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X.-M. ZHANG ET AL
from S. mukorossi are mainly sesquiterpene oligoglycosides and triterpenoidal saponins
of hederagenin, dammarane and tirucullane (Upadhyay & Singh 2012). These glycosides
have antimicrobial (Ibrahim et al. 2006), cytotoxic (Chen et al. 2010), molluscicidal (Huang
et al. 2003; Upadhyay & Singh 2011), insecticidal (Rahman et al. 2007), fungicidal (Supradip
et al. 2010) and hepatoprotective (Peng et al. 2014) properties. In this study, we isolated and
characterized two new glycosides and evaluated the in vitro antiproliferative activity of all
compounds against A549 cells.
2. Results and discussion
2.1. Structural elucidation
Compound 1 was a white amorphous powder. Its molecular formula was C63H102O28 based
on HR-ESI-MS and MS/MS data (Figure 1). The 1H NMR spectrum had signals characteristic
of seven tertiary singlet methyl groups [δH 0.87, 1.02, 1.04, 1.15, 1.33, 1.33, and 1.34], two
secondary methyl groups [δH 1.57 (3H, d, J = 5.4 Hz) and 1.57 (3H, d, J = 5.4 Hz)], an olefinic
proton [δH 5.50 (1H, br s)] and an oxygen-bearing methine proton [δH 3.32 (1H, overlapped
‘os’)]. The 1H NMR spectrum of 1 revealed the presence of six anomeric proton signals at δH
4.88 (1H, d, J = 7.8 Hz, Ara H-1′), 6.23 (1H, br s, Rha H-1′′), 5.36 (1H, d, J = 7.4 Hz, Xyl H-1′′′),
4.90 (1H, d, J = 8.0 Hz, Glc H-1′′′′), 5.30(1H, d, J = 6.8 Hz, Xyl H-1′′′′′) and 6.25 (1H, br s, Rha
H-1′′′′′′), which were correlated with the 13C NMR signals for anomeric carbons at δC 105.6
(C-1′), 102.0 (C-1′′), 108.0 (C-1′′′), 105.7 (C-1′′′′), 107.8 (C-1′′′′′) and 102.1 (C-1′′′′′′), respec-
tively. The corresponding seven angular methyl groups [δC 16.1, 17.7, 17.7, 24.3, 26.7, 28.7,
and 28.7], olefinic signal [δC 123.0, 145.3] and carboxyl group [δC 180.7] were detected. These
findings implied that compound 1 had an oleanane-type triterpene and six sugar moieties.
The linkage points of the sugar units to each other and to the aglycone were determined
by the following HMBC correlations (see supplementary material, Figure S1): δH 4.88 (Ara-1′)
with δC 89.3 (aglycon C-3), δH 6.23 (Rha-1′′) with δ 75.9 (Ara C-2′), δ 5.36 (Xyl-1′′′) with δC 83.5
(Rha C-3′′), δH 4.90 (Glc-1′′′′) with δC 89.3 (Xyl C-3′C′′), δH 5.30 (Xyl-1′′H′′′) with δC 67.9 (Glc C-6′′′′)
and δH 6.25 (Rha-1′′′′′′) with δC 83.3 (Xyl C-2′′′′′). The relative configuration of the aglucone
and the sugar linkages were confirmed by ROESY spectrum (Figure S2). The relatively large
coupling constants for the anomeric protons of compound 1 revealed an α-configuration
of the arabinose unit and a β-configuration of the glucose and xylose units. Even though
the anomeric protons of two rhamnose moieties were observed as singlets in the 1H NMR
spectrum, the 13C NMR shifts of Rha C-5 at δC 69.7 and 70.0 indicated an α-configuration
(Wang et al. 2013). Acid hydrolysis of compound 1 yielded L-arabinose, d-glucose, d-xylose
and l-rhamnose, which were detected by GC-MS analysis of their derivatives. Accordingly,
compound 1 was identified as 3-O-α-l-rhamnopyranosyl-(1→2)-β-d-xylopyranosyl-(1→6)-
β-d-glucopyranosyl-(1→3)-β-d-xylopyranosyl-(1→3)-α-l-rhamnopyranosyl-(1→2)-α-l-arab-
inopyranosyl oleanolic acid, and was named sapindoside G.
Compound 2 was a white powder. The negative ion HR-ESI-MS spectra of compound
2 showed quasimolecular ion peaks at m/z 1231.56188 [M−H]−; its molecular formula was
C55H92O30 (Figure 1). The 1H and 13C NMR spectra of compound 2 had signals characteristic
of three methyl groups [δH 1.01 (3H, d, J = 7.2 Hz, Me-15), 1.60 (3H, s, Me-14) and 1.63 (3H,
s, Me-13)]; two methylene groups bearing an oxygen function [δH 3.34 (1H, m, H-12α), 3.84
(1H, os, H-12β), 4.28 (1H, os, H-1α) and 4.56 (1H, os, H-1β)], two tri-substituted olefins [δH 5.21
(1H, m, H-6), 5.60 (1H, os, H-2)]; two β-d-glucopyranosyl moieties [δH 4.68 (1H, d, J = 7.2 Hz,
Glc H-1′′′′) and 4.73 (1H, d, J = 7.2 Hz, Glc H-1′)]; four α-l-rhamnopyranosyl moieties [δH 1.61