Substituted Methyl 5-BETA-Cholan-24-oATES .1. 0-17 NMR Spectral Characterization

Methyl esters of tour common bile acids, 3 alpha-hydroxy-5 beta-cholan-24-oic (lithocholic) acid, 3 alpha,7 alpha-dihydroxy-5 beta-cholan-24-oic (chenodeoxycholic) acid, 3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-oic (deoxycholic) acid and 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholan-24-oic (cholic) acid, and 14 acetylated, trifluoroacetylated, mesylated and ore derivatives of methyl 5 beta-cholan-24-oates were prepared and their O-17 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 O-17 NMR resonance lines at natural abundance in 0.25-0.5 M acetonitrile solutions at 75 degrees C. In agreement with the present O-17 NMR results, molecular mechanics calculations revealed that a hydroxy substituent located at the 3 alpha-position clearly differs from the hydroxyls at the 7 alpha- and 12 alpha-positions. This is due to the fact that the 3 alpha-hydroxyl possessing only two gamma-carbons at antiperiplanar positions is less shielded than the other hydroxyls influenced also by the shielding effects of gamma-gauche carbons. The spectral deconvolution of the overlapping signals of the 7 alpha- and 12 alpha-hydroxyls is based on a computer-aided method or on chemical substitutions. The oxo groups located at the longitudinal (fore) vs. transversal (7- and 12-oxo) axes of the steroid framework show very different quadrupolar relaxation properties and O-17 NMR linewidths owing to the strong anisotropy of overall molecular motion. In contrast, the O-17 NMR linewidths of all 3 alpha-, 7 alpha- and 12 alpha-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.