Comformational Analysis of Nonsteroidal Estrogens. The Effect of Conformer Populations on the Binding Affinity of Meso- and Dl-hexestrol to the Estrogen Receptor

The conformational preferences of three nonsteroidal estrogens, meso-hexestrol, dl-hexestrol, and 1,2-bis(4-hydroxyphenyl)butane (isobutestrol), have been studied by proton nuclear magnetic resonance spectroscopy (¹H-NMR) and force field calculations (on model compounds), and have been used to rationalize the strikingly different binding affnities of these compounds for the uterine estrogen receptor. The chemical shifts for the methyl and methylene resonances in meso-hexestrol appear at field strengths 0.2 ppm higher than those of the related diastereomer dl-hexestrol. This shielding effect suggests that meso-hexestrol exists predominantly in the antiperiplanar conformation and dl-hexestrol predominantly in synclinal conformations. The lack of a shielding effect in isobutestrol suggests that it too prefers a synclinal conformation. Analysis of the vicinal coupling constants of the benzylic protons in two diastereometric hexestrol analogues leads to similar predictions of conformer population and, together with the chemical shift data, suggests that the (-)-synclinal conformer may be the major cntributor in dl-hexestrol. Force field calculations that were done on meso- and dl-3,4-diphenylhexane and 1,2-diphenylbutane have confirmed the conformational preferences suggested by ¹H-NMR: the two synclinal conformations of meso-3,4-diphenylhexane lie 1.29 Kcal/mole above the antiperiplanar conformer, and the (+)-synclinal and antiperiplanar conformers lie above the (-)-synclinal conformation of dl-3,4-diphenylhexane by 0.79 Kcal/mole and 3.01 Kcal/mole, respectively. The (+)-synclinal comformation is preferred for 1,2-diphenylbutane, with the antiperiplanar and (-)-synclinal being more energetic by 0.73 Kcal/mole and 1.27 Kcal/mole, respectively. The binding affinities of the hexestrol diastereomers for the uterine estrogen receptor, measured relative to that for estradiol (100%), are 300% for the meso and 3.2% for the dl form. The ratio of receptor binding affinities for the two hexestrol diastereomers is comparable to the ratio of population of the antiperiplanar conformers of the diphenyl hexane diastereomers. This suggests a model in which the receptor binds only the antiperiplanar conformation of each diastereomer and does not distinguish between different configurations at the benzylic centers. The intermediate affinity of isobutestrol (25%) is thought to reflect both its lower lipophilicity and its intermediate preference for the antiperiplanar conformation.