Influence of an 8-trifluoroacetyl group on flavanol couplings

The effect of an electron attracting substituent in the Lewis acid catalyzed oligomerization of flavanols was investigated. The results showed that the presence of a COCF₃, at the 8 position of a (+)-catechin unit strongly influenced the attack of the 6 free nucleophile flavanol position by the electrophile generated from a 4-O-alkyloxy protected catechin unit. This was observed either with TiCl₄ or TMSOTf as Lewis acids in which the carbon-carbon bond formation was inhibited and the corresponding dimer was detected in small amount. On the contrary, the formation of a carbon-oxygen bond was observed and the corresponding C-4→O→C-3 ether linked procyanidin dimer was isolated in a good yield. In order to avoid the participation of the C-3 hydroxyl group in the dimerization reaction, the two flavanol units were forced into C-4→C-8 coupling by use of an internal link. The structural elucidation of the isolated compounds was achieved through MS and NMR spectroscopy.     

Hydrogenation of substituted aromatic aldehydes: nucleophilic trapping of the reaction intermediate, application to the efficient synthesis of methylene linked flavanol dimers

The synthesis of dimeric flavanols is the consequence of the possible trapping of the reaction intermediates generated in the course of the reductive hydrogenation of substituted benzaldehydes. The scope of this reaction is investigated.     

Sulfur-containing and dimeric flavanols from Glycosmis montana

Two novel sulfur-containing flavanols, glymontanines A and B, and two new flavanol dimers, montahomobisflavans A and B, were isolated from the twigs and leaves of Glycosmis montana. Their structures were elucidated by spectroscopic methods including 2D NMR analysis.     

Structure determination and colour properties of a new directly linked flavanol-anthocyanin dimer

The structure of catechin-(4α→8)-malvidin 3-O-glucoside obtained by reaction of taxifolin and malvidin 3-O-glucoside following a protocol adapted from proanthocyanidin dimer synthesis was determined by NMR spectrometry. Incorporation of the anthocyanin moiety into a covalent linked flavanol-anthocyanin dimer did not modify its colour properties (i.e., hydration equilibrium constant and copigmentation).     

Brainin A,a novel flavanol from Brainea insignis

A novel flavanol bearing phenylpropionic acid moiety at C-4 through a C-C bonding,named brainin A(1),was isolated from the rhizomes of Brainea insignis.Its structure was elucidated on the basis of spectral data including 2D NMR and HR-ESI-MS.     

Development and validation of an efficient ultrasound assisted extraction of phenolic compounds from flax (Linum usitatissimum L.) seeds

Flaxseed accumulates in its seedcoat a macromolecular complex composed of lignan (secoisolariciresinol diglucoside, SDG), flavonol (herbacetin diglucoside, HDG) and hydroxycinnamic acids (p-couramic, caffeic and ferulic acid glucosides). Their antioxidant and/or cancer chemopreventive properties support their interest in human health and therefore, the demand for their extraction. In the present study, ultrasound-assisted extraction (UAE) of flaxseed phenolic compounds was investigated. Scanning Electron Microscopy imaging and histochemical analysis revealed the deep alteration of the seedcoat ultrastructure and the release of the mucilage following ultrasound treatment. Therefore, this method was found to be very efficient for the reduction of mucilage entrapment of flaxseed phenolics. The optimal conditions for UAE phenolic compounds extraction from flaxseeds were found to be: water as solvent supplemented with 0.2 N of sodium hydroxide for alkaline hydrolysis of the SDG–HMG complex, an extraction time of 60 min at a temperature of 25 °C and an ultrasound frequency of 30 kHz. Under these optimized and validated conditions, highest yields of SDG, HDG and hydroxycinnamic acid glucosides were detected in comparison to other published methods. Therefore, the procedure presented herein is a valuable method for efficient extraction and quantification of the main flaxseed phenolics. Moreover, this UAE is of particular interest within the context of green chemistry in terms of reducing energy consumption and valuation of flaxseed cakes as by-products resulting from the production of flax oil.     

NMR structure characterization of a new vinylpyranoanthocyanin-catechin pigment (a portisin)

A new polyphenolic compound, the structure of which corresponds to a pyranomalvidin-3-glucoside linked to a (+)-catechin unit through a vinyl linkage, has been characterized by UV-vis, MS and NMR spectroscopy. This compound was obtained in an aqueous alcoholic solution from the reaction between a malvidin-3-glucoside-pyruvic acid derivative and (+)-catechin in the presence of acetaldehyde.     

Demonstration of the occurrence of flavanol-anthocyanin adducts in wine and in model solutions

Flavanol-anthocyanin (F-A⁺) adducts were detected in red wine. A mechanism involving acid-catalysed cleavage of flavanol oligomers followed by nucleophilic addition of the anthocyanin moiety (in its hemiketal form) to the resulting carbocation (F⁺) was postulated. To confirm this mechanism, reactions between malvidin 3-O-glucoside (Mv3glc) and procyanidin dimer epicatechin-(4-8)-epicatechin 3-O-gallate (B2-3′OG) were studied in a model solution system at pH 2. A new pigment with a UV-Vis spectrum similar to that of Mv3glc and a signal at m/z=781 in the positive ion mode was detected and was attributed to Ec-Mv3glc, in agreement with the proposed reaction pathway.A protocol adapted from the synthesis of procyanidin dimers, in which the terminal flavanol units was replaced with Mv3glc, was tested. Two new pigments were formed with a signal at m/z=781, in the positive ion mode. These ions were attributed to catechin-Mv3glc.Both adducts obtained by hemisynthesis showed exactly the same fragmentation pattern as that present in wine. In particular, the loss of a 126 amu fragment corresponding to the unsubstituted A-ring of the flavanol unit indicated that all of them were (epi)catechin-Mv3glc adducts.These results prove that reactions between the carbocations resulting from cleavage of tannin interflavanic bonds and anthocyanins occur in wine.     

New Flavanol Glucosides from Abacopteris aspera (Presl) Ching

Three new flavanol glycosides, 1 – 3 , and eight known compounds, 4 – 11 , were isolated from a MeOH extract of the fern Abacopteris aspera (Presl ) Ching . Their structures were elucidated on the basis of extensive spectroscopic analysis, including HSQC, HMBC, ¹H,¹H‐COSY, and NOESY experiments, acid hydrolysis, and by the comparison of their NMR data with those of related compounds.