Johan Coetzee

Structure and synthesis of ether-linked proteracacinidin and promelacacinidin proanthocyanidins from Acacia caffra

Two new ether-linked proanthocyanidins, epioritin-(4beta-->3)-epioritin-4beta-ol and epimesquitol-(4beta-->4)-epioritin-4beta-ol, were isolated from the heartwood of Acacia caffra. Their structures and absolute configurations were established by spectroscopic methods and syntheses.

Oligomeric flavanoids. Part 18. Dimeric prorobinetinidins from Robinia pseudacacia

Abstract The range of naturally occurring prorobinetinidins is extended by characterization of the novel robinetinidol-leucorobinetinidins 3, 5, 7, and 9, the robinetinidol-dihydrorobinetins 11, 13, and 15, a robinetinidol-robinetin 17, and a robinetinidol-flavone analogue 19. The diversity regarding the oxidation level of the chain terminating moieties suggests that the biflavanoids in Robinia pseudacacia may be interrelated via oxidation/reduction of these units. The conspicuous absence of the effects of dynamic rotational isomerism about the interflavanyl bond in the 1H NMR spectra of some of the derivatives is explained in terms of a preferred conformation of this bond rather than to ‘free rotation’.

Oligomeric flavanoids, part 29: Structure and synthesis of novel ether-linked [4-O-4] bis-teracacinidins

Abstract The rare series of proteracacinidin-type oligoflavanoids is extended by identification of two novel [4-O-4] ether-linked analogues 7 and 10 from the heartwood of Acacia galpinii . Their structures and absolute configuration were established via spectroscopic methods and by stereoselective synthesis from the appropriate leucoteracacinidin flavan-3,4-diol precursors.

Stereoselective synthesis of flavonoids. Part 7. Poly-oxygenated β-hydroxydihydrochalcone derivatives

Abstract Epoxidation of a series of poly-oxygenated chalcones with urea-hydrogen peroxide complex in THF in the presence of DBU and poly-( l )- or -( d )-leucine, followed by TBTH/AIBN catalysed ring opening, afforded β-hydroxydihydrochalcones in moderate to high enantiomeric excess and yield. This represents the first stereoselective route towards this group of flavonoids. Epoxidation of a series of poly-oxygenated chalcones with urea-hydrogen peroxide complex in THF in the presence of DBU andpoly-(L)-or-(D)-leucine, followed by TBTH/AIBN catalysed ring opening, afforded β-hydroxydihydrochalcones in moderate to high enantiomeric excess and yield. This represents the first stereoselective access to this group of flavonoids Download : Download full-size image

The novel flavan-3-ol, (2R, 3S)-guibourtinidol and its diastereomers

Abstract The novel (2 R ,3 S )-guibourtinidol, representing the first flavan-3-ol with 4′,7-dihydroxy phenolic substitution pattern, was identified in the heartwood of Cassia abbreviata . Asymmetric dihydroxylation of ( E )-1-(4′- O -methoxymethylphenyl)-3-(2″,4″-di- O -methoxymethylphenyl)-propene with AD-mix-α or AD-mix-β and subsequent acid-catalyzed cyclization afforded the four free phenolic guibourtinidol diastereomers, essentially enantiopure and in good yield.

Oligomeric flavanoids. Part 34: Doubly-linked proteracacinidin analogues from Acacia caffra and Acacia galpinii ☆

Abstract The rare series of doubly-linked proteracacinidin-type oligoflavanoids is extended by identification of four new analogues, oritin-(4α→7, 5→6)-epioritin-4α-ol 3, oritin-(4β→7, 5→6)-epioritin-4α-ol 5, epioritin-(4β→7, 5→6)-epioritin-4α-ol 7 and epioritin-(4β→7, 5→6)-oritin-4α-ol 9. The same sources also afforded epioritin-(4β→5, 3→4)-oritin-4α-ol 11 possessing a rare (4β→5)- as well as a unique (3→4)- ether linkage. Their structures and absolute configurations were established via spectroscopic methods.

Structure and stereochemistry of triflavanoids containing both ether and carbon-carbon interflavanyl bonds.

The first triflavanoids with both C-C and C-O-C interflavanyl bonds, epioritin-(4beta-->6)-epioritin-(4alpha-->4)-epioritin-4beta-ol, epioritin-(4beta-->3)-epioritin-(4beta-->6)-epioritin-4beta-ol and epioritin-(4beta-->3)-epioritin-(4beta-->6)-epimesquitol-4alpha-ol, were identified in the heartwood of Acacia caffra. The ethereal interflavanyl bond is readily susceptible to reductive cleavage with sodium cyanoborohydride in trifluoroacetic acid/dichloromethane which hence permits the unequivocal assignment of the absolute configuration of constituent flavanyl units.

Oligomeric Flavanoids. Part 28. Structure and Synthesis of Ether-linked (4-O-3)-Bis-teracacidins, a Novel Class of Naturally Occurring Proanthocyanidins

The range of natural dimeric proteracacinidins is extended by identification of the first (4-O-3)-linked bis-teracacidins, epioritin-(4β→3)-epioritin-4α-ol 13 and epioritin-(4β→3)-oritin-4α-ol 16 in the heartwood extract of Acacia galpinii.

The formation and stability of flavans with 2,3-cis-3,4-cis configuration

Abstract Treatment of 4β-chloroepioritin tri-O-methylether with phenylmethanethiol under neutral conditions affords both the 4α- and 4β-substituted epioritin derivatives. The stereochemical course of the reaction is of relevance to the thiolysis of 5-oxy (A-ring) proanthocyanidins and to the conspicuous stability of the 7,8-dihydroxy-2,3-cis-3,4-cis-flavan-3,4-diols, teracacidin and melacacidin, and of some of their all-cis (C-ring) oligomers.

Structure and synthesis of butiniflavan-epicatechin and -epigallocatechin probutinidins

Abstract The rare series of dimeric proanthocyanidins with flavan chain extender units is extended by characterization of butiniflavan-(4α→8)- and (4β→8)-epicatechins and butiniflavan-(4β→8)-epigallocatechin from the bark of Cassia petersiana. The structure and absolute configuration of the dimers were confirmed by synthesis via reduction of the racemic flavanone, (±)-7,3′,4′-tri-O-methylbutin, to the diastereomeric flavan-4-ols and condensation with 5,7,3′,4′-tetra-O-methylepicatechin and 5,7,3′,4′,5′-penta-O-methylepigallocatechin using titanium tetrachloride as Lewis acid.