C([BH3-])#N.[Na+].F[C:6]1[C:14](F)=[CH:13][CH:12]=[C:11]2[C:7]=1[CH:8]=[C:9]([CH3:16])[NH:10]2.[OH2:17]>C(O)(=O)C>[OH:17][C:6]1[CH:14]=[CH:13][CH:12]=[C:11]2[C:7]=1[CH2:8][CH:9]([CH3:16])[NH:10]2750 mg of sodium cyanoborohydride are gradually added to a solution of 1 g of 4,5-difluoro-2-methylindole in 20 ml of acetic acid under argon cooled to a temperature of about 5° C. The reaction mixture is stirred at ambient temperature for 3 hours and 5 ml of water are added. The mixture is then concentrated under reduced pressure and the residue is taken up in ethyl acetate, then treated with 60 ml of a saturated sodium hydrogen carbonate solution. After stirring for 1 hour and then settling out, the organic phase is separated and the aqueous phase is extracted with 50 ml of ethyl acetate. The organic phases are combined, washed with saturated brine, dried over anhydrous magnesium sulfate, filtered, and then concentrated to dryness under reduced pressure. The residue is purified by silica gel chromatography, elution being carried out with an 80/20 v/v heptane/ethyl acetate mixture. 0.37 g of 4-hydroxy-2-methylindoline is thus obtained in the form of a yellow oil, the characteristics of which are the following:
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