Boron trifluoride etherate → 6'-Bromospiro[cyclohexane-1,2'-indene]-1',4(3'H)-dione

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In a flame dried 20 mL vial was placed 6-bromo-2-methylene-2,3-dihydro-1H-inden-1-one (98 mg, 0.441 mmol) and it was dissolved in dichloromethane (4.5 mL). To this solution was added 2-trimethylsilyloxy-1,3-butadiene (98 μL, 0.565 mmol) and the solution was cooled down to −78° C. After stirring for 5 minutes, BF3.OEt2 (27 mL, 0.219 mmol) was slowly added. After 5 minutes of the BF3.OEt2 addition, TLC indicated consumption of the dienophile. The reaction was quenched with MeOH (300 μL), allowed to stir for 5 minutes at −78° C. and then warmed up to room temperature. Once at room temperature, 2M HCl (7 mL) was added. The phases were separated and the aqueous phase was back-extracted with dichloromethane twice (5 mL/each). The combined organic phases were dried over MgSO4, filtered and concentrated under reduce pressure. The crude material was purified by flash chromatography (ISCO, 12 g SiO2 cartridge, ethyl acetate/hexanes as the eluents). The corresponding fractions were combined and concentrated under reduce pressure yielding 6′-bromospiro[cyclohexane-1,2′-indene]-1′,4(3′H)-dione (62 mg, 0.212 mmol, 48% yield). 1H NMR=(CDCl3, 400 MHz) δ 7.68 (d, J=2.0 Hz, 1H), 7.51 (dd, J=8.0, 2.0 Hz, 1H), 7.16 (d, J=8.0 Hz, 1H), 2.94 (s, 2H), 2.48 (dt, J=15.2, 5.6 Hz, 2H), 2.22 (ddd, J=15.2, 10.8, 5.6 Hz, 2H), 1.98 (ddd, J=13.6, 11.2, 5.2 Hz, 2H), 1.65 (m, 2H) ppm.

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