To a solution of 1-benzenesulfonyl-5-methylthioindole (2 g) in tetrahydrofuran (70 ml), n-butyl lithium (5 ml, 1.59M) was added dropwise at −78° C. under nitrogen atmosphere, the mixture was stirred at the same temperature for 30 minutes, followed by drwopise addition of hexamethylphosphoric triamide (2.3 ml), and the mixture was stirred at the same temperature for 10 minutes. The stirred reaction mixture was heated to 15 to 30° C. over 30 minutes while carbon dioxide gas was blown therein at −78° C. To the reaction mixture was added 1N hydrochloric acid and the resulting mixture was extracted with ethyl acetate. The organic layer was washed with water and dried. The solvent was distilled off under reduced pressure. The resulting residue was separated using silica gel column chromatography (dichloromethane methanol:acetic acid=100:5:1) to obtain a crude product (4 g). To absolution of the resulting crude 1-benzenesulfonyl-5-methylthioindole-2-carboxylic acid (4 g) in tetrahydrofuran (70 ml) was added triethylamine (1.84 ml) and ethyl chlorocarbonate (1 ml) under ice-cooling, the mixture was stirred at the same temperature for 30 minutes, followed by addition of aqueous ammonia (2 ml) at the same temperature. The mixture was stirred for 15 minutes. The reaction mixture was diluted with ethyl acetate, washed with water and dried. The solvent was distilled off under reduced pressure. The resulting residue was recrystallized (hexane/ethyl acetate) to obtain 1.08 g of the desired product as a white powder.
