Br[CH2:2][CH2:3][CH2:4][CH2:5][C:6]([O:8][CH2:9][CH3:10])=[O:7].C(=O)(O)[O-:12].[Na+].[N+]1([O-])C=CC=CC=1>C1(C)C=CC=CC=1>[O:12]=[CH:2][CH2:3][CH2:4][CH2:5][C:6]([O:8][CH2:9][CH3:10])=[O:7]Ethyl 5-bromopentanoate (42 g), sodium bicarbonate (34 g) and pyridine N-oxide (38 g) in toluene (250 ml) are heated together under reflux in an atmosphere of nitrogen with vigorous stirring for 9 hours. After cooling, the product is partitioned with water (400 ml). The toluene layer is separated and the aqueous layer is extracted with a further amount of toluene (100 ml). The combined toluene extracts are dried over magnesium sulphate and the toluene removed by fractional distillation. The residue is distilled under vacuum to give ethyl 5-oxopentanoate as a colourless oil (14.1 g), m.s. M+ 144.
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