Configuration of Guibourtacacidin, and Synthesis of Isomeric Racemates.

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.