Xeractinol – A New Flavanonol C-glucoside from Paepalanthus argenteus var . argenteus ( Bongard ) Hensold ( Eriocaulaceae )

Um novo composto, isolado a partir do extrato metanólico das folhas de Paepalanthus argenteus var. argenteus (Bongard) Hensold foi caracterizado como xeractinol, um novo diidroflavonol C-glucosilado. Sua estrutura foi elucidada com base em extensiva análise espectroscópica (RMN unie bidimensionais, EM, HREIMS, IV, UV). Cálculos ab initio de estrutura eletrônica corroboram nossa proposição para a estrutura molecular. O diidroflavonol aqui isolado pode servir como marcador taxonômico de Paepalanthus subg. Xeractis, uma vez que este flavonóide não foi encontrado em nenhum outro taxon de Eriocaulaceae.


Introduction
Eriocaulaceae is a pantropical, predominantly herbaceous monocotyledonous family, comprising around 1200 species included in 10 genera. 1 They are frequent components of the vegetation in montane shallow pools or swamps, especially on sandy ground.3][4][5] We have already reported that flavonoids and naphthopyranones are of taxonomic significance to the systematic of Eriocaulaceae. 6,7We report here the isolation and characterization of xeractinol, a new flavanonol from the leaves of Paepalanthus argenteus var.argenteus (Bongard) Hensold.

Results and Discussion
The flavanonol C-glucoside 1 (xeractinol) was obtained directly from the fractionation of the methanolic extract of leaves of P. argenteus var.argenteus by column chromatography on Sephadex LH-20 (Pharmacia) eluted with MeOH.It gave a yellow spot on TLC under UV light.The IR spectrum exhibited bands at 3425 (OH) and at 1656 (C=O) and 1604 cm -1 (C=C).The UV spectra showed absorptions bands at 287 and 330 nm.The ES-MS spectrum of 1 showed a pseudo molecular ion [M+H] + at m/z 467, consistent with a molecular formula C 21 H 22 O 12 .
The [(M+H)-120] + ion was formed by an A-type cleavage, which is consistent with a C-6 glucoside substituent. 9The peak at m/z 177 is due to the ion with formula C 9 H 15 O 5 , resulting from the retro-Diels-Alder fragmentation of 1 in which part of the sugar moiety is bounded to the Aring, thus evidencing that the sugar moiety is linked to this part of the molecule.The formula.The 1 H NMR of 1 showed two doublets with J 11 Hz at δ 4.95 and at δ 4.46, related to the H-2 and H-3, respectively, of a 3-hydroxy substituted flavanone. 9The singlet at δ 5.92 was assigned to H-8.The doublet at δ 6.74 (2H, J 2.0) is related to H-2' and H-6' (magnetically equivalent) and the doublet at δ 6.87 (J 2. Considering the two hydrogen atoms bonded to carbon 2 and 3, there are four possible isomers.The isomer seen in Figure 1 was named trans a. Considering this structure, the isomer trans b has the hydrogen atoms with changed positions relative to the plane of the ring to which they are attached; to cis a the hydrogen attached to carbon 3 changes position with the hydroxyl group.Considering cis a, in cis b both hydrogen change position relative to the plane of the ring.In order to know which one is the most stable isomer, we made use of quantum chemistry theory.The results (Table 1) disfavor the cis configurations, once the difference in total energy between the trans and cis configurations is high enough to prevent the cis configurations to occur.So, at least is possible to conclude that one of the trans configurations is the most probable structure, once the difference in total energy between trans-a and trans-b is very low.Thus, 1 was identified as (2R, 3R)-2-(3,5dihydroxyphenyl)-6-β-D-glucopyranosyl-2,3-dihydro-3,5,7-trihydroxy-4H-1-benzopyran-4-one (xeractinol), Figure 1.
According to previous works, P. subg.Platycaulon is characterized by naphthopyranone derivatives and 7-methoxy flavonol derivatives; 3,4 P. sect.Actinocephalus presents mainly flavonol glycosides and acylated flavonols; 2 in P. subg.Xeractis 6-methoxyflavone derivatives are major constituents as well as in Leiothrix and Syngonanthus, which also contain flavones rather than flavonols. 6,7The dihydroflavonol herein isolated may serve as taxonomic marker of Paepalanthus subg.Xeractis, since this type of flavonoid have not been reported in any other taxon of Eriocaulaceae.

Extraction and isolation
The separated powdered leaves of the species were extracted with CHCl 3 and then with 80% MeOH [(M+H)-90] + ion corresponds to the cleavage of the B-ring.The negative HREIMS [M-H] -at m/z 465.1039 suggest C 21 H 22 O 12 to be the molecular Xeractinol -A New Flavanonol C-glucoside J. Braz.Chem.Soc.

Table 1 .
Differences of theoretical total energy (kJ mol -1 ), taking trans-a as the reference level