The structure and synthesis of oligoflavanoids and oligostilbenes from cassia abbreviata

[摘要] English: During the present study the acetone extract of the heartwood of Cassia abbreviataafforded the known flavan-f-ols, catechin, epicatechin, epiafzelechin, afzelechin and thenew (2R,3S) guibourtinidol. This was the first natural occuring 4',7-dihydroxy substitutedflavan-f-ol to be isolated. The protocol recently developed in our laboratories usingasymmetric dihydroxylation of 1,3-diarylpropenes and subsequent acid-catalyzedcyclization was used to yield the four diastereomers and amongst them the one isolatedfrom C. abbreviata.Proguibourtinidin dimers were identified as the permethyl ether acetates namely the fourknown dimers only isolated now for the second time, guibourtinidol-( 4α �?)-catechin, -epicatechin, -afzelechin and epiafzelechin. (2S,3R,4R)-Guibourtinidol-( 4 β �?)-afzelechin and (2R,3S,4S)-guibourtinidol-(4α �?)-afzelechin are two new dimers fromthe heartwood. With extensive NOESY and COSY NMR-experiments it was possible notonly to elucidate the structures and the stereochemistry of all the proguibourtinidindimers but also the preferred conformations of the two rotamers of each compound.The assignment of the structures and configuration of the two dimeric stilbenes(cyclobutane derivatives) required a special effort due to the similarity and very smalldifferences in their 1H NMR spectra, l3C NMR spectra, mass spectral fragmentationpattern and the inability to grow crystals suitable for X-ray studies.From the synthesis using photodimerization of the monomenc 3,3',4',5-tetramethoxystilbene and limited structural information from the literature, 1H NMR andMS it was possible to present structures for the two dimeric 3,3',4',5-tetramethoxystilbenes as α-trixillic and β-truxinic configurations.The monomeric 3,3',4',5-tetrahydroxystilbene, the assumed precursor for the cyclobutanecompound was also detected in the acetone extract. Partnering the stilbene were the twobibenzyls namely 3,3',4',5-tetrahydroxy- and 3,4',5-trihydroxydihydrostilbene.Extensive investigative attempts were conducted to synthesize the trimeric procassinidin,cassiaflavan-( 4�?)-epigallacatechin-( 4�?)-cassiaflavan isolated from Cassiapetersiana. From the model reactions executed excellent control regarding regio- andstereoselectivity during C4→C8 formation and good yields were obtained in thesynthesis of different cassiaflavan-flavan-3-o1 dimers. However, when the sameprocedure was appplied to accomplish the C4→C6 coupling between the top dimeric unitand the bottom cassiaflavan, it was not successful.

[发布机构] University of the Free State