[Br:1]Br.[C:3]1(=[O:10])[CH2:9][CH2:8][CH2:7][CH2:6][CH2:5][CH2:4]1.[CH2:11]([OH:14])[CH2:12]O.C(=O)(O)[O-].[Na+]>O1CCCC1>[CH2:11]1[O:14][C:3]2([CH2:9][CH2:8][CH2:7][CH2:6][CH2:5][CH:4]2[Br:1])[O:10][CH2:12]1Bromine (44.8 g, 0.28 mol) was added dropwise to a stirred solution of cycloheptanone (28.5 g, 0.25 mol) in a 1:1 solution (300 mL) of ethylene glycol and tetrahydrofuran at room temperature. The mixture was stirred for 5 hours, then poured into aqueous sodium bicarbonate and extracted 3× with diethyl ether. The combined organic layers were washed with brine, dried over sodium sulfate, filtered, and concentrated in vacuo to give 2-bromocycloheptanone ethylene ketal (60 g). A mixture of this crude bromo ketal and a methanolic solution of sodium methoxide (250 mL, 25 wt. % solution) was heated at reflux overnight. The mixture was cooled to ambient temperature, poured into water, and extracted with hexanes. The organic layer was washed with brine, dried over sodium sulfate, filtered, and concentrated in vacuo to afford 2-cycloheptenone ethylene ketal (39 g).
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