Total synthesis of Hermannioside A, a novel flavonoid triglycoside from Anthyllis hermanniae

Anthyllis hermanniae L. (Leguminosae) is a shrub typical of the Mediterranean region. Information concerning the phytochemical content of the genus Anthyllis is limited and even more so for the species A. hermanniae [1,2]. The methanol extract of the A. hermanniae yielded two novel flavonoid glycosides, Hermannioside A and B, characterized by an O-linked branched trisaccharide together with other flavonoid glycosides isoflavones, coumarins, sterols and phenolic acids. The structure of the novel glycosides were determined to be quercetin 3-O-[α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranoside]-7-O-α-L-rhamnopyranoside and kaempferol 3-O-[α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranoside]-7-O-α-L-rhamnopyranoside. These previously unreported natural products are very rare due to the presence of the arabinoside unit on position-3 of the aglycone and the (1→2) interglycosidic linkage within the disaccharide residue. Preparative HPLC was performed for the isolation of Hermannioside A & B and ESI-MS, UV-Vis and NMR spectroscopy (1 & 2D) were employed for the structural elucidation of the isolated constituents. Dispite of the major role in the growth of plants and at the interesting biological activities, the structural elucidation and chemical characterization of flavonoid glycosides with 2, 3 or even more sugar moieties is often incompetent due to the complexity of the NMR spectra [3] and at the same time only a few general and efficient methods have been described for their synthesis especially for flavonol 3-O-pentopyranosylpentopyranosides, 3-O-hexopyranosylpentopyranosides and 7-O-hexopyranosyl-3-O-hexopyranosylpentopyranosides. Hermannioside A was synthesized through a combination of stereospecific phase-transfer-catalysed conditions [4] and C-7 glycosylation reaction catalyzed by trimethylsilyl triflate [5].

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