Johann F. W. Burger

Phenolic metabolites from rooibos tea (Aspalathus linearis)

Abstract The processed leaves and stems of Aspalathus linearis contain hydroxylated benzoic and cinnamic acids, luteolin, chrysoeriol, quercetin, isoquercitrin, the C–C linked β- d -glucopyranosides based on four flavones and the dihydrochalcone aspalathin.

A-type proanthocyanidins from Prunus spinosa

Abstract The range of natural dimeric A-type proanthocyanidins is extended by identification of ent-epicatechin-(4α → 8; 2α → O → 7)-catechin and ent-epiafzelechin-(4α → 8;2α → O → 7)-epicatechin. They are accompanied in the flowers of Prunus spinosa by the structurally related metabolites ent-epicatechin-(4α → 8;2α → O-7)- epicatechin, ent-epiafzelechin-(4α → 8;2α → O → 7)-catechin and ent-epiafzelechin-(4a@8;2a@0-7)-epiafzelechin, these being reported from a plant source for the second time. Nuclear Overhauser effect difference spectroscopy facilitated assessment of both 3,4-relative stereochemistry of the heterocyclic ring C and absolute configuration of terminal units, as well as determination of the mode of interflavanyl linkage. Whereas methylation and subsequent acetylation of A-types with ‘lower’ catechin units produce the expected methyl ether acetates, those species with a ‘lower’ epicatechin moiety indicate restrictedaccess of diazomethane to 5-OH(A).

Iridoid and phenolic glycosides from Harpagophytum Procumbens

Abstract A novel bioside, β-(3′,4′-dihydroxyphenyl)ethyl- O -α- L -rhamnopyranosyl(1 → 3)-D-glucopyranoside, was obtained from the secondary roots of Harpagophytum procumbens . It is accompanied by the known iridoid glucosides harpagoside, procumbide, and its 6′- O -p-coumaroyl ester, and phenolic glycosides, acteoside and isoacteoside, the latter pair being obtained from H. procumbens for the first time. The structures of these metabolites were differentiated by high resolution NMR studies, while that of the bioside is additionally supported by synthesis.

Oligomeric flavanoids. Part W. base-catalyzed pyran rearrangements of procyanidin B-2, and evidence for the oxidative transformation of B- to A-type procyanidins☆

Abstract Procyanidin B-2 3 is subject to facile C-ring isomerizations in 0.1M NaHCO3 solution to form a novel series of 3,4,9, 10-tetrahydro-2H, 8-pyrano[2,3-h]chromenes 7 , 9 , and 10 . The low percentage conversion of B- to A-type procyanidin 2 is rationalized in terms of an initial oxidative removal of hydride ion at C-2 (C-ring).

Oligomeric flavanoids. Part 3. Structure and synthesis of phlobatannins related to (–)-fisetinidol-(4α,6)- and (4α,8)-(+)-catechin profisetinidins

Several members of the novel class of natural ‘phlobaphene’ condensed tannins, representing the products of c-ring isomerization of 2,3-trans-3,4-trans-(–)-fisetinidol units present in (4,6)- and (4,8)-biflavanoid profisetinidins, have been characterized by 1H n.m.r. n.o.e. difference spectroscopy. These include the functionalized 8,9-trans-9,1 0-cis-tetrahydropyrano[2,3-h]chromenes (9) and (12), and the [2,3-f]- and [2,3-g] regioisomers (14) and (19). Since the (4α,8)-biflavanoid (1) is subject to extensive base-catalysed rearrangement and epimerization, the protected 4-O-methyl ethers (E-ring)(2) and (6) were utilized to confirm the proposed structures of phlobatannins by stereospecific C-ring isomerization of (2) and (6) under basic conditions.

Assessment of 3,4trans and 3,4-cis relative configurations in the a-series of (4,8)-linked proanthocyanidins

Abstract — Proanthocyanidins of the A—type exhibit identical 1 H NMR coupling constants (J 3,4 = 3.5 Hz) irrespective of the relative configurations of their C-rings. The selective 1 H NOE association of 3—H (C-ring) to either 6—H(D) or 8—H(A) permits unequivocal differentiation of (4,8)-linked analogues with respectively 3,4- trans or 3,4- cis configurations of these heterocyclic rings.

A novel doubly-linked proteracacinidin analogue from Acacia caffra

Abstract A new proteracacinidin analogue with a unique double linkage, ent-oritin-(4β→7, 5→6)-epioritin-4α-ol was isolated from the heartwood of Acacia caffra .

Absolute configurations of flavan-3-ols and 4-arylflavan-3-olsvia a modified mosher's method

Abstract 1 H NMR Analysis of R-(+)-α-methoxy-α-trifluoromethylphenyl acetic acid (MTPA) esters of different sets of enantiomeric flavan-3-ols and 4-arylflavan-3-ols respectively, permits assessment of the absolute configurations at C-3 of these condensed tannin structural units.

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.

Oligomeric flavanoids. Part 4. Base-catalysed conversions of (–)-fisetinidol-(+)-catechin profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol constituent units

Additional novel members of the class of natural ‘phlobaphene’ condensed tannins, representing the products of C-ring isomerization of 2,3-trans-3,4-cis-(–)-fisetinidol units present in (4β,6)- and (4β,8)-biflavanoid profisetinidins have been characterized. These comprise the functionalized 8,9-trans-9,1 0-trans-3,4,9,10-tetrahydro-2H,8H-pyrano[2,3-h]chromenes (2) and (11), and 8,9-cis-9, 10-trans analogue (8), and a 6,7-cis-7,8-trans-[2,3-f]-regioisomer (27). Analogues (8) and (11) are prototypes of a unique class of phlobatannins in which the resorcinol A- and pyrocatechol B-rings are interchanged relative to their positions in the more common isomers. Their formation represents a novel rearrangement of profisetinidins with 2,3-trans-3,4-cis-flavan-3-ol units, e.g. (1) with concomitant inversion of absolute configuration at 3-C(C), under base catalysis. The proposed structures of the natural products were confirmed by synthesis via base-catalysed conversion of (–)-fisetinidol-(4β,8)- and (4β,6)-(+)-catechin O-methyl ethers (1) and (14).