Br[C:2](Br)([CH2:5][CH3:6])[CH2:3][CH3:4].[C:8]1(=[O:14])[CH2:13][CH2:12][CH2:11][CH2:10][CH2:9]1.[K].C([O-])(C)(C)C.Cl>C1(C)C=CC=CC=1>[C:8]1(=[O:14])[C:13]2([CH2:6][CH2:5][CH2:2][CH2:3][CH2:4]2)[CH2:12][CH2:11][CH2:10][CH2:9]1Dibromopentane (5 mmol) is added to a solution of cyclohexanone (5 mmol) and potassium-tert-butanolate (10 mmol) in toluene (7.5 ml) and the reaction mixture is refluxed for 48 hours. After cooling to room temperature 25% hydrochloric acid is added and extraction is carried out with diethyl ether. The combined organic phases produce, after drying over sodium sulphate, removal of the solvent in a vacuum and chromatography of the residue using silica gel (ethyl acetate/heptane, 1:5) pure spiro[5.5]undecan-1-one (Tetrahedron 1964, 20, 2553-2573): tR 1.90 min. (LC-2); ESI-MS (+): m/z 167.27 [M+H]+.
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