To a mixture of the 97 (100 mg, 0.41 mmol) and 4-maleimidobenzoic acid (101) (98 mg, 0.45 mmol) was added dichloromethane (5 mL), followed by TBTU (157 mg, 0.49 mmol) and DIPEA (212 uL, 1.23 mmol). The mixture was stirred at an ambient temperature for 16 h and then purified on a 2 mm radial chromatotron plate eluting with 50% ethyl acetate in hexanes to give 95 mg (51%) of 100: 1H-NMR (400 MHz, CDCl3) δ 7.85 (d, J=6.6 Hz, 2H), 7.42 (d, J=6.6 Hz, 2H), 6.81 (s, 2H), 6.38 (bs, 1H), 4.43 (m, 2H), 2.14 (sept, J=6.6 Hz, 1H), 1.41 (s, 9H), 1.31 (d, J=7.0 Hz, 3H), 0.98 (m, 6H); MS (ES−) m/z 441.90 [M−H]−.
