Conformational equilibria of eremophilanes, naturally occurring cis-decalin derivatives, studied by n.m.r. and c.d. spectroscopy

Abstract

Variable-temperature ¹H and ¹³C n.m.r. spectra of ligularol (1), 6-epi-ligularol (3), ligularone (4), isoligularone (5), and fukinone (6) have demonstrated that these compounds exist in solution as two conformational isomers in equilibrium, the so-called ‘steroid-like’ and ‘non-steroid-like’ conformers. The conformations of the major and minor isomers, frozen on the n.m.r. time scale at low temperatures, have been determined by ¹H n.m.r. spectral analyses. The ratios of the conformational isomers at equilibrium for the alcohols (1) and (3) are considerably affected by concentration and solvent, probably because of intermolecular hydrogen-bonding. The stable isomer adopts predominantly the ‘non-steroid-like’ conformation for compound (4) and ligularol acetate (2), but the ‘steroid-like’ conformation for compounds (5) and (6). Gibbs free-energy of activation has been determined for the conformational changes of compounds (1), (3), and (4)(ΔG_c‡ca. 46,51, and 52 KJ mol ^–1, respectively) from the signal-coalescence temperatures of the ¹³C dynamic n.m.r spectra in [²H₆]acetone. All the ¹³C n.m.r. signals have been assigned and the ¹³C n.m.r. chemical-shifts of the two conformers are compared. Variable-temperature c.d. spectra are also examined for the ketones (4)–(6) and 10β-hydroxyligularone (7). Temperature-dependent Cotton effects showed the same conformational equilibria as obtained by n.m.r. spectroscopy, and 10β-hydroxy-ligularone is assumed to have mainly the ‘steroid-like’ conformation.