Substituted methyl 5β‐cholan‐24‐oates. I—¹⁷O NMR spectral characterization

Abstract

Abstract Methyl esters of four common bile acids, 3α‐hydroxy‐5‐β‐cholan‐24‐oic (lithocholic) acid, 3α, 7α‐dihydroxy‐5β‐cholan‐24‐oic (chenodeoxycholic) acid, 3α,12α‐dihydroxy‐5β‐cholan‐24‐oic (deoxycholic) acid and 3α,7α,12α‐trihydroxy‐5β‐cholan‐24‐oic (cholic) acid, and 14 acetylated, trifluoroacetylated, mesylated and oxo derivatives of methyl 5β‐cholan‐24‐oates were prepared and their 17 O NMR spectra recorded. In spite of their relatively high molecular masses and the rigid molecular structure of the steroid skeleton, most of the oxygens included in these structures gave well resolved 17 O NMR resonance lines at natural abundance in 0.25–0.5 M acetonitrile solutions at 75°C. In agreement with the present 17 O NMR results, molecular mechanics calculations revealed that a hydroxy substituent located at the 3α‐position clearly differs from the hydroxyls at the 7α‐ and 12α‐positions. This is due to the fact that the 3α‐hydroxyl possessing only two γ‐carbons at antiperiplanar positions is less shielded than the other hydroxyls influenced also by the shielding effects of γ‐ gauche carbons. The spectral deconvolution of the overlapping signals of the 7α‐ and 12α‐hydroxyls is based on a computer‐aided method or on chemical substitutions. The oxo groups located at the longitudinal (3‐oxo) vs. transversal (7‐ and 12‐oxo) axes of the steroid framework show very different quadrupolar relaxation properties and 17 O NMR linewidths owing to the strong anisotropy of overall molecular motion. In contrast, the 17 O NMR linewidths of all 3α‐, 7α‐ and 12α‐hydroxyls are very similar and clearly smaller than those of the corresponding oxo groups, revealing that their quadrupolar relaxation is merely determined by their internal rotation rather than by the overall molecular motion.