Natural and synthetic diastereoisomeric (−)-3′,4′,7-trihydroxyflavan-3,4-diols

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Abstract
Rhodesian copalwood (Guibourtia coleosperma) contains three diastereo-isomeric leuco-fisetinidins. These consist of the (-)-2,3-cis-3,4-cis (2R,3R,4R) and (-)-2,3-cis-3,4-trans (2R, 3R4,S) 3[image],4[image],7-trihydroxyflavan-3,4-diols, and the third was shown to be a 2,3-trans-3,4-cid isomer by means of paper ionophoresis. There occurrence in similar proportions as tannin precursors also in the tropical hardwoods G. tessmannii and G. demeusii implies a close taxonomic relationship between these, and with G. coleosperma Epimerization of the natural (-)-3[image],4[image],7-trihydroxy-2,3-trans-flavan-3,4-trans-diol affords a mixture from which the (-)-2,3-cis-3,4-cis isomer was separated readily, but the (-)-2,3-trans-3,4-cis isomer was obtained with difficulty. These were formed by epimerization of the (-)-2,3-trans-3,4-trans isomer at C-2 and C-4, and at C-4, respectively.