Hendrik van Rensburg

OLIGOMERIC FLAVANOIDS. PART 27*. INTERFLAVANYL BOND FORMATION IN PROCYANIDINS UNDER NEUTRAL CONDITIONS

Abstract Dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) and silver tetrafluoroborate (AgBF4) activate the C4S bond in the 4-thioethers of flavan-3-ols toward carbon nucleophiles to permit formation of the interflavanyl bond in procyanidins under neutral conditions.

Synthesis of isoflavonoids. Enantiopure cis- and trans-6a-hydroxypterocarpans and a racemic trans-pterocarpan

Abstract Aldol condensation between phenylacetates and benzaldehydes affords 2,3-diaryl-3-hydroxypropanoates which serve as common precursors to both the first racemic trans-pterocarpan and enantiopure cis- and trans-6a-hydroxypterocarpans.

ENANTIOSELECTIVE SYNTHESIS OF THE FOUR CATECHIN DIASTEREOMER DERIVATIVES

Abstract Asymmetric dihidroxylation of (E)-1-(3,4-dimethoxyphenyl)-3-(2-methoxy-methyl-4,6-dimethoxy-phenyl)-propene with AD-mix-α or AD-mix-β in the presence of methanesulfonamide, followed by acid catalysed cyclization, afforded the four catechin diastereomers in high enantiomeric excesses and yields.

Enantioselective synthesis of flavonoids. Part 51. Poly-oxygenated β-hydroxydihydrochalcones

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 ringopening, afforded β-hydroxydihydrochalcones in moderate to high enantiomeric excess and yield.

DIRECT SYNTHESIS OF PTEROCARPANS VIA ALDOL CONDENSATION OF PHENYLACETATES WITH BENZALDEHYDES

Abstract Aldol condensation between phenylacetates and benzaldehydes affords 2,3-diaryl-3-hydroxy-propanoates which are converted into pterocarpans in moderate to high yields.

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.

Enantioselective synthesis of flavonoids. Part 3.1trans- and cis-Flavan-3-ol methyl ether acetates

Asymmetric dihydroxylation of a series of polyoxygenated 1,3-diarylpropenes with AD-mix-α or AD-mix-β in the presence of methanesulfonamide and subsequent acid-catalysed cyclization, affords for the first time synthetic access to trans- and cis-flavan-3-ol derivatives, essentially enantiopure and in good yield.

Circular Dichroic Properties of Flavan-3-ols

CD data of all four diastereoisomers of the permethylaryl ether 3-O-acetyl derivatives of a series of flavan-3-ols permit assignment of the absolute configuration at the stereocentres of the heterocyclic ring.

Stereoselective Synthesis of Flavonoids. Part 8. Free Phenolic Flavan-3-ol Diastereoisomers

Asymmetric dihydroxylation of a series of poly-O-methoxymethyl-1,3-diarylpropenes with AD-mix-α or AD-mix-β and subsequent acid-catalyzed cyclization of the intermediate syn-diols permits the first synthetic access to all four diastereoisomers of free phenolic flavan-3-ols in high enantiomeric excess and yield.