Elfranco Malan

Flavonoids and tannins of Acacia species

Abstract The heartwoods of Acacia giraffae and A. galpinii were selected from South African Acacias as representative of those with abnormally high and minimal tannin contents respectively. A. galpinii contains amongst other analogues, the first natural (+)-2,3-trans-3,4-trans-teracacidin (7,8,4′-trihydroxy-flavan-3,4-diol and novel 3-O-methyl-, 7,8-di-O-methyl- and 7,8,4′-tri-O-methylflavonol analogues. (−)-2,3-cis-3,4-cis-Melacacidin (7,8,3′,4′-tetrahydroxyflavan-3,4-diol) is also present, but tannins are absent. By contrast, from the large excess of leueofisetinidin tannins which characterizes the wood of A. giraffae, only (+)-catechin, (+)-2,3-trans-3,4-trans-leucofisetinidin (7,3′,4′,trihydroxyflavan-3,4-diol and all-trans-(+)-leueofisetinidin-(+)-catechin could be isolated.

The structure and synthesis of the first dimeric proteracacinidins from acacia galpinii

Abstract The first two proteracacinidin dimers were isolated from the heartwood of Acacia galpinii . Epioritin-4α-ol under mild acidic conditions yielded epioritin-(4β → 6)-epioritin-4α-ol and epioritin (4β → 6)-epioritin-4β-ol, confirming the structures of the two isolated dimers.

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.

Substituted bibenzyls, phenanthrenes and 9,10-dihydrophenanthrenes from the heartwood of Combretum Apiculatum

Abstract 3,5,4′-, 3,4,3′,5′-, 3,4,5,4′- and 3,4,5,3′,4′-substituted dihydrostilbenes, 2,3,4,6,7- substituted phenanthrenes and its 9,10-dihydro analogues were isolated and identified in the acetone extract from the heartwood of Combretum apiculatum . Growth inhibition and fungitoxic screening tests were conducted on 4,7-dihydroxy-2,3,6-trimethoxyphenanthrene, 2,7-dihydroxy-2,4,6-trimethoxy-9, 10-dihydrophenanthrene and 4,4′-dihydroxy-3,5-dimethoxydihydrostilbene.

Derivatives of (+)-catechin-5-gallate from the bark of Acacia nilotica

Abstract Four derivatives of (+)-catechin-5-gallate present in very low concentrations in the bark of Acacia nilotica were isolated and characterized from the acetone extract. An attempt was made to synthesize some of the (+)-catechin galloyl esters found to date.

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.

The structure and synthesis of proguibourtinidins from Cassia abbreviata

Abstract An acetone extract of the bark of Cassia abbreviata yielded guibourtinidol-(4β → 8)-epicfzelechin, guibourtinidol-(4α → 8)-epiafzelechin, guibourtinidol-(4α → 8)-catechin, guibourtinidol-(4β → 8)-epicatechin and ent-guibourtinidol-(4β → 8)-epicatechin. The structures of the proguibourtinidin dimers were confirmed by synthesis via mild acid-condensation. Guibourtinidol-(4α → 8)-afzelechin, previously isolated from Acacia leuderitzii, was also synthesized. Another two proguibourtinidol oligomers, namely, guibourtinidol-(4β → 8)-afzelechin and guibourtinidol-(4α → 6)-afzelechin were products of the previous synthesis but neither of them have been reported to date.

A (4β → 5)-linked proteracacinidin dimer from the heartwood of Acacia caffra

Abstract The proteracacinidin ent -oritin-(4 β → 5) epioritin-4β-ol with 8- O -methylepioritin-4α-ol, 3- O -methyl-7,8,4′-trihydroxy-flavone and other teracacidin analogues were isolated from the heartwood of Acacia caffra .

(+)-catechin-galloyl esters from the bark of Acacia gerrardii

Abstract Five (+)-catechin galloyl esters were isolated and characterized from the acetone extract of the bark of Acacia gerrardii .

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.