Three new monoterpene glucosides from Sibiraea angustata

Abstract Three new monoterpene glycosides, named sibiraglycoside I(1), J(2) and K(3), were isolated from an aqueous extract of the aerial portion of Sibiraea angustata. Their structures were elucidated on the basis of extensive spectroscopic data analysis (including 1D and 2D NMR and MS experiments) and compared to literature data. Three new monoterpene glucosides were isolated and identified from the plant of Sibiraea angustata.


Introduction
Sibiraea angustata (Rosaceae) is a shrub, widely distributed in the western part of China, including Qinghai, Gansu, Sichuan, Xizang and Yunnan provinces, whose leaves and twig could be used as a common and civil traditional medicine in Tibet, for the treatment of indigestion and upset stomach for many years, known as 'Liucha' . And long term usage of Liucha benefits human body and reduces lipids (Institute of Botany 1985). As for, thorough chemical and bioactive studies have been performed on this plant, leading to the isolation of many active constituents, such as terpenes, phenolic acid, phenolic ester, saponins, volatile oils and polysaccharides (Yan et al. 2007;Xie et al. 2011;Wang et al. 2014 papers, the identification of 13 monoterpenes and monoterpene glycosides (Ito et al. 2009;Li et al. 2010Li et al. , 2015Wang et al. 2013), as well as a dicaffeic acid ester (Li et al. 2015) from the plant were reported, and the monoterpene glycosides showed significant anti-obesity activity. In our continual research, the isolation and structural elucidation of three new monoterpene glycosides, sibiraglycoside I (1), sibiraglycoside J (2) and sibiraglycoside K (3) from the aerial part of S. angustat was presented.

Results and discussion
The powdered residue of the aqueous extract from the aerial part of S. angustata was extracted by the ethanol first, and the extraction was concentrated under reduced pressure to yield a solid brown material, which was applied to a Diaion HP-20 chromatography column eluting with a gradient of C 2 H 5 OH/H 2 O. Fortunately, three new monoterpene glycosides, sibiraglycoside I (1), sibiraglycoside J (2) and sibiraglycoside K (3) were obtained from the 20% Ethanol fraction by next various column chromatography (CC). Their structures were elucidated by extensive spectroscopic methods including 1D ( 1 H, 13 C NMR) and 2D NMR (HSQC, HMBC, 1 H-1 H COSY and ROESY) experiments as well as HRESI-MS and ESI-MS analysis and by comparison with those of the literature data.
1 contained a hexose moiety which was identified as a glucose from the methyl-1-Oglucoside obtained on methanolysis. The configuration of the glycosidic linkage was β as based on the coupling constant of the anomeric proton at δ H 4.15 (d, J = 7.8 Hz). The sugar β-d-glucose was also identified by GC analysing the trimethylsilyl ether derivatives of its acidic hydrolysis products with standard sugars (Ito et al. 2009). In the ROESY spectrum signal at δ H 2.37 (H-2) showed long correlations with signal at δ H 6.26 (H-4) declaring these two protons occupied the same sides, so the geometry of double bond is E. Thus, the structure of 1 was determined to be 3,7-dimethyl-7-hydroxy-3(E)-octene-5-one-1-O-β-d-glucopyranoside and named sibiraglycoside I (Figure 1).
Compound 2 was obtained as a colourless gum with an [ ] 20 D value of −9.0 (c 0.001, CH 3 OH), the molecular formula of which was determined to be C 16 H 28 O 8 from the molecular ion peak at m/z 371.5321 [M + Na] + (Calcd for 371.5336) in the HRESI-MS, indicating 3° of unsaturation. The IR spectrum showed absorption bands characteristic of a hydroxyl group (3397 cm −1 ), a conjugated carbonyl group (1673 and 1610 cm −1 ). The UV spectrum exhibited absorption maxima at 252 nm (log ε = 2.79). The spectral data are very similar to those of 1. The differences are a downfield shift of the allylic methylene group [δ H 2.78 (2H, t, H-2)] in 2 from [δ H 2.37 (2H, t, H-2)] in 1, and an upfield shift of the vinyl methyl group [δ H 1.90 (3H, s, H-10)] in 2 from [δ H 2.06 (3H, s, H-10)] in 1. Thus, 2 was presumed to have a structure in which the double bond was isomerised to the Z-configuration. In fact, the appearance of a signal between H-4 and H-10 in the NOESY spectrum suggested that these protons were long correlations, so the geometry of double bond is Z. Thus, the structure of 2 was elucidated as 3,7-dimethyl-7-hydroxy-3(Z)-octene-5-one-1-O-β-d-glucopyranoside and named sibiraglycoside J (Figure 1).

General procedures
Optical rotations were measured on a Perkin Elmer 241 automatic digital polarimeter using Ethanol as solvent. UV spectra were recorded on a Shimadzu UV-300 spectrophotometer. IR spectra were recorded on a Nicolet 5700 FT-IR spectrometer by a transmission microscope method. NMR spectra were obtained on an INOVA-500 and Mercury-400 spectrometers in DMSO-d 6 with solvent peaks as references (δ H 2.50, δ C 39.7). GC was conducted using an Agilent 7890A instrument (Agilent). HRESI-MS and ESI-MS spectra were measured on an Agilent 1100 Series LC/MSD ion trap mass spectrometer. Analytical HPLC was run on a Shimadazu LC-15C instrument with a SPD-10A detector using a YMC column (RP-C 18 , 4.6 × 250 mm, 5 μm). Preparative HPLC was performed on a Shimadzu LC-6AD instrument with an SPD-20A detector using a YMC-Pack ODS-A column (250 × 20 mm, 5 μm). The Sephadex LH-20 was obtained from Amersham Pharmacia Biotech AB Factory, Sweden. CC was performed with Diaion HP20 macroporous resin (Mitsuboshi Chemical Industries, Tokyo Japan), silica gel (60-100 and 200-300 mesh, Qingdao Marine Chemical Inc, PR China) and ODS (50 μm, YMC, Japan). TLC was carried out on glass precoated with silica gel GF254 plates (Qingdao Marine Chemical Inc, PR China). Spots were visualized under UV light or by spraying with 5% vanillin-sulphuric acid solution followed by heating.

Plant Material
S. angustata was collected in the Sichuan province, China in 2002 and identified by professor Wang Tianzhi in West China School of Pharmacy, Sichuan University, China, and a voucher specimen number specimen (001612) had been deposited in Herbarium of Jiangxi University of Traditional Chinese Medicine.

Acid hydrolysis and sugar analysis of compounds 1, 2 and 3
Compounds 1-3 (each 3 mg) were refluxed in 2 M HCl (3 mL) at 60 °C for 3 h and concentrated to give a brown material, then dissolved in water (3 mL) and partitioned by Ethyl Acetate (3 mL × 3), the aqueous layer was evaporated to dryness under reduced pressure to give the three monosaccharide residues. The absolute configuration of the monosaccharide was determined based on a reported literature (e.g. Kinjo et al. 1992). Authentic monosaccharide samples were trimethylsilylated by l-cysteine methyl ester hydrochloride and N-trimethylsilylimidazole. Then the derivatives were analysed by GC, and the retention times (t R ) were as follows: d-glucose at 19.83 min and l-glucose at 20.01 min. The monosaccharides obtained from the acid hydrolysis of compounds 1-3 were treated in a similar manner to the authentic monosaccharide samples and were both identified as d-glucose (19.83 min), which are matched to the standard d-glucose.

Conclusion
From an aqueous extract of the aerial portion of S. angustata, three new monoterpene glycosides named sibiraglycoside I, J and K were isolated and their structures were elucidated on the basis of the spectroscopic evidence.

Supplementary material
Supplementary material relating to this article is available online, alongside Figures S1-S2.