Stereochemistry and biogenesis of mopanols and peltogynols and associated flavanoids from Colophospermum mopane
(+)-Mopanol and its 4-epimer (mopanol B) from mopane heartwood are shown to have the same relative and absolute configurations as the accompanying isomeric and identically related pair, (+)-peltogynol and peltogynol B. The relative configurations of the peltogynols (2,3-trans-3,4-trans and 2,3-trans-3,4-cis, respectively) are confirmed by nuclear magnetic resonance spectroscopy of their derivatives. The absolute configurations of (+)-mopanol and (+)-peltogynol are revised to 2R,3S,4R, following comparison of (+)-mopanone with (+)-fustin by optical rotatory dispersion methods. Accordingly, (+)-mopanol B and (+)-peltogynol B have the 2R,3S,4S configuration. The difference between isomeric mopanols and peltogynols is confined to a variation in the C-linked substitution of their catechol nuclei.
This structural relationship, their concomitance, and the association with (–)-fisetinidol and the new compound (+)-epifisetinidol and (+)-3′,4′7,-trihydroxy-2,3-trans-flavan-3,4-cis-diol, indicates the common biogenetic origins of mopanols and peltogynols in the reaction of formaldehyde with mutual 2,3-trans-3,4-trans and 2,3-trans-3,4-cis leucofisetinidin precursors.