ORGANIC SPECTROSCOPY INTERNATIONAL: 1E-Methyl[4]dendralene

1E-Methyl[4]dendralene (10)

A freshly prepared solution of (3-methylenepenta-1,4-dien-2-yl)magnesium chloride (63) (25 mL, 0.16 M solution in THF, 4.0 mmol, 1.6 mol equiv) was added slowly into a stirred solution of ZnBr₂ (0.92 g, 4.1 mmol, 1.6 mol equiv) and THF (5.0 mL) at 0 °C. Once the addition was complete, the reaction mixture was allowed to warm to 25 °C and stirred for a further 20 min. To this was added 1E-bromopropene (19) (0.22 mL, 2.6 mmol, 1.0 mol equiv) followed by addition of Pd(PPh₃)₄ (0.15 g, 0.13 mmol, 0.050 mol equiv) and the reaction mixture was stirred at 25 °C for 16 h with the exclusion of light. The resulting solution was poured into water (0.15 L), stirred for 15 min and petroleum ether (30–40 °C) (0.15 L) was added. The organic phase was separated and aqueous phase was then extracted with petroleum ether (30–40 °C) (2 × 0.10 L). The organic phases were combined, washed with brine, dried over MgSO₄, filtered and concentrated under reduced pressure (50 mbar, 0 °C). Purification by flash column chromatography (SiO₂, petroleum ether (30–40 °C)) gave the title compound 10 (0.11 g, 0.92 mmol, 35%) as a colorless oil.

R_f 0.56 (petroleum ether (30–40 °C));

¹H NMR (300 MHz, CDCl₃) δ 6.41 (ddd, J = 17.4, 10.5, 0.8 Hz, 1H), 6.13 (ddd, J = 15.5, 1.9, 0.7 Hz, 1H), 5.65 (dq, J = 15.6, 5.0 Hz, 1H), 5.24–5.14 (m, 2H), 5.12–5.01 (m, 3H), 4.89 (dd, J = 2.2, 0.8 Hz, 1H), 1.74 (ddt, J = 6.7, 1.5, 0.7 Hz, 3H) ppm;

¹³C NMR (75 MHz, CDCl₃) δ 147.5 (C), 146.4 (C), 137.7 (CH), 132.1 (CH), 128.5 (CH), 117.4 (CH₂), 116.5 (CH₂), 115.0 (CH₂), 18.2 (CH₃) ppm;

IR (thin film) ν_max = 3083, 2954, 2923, 2852, 1635, 1456 cm^–1;

LRMS (70 eV, EI) m/z (%) 120 ([M]^+•, 15%), 105 (100), 91 (44), 79 (34);

HRMS calc for C₉H₁₂ [M]^+• 120.0939, found 120.0938.