Water → 5-Ethyl-2-thiophenecarboxylic acid, N-t-butyl amide

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To a solution of 28 g (0.18 mol) of 5-ethyl-2-thiophenecarboxylic acid, from step 3a above, in 130 mL of methylene chloride was added a large excess (130 mL, 10 mol) of thionyl chloride, and the solution was heated at reflux for 2 hours. The solvent and excess reagent were removed by evaporation, and the residue was dissolved in 130 mL of chloroform and cooled to 0° C. To this solution was added 94 mL (0.9 mol) of t-butylamine dropwise, and the solution was stirred at reflux for 2 hours. The reaction was cooled, the solution poured into water, and the mixture extracted with ethyl acetate. The extract was washed with water and brine, dried, and concentrated. The residue was dissolved in ethyl acetate and the crude product was precipitated by addition of hexane. The crude product was collected by filtration and purified by flash chromatography over silica gel, eluting with 1:6 ethyl acetate:hexane, to afford 13.3 g (80% yield) of the title product after drying. MS 212 (M+H)+. NMR (CDCl3) δ: 7.25 (d, 1H, J=3 Hz), 6,72 (d, 1H, J=3 Hz), 5.70 (bs, 1H), 2.84 (q, 2H, J=7 Hz), 1.45 (s, 9H), 1.31 (t, 3H, J=7 Hz).

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