[O:1]=[C:2]([CH2:6][CH3:7])[C:3]([OH:5])=[O:4].O.[C:9]1(C)C=CC(S(O)(=O)=O)=C[CH:10]=1>C(O)C>[CH2:9]([O:4][C:3](=[O:5])[C:2](=[O:1])[CH2:6][CH3:7])[CH3:10]2-Oxo-butyric acid (10.0 g, 97.2 mmol) was dissolved in ethanol (100 mL) and p-toluene sulfonic acid mono hydrate (0.1 g) was added. The reaction mixture was heated to reflux for 48 h. After cooling the reaction mixture to room temperature, the ethanol was removed under reduced pressure to give 11.4 g of 2-oxo-butyric acid ethyl ester.
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