Flavone C-glycosides from Trichuriella monsoniae (L.f.) Bennet

Abstract In the first phytochemical investigation of Trichuriella monsoniae, three known flavonoidal C-glycosides, isoswertisin 1, 2″-O-β-d-galactosyl isoswertisin 2 and 2″-O-β-d-xylosyl isoswertisin 3 were isolated from the methanolic extract of the whole plant. Their structures were elucidated by extensive NMR spectroscopic studies including 2D NMR and HRMS, and the structure of 2 was supported by single crystal X-ray data studies. Further, NMR assignments for 3 are being reported for the first time.


Introduction
Trichuriella monsoniae (L.f.) Bennet (syn: Aerva monsoniae (L.f.) Mart.), belonging to the family of Amaranthaceae is a perennial herb. It is found all over India and reported to be useful in the treatment of diabetes, urinary tract infections, wounds and sore throat. It is also used as a diuretic and hepatoprotective agent (Madhava et al. 2008), and has been found to possess anti-hyperglycemic activity (Rani et al. 2014). An aqueous extract of A. monsoniae exhibited angiogenesis, analgesic and anti-inflammatory activities (Sandhya et al. 2012). To our knowledge, this is the first phytochemical investigation on Trichuriella sps., yielding three known isoswertisin type C-glycosides. We herein discuss the isolation and structural elucidation of these isolates.

Results and discussion
All three of the isolated compounds gave a positive colour reaction in the Shinoda test (Jones & Kinghorn 2012) indicating that they were flavonoids and subsequent study of their 1 H and ABSTRACT In the first phytochemical investigation of Trichuriella monsoniae, three known flavonoidal C-glycosides, isoswertisin 1, 2-O-β-d-galactosyl isoswertisin 2 and 2-O-β-d-xylosyl isoswertisin 3 were isolated from the methanolic extract of the whole plant. Their structures were elucidated by extensive NMR spectroscopic studies including 2D NMR and HRMS, and the structure of 2 was supported by single crystal X-ray data studies. Further, NMR assignments for 3 are being reported for the first time. C NMR spectra indicated that they were flavonoidal glycosides. HMBC and COSY analysis of the aromatic protons revealed that the B-ring is 4-substituted and they were glycosylated at the 8th position (C-glycosylation, Figure 1). Thus, they belong to isoswertisin type glycosides and not swertisin glycosides. Structures and the NMR assignments of the three compounds are given in Figure 1 and Table S1 respectively.
Compound 2 was found to be a known bioside previously isolated from Peperomia dindygulensis (Chen et al. 2008), however, for the first time, the structure of 2 was further confirmed by single crystal X-ray diffraction (Figures S1-S2 and Table S2). To our knowledge this is also the first crystallographically characterised flavone C-diglycoside. The compound crystallizes from methanol to afford crystals with the formula 2·1.5(CH 3 OH)·H 2 O. A region of solvent in the lattice (methanol, water) is highly disordered and does not permit a complete analysis of the hydrogen bonding interactions throughout the crystal. An ORTeP diagram of 2 (solvent omitted, Figure S1) confirms the configuration at the 10 stereocentres; the O5-H5⋯O4 intramolecular hydrogen bond is indicated as a dotted line (H5⋯O4 1.857 Å, O5-H5 0.840 Å). Flavonoids tend to be stabilized in the solid state by intermolecular π-stacking interactions (Jiang et al. 2009), an arrangement that is also observed here (packing diagram, Figure S2). The pairwise, face-to-face packing places the flavone cores in close proximity (3.20-3.62 Å), supported by hydrogen bonding interactions between the pair.
The 13 C NMR spectrum of 3 showed only 11 carbon resonances in the glycone region instead of 12 as in 2. Therefore, it was concluded that it contained a pentose and a hexose ring. 1D and 2D NMR analysis and by comparing the 2-O-β-d xylosyl carbon resonances (Bilia et al. 1996), xylose was found to be attached to isoswertisin at the 2nd position of glucose. It is also a known bioside (Ouabonzi et al. 1983;Bakhtiar et al. 1990). The NMR data for this compound is being reported here for the first time (Table S1).

Conclusion
Though the three isolates were known compounds, it is the first time that this plant has been subjected to a phytochemical investigation, making it a new source for isoswertisin type glycosides.

Supplementary material
experimental details, Tables S1 and S2 and Figures S1-S16 are available online.