Toshichika Sato

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

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)(ΔGc‡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.