Abstract
Our study on neurotrophic substances in the metanol extract of a medicinal plant, Magnolia obovata THUNB. led to the isolation of a number of novel sesquiterpene-neolignans named eudesobovatols A (1) and B (2), eudeshonokiols A (3) and B (4), clovanemagnolol (5), and caryolanemagnolol (6). These new compounds belong to unique class of natural products made up of sesquiterpene and biphenyl-type neolignan via an ether bond. The structures of eudesobovatols A (1) and B (2), eudeshonokiols A (3) and B (4) were elucidated on the basis of the extensive spectroscopic analyses and confirmed by identifying (+)-γ-eudesmol as terpene units, and obovatol and honokiol corresponding to respective aromatic units produced on treatment of CF_3COOH. Clovanemagnolol (5) was clarified to have the structure consisted of tricyclic sesquiterpene clovanediol and magnolol linked through an ether bond by 2D-NMR, in particular, HMBC and MS spectra and its tentative structure was established by a biomimetic synthesis starting from caryophyllene β-oxide and magnolol, whereas caryolanemagnolol was assigned as the structure of 2 based on a combination of various 2D-NMR methods and is most likely to be biosynthesized from caryophyllene α-oxide and magnolol. Compound 1, 5, and 6 exhibited interesting neurotrophic properties on neuronal cell culture system of fetal rat cerebral hemisphere, which not only accelerate neurite sprouting but also enhance choline acetyltransferase activity at 10^<-5>〜10^<-7>M.
