Hydrochloric Acid → 6-Chloro-5-fluoroindolin-2-one

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Reaction Steps

Compound (III) is obtained by cyclizing (II) with an acid as shown in step [b] of Scheme 1. Compound (II) is dissolved in a mixture of an acid and a cosolvent, preferably the acid is glacial acetic acid in which instance the preferred cosolvent is water or the acid is hydrochloric acid in which instance the preferred cosolvent is methylene chloride. With acetic acid the ratio of the acid and cosolvent mixture ranges from 2:1 to pure acetic acid respectively, the preferred ratio being 6:1. With HCl, much less acid is required, preferably 1:4 3N HCl to CH2Cl2. Once compound (II) is dissolved, the reaction mixture is stirred at ambient temperature for from 6 to 7 hours to produce 5-fluoro-6-chlorooxindole (III). Compound (III) is readily purified by recrystallization from ethyl acetate to yield an off-white solid.

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