Synthesis and characterization of N-demethylated metabolites of malachite green and leucomalachite green

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 H-1 and C-13 spectral assignments of the metabolites were aided by the availability of C-13(7)-labeled MG and LMG. The results indicate the existence of a resonance structure with the cationic charge located in the central methane carbon (C-7). The synthetic procedure is general in scope so that it can be extended to the preparation off-demethylated metabolites of other structurally related N-methylated triphenylmethane dyes.