A mixture of 1.30 g of 4-chloro-5-fluoro-1H-pyrrolo[2,3-b]pyridine, 1.6 g of 4-methylbenzenesulfonyl chloride, 3.40 g of sodium hydroxide dissolved in 16 cm3 of water, and 0.052 g of tetrabutylammonium hydrogen sulfate in 200 cm3 of toluene is stirred for about 24 hours in the region of 20° C. The mixture is diluted with 500 cm3 of ethyl acetate; the organic phase is washed with three times 200 cm3 of water, dried over magnesium sulfate, filtered and concentrated to dryness under reduced pressure (13 kPa). The residue is purified by flash chromatography on a column of silica [eluent: dichloromethane]. 1.90 μg of 4-chloro-5-fluoro-1-(toluene-4-sulfonyl)-1H-pyrrolo[2,3-b]pyridine are thus obtained in the form of a powder, the characteristics of which are as follows:
