Reaction Steps
To a solution of 2.2 mL (20.1 mmol) of (S)-(+)-2-methylbutanoic acid {Aldrich, 99% ee, [α]24D+24.0 (c 0.1, MeOH)} in 15 mL of benzene was treated with 2.2 mL (25.2 mmol) of oxalyl chloride and 10 μl of DMF. After 1.5 h at room temperature, the benzene and excess oxalyl chloride were removed followed by the addition of another 15 mL portion of benzene. The acid chloride residue was dissolved in 15 mL of benzene, and then a solution of 1.65 g (10.7 mmol) of 6a and 18 ml (22.2 mmol) of pyridine in 15 mL of benzene were added dropwise. After stirring at room temperature for 1 h, the solvent was removed. The residue was treated with 20 mL of water, and extracted with ether. The organic solution was washed with water, 1N hydrochloric acid, 1N aqueous sodium bicarbonate, and brine, dried over sodium sulfate, and concentrated to afford yellow oil. Pure 2a (2.0 g, 79% yield) was obtained by chromatography (silica gel, 2% of ethyl acetate in hexanes). Optical purity: 99% ee; [α]24D−0.79 (c 2.03, MeOH); 1H NMR (C6D6): δ 0.84 (3H, t, J=7.19 Hz), 1.08 (3H, d, J=6.82 Hz), 1.12 (3H, s), 1.24 (3H, s), 1.36 (1H, m), 1.40 (3H, bs), 1.51 (3H, t, J=1.89 Hz), 1.67 (2H, m), 2.05 (1H, m), 2.17 (1H, m), 2.27 (1H, m), 2.48 (1H, ddt, J=14.75, 8.79, 1.52 Hz), 4.15 (2H, d, J=7.57 Hz); 13C NMR (C6D6): δ 175.76, 137.48, 122.55, 65.30, 44.43, 41.39, 39.71, 28.62, 27.79, 27.13, 21.17, 19.60, 18.64, 16.85, 11.80; EI-MS m/z (%): 238 [M]+ (2), 136 (29), 121 (100), 107 (12), 93 (20), 81 (26), 67 (5), 57 (16), 41 (10); HREIMS: obsd. 238.1929, calcd. for C15H26O2 (M+): 238.1933.
