Trifluoroacetic acid → 2-[(3R*,4S*)-1-{[2-Chloro-6-(difluoromethyl)phenyl]methyl}-3-{[1-(cyclohex-1-en-1-ylmethyl)piperidin-4-yl]carbamoyl}-4-methylpyrrolidin-3-yl]acetic acid

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tert-Butyl 4-[(3R*,4S*)-3-[2-(tert-butoxy)-2-oxoethyl]-1-{[2-chloro-6-(difluoromethyl)phenyl]methyl}-4-methylpyrrolidin-3-amido]piperidine-1-carboxylate obtained in Example 12a (332 mg, 0.553 mmol) was dissolved in dichloromethane (3.0 ml), trifluoroacetic acid (3.0 ml) was added thereto, and the mixture was then left to stand at mom temperature. After two hours and 30 minutes, the reaction liquid was concentrated and azeotropically distilled with dichloromethane twice to give an intermediate. 186 mg of the resulting intermediate (372 mg) was dissolved in tetrahydrofuran (5 ml), triethylamine (116 μl, 0.831 mmol) and cyclohex-1-ene-1-carbaldehyde (91.5 mg, 0.831 mmol) were added, and the mixture was left to stand at room temperature. After 30 minutes, sodium triacetoxyborohydride (176 mg, 0.831 mmol) was added thereto, and the mixture was stirred at mom temperature. After 10 hours and 50 minutes, the mixture was concentrated. The residue was purified by silica gel column chromatography (ODS, elution solvent: water/methanol) to give the title compound (80 mg).

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