The structure and synthesis of oligoflavanoids and oligostilbenes from cassia abbreviata

English: During the present study the acetone extract of the heartwood of Cassia abbreviata afforded the known flavan-f-ols, catechin, epicatechin, epiafzelechin, afzelechin and the new (2R,3S) guibourtinidol. This was the first natural occuring 4',7-dihydroxy substituted flavan-f-ol to be isolated. The protocol recently developed in our laboratories using asymmetric dihydroxylation of 1,3-diarylpropenes and subsequent acid-catalyzed cyclization was used to yield the four diastereomers and amongst them the one isolated from C. abbreviata. Proguibourtinidin dimers were identified as the permethyl ether acetates namely the four known dimers only isolated now for the second time, guibourtinidol-( 4α →8)-catechin, - epicatechin, -afzelechin and epiafzelechin. (2S,3R,4R)-Guibourtinidol-( 4 β →8)- afzelechin and (2R,3S,4S)-guibourtinidol-(4α →6)-afzelechin are two new dimers from the heartwood. With extensive NOESY and COSY NMR-experiments it was possible not only to elucidate the structures and the stereochemistry of all the proguibourtinidin dimers 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 small differences in their 1H NMR spectra, l3C NMR spectra, mass spectral fragmentation pattern 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 and MS 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 cyclobutane compound was also detected in the acetone extract. Partnering the stilbene were the two bibenzyls namely 3,3',4',5-tetrahydroxy- and 3,4',5-trihydroxydihydrostilbene. Extensive investigative attempts were conducted to synthesize the trimeric procassinidin, cassiaflavan-( 4→8)-epigallacatechin-( 4→6)-cassiaflavan isolated from Cassia petersiana. From the model reactions executed excellent control regarding regio- and stereoselectivity during C4→C8 formation and good yields were obtained in the synthesis of different cassiaflavan-flavan-3-o1 dimers. However, when the same procedure was appplied to accomplish the C4→C6 coupling between the top dimeric unit and the bottom cassiaflavan, it was not successful.