Synthesis of Dibrominated Compound 31

Compound 31 (16.8 mg, 0.033 mmol), butyric acid (0.02 mL, 0.21 mmol), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (31.5 mg, 0.16 mmol), 4- (dimethylamino)pyridine (4.1 mg, 0.033 mmol) were mixed together in CH₂Cl₂ overnight. The reaction mixture was subjected to preparative TLC to give the desired compound, which was recrystallized from CH₂Cl₂ -hexane. Yield 86 %; colorless prisms; mp: 109–110 °C; ¹H NMR (CDCl₃) δ 5.94 (d, J = 1.2 Hz, 2H), 5.92 (d, J = 1.6 Hz, 2H), 4.97 (d, J = 1.2 Hz, 2H), 4.89 (d, J = 1.2 Hz, 2H), 4.06 (s, 6H), 2.18 (t, J = 7.6 Hz, 6H), 1.57 (sex, J = 7.6 Hz, 4H), 0.88 (t, J = 7.2 Hz, 6H); HRMS calcd for C₂₈H₃₂Br₂NaO₁₀ (M + Na)⁺ 682.9926, found 682.9916.