The structure and synthesis of trimeric and related dimeric Proteracacinidins from Acacia heteroensis.

[摘要] English: The presence of 4', 7,8-trihydroxyl substituted flavanoids in plant material is limited to Acacia galpinii and Acacia caffra in Southern Africa. The existence of pyrogallol-type A-ring oligomers in these trees was for a long time believed not to be possible. During this investigation the presence of a number of dimeric and trimeric proteracacinidins in the heartwood of Acacia hereroensis were manifested and synthesized.Column separations of the extract resulted in the enriched complex phenolic fractions which subsequently were derivatized and the compounds isolated as the methyl ether acetate derivatives. Extensive 1H and 13C (300 MHz) work, which entailed COSY, NOESY, HMQC and HMBC experiments were conducted to enable structure elucidations.The monomeric leucoanthocyanidins epioritin-4α-ol, epioritin-4β-ol and oritin-4α-ol, are present in large quantities in the heartwood of A. hereroensis and were also accompanied by a variety of dimeric and trimeric proteracacinidins in lower concentrations. The two, by now well-known dimeric C-4 (C-ring) �?C-6 (D-ring) linked compounds, e.g. epioritin-(4β �?)-epioritin-4α -ol and epioritin-(4β �?)-epioritin-4β-ol, were isolated.The rare group of proteracacinidins with a C-4 (C-ring) �?C-6 (D-ring) interflavanyl bond was extended with the isolation of ent-oritin-(4α �?)-epioritin-4α-ol, ent-oritin- (4β �?)-epioritin-4α -ol, epioritin-(4β �?)-oritin-4α-ol and the novel promela- /proteracacinidin dimer viz. epimesquitol-(4β �?)-epioritin-4α-ol. The C-4 (C-ring) �?C- 4 (F-ring) ether linked dimer ent-oritin-(4α �?)-epioritin-4α -ol was isolated, together with the unique C-C and C-O-C linked trimer epioritin-(4β �?)-epioritin-(4β �?)- epioritin-4β-ol.The difficult and sensitive synthetic approach during which the 4β-benzylthioether derivatives, epioritin-4β-benzylthioether and ent-oritin-4α -benzylthioether together with their corresponding underivatized monomers, e.g. epioritin-4β-ol and epioritin-4α-ol as nucleophiles in the presence of DMTSF as thiophilic Lewis acid were used to synthesize ent-oritin-(4α �?4)-epioritin-4α-ol and epioritin-(4β �?3)-epioritin-(4β �?)-epioritin-4β-ol, using different reaction conditions. The use of DMTSF and AgBF4 as thiophilic Lewis acid catalysts were tested with the successful synthesis of the two dimers, epioritin-(4β �?)-epioritin-4α-ol and epioritin-(4β �?)-epioritin-4β-ol. Extensive use of these reactions were utilized to optimise the reaction conditions required during the other procedures.

[发布机构] University of the Free State