5-Methoxyindole → 5-methoxy-2,3-dihydro-1H-indole

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Reaction Steps

5-Methoxyindole (50 g, 0.34 mol) was dissolved in glacial acetic acid (500 mL) in a 3 L 3-necked flask fitted with a mechanical stirrer, a dropping funnel and a thermometer. The solution was cooled to 10–12° C. with an ice bath and sodium cyanoborohydride (64 g, 1.0 mol) was added in portions while ensuring the temperature remained at or below 15–16° C. After the addition was complete the cooling bath was removed and the reaction was warmed to ambient temperature for 0.5 hour. TLC (1:1 EtOAc/hexane) confirmed a complete reaction. The reaction was cooled to 5–10° C. and 50% aqueous sodium hydroxide was added until the pH was 8–10. The product oiled out and was extracted with ethyl acetate (3×700 mL). The combined organic layers were washed with water (2×500 mL) and brine (400 mL), dried over anhydrous potassium carbonate, filtered and concentrated to afford 5-methoxyindoline (50 g, 0.337 mol, 99% yield) as a thick oil. This product was used without further purification.

Provenance & Evidence