2-Thiophenesulfonyl chloride → ethyl 6-chloro-7-[(2-thienylsulfonyl)amino]-1H-indole-2-carboxylate

To a mixture of ethyl 7-amino-6-chloro-1H-indole-2-carboxylate (1.59 g) and pyridine (10 ml) was added thiophene-2-sulfonyl chloride (1.46 g), and the mixture was stirred at 50° C. for 3 hr. The reaction mixture was concentrated, 10% aqueous citric acid solution was added, and the mixture was extracted with ethyl acetate. The ethyl acetate layer was washed with saturated brine, dried (MgSO4), and concentrated. The obtained residue was subjected to silica gel column chromatography to give the title compound (2.17 g, yield 85%) as colorless crystals from a fraction eluted with tetrahydrofuran-hexane (1:2, volume ratio). The crystals were recrystallized from ethyl acetate. melting point 191-192° C.