[CH3:1][O:2][C:3](=[O:9])[CH2:4][C:5]([O:7][CH3:8])=[O:6].Br[CH2:11][CH2:12][CH2:13][CH2:14]Br.C([O-])([O-])=O.[K+].[K+].F[B-](F)(F)F.C([N+]1C=CN(C)C=1)CCC>CN(C=O)C.O>[CH3:1][O:2][C:3]([C:4]1([C:5]([O:7][CH3:8])=[O:6])[CH2:14][CH2:13][CH2:12][CH2:11]1)=[O:9]To a solution of malonic acid dimethyl ester (5.28 g, 40 mmol) in DMF (100 ml), 1,4-dibromo-butane (5.26 ml, 44 mmol), K2CO3 (13.8 g, 100 mmol), 1-butyl-3-methylimidazolium tetrafluoroborate (0.904 g, 4.0 mmol) are added at room temperature. The mixture is stirred at room temperature for 15 hours. To the mixture, water is added and the solution is extracted with AcOEt. The organic layer is washed with H2O and brine, dried over MgSO4, and concentrated under reduced pressure. The residue is purified by silica gel column chromatography to give cyclopentane-1,1-dicarboxylic acid dimethyl ester (6.13 g, 82%); TLC (hexane/AcOEt, 5:1) Rf 0.48, 1H NMR (400 MHz, chloroform-d) δ ppm 1.67-1.71 (m, 4H), 2.17-2.21 (m, 4H), 3.72 (s, 6H).
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