... Press plc STRUCTURE AND SYNTHESIS OF PHLOBATANNINS RELATED TO (-)-FISETINIDOL-(-)-EPICATECHIN... more ... Press plc STRUCTURE AND SYNTHESIS OF PHLOBATANNINS RELATED TO (-)-FISETINIDOL-(-)-EPICATECHIN PROFISETINIDINS* JAN P. STEYNBERG, JOHANN FW BURGER, ANNEMARIE CRONJ , SUSAN ... I. 235. 2. Botha, JJ, Ferreira, D. and Roux, DG (1981) ...
Journal of Photochemistry and Photobiology A: Chemistry, 2012
ABSTRACT A new method to open the heterocyclic ring of flavan-3-ols via photolytic cleavage of th... more ABSTRACT A new method to open the heterocyclic ring of flavan-3-ols via photolytic cleavage of the ether bond, with stereoselective trapping of the intermediates with phloroglucinol to obtain phloroglucinol grafted derivatives of flavan-3-ols, was developed. Photolysis of catechin and epicatechin, respectively, in the presence of phloroglucinol yielded the enantiomeric (1S,2S)- and (1R,2R)-1,3-di(2,4,6-trihydroxyphenyl)-1-(3,4-dihydroxyphenyl)propan-2-ols, respectively. The absolute configuration at C-1 and C-2 was determined by electronic circular dichroism (experimental and calculated) and these results confirmed that the trapping mechanism is controlled by the C-3 configuration of the flavan-3-ol.
The benzophenone, iriflophenone-3-C-glucoside, was isolated from Cyclopia genistoides using a com... more The benzophenone, iriflophenone-3-C-glucoside, was isolated from Cyclopia genistoides using a combination of fluid-fluid extraction, high performance counter-current chromatography (HPCCC) and semi-preparative high performance liquid chromatography (HPLC). The microplate oxygen radical absorbance capacity (ORAC) assay, with fluorescein as probe, was adapted for use in an on-line HPLC configuration. The method was validated using a mixture of authentic standards including iriflophenone-3-C-glucoside, and the xanthones, mangiferin and isomangiferin. Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) was included in the mixture for calculation of Trolox equivalent antioxidant capacity (TEAC) values. Using the on-line HPLC-ORAC assay, as well as 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS(+)) on-line assays, the antioxidant activity of iriflophenone-3-C-glucoside and isomangiferin was demonstrated for the first time. Iriflophenone-3-C-glucoside presented no radical scavenging ability against DPPH, but scavenged ABTS(+) and peroxyl radicals (TEACABTS of 1.04 and TEACORAC of 3.61). Isomangiferin showed slightly lower antioxidant capacity than mangiferin against DPPH (TEACDPPH of 0.57 vs. 0.62), but higher capacity against ABTS(+) (TEACABTS of 1.82 vs. 1.67) and peroxyl radical (TEACORAC of 4.14 vs. 3.69) than mangiferin. The on-line HPLC-ORAC assay was shown to be more sensitive for radical scavengers, but at the same time less selective for rapid radical scavengers than the DPPH assay.
The natural class of phlobaphene condensed tannins is complemented by two functionalized hexahydr... more The natural class of phlobaphene condensed tannins is complemented by two functionalized hexahydrodipyrano[2,3-f:2′,3′-h]chromenes representing the products of stereoselective pyran ring rearrangement of the 2,3-trans-3,4-trans- and 2,3-trans-3,4-cis-flavan-3-ol moieties in the bis-fisetinidol-(4α,6:4β,8)-catechin triflavanoid. Structural eludication of these complex natural products was effected by synthesis via base-catalysed conversion of the 4-O(E)-methyl ether of their presumed profisetinidin triflavanoid precursor.
Z-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid (PPAG), a compound postulated to contribute to... more Z-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid (PPAG), a compound postulated to contribute to the taste and mouthfeel of fermented rooibos tea (Aspalathus linearis), was isolated from unfermented rooibos plant material. Its structure was unequivocally confirmed by LC-MS, -MS(2), FT-IR and NMR of the underivatised natural product, and optical rotation measurements of the hydrolysed sugar moiety. A similar compound, postulated to be E-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid, was also detected. Analysis of the leaves of a large number of rooibos plants (n=54), sampled at commercial plantations, showed that PPAG is not ubiquitously present in detectable quantities in the leaves of different plants. This leads to large variation in the fermented plant material, infusions and food-grade extracts. PPAG was shown to have a slightly bitter to astringent taste and a detection threshold of 0.4 mg/l in water.
A novel and efficient method for the oxidative condensation of tetra-O-methyl-3-oxocatechin 4 wit... more A novel and efficient method for the oxidative condensation of tetra-O-methyl-3-oxocatechin 4 with tetra-O-methylcatechin is described. Treatment of a solution of 3 (2 equiv) and 4 (1 equiv) with silver tetrafluoroborate readily affords the phenolic per-O-methyl ethers of 3-oxocatechin(4-8)-catechin 18 and 19. Subsequent metal hydride reduction provides access to procyanidin B-3 analogues with the 3,4-cis diastereomers predominating.
... Press plc STRUCTURE AND SYNTHESIS OF PHLOBATANNINS RELATED TO (-)-FISETINIDOL-(-)-EPICATECHIN... more ... Press plc STRUCTURE AND SYNTHESIS OF PHLOBATANNINS RELATED TO (-)-FISETINIDOL-(-)-EPICATECHIN PROFISETINIDINS* JAN P. STEYNBERG, JOHANN FW BURGER, ANNEMARIE CRONJ , SUSAN ... I. 235. 2. Botha, JJ, Ferreira, D. and Roux, DG (1981) ...
Journal of Photochemistry and Photobiology A: Chemistry, 2012
ABSTRACT A new method to open the heterocyclic ring of flavan-3-ols via photolytic cleavage of th... more ABSTRACT A new method to open the heterocyclic ring of flavan-3-ols via photolytic cleavage of the ether bond, with stereoselective trapping of the intermediates with phloroglucinol to obtain phloroglucinol grafted derivatives of flavan-3-ols, was developed. Photolysis of catechin and epicatechin, respectively, in the presence of phloroglucinol yielded the enantiomeric (1S,2S)- and (1R,2R)-1,3-di(2,4,6-trihydroxyphenyl)-1-(3,4-dihydroxyphenyl)propan-2-ols, respectively. The absolute configuration at C-1 and C-2 was determined by electronic circular dichroism (experimental and calculated) and these results confirmed that the trapping mechanism is controlled by the C-3 configuration of the flavan-3-ol.
The benzophenone, iriflophenone-3-C-glucoside, was isolated from Cyclopia genistoides using a com... more The benzophenone, iriflophenone-3-C-glucoside, was isolated from Cyclopia genistoides using a combination of fluid-fluid extraction, high performance counter-current chromatography (HPCCC) and semi-preparative high performance liquid chromatography (HPLC). The microplate oxygen radical absorbance capacity (ORAC) assay, with fluorescein as probe, was adapted for use in an on-line HPLC configuration. The method was validated using a mixture of authentic standards including iriflophenone-3-C-glucoside, and the xanthones, mangiferin and isomangiferin. Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) was included in the mixture for calculation of Trolox equivalent antioxidant capacity (TEAC) values. Using the on-line HPLC-ORAC assay, as well as 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS(+)) on-line assays, the antioxidant activity of iriflophenone-3-C-glucoside and isomangiferin was demonstrated for the first time. Iriflophenone-3-C-glucoside presented no radical scavenging ability against DPPH, but scavenged ABTS(+) and peroxyl radicals (TEACABTS of 1.04 and TEACORAC of 3.61). Isomangiferin showed slightly lower antioxidant capacity than mangiferin against DPPH (TEACDPPH of 0.57 vs. 0.62), but higher capacity against ABTS(+) (TEACABTS of 1.82 vs. 1.67) and peroxyl radical (TEACORAC of 4.14 vs. 3.69) than mangiferin. The on-line HPLC-ORAC assay was shown to be more sensitive for radical scavengers, but at the same time less selective for rapid radical scavengers than the DPPH assay.
The natural class of phlobaphene condensed tannins is complemented by two functionalized hexahydr... more The natural class of phlobaphene condensed tannins is complemented by two functionalized hexahydrodipyrano[2,3-f:2′,3′-h]chromenes representing the products of stereoselective pyran ring rearrangement of the 2,3-trans-3,4-trans- and 2,3-trans-3,4-cis-flavan-3-ol moieties in the bis-fisetinidol-(4α,6:4β,8)-catechin triflavanoid. Structural eludication of these complex natural products was effected by synthesis via base-catalysed conversion of the 4-O(E)-methyl ether of their presumed profisetinidin triflavanoid precursor.
Z-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid (PPAG), a compound postulated to contribute to... more Z-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid (PPAG), a compound postulated to contribute to the taste and mouthfeel of fermented rooibos tea (Aspalathus linearis), was isolated from unfermented rooibos plant material. Its structure was unequivocally confirmed by LC-MS, -MS(2), FT-IR and NMR of the underivatised natural product, and optical rotation measurements of the hydrolysed sugar moiety. A similar compound, postulated to be E-2-(β-d-glucopyranosyloxy)-3-phenylpropenoic acid, was also detected. Analysis of the leaves of a large number of rooibos plants (n=54), sampled at commercial plantations, showed that PPAG is not ubiquitously present in detectable quantities in the leaves of different plants. This leads to large variation in the fermented plant material, infusions and food-grade extracts. PPAG was shown to have a slightly bitter to astringent taste and a detection threshold of 0.4 mg/l in water.
A novel and efficient method for the oxidative condensation of tetra-O-methyl-3-oxocatechin 4 wit... more A novel and efficient method for the oxidative condensation of tetra-O-methyl-3-oxocatechin 4 with tetra-O-methylcatechin is described. Treatment of a solution of 3 (2 equiv) and 4 (1 equiv) with silver tetrafluoroborate readily affords the phenolic per-O-methyl ethers of 3-oxocatechin(4-8)-catechin 18 and 19. Subsequent metal hydride reduction provides access to procyanidin B-3 analogues with the 3,4-cis diastereomers predominating.
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