Reaction Steps
To a solution of 14c (6.6 g; 28.2 mmol), freshly sublimed carbon tetrabromide (10.77 g; 32.5 mmol) and anhydrous collidine (4.3 ml; 3.9 g; 32.5 mmol) in dry dichloromethane (135 ml) at -20° C. was added solid triphenylphosphine (10.83 g; 41.3 ml). The reaction was complete within 30 min as demonstrated by thin-layer chromatographic analysis (silica 5% and 50% ethyl acetate in hexanes), and was quenched with the addition of methanol (2 ml) and the solvent removed. The crude mixture was chromatographed on silica (10% ethyl acetate in hexanes) to provide 14d as a colorless oil. (5.93 g; 71% yield). 1H NMR (CDCl3, 500 MHz) δ 7.60 (dd, J=7.5, 1.5 Hz, 1 H, H at C-6, 7.25 (m, 2 H, H at C-4 and C-5), 7.12 (dd, J=7.5, 1.5 Hz, 1 H, H at C-3), 5.92 (dt, J=15.0, 6.5 Hz, 1 H, H at C-3 of butene), 5.71 (m, 1 H, H at C-2 of butene), 5.56 (s, 1 H, H at C-2 of dioxane), 4.24 (dd, J=11.5, 5.0 Hz, 2 H, He at C-4 and C-6 of dioxane), 3.96 (m, 4 H, H at C-4 of butene and Ha at C-4 and C-6 of dioxane), 3.53 (d, J=6.5 Hz, 2 H, H at C-1 of butene), 2.22 (m, 1 H, Ha at C-5 of dioxane), 1.42 (d, J=13.5, 1 H, H at C-5 of dioxane) 13C NMR (CDCl3, 125 MHz) δ 136.5, 134.9 (4o carbons of aromatic ring, 129.8, 129.0, 127.4, 126.7, 126.5, 126.4 (alkene and aromatic C-H's), 100.0 (C-2 of dioxane), 67.6 (C-4 and C-6 of dioxane), 35.2 (C-4 of alkene), 33.2 (C-1 of alkene), 25.8 (C-5 of dioxane) IR 2985, 2860 (aromatic C--H), 1100 (C--O) cm-1 ; Mass Spec. (70 ev) m/z (rel abund 297 (M+ -, 0.5), 217 (M+ -Br, 86), 159 (M+ -C3H7O and Br, 37), 143 (M+ -C3H7O2 and Br, 53), 131 (M+ -1,3 dioxane and Br, 100); HRMS calcd for C14H16BrO2 : M+ -H=297.0317. Found: 297.0340.
