Ferrocene-based anilides: synthesis, structural characterization and inhibition of butyrylcholinesterase

Twenty-three compounds in two series of ferrocene-based anilides, with the general formula C5H5-Fe-C5H4-C6H4-NH-CO-C6H4-R (where R = H, F, Cl, CH3 and OCH3), have been successfully synthesized. The compounds were characterized by elemental analysis and FTIR, 1H NMR and 13C NMR spectroscopy. Two compounds (M07 and P09) were characterized by X-ray crystallography. Solid state studies indicate that ferrocene derivatives with the conformation of meta amide substituents engage in intermolecular H-bonding, which stabilizes the meta derivatives over their para analogues. The H-bonding takes place when the conformation of the ferrocene changes by rotation around the C-N bond, favoring interactions between two molecules in adjacent layers in the solid state. The potential importance of this H-bonding to the biological effects of these molecules was investigated using both experimental and computational studies. All the compounds were found to inhibit butyrylcholinesterase. The most active compound shows 50% inhibition at a concentration of 9 ± 0.2 μM, similar to the known drug galantamine (with an IC50 of 8 μM). Compounds with the ferrocene moiety meta to the amide linkage were consistently found to be slightly more active than the other structural isomers, suggesting that the H-bonding may only slightly increase the overall affinity for the protein. Computational studies confirmed the limited effects of the H-bonding in the presence and absence of water in the active site of butyrylcholinesterase, supporting the importance of hydrophobicity for inhibitors of this enzyme.