H), 3.48–3.44 (m, 1H), 3.42–3.38 (m, 1H), 3.11 (dd, J = 14.3, 5.9 Hz, 1H), 3.06 (s, 3H), 2.93 (dd, J = 14.3, 10.0 Hz, 1H), 2.41–2.33 (m, 2H), 2.28–2.23 (m, 1H), 2.11–2.07 (m, 1H), 1.96–1.89 (m, 2H), 1.88–1.81 (m, 2H), 1.79–1.74 (m, 1H), 1.59–1.53 (m, 2H), 1.51–1.43 (m, 2H), 1.33–1.27 (m, 2H), 1.18–1.10 (m, 1H), 1.01 (d, J = 6.9 Hz, 3H), 0.97 (d, J = 6.9 Hz, 3H), 0.95 (t, J = 7.5 Hz, 3H), 0.92 (d, J = 6.4 Hz, 3H), 0.92 (d, J = 6.8 Hz, 3H), 0.89 (d, J = 6.2 Hz, 3H), 0.87 (t, J = 7.4 Hz, 3H), 0.86 (d, J = 6.8 Hz, 3H), 0.82 (d, J = 7.1 Hz, 3H); 13C-NMR (200 MHz, CD3OD, a mixture of rotamers) Major rotamer δ 175.7, 174.6, 173.9, 173.8, 173.0, 171.9, 170.3, 138.3, 130.8 129.3 127.6, 78.3, 76.4, 71.4, 60.9, 58.7, 57.2, 52.6, 52.3, 48.5, 46.6, 41.5, 41.4, 38.4, 38.3, 35.6, 33.3, 31.0, 30.0, 27.2, 26.2, 26.1, 25.7, 23.8, 22.2, 19.3, 17.1, 16.6, 15.8, 14.7, 11.9, 11.5; IR (thin film, neat) νmax 3308, 2960, 2877, 1743, 1636, 1533, 1448, 1361, 1265, 1195, 1038, 753 cm−1; LR-MS (ESI+) m/z 846 (M + H+); HR-MS (ESI+) calcd for C44H72N5O11 (M + H+) 846.5223; found 846.5229.