Boron trifluoride etherate → 2,3-dihydro-1H-indol-5-ol

Reaction equation

[OH:1][C:2]1[CH:3]=[C:4]2[C:8](=[CH:9][CH:10]=1)[NH:7][CH:6]=[CH:5]2.C([BH3-])#N.[Na+].B(F)(F)F.CCOCC>CO>[OH:1][C:2]1[CH:3]=[C:4]2[C:8](=[CH:9][CH:10]=1)[NH:7][CH2:6][CH2:5]2

Participants

Procedure

5-Hydroxyindole (2 g, 15 mmol) was dissolved in methanol (60 ml) under argon. Sodium cyanoborohydride (1.89 g, 30 mmol) and trifluoroboron etherate (4.2 ml, 33 mmol) were added and the mixture was heated at reflux for 3 hours then left to cool to ambient temperature. The solvent was evaporated and the residue was partitioned between ethyl acetate and water. Ammonia was added to adjust the pH to 10 and the aqueous phase was extracted with more ethyl acetate. The combined organic phases were washed with water, brine, dried (MgSO4), filtered and the solvent evaporated. The residue was purified by flash chromatography using dichloromethane/methanol (95/5) as solvent. Evaporation of the solvent gave 5-hydroxy-2,3-dihydro-1H-indole (1.45 g, 73%) as an off white solid.