Isolation and structural elucidation of two impurities from a diacerein bulk drug

Two impurities were found in the crude sample of diacerein. The level of these impurities 1.14% and 1.24% were detected by isocratic reverse-phase high performance liquid chromatography (HPLC). The molecular weights of the impurities were determined by liquid chromatography–mass spectroscopy (LC–MS) analysis. These impurities were isolated from crude sample of diacerein by reverse-phase preparative liquid chromatography. These impurities were characterized as 5-acetoxy-4-hydroxy-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid (Impurity-1) and 4-acetoxy-5-hydroxy-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid (Impurity-2) respectively. Structural elucidation of both the impurities were carried out by 1H NMR, 13C NMR, DEPT, 1D NOESY, MS and IR spectroscopy.