STEREOCHEMICAL AND C-13 NMR SPECTROSCOPICAL INVESTIGATIONS .25. CONFORMATIONAL RELAXATION AS LIMITATION OF CHEMICAL-MODELS - EMPIRICAL FORCE-FIELD CALCULATIONS AND C-13 NMR SHIELDING EFFECTS FOR SOME CYCLOHEXANES, BICYCLO[2.2.1]HEPTANES, BICYCLO[3.3.1]NONANE, AND 11 BETA-SUBSTITUTED ESTRENES

Molecular mechanics calculations on the title compounds demonstrate the redistribution of steric effects of conceptual single origin over the whole molecule. Sterically induced substituent effects on 13C NMR shifts are obtained as the sum of up to ten single forces; use of nonrelaxed structures leads to gross overestimations of the interactions. A potential surface comparison between bicyclo[3.3.1]nonane and cyclohexane reveals that introduction of the bridge into cyclohexane rather extends than limits the number of conformations with similar energy in the chair inversion transition state. Considerable differences are found between published X-ray and force field derived structures of estrene derivatives, although the reflex angle between diaxial methyl groups is similar and comparable to that in isolated cyclohexanes. A potential surface calculation shows that both the C-ring distortion and the skeleton curvature brought about by axial substituents on the steroidal β side induce little strain energy variation in comparison to the binding energy to steroid hormone receptors. © 1979, American Chemical Society. All rights reserved.