Ethylene Glycol → 5'-Bromospiro[1,3-dioxolane-2,3'-[3H]indol]-2'(1'H)-one

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A magnetically stirred solution of 5-bromoindoline-2,3-dione (226 g, 1.0 mol, 1 eq.), ethylene glycol (186 g, 3.0 mol, 3 eq.), p-TSA (30 g, 0.16 mol, 16 mol %) and toluene (1500 mL) was refluxed overnight using a Dean-Stark apparatus. After total consumption of 5-bromoindoline-2,3-dione, EtOAc (1500 mL) and water (1000 mL) were added to remove excess of diol. The aqueous layer was extracted twice with EtOAc (500 mL). The collected organic layers were collected, dried over Na2SO4, filtered and concentrated under reduced pressure. The yellowish solid was washed with diethyl ether to give the desired compound as a pale yellow solid (236 g, 87%). 1H NMR (400 MHz, DMSO-d6): δH 10.56 (s, 1H), 7.31-7.33 (m, 2H), 6.93 (d, J=8.0 Hz, 1H), 4.27-4.35 (m, 4H), 2.35 (s, 3H), 2.18 (s, 3H). MS (ESI) m/e [M+1]+ 270, 272.

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