18 オタカラコウの成分Liguloxide,LiguloxidolおよびLiguloxidol acetateの構造

抄録

Three closely related sesquiterpenes, liguloxide, liguloxidol and liguloxidol acetate, were isolated from the aerial part of Ligularia Fischeri Turcz. On the basis of spectral evidences and chemical transformations, such as dehydrogenation of liguloxide to S-guaiazulene and conversion of liguloxidol to a keto-acid 9, the plane structures 1, 2 and 3 were assigned to liguloxide, liguloxidol and liguloxidol acetate, respectively. The stereoformulae 12, 13 and 14 were given to these three compounds from the following observations: (a) BF_3 treatment of 10-epiliguloxide gave (+)-guaiol. (b) The i.r. spectrum of liguloxidol taken in dilute solution showed the presence of strong intramolecular hydrogen bonding (v_<max>. 3472cm.^<-1>). (c) o.r.d. determination of a ketone 4 showed a negative Cotton effect curve(a=-76). (d) LAH reduction of the ketone 4 predominantly produced liguloxidol. However, these assignments have proved to be incorrect in connection with structure elucidation of guaioxide: we have recently confirmed the absolute configuration of guaioxide, a constituent of guaiac wood oil, to be represented by the formula 16, and have found that 4-epiguaioxide 34 corresponds to liguloxide. According to this finding and the observation (b), the structures of liguloxidol and its acetate should be revised to 36 and 37 respectively. The observation (a) is compatible with the new structure 35 for 10-epiliguloxide, because it becomes reasonable to consider that there had occurred inversion of the configuration at C-7 during the transformation of this compound to (+)-guaiol. The other observations (c) and (d) also explain satisfactorily all the revised structures.