木棉根皮之化學成分研究

Abstract

木棉(Bombax malabarica DC.)屬於木棉科(Bombacaceae)木棉屬(Bombax L.)，原產地在印度、馬來西亞、及菲律賓等地，引進臺灣後已本土化成為本島常見的大型落葉喬木。木棉的花、樹皮、及根部常用於民間用藥中，而過去的科學研究也顯示木棉含有許多具生物活性的化合物，基於對天然物研究的興趣，以及希望發掘具生物活性之成分，我們利用木棉根皮為材料，進行了一系列的分離、純化、及結構鑑定等工作。 在實驗過程中，首先將酒精萃取物依極性劃分為二氯甲烷、正丁醇、和水可溶的三部分，經活性試驗發現具抑制甲型葡萄糖水解酶 (α-glucosidase) 功能之成分集中在正丁醇可溶部分，因此以Sephadex LH-20、Lobar RP-18、及半製備型HPLC等對正丁醇可溶部分進行分離，最後得到六個不同類型的化合物，分別是shorealactone (1)、adenosine (2)、(-)-epicatechin 5-O-β-D-xylopyranoside (3)、2-C-[β-D- apiofuranosyl-(1 → 6)]-β-D-glucopyranosyl]-1, 3, 6-trihydroxy-7-methoxyxanthone (4)、3, 4, 5-trimethoxyphenol-1-(6-xylopyranosyl) glucopyranoside (5)、及4-hydroxy-3, 5-dimethoxybenzoic acid (6)。二氯甲烷可溶部分經過相似的分離程序也得到四個化合物，分別是lupeol (7)、β-sitosterol (8)、isohemigossypol-1-dimethyl ether (9)、及化合物10。上述化合物使用包括核磁共振儀(NMR)、質譜儀(MS)等儀器進行結構解析。 在這十個化合物中，我們針對從正丁醇部分分出的化合物及化合物10做了β-glucosidase assay，僅化合物1、3、4對α-glucosidase有弱抑制活性，其餘均無活性。此外，化合物1推測是由bis-stilbenoid及ascorbic acid經植物酵素催化合成，經資料庫搜尋，發現此化合物為相當少見的化合物，首次發表是在2003年從Shorea hemsleyana樹皮中分離出來，本實驗則是第二次發現該化合物。有趣的是，S. hemsleyana與B. malabarica在傳統植物分類中屬不同科，化合物1的發現意味這兩種植物在化學分類法中有著某種程度的關聯。除此之外，在仔細地分析核磁共振光譜後，發現化合物10為新天然物，其部份架構與isohemigossypol-1-dimethyl ether (9)相似。

Bombax malabarica DC. (Bombacaceae), commonly known as silk cotton tree, is native to India, Malaya, and Philippines, and has been naturalized in Taiwan. Its flower, stem bark, and root are common materials for folklore medical use. Past studies had led to the isolation of some bioactive constituents. Being interested in the exploration of bioactive natural ingredients, we reinvestigated this plant. The EtOH extract of the root bark of B. malabarica was divided into fractions soluble in CH2Cl2, n-BuOH, and H2O by liquid-liquid partitioning. Through α-glucosidase assay, we observed that the n-BuOH-soluble fraction showed strong inhibitory effects. Therefore, n-BuOH-soluble fraction was chromatographed over Sephadex LH-20, Lobar RP-18, and semi-preparative HPLC to give six compounds. They are shorealactone (1), adenosine (2), (-)-epicatechin 5-O--D-xylopyranoside (3), 2-C-[β-D-apiofuranosyl-(1→ 6)]-β-D-glucopyranosyl]-1, 3, 6-trihydroxy-7- βmethoxyxanthone (4), 3, 4, 5-trimethoxyphenol-1-(6-xylopyranosyl) glucopyranoside (5), and 4-hydroxy-3, 5-dimethoxybenzoic acid (6). Four compounds, i.e., lupeol (7), β-sitosterol (8), isohemigossypol-1-dimethyl ether (9), and compound 10, were isolated from the CH2Cl2-soluble fraction in a similar manner. The structures of these compounds were elucidated based on elaborated spectroscopic analysis. The compounds isolated from the n-BuOH-soluble fraction and compound 10 had been assayed against α-glucosidase. Only compound 1, 3, and 4 showed weak inhibitory effects. Of these compounds isolated, shorealactone (1) is an adduct of bis-stilbenoid and ascorbic acid, and represents the second appearance of such compound from the natural sources, the first one being isolated from Shorea hemsleyana in 2003. Interestingly, both species belong to different plant families. This discovery may imply a certain relationship between S. hemsleyana and B. malabarica on the basis of chemical taxonomy. In addition, compound 10 is a new natural product, possessing an unsaturated moiety the same with isohemigossypol-1-dimethyl ether (9).