New ferrocenic pyrrolo[1,2-a]quinoxaline derivatives: Synthesis, and in vitro antimalarial activity

Following our search for antimalarial compounds, novel series of ferrocenic pyrrolo[1,2-a]quinoxaline derivatives 1–2 were synthesized from various substituted nitroanilines and tested for in vitro activity upon the erythrocytic development of Plasmodiumfalciparum strains with different chloroquine-resistance status. The pyrrolo[1,2-a]quinoxalines 1 were prepared in 6–8 steps through a regioselective palladium-catalyzed monoamination by coupling 4-chloropyrrolo[1,2-a]quinoxalines with 1,3-bis(aminopropyl)piperazine or -methylamine using Xantphos as the ligand. The ferrocenic bispyrrolo[1,2-a]quinoxalines 2 were prepared by reductive amination of previously described bispyrrolo[1,2-a]quinoxalines 9 with ferrocene-carboxaldehyde, by treatment with NaHB(OAc)3. The best results were observed with ferrocenic pyrrolo[1,2-a]quinoxalines linked by a bis(3-aminopropyl)piperazine. Moreover, it was observed that a methoxy group on the pyrrolo[1,2-a]quinoxaline nucleus and no substitution on the terminal N-ferrocenylmethylamine function enhanced the pharmacological activity. Selected compounds 1b, 1f–h, 1l and 2a were tested for their ability to inhibit β-haematin formation, the synthetic equivalent of hemozoin, by using the HPIA (heme polymerization inhibitory activity) assay. Of the tested compounds, only 2a showed a β-haematin formation inhibition, but no inhibition of haem polymerization was observed with the other selected ferrocenic monopyrrolo[1,2-a]quinoxaline derivatives 1b, 1f–h and 1l, as the IC50 values were superior to 10 equivalents.

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