3-Methoxycarbonyl-2,2,5,5-tetramethyl-3-pyrrolidin-1-oxyl (3-carboxy-2,2,5,5-tetramethyl-3-Pyrrolin-1-yloxy, methyl ester, 2,2,5,5-Tetramethyl-4-methoxycarbonylpyrroline-1-oxyl, 3-Methoxycarbonyl-2,2,5,5-tetramethylpyrroline-1-oxyl, CMPO)