Conformational Studies on (17α,20Z)-21-(X-Phenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17β-diols Using 1D and 2D NMR Spectroscopy and GIAO Calculations of ¹³C Shieldings

Differences in agonist responses of the novel estrogen receptor ligands (17α,20Z)-(p-methoxyphenyl)vinyl estradiol (1), (17α,20Z)-(o-α,α,α-trifluoromethylphenyl)vinyl estradiol (2), and (17α,20Z)-(o-hydroxymethylphenyl)vinyl estradiol (3) led us to investigate their solution conformation. In competitive binding assay studies, we observed that several phenyl-substituted (17α,20E/Z)-(X-phenyl)vinyl estradiols exhibited significant estrogen receptor binding, but with variation (RBA (1) = 20; RBA (2) = 23; RBA (3) = 140 where estradiol RBA = 100) depending on the phenyl substitution pattern. Because the 17α-phenylvinyl substituent interacts with the key helix-12 of the ligand binding domain, we considered that differences in the preferred conformation of 1−3 could account for their varying binding affinity. 2D NMR experiments at 500 MHz allowed the complete assignment of the ¹³C and ¹H spectra of 1−3. The conformations of these compounds in solution were established by 2D and 1D NOESY spectroscopy. A statistical approach of evaluating contributing conformers of 1−3 from predicted ¹³C shifts correlated quite well with the NOE data. The 17α substituents of 1 and 2 exist in similar conformational equilibria with some differences in relative populations of conformers. In contrast, the 17α substituent of 3 exists in a different conformational equilibrium. The similarity in solution conformations of 1 and 2 suggests they occupy a similar receptor volume, consistent with similar RBA values of 20 and 23. Conversely, the different conformational equilibria of 3 may contribute to the significant binding affinity (RBA = 140) of this ligand.