Ferrocenylmethylation of estrone and estradiol: Structure, electrochemistry, and antiproliferative activity of new ferrocene–steroid conjugates

Abstract

Conjugates of ferrocene with steroidal estrogens as selective antiproliferative agents against hormone‐dependent breast cancer cells are believed to be limited by the inherent estrogenicity of the conjugates. Motivated by a significant cytotoxicity of the ester of ferrocenecarboxylic acid and the phenolic group of estradiol toward such a cell line, we decided to explore other a‐ring‐tethered ferrocene–estra‐1,3,5(10)‐triene conjugates; in this study, ferrocenylmethylation of estradiol and estrone with (ferrocenylmethyl)trimethylammonium iodide in the presence of potassium carbonate yielded five new compounds (1–5). In dimethylformamide, only O‐alkylated products formed (1 and 3), while a mixture of O‐ and C‐alkylated products was obtained when methanol was used (2, 4, and 5 in addition to 1 and 3). All compounds were characterized using 1D and 2D NMR, IR, UV–Vis, and high‐resolution mass spectrometry. Two of the conjugates, a 3‐O‐ and a 4‐C‐alkylated derivative of estrone (3 and 4, respectively), were also analyzed using single‐crystal X‐ray diffraction. A cyclic voltammetric investigation of the electrochemical properties of 1–5 was performed. While some of the compounds were shown to have a slight‐to‐moderate antiproliferative activity against at least one of the six tested human tumor cell lines and were nontoxic to (the noncancerous) fetal human fibroblasts, compound 2 (4‐(ferrocenylmethyl)estra‐1,3,5(10)‐triene‐3,17β‐diol) with an IC50 value of 0.34 μM was found to be more active against the hormone‐dependent breast cancer cell line MCF‐7 than doxorubicin. These results suggest that a‐ring substitution of steroidal estrogens is a plausible strategy for preparing other ferrocene–steroid conjugates acting against tumor cells.