New Type of Ferrocene‐Betulonic Acid Hybrids. Bioactivity, Synthesis, Structure, IR and CD versus DFT Calculations

Abstract Ferrocene‐modified natural compounds are of interest for the development of new pharmaceuticals due to improved bioavailability, increased bioactivity, and also as redox markers or vectors. Betulin is a natural plant‐derived product with a wide range of pharmacological properties. However, it is limited in its bioavailability and efficacy. In order to enhance its pharmacological profile, the ferrocene modification of betulin was used in this study. A hybrid of the betulin core (up to 30 % betulin in birch bark) with a ferrocene moiety combined through a piperazine linker (compound 12 ) was prepared in good yield from readily available ferrocene carboxylic acid and betulonic acid, a natural betulin derivative, via coupling reactions. Spectroscopic techniques, including 1 H and 13 C NMR, IR and UV‐vis spectroscopy, as well as circular dichroism (CD), were applied to characterize the physicochemical and chiroptical properties. The cytotoxicity of the synthesized hybrid 12 and its ferrocene precursor 13 on MCF‐7 human cancer cell lines were in vitro evaluated by the MTT assay. The IC 50 cytotoxicity index of 12 was found to be 26 μM. The theoretically calculated energies of the HOMO‐LUMO orbitals and electronic characteristics of betulin 10 , betulonic acid 11 , hybrid 12 and ferrocenyl piperazine 13 were determined by the DFT method with hybrid functionals ωB97XD, LC‐ωPBE and B3LYP‐D3 and pure M06 L functionals, with a 6‐311+G(d), 6‐31G(d) or TZV basis set.