Condensed tannins. 18. Stereochemistry of flavan-3,4-diol tannin precursors: (+)-mollisacacidin, (−)-leuco-fisetinidin and (+)-leuco-robinetinidin

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Abstract
The relative configurations of (+)-mollisaca-cidin, (-)-leuco-fisetinidin and (+)-leuco-robinetinidin have been revised to the 2,3-trans-3,4-trans arrangement by comparison of their oxidation rates and nuclear-magnetic-resonance spectra with synthetic 2,3-trans-3,4-trans and 2,3-trans-3,4-cis isomers. The natural 2,3-trans-3,4-trans-flavan-3,4-diols form 2,3-trans-3,4-cis-isopropylidene derivatives in high yield, and such reaction is, therefore, an unreliable criterion of 3,4-cis configuration. Chemical shifts of the 5-protons in flavan-3,4-diol and flavanone derivatives are recorded, and differences between the 3,4-cis- and 3,4-trans-diacetates apparently support the half-chair conformation of the heterocyclic ring. The revised absolute configurations of (+)-mollisacacidin and (+)-leuco-robinetinidin (2R, 3S,4R), of their (+)-flavan-4[beta]-ol analogues (2S,4S) and of ([long dash])-leuco-fisetinidin (2S, 3R, 4S) may be deduced from these and from previous correlations.

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