88(P-24) タイ産アカネ科植物Uncaria attenuata含有新規オキシインドールアルカロイド類の合成研究(ポスター発表の部)

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A number of indole and oxindole alkaloids have been isolated from the Uncaria plants (Rubiaceae) and found to possess pharmacologically significant activities. The chemical investigation of Uncaria attenuata Korth in Thailand was first carried out by UK-Thai researchers, resulting in the isolation of many Corynanthe- and heteroyohimbine-type indole and oxindole alkaloids. By the chemical re-investigation of this plant, we found novel types of oxindole alkaloids, i.e., Us-7 (1), Us-8 (2), and Us-9 (3a, 3b). Their structures have been deduced based on spectroscopic analysis and biogenetic speculation. In order to elucidate the structures unambiguously, we carried out the synthesis of these alkaloids. I. Based on the biogenetic speculation, Us-7 (1) and -8 (2), D-seco Corynanthe-type oxindole alkaloids, would be formed from normal corynanthe-type compounds through an oxidative cleavage at the enamine double bond at the C_<20>-C_<21> positions. According to this hypothesis, the stereochemistry at the C15 is most likely to be R. To elucidate the structures including the absolute stereochemistry at C15, we planned the asymmetric total synthesis of Us-7 (1) and -8 (2). Starting from (S)-glycidol (4), a chiral (R)-γ-butenolide (10) was prepared via 7 steps reactions, which was then converted to a key intermediate (14), corresponding to the monoterpenoid part of the target molecules, through a stereoselective addition of a malonate derivative (11) to the butenolide (10). Condensation of 14 with tryptamine (15) gave the lactam (16), which was transformed to the tetrahydro-β-carboline derivatives (20a, 20b) via Bischler-Napieralski reaction and N_b-formylation. The structure of 20a was determined by X-ray crystallographic analysis. The thus obtained indoles were respectively converted to oxindole derivatives. Their stereochemistry at C7 was elucidated by the comparison of CD spectra as well as NOE experiments. The final stage, i.e., hydrolysis of the acetyl ester at C20 and oxidation of the resulting secondary hydroxy group leading to Us-7 (1) and -8 (2), is in progress. II. Us-9 (3a, 3b) was deduced to be 3-oxo-7-hydroxy-3,7-secorhynchophylline by spectroscopic analysis. Based on the biogenetic speculation, Us-9 was prepared from isorhynchophylline via modified Polonovski reaction of its N-oxide and then the introduction of a hydroxy group at the C7. Two diastereomers ascribable to C7 of the natural and semi-synthetic (3a, 3b) were separated by chiral column chromatography. The absolute configulation at C7 was elucidated by comparison of the CD spectra with those of the known oxytryptophan derivatives (28a, 28b), which were prepared from L-tryptophan by Labroo's procedure.
天然有機化合物討論会の論文
2002-09-01

88(P-24) タイ産アカネ科植物Uncaria attenuata含有新規オキシインドールアルカロイド類の合成研究(ポスター発表の部)

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