[Br:1][C:2]1[C:3](=[O:13])[C:4]2([CH3:12])[O:10][C:7]([CH3:11])([C:8]=1Br)[CH:6]=[CH:5]2.[CH:14]([O-:17])([CH3:16])[CH3:15].[Li+].CCCCCC.C(OCC)(=O)C.CCCCCC.P([O-])(O)(O)=O.[Na+]>O1CCCC1.O>[Br:1][C:2]1[C:3](=[O:13])[C:4]2([CH3:12])[O:10][C:7]([CH3:11])([C:8]=1[O:17][CH:14]([CH3:16])[CH3:15])[CH:6]=[CH:5]2A solution of 2.74 g (8.9 mmol) of 3,4-dibromo-1,5-dimethyl-8-oxa-bicyclo[3.2.1]octa-3,6-dien-2-one (prepared according to Organic Lett. 4(12), 1997 (2002)) dissolved in 10 ml of tetrahydrofuran is added dropwise at ambient temperature to a solution of 5.4 ml (10.7 mmol) of 2M lithium isopropanolate diluted with 10 ml of tetrahydrofuran. The mixture is stirred for 3 hours at ambient temperature until the starting material has reacted completely (TLC monitoring: hexane/ethyl acetate/hexane 4:1). The reaction solution is then treated at a temperature of 0° C. with a 10% sodium dihydrogen phosphate solution (20 ml) and water (30 ml) and extracted three times with ethyl acetate. Drying over sodium sulfate and concentration by evaporation are carried out. For further purification, the dark oil so obtained is purified by chromatography over silica gel with 5% ethyl acetate in hexane. 1.73 g (68% of theory) of pure 3-bromo-1,5-dimethyl-4-isopropoxy-8-oxa-bicyclo[3.2.1]octa-3,6-dien-2-one are isolated.
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