Hydrogen chloride-ethyl acetate → methyl(3R,5S)-5-[{[1-(4-methoxybutyl)-1H-indol-2-yl]carbonyl}(2-methylpropyl)amino]piperidine-3-carboxylate

1-(4-Methoxybutyl)-1H-indole-2-carboxylic acid (247 mg), 1-tert-butyl 3-methyl (3R,5S)-5-[(2-methylpropyl)amino]piperidine-1,3-dicarboxylate (314 mg) and diisopropylethylamine (862 μl) were dissolved in methylene chloride (5 ml), chloro-N,N,N′,N′-tetramethylformamidinium hexafluorophosphate (337 mg) was added, and the mixture was stirred at room temperature for 15 hr. The reaction mixture was subjected to silica gel column chromatography, and a fraction eluted with ethyl acetate-hexane (3:7) was concentrated under reduced pressure to give 1-tert-butyl 3-methyl (3R,5S)-5-[{[1-(4-methoxybutyl)-1H-indol-2-yl]carbonyl}(2-methylpropyl)amino]piperidine-1,3-dicarboxylate (40 mg) as an oil. The obtained 1-tert-butyl 3-methyl (3R,5S)-5-[{[1-(4-methoxybutyl)-1H-indol-2-yl]carbonyl}(2-methylpropyl)amino]piperidine-1,3-dicarboxylate (40 mg) was dissolved in methanol (2 ml), 4M hydrogen chloride-ethyl acetate (2 ml) was added, and the mixture was stirred at room temperature for 15 hr. The reaction mixture was concentrated, and the residue was purified by reversed-phase preparative HPLC, and the object fraction was concentrated under reduced pressure. An aqueous sodium bicarbonate solution was added to the residue, and the mixture was extracted with ethyl acetate. The extract was washed with saturated brine, dried over anhydrous magnesium sulfate, and concentrated to give the object product (13 mg).