30 ノボタン科植物のエラジタンニンオリゴマーの構造(口頭発表の部)

抄録

In our continuing study on polyphenolic constituents of Melastomataceous plants, we have reported the isolation and structure determination of ellagitannin oligomers such as nobotanin K (1). A chromatographic survey of the tannins in this plant family revealed that Tibouchina multiflora and Monochaetum multiflorum, endemic to Colombia, are rich in ellagitannin oligomers. We report herein the structure elucidation of 16 new ellagitannins and polyphenols isolated from leaves of the above two plants. The aqueous acetone homogenates of dried leaves of T. multiflora and M. multiflorum were concentrated and extracted successively with Et_2O, EtOAc and n-BuOH, respectively. Each extract was submitted to column chromatography over Diaion HP-20, Toyopearl HW-40, MCI GEL CHP-20P and/or preparative HPLC to yield two new ellagitannins [nobotanins O and P (2)] along with 15 known tannins and related polyphenols from the former, and 41 ellagitannins and related polyphenols including 14 new compounds [benzyl 6'-O-galloyl-β-D-glucopyranoside, 4-O-(6'-O-galloyl-β-D-glucopyranosyl)-cis-p-coumaric acid, 6'-O-galloylprunasin, monochaetin (3), nobotanins Q(4), R, S(5), T(6), U and V, and melastoflorins A(7), B(8), C(9) and D(10)] from the latter. Sixteen new compounds including four ellagitannin tetramers (2, 4-6) and four pentamers (7-10) were characterized by spectroscopic (^1H-^1H COSY, TOCSY, ^1H J-resolved, DEPT, NOESY, HMQC and/or HMBC) and chemical methods. Although approximately 200 oligomeric ellagitannins have been found in nature to date, most of them were dimers, and the number of trimers-tetramers was limited. Melastoflorins A (7), B(8), C(9) and D(10) are the first examples of pentameric hydrolyzable tannins composed of different monomeric units (casuarictin, pterocaryanin C, and their derivatives) with C1 glucopyranose cores. The present study should prompt a further search for this class of higher oligomers in nature.