Chlorosuccinimide → Ethyl 2-chloro-2-[2-(4-methoxyphenyl)hydrazono]acetate

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Method A. A solution of [(4-methoxyphenyl)hydrazono]acetic acid ethyl ester (33, 4.0 g, 18.0 mmol) in anhydrous THF (40 mL) was treated with N-chlorosuccinimide (NCS, 2.52 g, 18.9 mmol, 1.05 equiv) at 0-5° C. under N2, and the resulting reaction mixture was stirred at 0-5° C. for an additional 30 min before being gradually warmed to 25° C. for 12 h. When HPLC and TLC showed the reaction was deemed complete, the solvent was removed in vacuo. The residue was directly purified by column chromatography (SiO2, 0-25% EtOAc/hexane gradient elution) to afford the desired chloro[(4-methoxyphenyl)hydrazono]acetic acid ethyl ester (34, 3.51 g, 4.62 g theoretical, 76%) as pale-yellow solids, which was found to be essentially pure to do the following reaction without further purification. For 34, CIMS m/z 255/257 (M+−H, C11H13ClN2O3).

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