72(P-29) シキミ科植物(Illicium merrillianum)のアニスラクトン型セスキテルペンの構造と神経突起伸展活性(ポスター発表の部) 72(P-29) Structure and Neurotrophic Activity of Anislactone-type Sesquiterpenes from Illicum merrillianum
There are approximately 34 Illicium plants in the world, 24 of which are believed to be indigenous to China. A variety of seco-prezizaane-type sesquiterpenes have been found in the Illicium genus by Kouno, Schmidt and Yang since the structure of a neurotoxic anisatin was established. We have also been involved in chemical studies on the bioactive substances in I. tashiroi, and thus have turned our attention to chemicals in the pericarps of I. merrillianum indigenous to China. Extensive fractionation of the methanol extract of the title plant led to the isolation of four novel sesquiterpenes named merrilactone A (1), merrilactone B (2), merrilactone C (3) and 7-O-acetylanislactone B (4) along with the previously known anislactones A (5) and B (6). Merrilactone A was unambiguously determined as the rare structure having an oxetane ring in a molecule by X-ray crystallographic analysis and its absolute configuration was assigned by applying modified Mosher's method to the secondary hydroxyl group on the C-2 position. Additionally, merrilactone A was effectively synthesized from anislactone A (6), a main component of I. merrillianum. The structure of merrilactone B (2) was elucidated by extensive analysis of 2D NMR data and comparison of ^<13>C NMR data with those of anislactone B. In order to confirm the elucidated structure of 2, anislactone B was submitted for transesterfication under basic conditions giving rise to anislactone A (5), and the C-1 epimer mixtures of 2, but not giving 2. This transformation was rationally explained due to repeating retro-aldol and aldol-type reactions. Merrilactone C (3) was obtained as an equilibrated mixture, and thus was treated with diazomethane to afford a single crystaline, which was submitted to X-ray crystallographic analysis. As a result, 3 was established as the first anislactone-type sesquterpene having an intriguing lactol ring on the C-5 and C-6 positons. Taking some chemical feature of anilactones into consideration, we have proposed new biosynthetic route of anislactone-type sesquiterpenes, which should be derived from a common tricyclic intermdeiate F by the bond clevage between C-10 and C-11. This new route is not likely to be in conflict with the inversion of the C-9 configuration, but also can expain the origin of the C-8 methyl group. In fact, tashironin (12) and its debenzoate derivative which consist of the carbon skeleton corresponding to F had been isolated from the Illicum species. Merrilactone A (1) has been found to exhibit both potent neurite outgrowth promoting activity and neuroprotective activty at 0.01μM〜1μM in the primary cultures of 18 D fetal rat cortical neurons in the serum-free medium supplemented with B27.
天然有機化合物討論会講演要旨集 43(0), 425-430, 2001