To a mixture of No. 5317776 (Compound E; 60 mg, 0.08 mmol), DBU (19 μL, 0.126 mmol) and dichloromethane (0.84 mL) was added 2-bromomethyl-isoindole-1,3-dione (30 mg, 0.126 mmol) commercially available from Aldrich (cat. No. 252611). After the mixture was stirred at room temperature for 6 hours, further 2-bromomethyl-isoindole-1,3-dione (10 mg, 0.04 mmol) and DBU (6.3 μL, 0.04 mmol) were added. Then, 4-(1,3-dioxo-1,3-dihydro-isoindole-2-ylmethyl) 1-butyl (S)-2-{(E)-(S)-1-[(S)-2-(4-but-2-ynyloxy-phenyl)-1-methoxycarbonyl-ethylcarbamoyl]-10-oxo-heptadec-2-enyl}-2-hydroxy-succinate (56 mg, 76%) was obtained (ESI (LC/MS positive mode) m/z 873 (M+H); Rt 3.38 min.).
