Synthesis and characterization of N-demethylated metabolites of malachite green and leucomalachite green
Date of Original Version
Malachite green (MG), a triphenylmethane dye used to treat fungal and protozoan infections in fish, undergoes sequential oxidation to produce various N-demethylated derivatives (monodes-, dides(sym)-, dides(unsym)-, trides-, and tetrades-) both before and after reduction to leucomalachite green (LMG). The close structure resemblance of the metabolites with aromatic amine carcinogens implicates a potential genotoxicity from exposure to MG. The availability of the synthetic standards is important for metabolic and DNA adduct studies of MG. This paper describes a simple and versatile method for the synthesis of MG, LMG, and their N-demethylated metabolites. The synthesis involves a coupling of 4-(dimethylamino)-benzophenone or 4-nitrobenzophenone with the aryllithium reagents derived from appropriately substituted 4-bromoaniline derivatives, followed by treatment with HCl in methanol. The resulting cationic MG and their leuco analogues showed systematic UV/vis spectral and tandem mass fragmentation patterns consistent with sequential N-demethylation. The extensive 1H and 13C spectral assignments of the metabolites were aided by the availability of 13C7-labeled MG and LMG. The results indicate the existence of a resonance structure with the cationic charge located in the central methane carbon (C7). The synthetic procedure is general in scope so that it can be extended to the preparation of N-demethylated metabolites of other structurally related N-methylated triphenylmethane dyes.
Chemical Research in Toxicology
Cho, Bongsup P., Tianle Yang, Lonnie R. Blankenship, Joanna D. Moody, Mona Churchwell, Frederick A. Beland, and Sandra J. Culp. "Synthesis and characterization of N-demethylated metabolites of malachite green and leucomalachite green." Chemical Research in Toxicology 16, 3 (2003): 285-294. doi:10.1021/tx0256679.