[C:1]([O:9][CH2:10][CH3:11])(=[O:8])[CH2:2][C:3]([O:5][CH2:6][CH3:7])=[O:4].[O-]CC.[Na+].[Na].Cl[CH2:18][CH2:19][O:20][CH2:21][CH2:22]Cl>C(O)C>[CH2:10]([O:9][C:1]([C:2]1([C:3]([O:5][CH2:6][CH3:7])=[O:4])[CH2:22][CH2:21][O:20][CH2:19][CH2:18]1)=[O:8])[CH3:11]A solution of diethyl malonate (15.2 mL, 99.8 mmol) in ethanol (10 mL) was added dropwise to a solution of sodium ethoxide in ethanol [freshly prepared from sodium (2.3 g, 100 mmol) and ethanol (30 mL)] at ambient temperature and stirred for 10 min. Bis(2-chloroethyl)ether (12 mL, 102 mmol) was added dropwise then the mixture was heated at reflux overnight. It was then cooled to 10° C. before another portion of freshly-prepared sodium ethoxide in ethanol [prepared from sodium (2.3 g, 100 mmol) and ethanol (30 mL)] was added. The mixture was heated at reflux for 48 h then cooled, filtered to remove the precipitated sodium chloride then the filtrate was concentrated to dryness. Water was added to the residue which was then extracted with ether (3×25 mL). The combined ether layers were washed with water, brine and dried over anhydrous sodium sulfate. Concentration under reduced pressure yielded tetrahydropyran-4,4-dicarboxylic acid diethyl ester (10.1 g, 44%) as a mobile oil.
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