C-Glycosylflavones from Avena sativa

Avena sativa leaves, stems and inflorescences contain a range of new C-glycosylflavone 2″-O-glycosides, including vitexin and isoswertisin 2″-rhamnosides, isovitexin and isoorientin 2″-arabinosides. The structure of ‘vitexin 4′-rhamnoside’ from Crataegus oxyacantha is revised in vitexin 2″-rhamnoside.     

C-glycosylflavones from Oryza sativa

Eight flavone C-glycosides isolated from rice plant were found to act as probing stimulants for planthoppers. They have been identified as the known compounds schaftoside, neoschaftoside, carlinoside, isoorientin 2″-glucoside and the new constituents neocarlinoside (6-C-β-D-glucopyranosyl-8-C-β-L-arabinopyranosylluteolin), isoscoparin 2″-glucoside (chrysoeriol 6-C-β-D-(2-O-β-D-glucopyranosyl)glucopyranoside) and its 6?-p-coumaric and ferulic acid esters.     

C-glycosylanthocyanidins synthesized from C-glycosylflavones

Nine C-glycosyldeoxyanthocyanidins, 6-C-β-glucopyranosyl-7-O-methylapigeninidin, 6-C-β-glucopyranosyl-7-O-methylluteolinidin, 6-C-β-(2″-O-β-glucopyranosylglucopyranosyl)-7-O-methylapigeninidin, 6-C-β-(2″-O-β-glucopyranosylglucopyranosyl)-7,4′-di-O-methylapigeninidin, 8-C-β-glucopyranosylapigeninidin, 8-C-β-(2″-O-α-rhamnopyranosylglucopyranosyl)apigeninidin, 8-C-β-(2″-O-α-(4″′-O-acetylrhamnopyranosyl)glucopyranosyl)apigeninidin, 6,8-di-C-β-glucopyranosylapigeninidin (8), 6,8-di-C-β-glucopyranosyl-4′-O-methylluteolinidin (9), have been synthesized from their respective C-glycosylflavones (yields between 14% and 32%) by the Clemmensen reduction reaction using zinc-amalgam. The various precursors (C-glycosylflavones) of the C-glycosylanthocyanidins were isolated from either flowers of Iris sibirica L., leaves of Hawthorn ‘Crataegi Folium Cum Flore’, or lemons and oranges. This is the first time C-glycosylanthocyanidins have been synthesized. The structures of all flavonoids including the flavone rotamers were elucidated by 2D NMR techniques and high-resolution electrospray MS. The distribution of the various structural forms of 8 and 9 are different at pH 1.1, 4.5, and 7.0, however, the two pigments undergoes similar structural transformations at the various pH values. Pigments 8 and 9 with C-C linkages between the sugar moieties and the aglycone, were found to be far more stable towards acid hydrolysis than pelargonidin 3-O-glucoside, which has the typical anthocyanidin C-O linkage between the sugar and aglycone. This stability may extend the present use of anthocyanins as nutraceuticals, pharmaceuticals or colorants.     

Nouvelles C-glycosylflavones extraites de Spergularia rubra

Three 6,8-di-C-glucosylflavones: 6,8-di-C-β-d-glucopyranosylapigenin, luteolin and chrysoeriol were isolated from the whole plant of Spergularia rubra. Two new compounds, 7,2″-di-O-β-d-glucopyranosyl-6-C-α-l-arabinopyranosylapigenin and 6-C-α-l-arabinopyranosylapigenin (isomollupentin), were also characterized. Structural assignments were based on ¹H NMR and MS spectra and on comparison with synthetic samples. MS fragmentation patterns of the new di-O-glucosyl compound PM derivative and of its acid hydrolysis product are given.     

Structure of a 6,8-di-C-pentosylapigenin from Mollugo pentaphylla

One of the di-C-pentosylflavones isolated from Mollugo pentaphylla was identified as 6-C-β-d-xylopyranosyl-8-C-α-l-arabinop     

Identification of the glycosylflavones of Ephedra and Briza by mass spectrometry of their permethyl ethers

The application of permethylation and mass spectrometry to several glycoflavone samples homogenous by paper chromatography showed them in most cases to be mixtures of closely related isomers. A sample isolated from Ephedra andina leaves was identified by these means as a mixture of vicenin-1 and -2. Application of the procedure to a sample from Briza media thought to be vitexin showed it to be 8-C-galactosylapigenin and this was confirmed by co-chromatography with a synthetic specimen; an acylated 8-C-galactosylapigenin was also detected in the same source.     

Structural determination of 6-C-diglycosyl-8-C-glycosyl-flavones and 6-C-glycosyl-8-C-diglycosylflavones by mass spectrometry of their permethyl ethers

Permethylated 6-C-diglycosyl-8-C-glycosylflavones and 6-C-glycosyl-8-C-diglycosylflavones gave well defined EIMS including the molecular peak and a fragmentation pattern characteristic of the 6-C-glycosyl residue. X″′-O-glycosides (8-C-disaccharides) are thus easily distinguished from X″-O-glycosides (6-C-disaccharides) and, in the latter, the position of the O-glycosidic bond should be deduced from the MS, after acid hydrolysis. Three new C-glycosylflavones have been characterized in this way from Spergularia rubra and Stellaria holostea.     

Sugar ring isomerization in C-arabinosylflavones

6-C-α-l-Arabinopyranosyl- and furanosylacacetins have been synthesized. They are isomerized by short acid treatment to give a mixture of the four anomer/ring size combinations without any Wessely-Moser isomerization. In the same conditions molludistin (8-C-α-l-arabinopyranosylgenkwanin) led only to a mixture of molludistin and 8-C-α-l-arabinofuranosylgenkwanin. This is the first demonstration of ring sugar isomerization in C-glycosylflavones. In usual solvent systems, α-anomers are easily separated from β-anomers, whereas corresponding pyranosyl and furanosyl anomers are not. However, they are easily separated after permethylation and characteristic features are found in the mass spectra of PM 6-C-arabinofuranosyl isomers.     

Flavonoids of Plagiochila asplenioides

Besides an apigenin- and a luteolin-di-C-glycoside, 5 previously unknown di-C-glycosides of tricetin were identified in the gametophytic and sporophytic tissues of Plagiochila asplenioides. Two of them were new 6,8-di-C-hexopyranosyltricetins, and two were new 6-C- hexopyranosyl-8-C-pentopyranosyltricetins. 6-C-Hexopyranosyl-8-C-pentopyranosyltricetin-5′-methyl ether was also found.     

Biologically-active flavonoids from Gossypium arboreum

A new flavonol glycoside, gossypetin 8-O-rhamnoside, was isolated from flower petals of Gossypium arboreum along with quercetin 7-O-glucoside, quercetin 3-O-glucoside and quercetin 3′-O-glucoside. These compounds showed antibacterial activity against Pseudomonas maltophilia and Enterobacter cloacae.     

C-galactosylflavones from Polygonatum multiflorum

8-C-Galactosylapigenin and 6-C-galactosyl-8-C-arabinosylapigenin were isolated from the leaves of Polygonatum multiflorum (L.) All. Structural assignments for the latter compound were made on the basis of mass, CD and ¹³C-NMR spectra.     

Structural determination of C-glycosylflavones by mass spectrometry of their permethyl ethers

Permethylated C-glycosylflavones give well defined MS, including in all cases an important molecular peak. The observed fragmentations are characteristic for the nature and position of the sugar. The 6-C and 8-C glycosylated derivatives are clearly differentiated. In dissymmetrical 6,8-di-C-glycosylflavones, the natures of the sugar in both the 6- and 8- positions can be determined. The structures of several natural compounds are discussed.     

A stereospecific synthesis of l-dendroketose derivatives

The 3,4-O- and 1,2:3,4-di-O-isopropylidene derivatives (7 and 8) of l-dendroketose [4-C-(hydroxymethyl)-l-glycero-pentulose] (1) have been synthesized stereo-specifically from 4-C-(hydroxymethyl)-1,2:3,4-di-O-isopropylidene-l-erythro-pentitol (2).     

A method to differentiate isomeric C-glucosyl chromones,isoflavones and xanthones

6-C- and 8-C-glucosyl isomeric chromones and isoflavones can be readily distinguished by a study of the NMR signals of their acetates. In a similar manner 2-C- and 4-C-glucosylxanthones can be distinguished.     

Flavonoids of the primitive liverwort Takakia and their taxonomic and phylogenetic significance

The flavonoid chemistry of Takakia is described for the first time. T. lepidozioides, thought to be amongst the most primitive of extant liverworts, contains a high level and wide variety of flavone C- and O-glycosides, many of which are unique. New flavonoids include the 8-O-glucuronide and 8-O-xylosylglucoside of takakin (8-hydroxyacacetin), luteolin 6-C-arabinoside-8-C-pentoside, kaempferol 3-O-glucoside-7-O-xyloside and a number of tricetin C-glycosides. The only other known Takakia species, T. ceratophylla, contains the same 4 major constituents but significantly lacks flavonols. The often suggested relationship of Takakia with the order Calobryales is not supported by the available flavonoid data. Biochemical affinities of Takakia with all major liverwort orders are noted and the flavonoid data are interpreted as supporting the concept of Takakia as an isolated branch among the ancestors of modern bryophytes.     

Structural determination of C-glycosylflavones by mass spectrometry of their permethyl ethers: O-glycosyl-6-C-glycosylflavones

Permethylated O-glycosyl-C-glycosylflavones give well defined MS including an important molecular peak. Permethyl 6-C-glycosylflavones O-glycosylated on a phenolic hydroxyl group are easily distinguished from the isomeric permethyl 6-C-diholosylflavones. In both types, the position of the O-glycosidic bond can be deduced from the MS, eventually after acid hydrolysis. 2″-O-glycosyl-6-C-glycosylflavones can be differentiated from their 8-C isomers.     

Unprecedented furan-2-carbonyl C-glycosides and phenolic diglycosides from Scleropyrum pentandrum

Five unprecedented furan-2-carbonyl C-glycosides, scleropentasides A–E, and two phenolic diglycosides, 4-hydroxy-3-methoxybenzyl 4-O-β-d-xylopyranosyl-(1 → 6)-β-d-glucopyranoside and 2,6-dimethoxy-p-hydroquinone 1-O-β-d-xylopyranosyl-(1 → 6)-β-d-glucopyranoside, were isolated from leaves and twigs of Scleropyrum pentandrum together with potalioside B, luteolin 6-C-β-d-glucopyranoside (isoorientin), apigenin 8-C-β-d-glucopyranoside (vitexin), apigenin 6,8-di-C-β-d-glucopyranoside (vicenin-2), apigenin 6-C-α-l-arabinopyranosyl-8-C-β-d-glucopyranoside (isoschaftoside), apigenin 6-C-β-d-glucopyranosyl-8-C-β-d-xylopyranoside, adenosine and l-tryptophan. Structure elucidations of these compounds were based on analyses of chemical and spectroscopic data, including 1D and 2D NMR. In addition, the isolated compounds were evaluated for their radical scavenging activities using both DPPH and ORAC assays.     

Flavone C-glycosides of two Metzgeria species

Six known tricin and apigenin di-C-glycosides, including 2″-O-ferulylisoschaftoside, have been identified in gametophytic material of Metzgeria conjugata. M. leptoneura contains a new di-C-glycoside, tricin 6-C-xyloside-8-C-hexoside. The chemotaxonomic relevance of the flavonoid patterns is briefly discussed.     

Flavone C-glycosides of Apometzgeria pubescens

Eleven flavone di-C-glycosides, including nine which are new, have been identified in gametophytic material of Apometzgeria pubescens. Tricetin 6,8-di-C-glucoside and tricin 6-C-arabinoside-8-C-pentoside are the major compounds. Another identified was ferulylisoschaftoside. The chemotaxonomic relevance of the flavonoid pattern of Apometzgeria pubescens is briefly discussed.     

C-Glycosylflavones from Gnetum buchholzianum and Gnetum africanum

C-Glycosylflavones, including two new compounds 2″-xylosylisoswertisin and 2″-glucosylisoswertisin, were isolated from the leaves of Gnetum buchholzianum and G. africanum. The occurrence of apigenin 7-neohesperidoside distinguishes the latter from the first species.