Under an argon atmosphere, to a solution of 2-(5-amino-6-benzyl-3-chloropyrazin-2-yl)-5-methoxybenzaldehyde (7) (2.28 g, 6.44 mmol) in deaerated toluene (30 mL) were successively added dichlorobis(triphenylphosphine)palladium(II) (226 mg, 322 μmol), tributyl(vinyl)tin (3.75 mL, 12.8 mmol) and tetra-n-butylammonium chloride (3.50 g, 12.6 mmol) at room temperature. The mixture was heated to reflux for 1.5 hours (oil bath temperature at 110° C.). After cooling to room temperature, to the mixture was added water and the product was extracted with ethyl acetate (×3). The combined organic extract was washed successively with water (×1) and brine (×1) followed by drying over anhydrous sodium sulfate. After filtration and concentration under reduced pressure, the residue was purified by silica gel flash column chromatography (silica gel mixed with 10 wt % potassium fluoride, n-hexane/ethyl acetate=3/1) to give 2-(5-amino-6-benzyl-3-vinylpyrazin-2-yl)-5-methoxybenzaldehyde (8) (1.24 g, 3.59 mmol, 55.5%) as a reddish brown oily substance. TLC Rf=0.27 (n-hexane/ethyl acetate=3/1); 1H NMR (500 MHz, CDCl3) δ 3.92 (s, 3H), 4.15 (s, 2H), 4.50 (s, 2H), 5.39 (dd, 1H, J=2.0, 11.0 Hz), 6.33 (dd, 1H, J=2.0, 17.0 Hz), 6.57 (dd, 1H, J=11.0, 17.0 Hz), 7.20-7.28 (m, 4H), 7.30-7.36 (m, 2H), 7.41 (d, 1H, J=8.5 Hz), 7.55 (d, 1H, J=3.0 Hz), 9.88 (s, 1H); 13C NMR (126 MHz, CDCl3) δ 41.0, 55.7, 110.6, 120.5, 121.0, 127.2, 128.5 (2C), 129.1 (2C), 132.3, 132.9, 134.4, 136.1, 136.5, 138.7, 140.2, 145.2, 151.5, 159.8, 191.5.
