Structure–Activity Relationship of Oligomeric Flavan-3-ols: Importance of the Upper-Unit B-ring Hydroxyl Groups in the Dimeric Structure for Strong Activities

Proanthocyanidins, which are composed of oligomeric flavan-3-ol units, are contained in various foodstuffs (e.g., fruits, vegetables, and drinks) and are strongly biologically active compounds. We investigated which element of the proanthocyanidin structure is primarily responsible for this functionality. In this study, we elucidate the importance of the upper-unit of 4–8 condensed dimeric flavan-3-ols for antimicrobial activity against Saccharomyces cerevisiae (S. cerevisiae) and cervical epithelioid carcinoma cell line HeLa S3 proliferation inhibitory activity. To clarify the important constituent unit of proanthocyanidin, we synthesized four dimeric compounds, (−)-epigallocatechin-[4,8]-(+)-catechin, (−)-epigallocatechin-[4,8]-(−)-epigallocatechin, (−)-epigallocatechin-[4,8]-(−)-epigallocatechin-3-O-gallate, and (+)-catechin-[4,8]-(−)-epigallocatechin and performed structure–activity relationship (SAR) studies. In addition to antimicrobial activity against S. cerevisiae and proliferation inhibitory activity on HeLa S3 cells, the correlation of 2,2-diphenyl-l-picrylhydrazyl radical scavenging activity with the number of phenolic hydroxyl groups was low. On the basis of the results of our SAR studies, we concluded that B-ring hydroxyl groups of the upper-unit of the dimer are crucially important for strong and effective activity

The structure proposed for apteniol D is different from that of the compound obtained by total synthesis

<p>We describe the synthesis of 4,4′-oxyneolignan, the proposed structure for naturally occurring apteniol D. The diphenyl ether moiety in 4,4′-oxyneolignan was formed via classical Ullmann ether synthesis using excess copper powder in <i>N,N</i>-dimethylacetamide. The spectral data of synthesised apteniol D show differences compared to those of naturally occurring apteniol D.</p