[Mg].Cl[CH:3]1[CH2:8][CH2:7][O:6][CH2:5][CH2:4]1.CN(C)[CH:11]=[CH:12][C:13](=[O:15])[CH3:14].Cl>O1CCCC1>[O:6]1[CH2:7][CH2:8][CH:3]([CH:11]=[CH:12][C:13](=[O:15])[CH3:14])[CH2:4][CH2:5]1
5.0 g of magnesium were introduced into 100 ml of anhydrous tetrahydrofuran. 25 g of 4-chlorotetrahydropyran were added dropwise under reflux under a nitrogen atmosphere, and the mixture was refluxed for 2 hours. A solution of 23.5 g of 1-(dimethylamino)but-1-en-3-one in 20 ml of anhydrous tetrahydrofuran was subsequently added dropwise to the reaction mixture, cooled to from 0° to 5° C., at a rate such that the internal temperature did not exceed 50° C., and the mixture was stirred at room temperature for a further 2 hours. For work-up, the reaction mixture was poured into a mixture of ice and dilute HCl, and extracted 3 times with 200 ml of chloroform, and the extracts were washed with saturated sodium chloride solution and dried over MgSO4. Removal of the solvent gave 24.0 g (75%) of 1-(4-tetrahydropyranyl)but-1-en-3-one having a boiling point of 117° C./10 mmHg.
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