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In a 250 mL of round bottom flask were added 2 (2.0 g, 9.55 mmol) and Et3N (1.44 mL, 10.54 mmol) in 100 mL of THF. The mixture was cooled to 0° C., and a solution of 2-bromo isobutyryl bromide (2.34 g, 10.0 mmol) in 25 mL of THF was added dropwise (30 min) to the reaction mixture. The white suspension was stirred for 3 h at 0° C. and subsequently at ambient temperature for overnight. The ammonium salt was filtered off and the solvent was removed under reduced pressure to give a pale-yellow residue that was further purified by column chromatography over silica gel eluting with EtOAc/hexane (1:4) to give compound as a white solid. Yield: 2.5 g (91%). EIMS m/z 338.0 for [M+Na]+C14H16NO5BrNa calculated 380.02. 1H NMR (500 MHz, CDCl3, δ) 6.52 (s, 2H, CH═CH, bridge protons), 5.27 (s, 2H, —CHO, bridge-head protons), 4.33 (t, J=10.5 Hz, 2H, NCH2CH2OC═O), 3.77 (t, J=10.5 Hz, 2H, NCH2CH2OC═O), 2.87 (s, 2H, CH—CH, bridge protons), 1.90 (s, 6H, C(CH3)2—Br). 13C NMR (500 MHz, CDCl3, δ): 175.8, 171.3, 136.5, 80.8, 62.1, 55.6, 47.4, 37.5, 30.5.

1. filtrationThe ammonium salt was filtered off
2. customthe solvent was removed under reduced pressure
3. customto give a pale-yellow residue that
4. customwas further purified by column chromatography over silica gel eluting with EtOAc/hexane (1:4)
5. customto give compound as a white solid