Ligands for the estrogen receptor, containing cyclopentadienyltricarbonylrhenium units

As model systems for estrogens labeled with technetium-99m that might be used as in vivo imaging agents for estrogen receptor (ER)-positive breast tumors, we have prepared and determined the ER binding affinity of a series of nonsteroidal and steroidal estrogens substituted with a cyclopentadienyltricarbonylrhenium unit. While this organometallic unit interfered with ER binding when it was tethered close to the ligand, those analogs in which it was attached through a 17 alpha-ethynyl link (provided that the link was not polar) showed high ER affinity.