Trifluoroacetic acid → [5-Fluoro-2-methyl-3-(2-sulfobenzyl)indol-1-yl]acetic acid methyl ester

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A mixture of (5-fluoro-2-methylindol-1-yl)acetic acid methyl ester (1.0 g), 2-formylbenzensulfonic acid sodium salt (0.94 g) and 1,2-dichloroethane (15 mL) at 0° C. was treated dropwise with a mixture of triethylsilane (4.3 mL), trifluoroacetic acid (1.0 mL) and 1,2-dichloroethane (10 mL), and the resulting mixture was stirred at room temperature for 6 hours. The mixture was extracted with ethyl acetate and the combined organic extract was dried over sodium sulfate. The solvent was removed under reduced pressure and the residue purified by column chromatography on silica gel, eluting with a mixture of methanol and dichloromethane (0:1 to 1:9 by volume) to afford the title compound as a purple gum (1.3 g).

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