Sulfuric Acid → 2,2-Dimethyl-3-[4-({[(2'-oxospiro[cyclopentane-1,3'-indol]-1'(2'H)-yl)carbonyl]amino}methyl)piperidin-1-yl]propanoic acid

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To a stirred solution of methyl 2,2-dimethyl-3-[4-({[(2′-oxospiro[cyclopentane-1,3′-indol]-1′(2′H)-yl)carbonyl]amino}methyl)piperidin-1-yl]propanoate (170 mg, 0.38 mmol, step 3 of Example 2) in acetic acid (2 mL), H2SO4 (113 mg) in water (2 mL) was added, and the mixture was refluxed for 36 h. The mixture was cooled to room temperature and solid NaHCO3 (500 mg) was added slowly. The resulting mixture was concentrated to give white solid. To the solid, CH2Cl2 (40 mL) was added and stirred for 10 min, dried over MgSO4. The solution was filtered and concentrated and then resulted yellow oil. The oil was chromatographed on a column of silica gel eluting with CH2Cl2/methanol (14:1) to give 130 mg (80%) of the title compound as white solid. Recrystallization from ethyl acetate/diethyl ether gave white powder.

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