[CH2:1]([Si](C)(C)C)[CH:2]=[CH2:3].[CH3:8][C:9]1([CH3:23])[CH2:14][C:13]([C:16]2[CH:21]=[CH:20][C:19]([CH3:22])=[CH:18][CH:17]=2)(O)[CH2:12][CH2:11][O:10]1.B(F)(F)F.O(CC)CC>C(Cl)Cl>[CH3:8][C:9]1([CH3:23])[CH2:14][C:13]([C:16]2[CH:21]=[CH:20][C:19]([CH3:22])=[CH:18][CH:17]=2)([CH2:3][CH:2]=[CH2:1])[CH2:12][CH2:11][O:10]1Allyltrimethylsilane (4.34 ml, 27.2 mmol) was added to a solution of 2,2-dimethyl-4-(4-methylphenyl)oxan-4-ol (3.0 g, 13.6 mmol) in dry CH2Cl2 (100 ml) at 0° C., followed by BF3—OEt2 (3.42 ml, 27.2 mmol). The resulting mix was stirred at 0° C. for 1 h. The reaction was quenched with H2O (10 ml) and diluted with CH2Cl2 (10 ml), and washed with sat. NaHCO3 (20 ml), brine (20 ml), dried and concentrated. Purified by normal phase SiO2 chromatography (5 to 40% EtOAc/hexanes) to give 2,2-dimethyl-4-(4-methylphenyl)-4-(prop-2-en-1-yl)oxane as a colorless oil, which was used crude (2.49 g, 75%).
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