To a mixture of N-[2-(5-formyl-4,5-dihydro-1,3-thiazol-2-yl)-1H-indol-7-yl]-N-methylthiophene-2-sulfonamide (550 mg), thiomorpholine 1-oxide hydrochloride (310 mg), triethylamine (200 μL) and tetrahydrofuran (15 mL) was added sodium triacetoxyborohydride (420 mg) at room temperature, and the mixture was stirred for 10 min. Saturated aqueous sodium hydrogencarbonate solution was added to the reaction mixture, 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 a colorless oil from a fraction eluted with ethyl acetate:methanol=90:10. The obtained colorless oil was purified by preparative HPLC to give the title compound (230 mg, yield 35%) as white crystals. melting point 204° C.
