Reaction Steps
To a solution of compound 36 (90 mg, 0.23 mmol) in anhydrous DMF (5 mL) at room temperature under an atmosphere of argon were added carboxylic acid 20 (68 mg, 0.25 mmol), PyBOP (132 mg, 0.25 mmol), HOBt (34 mg, 0.25 mmol), and DIPEA (128 μL, 0.74 mmol). The solution was subsequently stirred overnight at room temperature. The resulting solution was then poured into water and extracted with EtOAc (2×30 mL), washed with brine, dried with MgSO4 and evaporated to dryness. Purification by two successive flash chromatography procedures using (DCM/MeOH: 95:5) and (Hexanes/Acetone: 1:1) respectively and trituration with diethyl ether yielded 35 mg (23%) of compound 37. 1H NMR (CDCl3) δ: 0.73 (s, 18-CH3), 0.84 (s, 19-CH3), 0.65-2.25 (residual CH and CH2), 2.40-2.75 (broad m, 2×CH2N and 2α-CH), 3.19 (t, J=8.3 Hz, 17α-CH), 3.32 (s, OCH3), 3.45-3.95 (broad m, 2×CH2NCO, CH2N of proline and 3β-CH), 5.14 (t, J=7.6 Hz, NCHCO of proline), 7.52 (m, 2×CH of naphthalene), 7.67 (d, J=8.4 Hz, CH of naphthalene), 7.85 (m, 3×CH of naphthalene), 8.09 (s, CH of naphthalene). 13C NMR (CDCl3) δ: 11.7, 17.1, 21.0, 23.2, 27.6, 28.1, 29.4, 33.0, 35.2, 35.8, 38.4, 43.0, 48.0, 49.0, 50.3, 51.1, 55.7, 56.0, 56.2, 57.8, 59.3, 63.6, 63.8, 64.1, 64.7, 65.0, 66.9, 90.7, 126.6, 127.2, 127.4, 127.7, 127.8, 128.0, 128.6, 132.5, 133.6, 133.9, 169.5, 170.4 HRMS: calcd for C40H55N3O4: 642.4265, found [M+H]+ 642.4274.
