- OLIGOMERIC FLAVANOIDS. PART 13. SYNTHESIS OF PROFISETINIDINS BASED ON (-)-ROBINETINIDOL AND (+)-EPIFISETINIDOL
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Acid-catalyzed condensation of (+)-mollisacacidin with an excess of (-)-robinetinidol afforded a novel series of bi-, tri-, and tetraflavanoid profisetinidins.They are accompanied by (-)-fisetinidol-(4α,2')-(-)-robinetinidol which results from the pyrogallol B-ring of (-)-robinetinidol serving as nucleophile competing with its resorcinol A-ring in coupling with a C-4 carbocationic intermediate.Similar condensation with (+)-epifisetinidol led to the exclusive formation of -interflavanyl bonds, these units being 'linearly' arranged in the tetraflavanoid analogue in contrast to the 'branched' nature of the (-)-robinetinidol homologue.
- Malan, Johannes C. S.,Steenkamp, Jacobus A.,Young, Desmond A.,Ferreira, Daneel
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p. 7859 - 7868
(2007/10/02)
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- Synthesis of Condensed Tannins. Part 8. The First 'Branched' -Tetraflavanoid. Coupling Sequence and Absolute Configuration
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Condensation of (+)-(2R)-2,3-trans-3,4-trans-flavan-3,3',4,4',7-pentaol with excess of (-)-(2R)-2,3-trans-flavan-3,3',4',7-tetraol proceeds beyond the expected biflavanoid range to generate significant yields of both the 'linear' -2,3-trans-3,4-cis:2',3'-trans-3',4'-trans:2'',3''-trans-trifisetinidol and the first 'branched' -2,3-trans-3,4-cis:2',3'-trans-3',4'-trans:2'',3''-trans:3'',4''-trans:2''',3'''-trans-tetrafisetinidol.
- Steenkamp, Jacobus A.,Ferreira, Daneel,Roux, David G.,Hull, William E.
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- Synthesis of Condensed Tannins. Part 5. The First Angular -Triflavanoids and Their Natural Counterparts
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Angular triflavanoids comprising a group of four diastereoisomeric -bi--(+)-catechins with definable absolute configurations (2R,3S,4S-and 2R,3S,4R-2R,3S-2R,3S,4S; 2R,3S,4S- and 2R,3S,4R-2R,3S-2R,3S,4R) result from flavanyl-4-carbocation mediated condensation of (2R,3S,4R)-flavan-3,3',4,4',7-pentaol at C-6 of the (+)-catechin units of all-trans-(2R,3S,4S-2R,3S)--(-)-fisetinidol-(+)-catechin and its 3,4-cis-(2R,3S,4R-2R,3S) diastereomer respectively.The natural coexistence of some of the forementioned species of bi- and tri-flavanoids and their precursors indicate a comparable sequence of 4,8 preceding 4,6 condensation with (+)-catechin as common nucleophile.
- Botha, Jacobus J.,Vivers, Phillip M.,Young, Desmond A.,Preez, I. Cornelius du,Ferreira, Daneel,et al.
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p. 527 - 533
(2007/10/02)
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- Synthesis of Condensed Tannins. Part 4. A Direct Biomimetic Approach to - and -Biflavanoids
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The generation of flavanyl-4-carbo-cations from flavan-3,4-diols and their condensation with nucleophilic flavan-3-ols to form - and -biflavanoids at ambient temperatures and mildly acidic aqueous conditions apparently simulates the initial step in condensed tannin formation in a number of natural sources.The stereospecificity (or stereoselectivity) of the reaction is conditioned mainly by the 2,3-cis or 2,3-trans stereochemistry of the parent flavan-3,4-diol, but also by the nuclephilicity of the flavan-3-ol, and its regiospecific (or regioselective) course by steric factors arising from variation in substitution of the receptive A-ring of the flavan-3-ol.
- Botha, Jacobus, J.,Ferreira, Daneel,Roux, David G.
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p. 1235 - 1245
(2007/10/02)
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