The ferrocenylethynyl unit: A stable hormone tag

We prepared the organometallic complex 17α-(ferrocenylethynyl)estradiol (= [(3,17β- dihydroxyestra-1,3,5(10)-trien-17α-yl)ethynyl]ferrocene; FcEE; 1) by a novel synthetic method. This metallocene possesses sufficient stability in aqueous media to permit the study of its biological properties. Thus, we were able to show that, despite the addition of a bulky substituent at the 17α position of the steroid, the metallocene is still well-recognized by an antibody specific to estradiol (CR = 40%) and by both subtypes (ERα, ERβ) of the estrogen receptor (at 0°, RBA = 28 and 37%, resp.). A DCI-MS study of the stability of the carbocation [FcEE - OH]⁺ showed moderate stabilization of the carbocation, in agreement with the pK_R^- value of -0.72 found for the metallocene by means of Deno's method. The presence of the ferrocene allows the electrochemical detection of FcEE (1) by HPLC-ED, with a detection limit of ca. 1 nM, suitable for quantitative pharmacological analysis.