MOLECULAR-STRUCTURES, CONFORMATIONAL-ANALYSIS, AND PREFERENTIAL MODES OF BINDING OF 3-AROYL-2-ARYLBENZO[B]THIOPHENE ESTROGEN-RECEPTOR LIGANDS - LY117018 AND ARYL AZIDE PHOTOAFFINITY-LABELING ANALOGS

Structural and computational modeling studies were performed on the antiestrogen LY117018 (3) and two photoaffinity labeling analogs, in which an azide replaces the basic ether side chain (methyl ether tetrafluoro azide 7 and its protio analog 8). These studies were undertaken in order to determine the conformational preferences of these compounds and to propose favorable orientational modes for their binding to the estrogen receptor. In the crystallographic studies, we found that, unlike tetrafluoro azide 7, which adopts a face-to-face stacking of the p-hydroxyphenyl and benzoyl groups in the solid state, the pendant rings in the corresponding protio analog 8 are found in a predominantly offset pi-stacked array. In LY117018, which has an ether on the benzoyl ring, stacking of the pendant rings does not occur in the crystal structure; it assumes a T-shape, with the benzoyl group oriented perpendicular to the benzo[b]thiophene nucleus. In modeling studies, analogs of LY117018, 7, and 8 were subjected to a conformational grid search by molecular mechanics, and for each compound, three low-energy conformers (and their atropisomers) were obtained. These conformers were further geometry optimized by semiempirical molecular orbital calculations. For each compound, one of the three minimum-energy conformers is quite similar to the solid-state geometry. The computational structure of the tetrafluoro azide showed the greatest stacking between the benzoyl group and the p-methoxyphenyl ring, but less stacking than was observed in the crystallographic structure. The orientational preferences of these benzo[b]thiophene ligands with the estrogen receptor were analyzed with the receptor volume mapping technique, a method based on the correspondence of the hydroxyl groups and the volume that the benzo[b]thiophene compound shares with a composite molecular volume of high-affinity estradiol-type ligands (the receptor excluded volume, RExV). If the benzo[b]thiophene nucleus is overlapped with the steroid AB rings, the best overlap with the RExV is achieved, but there is poor correspondence of the hydroxyl groups. An orientation and conformation in which the benzoyl group of the 3-benzoyl-2-aryl-benzo[b]thiophenes occupies a 7 alpha-like position relative to the steroid produces both ample volume overlap with the RExV and close approximation of the hydroxyl groups and is presented as the putative bioactive conformation.