Glycosides of polygalacic acid from the stem barks of Piper guineense Schum and Thonn.

In a continuation of our study on constituents of P. guineense now focusing on the search for saponins, phytochemical investigation of the n-BuOH fraction of P. guineense stem bark led to the isolation of three previously undescribed triterpenoid saponins, named guineenosides A─C (1─3). Their structures were established on the basis of extensive analysis of 1D and 2D NMR (1H, 13C NMR, DEPT, COSY, TOCSY, NOESY, HSQC, HSQC-TOCSY and HMBC) and HRESIMS experiments, and by chemical evidence as 3-O-{α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-xylopyranosyl-(1 â†’ 2)-α-l-arabinopyranosyl-(1 â†’ 4)-α-l-rhamnopyranosyl-(1 â†’ 3)-[α-l-arabinofuranosyl-(1 â†’ 4)]-α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-fucopyranosyl} polygalacic acid 28-O-α-l-rhamnopyranosyl-(1 â†’ 4)-α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-xylopyranosyl ester (1), 3-O-{α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-xylopyranosyl-(1 â†’ 2)-α-l-arabinopyranosyl-(1 â†’ 4)-α-l-rhamnopyranosyl-(1 â†’ 3)-[α-l-arabinofuranosyl-(1 â†’ 4)]-α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-fucopyranosyl} polygalacic acid 28-O-ß-d-glucopyranosyl-(1 â†’ 3)-α-l-rhamnopyranosyl-(1 â†’ 4)-α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-xylopyranosyl ester (2), and 3-O-{α-l-rhamnopyranosyl-(1 â†’ 2)-ß-d-xylopyranosyl-(1 â†’ 3)-ß-d-fucopyranosyl} polygalacic acid 28-O-[α-l-rhamnopyranosyl-(1 â†’ 4)-α-l-rhamnopyranosyl-(1 â†’ 3)-ß-d-xylopyranosyl ester (3). This is the first report of triterpenoid saponins from P. guineense.

Analysis of Commercial Proanthocyanidins. Part 6: Sulfitation of Flavan-3-Ols Catechin and Epicatechin, and Procyanidin B-3.

Proanthocyanidins (PACs) are natural plant-derived polymers consisting of flavan-3-ol monomers. Quebracho (Schinopsis lorentzii and balansae) heartwood and mimosa (Acacia mearnsii) bark extracts are the major industrial sources of PACs. These commercial extracts are often sulfited to reduce their viscosity and increase their solubility in water. The chemical process of sulfitation is still poorly understood regarding stereochemical influences during the reaction and during the cleavage of the interflavanyl bond of oligomers. To gain a better understanding of sulfitation, two diastereomeric flavan-3-ol monomers were sulfited under industrial conditions, and procyanidin B-3 (catechin-4α→8-catechin) were sulfited to investigate interflavanyl bond cleavage with sulfitation at C-4. Treatment of diastereomeric flavan-3-ols 2R,3S-catechin and 2R,3R-epicatechin with NaHSO3 at 100 °C in aqueous medium afforded the enantiomeric (1R,2S)- and (1S,2R)-1-(3,4-dihydroxyphenyl)-2-hydroxy-3-(2,4,6-trihydroxyphenyl)propane-1-sulfonic acid, respectively. Utilizing computational NMR PD4 calculations it was determined that the direction of stereoselective nucleophilic attack is controlled by the C-3 configuration of the flavan-3-ols catechin and epicatechin. Sulfitation of the catechin-4α→8-catechin dimer 7 (procyanidin B-3) under the same conditions led to the cleavage of the interflavanyl bond yielding the C-4 sulfonic acid substituted catechin momomer. From the heterocyclic ring coupling constants it was determined that nucleophilic attack occurs from the ß-face of the dimer leading to the 2,3-trans-3,4-cis isomer as product.

Analysis of commercial proanthocyanidins. Part 5: A high resolution mass spectrometry investigation of the chemical composition of sulfited wattle (Acacia mearnsii De Wild.) bark extract.

Wattle (Acacia mearnsii De Wild., Leguminosae) bark extract is used commercially to tan leather and manufacture adhesives. The extract is treated with sodium hydrogen sulfite (sulfited) to improve its tanning properties. These include reduced viscosity, improved solubility, and better raw skin penetration. High resolution ESIMS allows unambiguous assignment of sulphur-containing monomeric and oligomeric products from sulfitation. It reveals that during sulfitation the constituent flavan-3-ol building blocks are sulfited at both C-2 and C-4 by a sulfite ion. A sulfonic acid moiety is introduced at C-2 to open the pyran ring, and C-4 to cleave the interflavanyl bond and reduce the degree of polymerization, respectively, explaining the improved tanning properties. MS2 fragmentation spectra and comparison with unsulfited extract support the interpretation of peaks and composition of sufited wattle bark extract. It also supports our published work that mimosa bark extract consists of a catechin or gallocatechin starter unit and fisetinidol or robinetinidol extender units.

Syntheses and in Vitro Antiplasmodial Activity of Aminoalkylated Chalcones and Analogues.

A series of readily synthesized and inexpensive aminoalkylated chalcones and diarylpropane analogues (1-55) were synthesized and tested against chloroquinone-sensitive (D10 and NF54) and -resistant (Dd2 and K1) strains of Plasmodium falciparum. Hydrogenation of the enone to a diarylpropane moiety increased antiplasmodial bioactivity significantly. The influence of the structure of the amine moiety, A-ring substituents, propyl vs ethyl linker, and chloride salt formation on further enhancing antiplasmodial activity was investigated. Several compounds have IC50 values similar to or better than chloroquine (CQ). The most active compound (26) had an IC50 value of 0.01 µM. No signs of resistance were detected, as can be expected from compounds with structures unrelated to CQ and other currently used antimalarial drugs. Toxicity tests (in vitro CHO cell assay) gave high SI indices.

Iriflophenone-3-C-glucoside from Cyclopia genistoides: isolation and quantitative comparison of antioxidant capacity with mangiferin and isomangiferin using on-line HPLC antioxidant assays.

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.

Concise and scalable synthesis of aspalathin, a powerful plasma sugar-lowering natural product.

Aspalathin (1), a dihydrochalcone C-glucoside, exhibits powerful plasma sugar-lowering properties and thus potentially could be used to treat diabetes. Small quantities occur in rooibos tea, manufactured via fermentation of the leaves of Aspalathus linearis, hence necessitating the need for an efficient and concise synthesis. Efforts to synthesize aspalathin (1) via coupling of a glucose donor to the nucleophilic phloroglucinol ring of the dihydrochalcone moiety have invariably failed, presumably because of ring deactivation by the electron-withdrawing carbonyl group. Reduction of the carbonyl group of a chalcone (15) and coupling of the resulting 1,3-diarylpropane (16) to tetra-O-benzyl-ß-D-glucopyranose afforded the C-glucosyl-1,3-diarylpropane (17). Regiospecific benzylic oxidation regenerated the carbonyl group and afforded the per-O-methylaspalathin (1a) quantitatively. This method was not successful with the per-O-benzyl-protected dihydrochalcone. However, the nucleophilicity of the phenolic hydroxy groups of the dihydrochalcone or its acetophenone precursor is not diminished by the carbonyl group. Thus, glucosylation of the di-O-benzylacetophenone (5c) at -40 °C afforded the α-O-glucoside (19) in 86% yield. Raising the temperature allowed facile BF3-catalyzed rearrangement to the ß-C-glucoside (6b), which upon hydrogenation, afforded aspalathin (1) in 80% overall yield [based on the usage of di-O-benzylphloroacetophenone (5c) and tetra-O-benzyl-1α-fluoro-ß-D-glucose (2e)].

Analysis of commercial proanthocyanidins. Part 4: solid state (13)C NMR as a tool for in situ analysis of proanthocyanidin tannins, in heartwood and bark of quebracho and acacia, and related species.

(13)C NMR is an effective method of characterizing proanthocyanidin (PAC) tannins in quebracho (Schinopsis lorentzii) heartwood and black wattle (Acacia mearnsii) bark, before and after commercial extraction. The B-rings of the constituent flavan-3-ols, catechols (quebracho) or pyrogallols (wattle), are recognized in unprocessed source materials by "marker" signals at ca. 118 or 105ppm, respectively. NMR allows the minimum extraction efficiency to be calculated; ca. 30%, and ca. 80%, for quebracho heartwood and black wattle bark, respectively. NMR can also identify PAC tannin (predominantly robinetinidin), and compare tannin content, in bark from other acacia species; tannin content decreases in the order A. mearnsii, Acacia pycnantha (87% of A. mearnsii), Acacia dealbata and Acacia decurrens (each 74%) and Acacia karroo (30%). Heartwood from an underexploited PAC tannin source, Searsia lancea, taxonomically close to quebracho, shows abundant profisetinidin and catechin PACs. NMR offers the advantage of being applicable to source materials in their native state, and has potential applications in optimizing extraction processes, identification of tannin sources, and characterization of tannin content in cultivar yield improvement programmes.

Occurrence and sensory perception of Z-2-(ß-d-glucopyranosyloxy)-3-phenylpropenoic acid in rooibos (Aspalathus linearis).

Z-2-(ß-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-(ß-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.

Analysis of commercial proanthocyanidins. Part 3: the chemical composition of wattle (Acacia mearnsii) bark extract.

Wattle (Acacia mearnsii) bark extract is an important renewable industrial source of natural polymers for leather tanning and adhesive manufacturing. The wattle bark proanthocyanidin oligomers have 5-deoxy extender units that render the interflavanyl bonds resistant to acid catalysed hydrolysis and their composition cannot be determined via conventional thiolysis. We combined established phyto- and synthetic chemistry perspectives with an electrospray mass spectrometry investigation to establish that the flavan-3-ol based oligomers consist of a starter unit which is either catechin or gallocatechin, angularly bonded to fisetinidol or predominantly robinetinidol extender units.

Analysis of commercial proanthocyanidins. Part 2: An electrospray mass spectrometry investigation into the chemical composition of sulfited quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood extract.

Proanthocyanidins (PACs) are natural plant-derived polymers used in leather tanning, wood adhesives, water purification, and mud additives for oil drilling. Quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood and mimosa (Acacia mearnsii) bark extracts are the major industrial sources of PACs. These commercial extracts are often sulfited via treatment with sodium hydrogen sulfite to reduce their viscosity and increase their solubility in water. An ESI-MS investigation into the molecular composition of sulfited (cold-water-soluble) quebracho heartwood extract indicates that sulfitation of the PACs occurs via S(N)2 attack of a sulfite ion at both C-2 and C-4 of the constituent flavan-3-ol monomer extender units. Attack at C-2 leads to the opening of the pyran ring. This releases an additional electron-donating phenolic hydroxy group on the A-ring and renders the extract more nucleophilic and suitable for the manufacturing of adhesives. Attack at C-4 leads to interflavanyl bond fission and decrease of the PAC oligomer chain length. The introduction of sulfonic acid moieties at C-2 or C-4 increases the polarity and water solubility of the hot water soluble (unsulfited) extract and transforms it into a cold-water-soluble extract.

Analysis of commercial proanthocyanidins. Part 1: the chemical composition of quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood extract.

Quebracho (Schinopsis lorentzii and Schinopsis balansae) extract is an important source of natural polymers for leather tanning and adhesive manufacturing. We combined established phyto- and synthetic chemistry perspectives with electrospray mass spectrometry experiments to prove that quebracho proanthocyanidin polymers consist of an homologous series of flavan-3-ol based oligomers. The starter unit is always catechin which is angularly bonded to fisetinidol extender units. By comparison of the MS(2) fragmentation spectra of the oligomer with product ion scans of authentic catechin and robinetinidol samples, we proved that quebracho extract contains no robinetinidol, as is often reported. Quebracho proanthocyanidins have acid resistant interflavanyl bonds, due to the absence of 5-OH groups in fisetinidol, and the aDP cannot be determined via conventional thiolysis and phloroglucinolysis. We used the MS data to estimate a conservative (minimum value) aDP of 3.1.

Tannin fingerprinting in vegetable tanned leather by solid state NMR spectroscopy and comparison with leathers tanned by other processes.

Solid state ¹³C-NMR spectra of pure tannin powders from four different sources--mimosa, quebracho, chestnut and tara--are readily distinguishable from each other, both in pure commercial powder form, and in leather which they have been used to tan. Groups of signals indicative of the source, and type (condensed vs. hydrolyzable) of tannin used in the manufacture are well resolved in the spectra of the finished leathers. These fingerprints are compared with those arising from leathers tanned with other common tanning agents. Paramagnetic chromium (III) tanning causes widespread but selective disappearance of signals from the spectrum of leather collagen, including resonances from acidic aspartyl and glutamyl residues, likely bound to Cr (III) structures. Aluminium (III) and glutaraldehyde tanning both cause considerable leather collagen signal sharpening suggesting some increase in molecular structural ordering. The ²7Al-NMR signal from the former material is consistent with an octahedral coordination by oxygen ligands. Solid state NMR thus provides easily recognisable reagent specific spectral fingerprints of the products of vegetable and some other common tanning processes. Because spectra are related to molecular properties, NMR is potentially a powerful tool in leather process enhancement and quality or provenance assurance.

Photochemistry of flavonoids.

Flavonoids and their photochemical transformations play an important role in biological processes in nature. Synthetic photochemistry allows access to molecules that cannot be obtained via more conventional methods. This review covers all published synthetic photochemical transformations of the different classes of flavonoids. It is first comprehensive review on the photochemistry of flavonoids.

Synthesis of proanthocyanidins. Part 1. The first oxidative formation of the interflavanyl bond in procyanidins.

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.