A solution of m-CPBA (65 mg) in chloroform (1.5 ml) was added to a solution of ethyl 5-(benzo[b]thiophen-4-ylamino)-3-(methylthio)-1,2,4-triazine-6-carboxylate obtained in the above-described Step 1 (33 mg) in chloroform (1.5 ml), the reaction solution was stirred at room temperature for 10 minutes. Thereafter, tert-butyl((1S,2R)-2-aminocyclohexyl)carbamate (60 mg) and diisopropylethylamine (0.10 ml) were added, and then the reaction solution was stirred at room temperature for 3 hours. The reaction solution was diluted with ethyl acetate and then washed successively with an aqueous sodium bicarbonate solution and a saturated saline solution. After drying over anhydrous sodium sulfate, the solvent was evaporated under vacuum, and then the resultant residue was purified by column chromatography on silica gel (developing solvent:hexane/ethyl acetate) to obtain ethyl 5-(benzo[b]thiophen-4-ylamino)-3-((1R,2S)-2-(tert-butoxycarbonylamino)cyclohexylamino)-1,2,4-triazine-6-carboxylate.
