[CH3:1][C:2]1[CH:3]=[CH:4][CH:5]=[C:6]2[C:10]=1[NH:9][CH:8]=[CH:7]2.C([BH3-])#N.[Na+].[OH-].[Na+]>C(O)(=O)C.O>[CH3:1][C:2]1[CH:3]=[CH:4][CH:5]=[C:6]2[C:10]=1[NH:9][CH2:8][CH2:7]2To a stirred solution of 7-methylindole (10.27 g, 78.3 mmol) in glacial acetic acid (50 mL) is added sodium cyanoborohydride (6.33 g, 100.7 mmol) portionwise over a 5 minute period. The reaction solution is allowed to stir for 1 hour. The solution is diluted with water (500 mL) and basified with 50% sodium hydroxide solution. The basic solution is extracted with three 250 mL portions of diethyl ether. The organic fractions are combined, dried over anhydrous potassium carbonate, filtered and evaporated to give a pink oil. The oil is purified by silica gel column chromatography using 20% ethyl acetate/hexanes as eluent to give 11.63 g of 2,3-dihydro-7-methylindole as a colorless oil (94% yield).
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