Configuration of guibourtacacidin, and synthesis of isomeric racemates |
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Authors: | H M Saayman and D G Roux |
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Institution: | Leather Industries Research Institute, Rhodes University, Grahamstown, South Africa |
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Abstract: | 1. Diacetates of the four possible racemates of 4′,7-dimethoxyflavan-3,4-diol have been synthesized. 2. Comparison of their nuclear-magnetic-resonance spectra and the ionophoretic mobilities of the diols in borate buffer with those of the corresponding derivatives of guibourtacacidin shows that the natural 4′,7-dihydroxyflavan-3,4-diol has a 2,3-cis–3,4-trans configuration, but is accompanied by 2,3-trans–3,4-trans and 2,3-trans–3,4-cis isomers. These occur in the approximate proportions 5:1:1. 3. The occurrence of guibourtacacidins in Guibourtia coleosperma appears to be of taxonomic significance. Their association with a large excess of related tannins in the heartwood suggests that flavan-3,4-diols with these configurations are suitable precursors in tannin biosynthesis. |
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