Synthesis of condensed tannins. Part 18. Stilbenes as potent nucleophiles in regio- and stereo-specific condensations: novel guibourtinidol-stilbenes from Guibourtia coleosperma
(E)-3,3′,4,5′-Tetrahydroxy- and 3′,4,5′-trihydroxy-stilbenes function as strong nucleophiles in regio- and stereo-specific condensations with carbenium ions generated from 2,3-trans- and 2,3-cis-flavan-3,4,4′,7-tetraols (leucoguibourtinidins) to generate those [4,2]-‘dimeric’ and [4,2 : 4,6]-‘trimeric’ proguibourtinidin oligomers which occur in Guibourtia coleosperma. Problems of structural analysis associated with dynamic rotational isomerism and exceptional energy requirements for the coalescence of 1H n.m.r. resonances have been surmounted by synthesis. The requisite 3,3′,4,5′-tetrahydroxystilbene was prepared via a modified Reimann procedure.