[O:1]=[C:2]1[CH2:6][CH2:5][CH:4]([C:7]([OH:9])=[O:8])[CH2:3]1.S(=O)(=O)(O)O.[CH3:15]O>>[O:1]=[C:2]1[CH2:6][CH2:5][CH:4]([C:7]([O:9][CH3:15])=[O:8])[CH2:3]1A solution of 3-oxocyclopentanecarboxylic acid (13.2 g, 103 mmol), MeOH (130 ml) and 98% sulfuric acid (0.1 ml) was heated under reflux with stirring for 1.5 h. The MeOH was evaporated in vacuo. The residue was dissolved in CH2Cl2 (100 ml) and was washed with 1% sodium bicarbonate (50 ml). The solvent was evaporated in vacuo to give the title compound as a liquid, 12.2 g (84% yield); ir (liq.) νmax : 1730-1740 cm-1 ; 1Hmr (CDCl3) δ: 3.71 (s, 3H, OMe), 3.2 (m, 1H, H-1) and 2.5-1.9 ppm (m, 6H, CH2).
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