80(P-8) Procyanidin Oligomerの立体選択的合成研究(ポスター発表の部)

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Proanthocyanidins are a ubiquitous family of polyphenolic natural products and are characterized by the oligomeric structures including an interflavanyl bond between the C_4-C_8 or C_4-C_6 position of the tlavan-3-ol units. The biological activities of proanthocyanidins are very interesting in that they have powerful free-radical scavenging activity, antioxidant activity and anti-tumor-promoting effect. Proanthocyanidins have been obtained from many kinds of plants, but because they are usually obtained as a complex mixture of structurally related compounds, it is very difficult to isolate them in a pure form. Consequently, we planned to develop efficient synthetic methods leading to procyanidins with a high level of purity and stereoselectivity. First, we examined in detail the stereoselective synthesis of procyanidin-B3, (+)-catechin-(4→8)-(+)-catechin dimer. Condensation of benzylated catechin with various 4-O-alkylated flavan-3,4-diol derivatives as electrophiles in the presence of a Lewis acid led to protected procyanidin-B3 and its diastereomer. In particular, the reaction using (2R,3S,4S)-3-acetoxy-5,7,3',4'-tetrabenzyloxy-4-(2"-ethoxyethoxy)flavan 16 as an electrophile in the presence of TMSOTf at -78℃ afforded octa-O-benzylated procyanidin-B3 with high levels of stereoselectivity and in excellent isolation yields. In addition, we succeeded in a stereoselective gram-scale synthesis of benzylated procyanidin-B3 and also in synthesese of benzylated procyanidin-B1, B2 and B4 in excellent stereoselectivities and in good yields using the same method. And we obtained a condensed catechin trimer, procyanidin-C2 in a pure form using the same method as that described above. Thus, condensation of benzylated procyanidin-B3 with 16 as an electrophile in the presence of TMSOTf at 78℃ afforded a benzylated catechin trimer derivative in a good yield. The stereochemistry of this oligomer was determined compared with the stereochemistry of the procyanidin-C2 obtained by another synthetic method using benzylated 4'''-ethoxyethoxy-B3 derivative 21 as an electrophile. The synthesis of a condensed catechin tetramer was also achieved by the condensation of 21 as an electrophile in the presence of TMSOTf at 78℃. Determination of the stereochemistry of this tetramer was now underway. Further more, we developed the stereoselective synthesis of diastereomer of procyanidin-B3 with intramolecule condensation method. On the details of this experiment, the examination is being carried out now.
天然有機化合物討論会の論文
2002-09-01

80(P-8) Procyanidin Oligomerの立体選択的合成研究(ポスター発表の部)

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