2-Acetyl-5-chlorothiophene → (NE)-N-[1-(5-chlorothiophen-2-yl)ethylidene]hydroxylamine

Combined 1-(5-chlorothiophen-2-yl)ethanone (100 g, 622 mmol), hydroxylamine hydrochloride (47.6 g, 685 mmol, 1.1 eq) and sodium acetate (56.2 g, 685 mmol, 1.1 eq) in 1 L of ethanol and 300 mL of water. The reaction was heated to reflux. After 3 h, the reaction was cooled to rt and concentrated. The residue was slurried in 500 mL of water overnight, filtered and dried to afford the crude solid. Purified by extraction of the solid with hot dichloromethane, cooling to rt and filtering to remove unreacted starting material. The solid was slurried in hot dichloromethane several times to extract the solid enriched in the first eluting (E-oxime) compound (100% DCM) after concentration. The highly crystalline material was purified by column chromatography using 100% dichloromethane to afford (E)-1-(5-chlorothiophen-2-yl)ethanone oxime. NMR (400 MHz, DMSO-d6) δ 11.31 (s, 1H), 7.21 (d, J=3.9 Hz, 1H), 7.07 (d, J=3.9 Hz, 1H), 2.12 (s, 3H) ppm.