fexofenadine Imp-C

Fexofenadine Imp-C is a methoxy-substituted aromatic byproduct arising from O-methylation of the phenolic hydroxyl group in the parent antihistamine fexofenadine. It retains the core tertiary amine and ether-linked pivaloyl moiety but features a methoxy group in place of the native hydroxyl, altering its polarity and H-bonding capacity. This impurity forms via nucleophilic alkylation during synthetic oxidation steps, stabilizing through conjugation with the aromatic ring system. It serves as a critical HPLC reference standard for quantifying process-related impurities in fexofenadine active pharmaceutical ingredient batches.