1H NMR Analyses, Shielding Mechanisms, Coupling Constants, and Comformations is Steroids Bearing Halogen, Hydroxy, Oxo Groups, and Double Bonds

Article abstract of DOI:10.1021/ja00310a046

The 1H NMR analyses of 16 5αH-androstanes and one progesterone analogue furnish shifts and coupling constants for the basic steroid skeleton and substituent-induced shifts (SIS) for oxo, hydroxy, and halogen groups as well as for a Δ double bond.It is shown how a single 2D experiment complemented by a NOE difference spectrum can lead to complete assignments even with the most complicated spin systems compirising, e.g., 29 strongly coupled protons within only 1 ppm; the accurancy of information from 2D techniques is evaluated by comparison to some 1D and computer-simulated spectra.On the basis of up to six simultaneously observable couplings, a special approach is used to scan the conformational space of particularly flexible parts.Intermediate conformations between half-chair and twist are obtained with a torsional C14-C15-C16-C17 angle of φ ca. 20 deg for the D ring with a sp² (17-oxo) carbon and of φ ca. 10 deg with only sp³ carbon atoms; the observed flat profiles, however, allow also for mixtures of different conformations, which is supported by MM2 calculations.For the Δ-3-oxo A ring, a sofa conformation is favored compared to a half-chair geometry.The observed shielding effects of heterosubstituents are partially at variance with the few earlier observations, which were mostly based on polysubstituted compounds.Classical shielding mechanisms were evaluated with the program SHIFT, based on force-field-minimized structures.Steric-induced shielding dominates in the hydrocarbon, leading to upfield shifts increasing with the number of 1,3-diaxial interactions.Linear electric-field effects predict, e.g., the shielding difference between equatorial and axial protons vicinal to C-Hal bonds and the deshielding observed for diaxial C-Hal/C-H bond arrangements.A combination of anisotropy and electric-field effects explains all shifts observed in the ketones with the exception of protons vicinal to C=O; a multilinear regression analysis leads to Δ_χ1^C=O = -36 (-27) and Δ_χ2^C=O = -24 (-21) (10^-3 cm³/molecule, old ApSimon values in parentheses); it is, however, demonstrated, that an analysis on the basis of NMR shifts alone leads to broad ranges of parameters.Parallels between 1H and 13C NMR shifts are drawn, particularly at γ and θ positions to C-Hal bonds.