CONFIGURATION OF GUIBOURTACACIDIN, AND SYNTHESIS OF ISOMERIC RACEMATES

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Abstract
Diacetates of the 4 possible racemates of 4[image], 7-dimethoxy-flavan-3, 4-diol were synthesized. 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[image], 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. The occurrence of gujbourtacacidins in Gulbourtia 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.