Synthesis of a protein modifying agent. A water-soluble stable free radical

Two synthetic routes to prepare a water-soluble stable free radical were investigated. The first product, sodium-2,4-dinitro-5-(4′-amino-2′,2′,6′,6′-tetramethylpiperidine-1-oxyl)benzene sulfonate, was water soluble but the sulfonate group was not readily replaceable with nucleophiles. The second synthetic route yielded N(4′-(2′,2′,6′,6′-tetramethylpiperidine-1-oxyl))-3,5-dinitro-4-sodium sulfonate benzamide, which was water soluble and its sulfonate group was readily replaced by nucleophiles.