Trifluoroacetic acid → 1-(1-Methylpiperidin-4-yl)-1H-pyrazole-4-carboxylic acid

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According to the previously reported procedure, before the basic hydrolysis (KOH/MeOH/water), 4-(4-ethoxycarbonyl-pyrazol-1-yl)-piperidine-1-carboxylic acid tert-butyl ester (776 mg, 2.4 mmol) was dissolved in DCM (24 mL) and treated with TFA (1.85 mL, 24 mmol) for 3 hours. The reaction was poured into saturated NaHCO3 solution (100 mL), extracted with DCM (2×200 mL). The combined organic layers were dried over anhydrous sodium sulfate and evaporated to dryness to afford 550 mg of crude product that was dissolved in DCM (20 mL) and treated first with TEA (1.058 mL, 7.595 mmol), then with 37 wt. % in water formaldehyde solution (0.22 mL, 2.95 mmol) and finally with sodium triacetoxyborohydride (782 mg, 3.69 mmol) at room temperature, with stirring, under a nitrogen atmosphere. After 1 hour, DCM was added (20 mL) and the organic layer was washed with saturated NaHCO3 solution (25 mL), dried over anhydrous sodium sulfate and evaporated to leave an oil that was employed in the basic hydrolysis step with no further purification affording title compound.

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