New Triterpene Glucosides from the Roots of Rosa laevigata Michx

Two new ursane-type triterpene glucosides, 2α,3α,24-trihydroxyurs-12,18-dien-28-oic acid β-d-glucopyranosyl ester (1) and 2α,3α,23-trihydroxyurs-12,19(29)-dien-28-oic acid β-d-glucopyranosyl ester (2), were isolated from the roots of Rosa laevigata, together with three known compounds: 2α,3β,19α-trihydroxyurs-12-en-28-oic acid β-d-glucopyranosyl ester (3), 2α,3α,19α-trihydroxyurs-12-en-28-oic acid β-d-glucopyranosyl ester (4) and 2α,3β,19α,23-tetrahydroxyurs-12-en-28-oic acid β-d-glucopyranosyl ester (5). The structures of new compounds were established on the basis of detailed 1D and 2D NMR spectroscopic analyses. Compounds 2 and 5 exhibited modest in vitro antifungal activities against Candida albicans and C. krusei.


Results and Discussion
Compound 1 was isolated as an amorphous powder. The molecular formula C 36 H 56 O 10 was established from the quasi-molecular ion [M+Na] + at m/z 671.3779 in the HR-ESI-MS. The IR absorptions at 3428, 1731 and 1645 cm -1 indicated the presence of hydroxyl, carbonyl and olefinic groups, respectively. The UV spectrum showed the absorption of a heteroannular diene at 220 nm [9].
The 1 H-NMR spectrum of 1 (Table 1) displayed signals corresponding to five tertiary methyls at δ H 0.98, 1.05, 1.14, 1.68, and 1.71, a secondary methyl at δ H 1.03, an olefinic proton at δ H 5.61 (br. s) and oxygenated methine and methylene protons, ascribed to a sugar moiety. The 13 C-NMR spectrum showed 36 signals, including 6 primary, 10 secondary, 11 tertiary, and 9 quaternary carbons. These NMR data suggested that 1 was a triterpene monoglycoside. A careful analysis of the 1 H-and 13 C-NMR data, assigned to the aglycon moiety from its 1 H-1 H correlated spectroscopy ( 1 H, 1 H COSY), heteronuclear single quantum coherence (HSQC), and 1 H-detected heteronuclear multiple-bond correlation (HMBC) spectra, suggested that the aglycon was an ursane-type triterpenoid with a heteroannular diene, three hydroxyls and a carboxyl group (C-28). The heteroannular diene was assigned at C-12(13) and C-18(19) by the HMBC correlation from the olefinic proton at δ H 5.61 to the carbon at δ C 135.2 (C-18), as well as the correlations from the methyls at δ H 19.5 and 18.6, assigned to CH 3 -29 and CH 3 -30, respectively, to the same carbon at δ C 133.7 (C-19) ( Figure 1). Two oxymethine protons at δ H 4.46 and 4.60 were observed to correlate to carbons C-10 and C-1, respectively, suggesting that the two hydroxyls were attached to C-2 and C-3. Similarly, the third hydroxyl was determined to be located at C-24 by the HMBC correlation from CH 3 -23 to the oxymethylene carbon (C-24). The sugar moiety was determined to be a D-glucose based on the coupling constants of each proton and the carbon chemical shifts. It was verified by a complete acid hydrolysis with HCl and then comparison with an authentic sample by GC analysis. The chemical shift of the anomeric proton at δ H 6.27 (d, J = 7.8 Hz) revealed that the glucose was attached to the carbonyl carbon at 174.8 (C-28). This was confirmed by a long-range correlation between the anomeric proton and the carbonyl carbon. The relative stereochemistry of 1 was established by analysis of its coupling constants and ROESY data ( Figure 2). The ROESY correlation between CH 3 -23 and H-5 showed the methyl at C-23 was αoriented, and thus the hydroxylmethylene group was in the β-orientation. The signal of H-2 was observed as a ddd splitting with the coupling constants of 10.5, 4.3 and 3.2 Hz, respectively, indicating a diaxial and two axial-equatorial couplings. Furthermore, the coupling constant of 3.2 Hz between H-2 and H-3 revealed an axial-equatorial coupling. Thus, the orientations of both 2-OH and 3-OH were defined as 2α,3α, which was confirmed by the ROESY correlations from H-2 and H-3 to CH 2 -24. Therefore, the structure of 1 was determined to be 2α,3α,24-trihydroxyurs-12,18-dien-28-oic acid β-Dglucopyranosyl ester.  Compound 2 was also isolated as an amorphous powder. The molecular formula was established as C 36 H 56 O 10 , the same as that of 1, by the HR-ESI-MS spectrum. Its UV, IR and 1 H-NMR spectra strongly resembled those of 1, suggesting that 2 shared the same structural skeleton with 1. The 1 H-NMR spectrum of 2 (Table 1) showed the characteristic signals for an exo-methylene at δ H 4.95 (br. s) and 5.10 (br. s), instead of a tertiary methyl in 1, which indicated that the double bond was transferred from C-18(19) to C-19(29) [10]. Another difference observed was the chemical shift value of C-23, which downfield shifted to δ C 71.2 in 2 instead of δ C 65.1 in 1. This evidence suggested that the configuration of CH 3 -24 in 2 might be opposite to that in 1. The relative stereochemistry of 2 was also established by analysis of its coupling constants and ROESY data ( Figure 2).The ROESY correlations ( Figure 2) between CH 2 -23 at δ H 3.88 (d, J = 10.2 Hz) and 3.73 (d, J = 10.2 Hz) and H-5 at δ H 2.03 (m) revealed that the hydroxylmethylene exhibited α-oriented, and CH 3 -24 was then β-oriented. Signals corresponding to H 2 -1, H-2 and H-3 showed the similar chemical shifts and the same multiplicities as 1 in the 1 H-NMR spectrum, indicating that 2 has the same 2α,3α oriented hydroxyls as 1. The ROESY correlations from H-2 and H-3 to CH 3 -24 further supported this stereochemistry assignment. Complete acid hydrolysis with HCl yielded D-glucose, which was determined by GC analysis. Thus, the structure of 2 was established as 2α,3α,23-trihydroxyurs-12,19(29)-dien-28-oic acid β-Dglucopyranosyl ester.
The known compounds 3-5 were identified by comparison with the NMR and MS data with the literature values [6][7][8].

Biological activity
All the isolates were subjected to the dilution assay for in vitro antimicrobial activities against Staphylococcus aureus (ATCC 25923), S. epidermidis (ATCC 26069), Bacillus subtilis (ATCC 6633), Escherichia coli (ATCC 25922), Candida albicans (ATCC 64550), C. krusei (ATCC 6258), C. parapsilosis (ATCC 22019), Klebsiella pneumoniae, Torulopsis glabrata, and Cryptococcus neoformans. The tests were carried out according to the protocols described in the literature [11]. K. pneumoniae, T. glabrata and C. neoformanin were obtained from Huashan Hospital, Shanghai, P. R. China. Two antimicrobial agents, chloroamphenicol and fluconazole, were used as positive controls in these tests. Among the tested compounds, compounds 2 and 5 showed modest antifungal activities against C. albicans and C. krusei with MIC 12.5-25 μg/mL (Table 2). It was observed that the presence of the hydroxymethylene group at C-23 in the ursane-type triterpenoid has a substantial contribution to the antifungal activity. Compounds 2 and 5 (containing such a 23α-hydroxymethylene group) show stronger antifungal activity than compound 1 (24β-hydroxymethylene group) or compounds 3 and 4 (without such functional groups at C-23 positions). Pentacyclic triterpenoids are distributed widely in plants and reported to exhibit extensive bioactivities, such as antimicrobial, anti-tumor, and anti-HIV properties. In this study, ursane-type triterpene glucosides 1-5 were identified from the roots of R. laevigata, and compounds 2 and 5 showed moderate antifugal activities. As the main components, they can account for the bioactivity of the EtOAc extract to some extent. These compounds are the chemical constituents reported for the first time from this part of R. laevigata except for tannins. They can be further considered as the chemical fingerprints of this folk medicine.

Determination of the Sugar Components [10]
Compounds 1-2 (4 mg) in 10% HCl soln./dioxane (1:1, 1 mL) was heated separately at 80 °C for 4 h in a water bath. The mixture was neutralized with Ag 2 CO 3 , filtered, and then extracted with CHCl 3 (30 mL). The aqueous layer was evaporated, and then the residue was treated with L-cysteine methyl ester hydrochloride (4 mg) in pyridine (0.5 mL) at 60 °C for 1 h. After reaction, the solution was treated with acetic anhydride (3 mL) at 60 °C for 1 h. Authentic samples were prepared by the same procedure. The acetate derivatives were subjected to GC analysis to identify the sugars (column temperature 210 °C; injection temperature 250 °C; carrier gas N 2 at a flow rate of 25 mL/min). Dglucose (t R 1.8 min) was observed from 1 and 2.

Antimicrobial activity
Ths was determined by the broth dilution technique as previously described [11]. The solutions (maximum concentration) of the compounds (i.e. the compounds that induced zones of inhibition) were prepared in DMSO, serially (2-fold) diluted and 0.5 mL of each dilution was introduced into a test tube containing 4.4 mL of Selenite broth; then 0.1 mL of microbial suspension (5 × 10 5 cfu/mL) was added and the mixture was homogenized. The total volume of the mixture was 5 mL, with the test compound concentrations in the tube ranging from 200 to 12.5 μg/mL and those of the standard compounds, i.e. Chloroamphenicol and Fluconazole, ranging from 8.0 to 2.0, and 50 to 1.56 μg/mL, respectively. After 24 h of incubation at 37 °C, the MIC was reported as the lowest concentration of a compound that prevented visible growth.