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.