Beilstein Journal of Organic Chemistry

Two new cycloartane-type triterpene glycosides, namely cimicifoetisides A (1) and B (2), along with seven known compounds cimigenol, 25-O-acetylcimigenol, cimigenol 3-O-β-D-xylopyranoside, 12β-hydroxycimigenol 3-O-β-D-xylopyranoside, cimigenol 3-O-α-L-arabinopyranoside, 25-deoxyshengmanol 3-O-β-D-xylopyranoside and cimilactone A, were isolated from the rhizomes of Cimicifuga foetida. Their structures were elucidated as cimigenol 3-O-(2'-O-acetyl)-α-L-arabinopyranoside (1) and 25-O-acetylcimigenol 3-O-(2'-O-acetyl)-α-L-arabinopyranoside (2). Both compounds 1 and 2 exhibited potent cytotoxicity against rat EAC (Ehrlich ascites carcinoma) and MDA-MB-A231 (human breast cancer) cells with IC 50 values of 0.52 and 6.74 μM for 1, and 0.19 and 10.21 μM for 2, suggesting their potential for further investigation as anti-cancer agents. Black cohosh (Cimicifuga racemosa (L.) Nutt.), [1] also known as bugbane, is widely used in the United States and the European Union as a herbal dietary supplement for the relief of symptoms related to menopause, [2,3] with a clinical history spanning over the last forty years. [4] Due to the large demand of the plant material to meet the ever-increasing American and European market, the alcoholic extract made from the rhizomes of several species of the same genus native to China, particularly C. heracleifolia, C. dahurica, C.simplex, and C. foetida, which are used as anti-pyretic and analgesic agents in traditional Chinese medicines , have been imported into Western markets. [5] In our continuing search for novel anti-cancer agents from natural products, we found that a methanol extract from the rhizomes of C. foetida exhibited considerable cytotox-icity to human cancer cell lines. To date, more than 30 trit-erpene glycosides have been isolated from C. foetida collected from different geographic regions. [6-9] In the present investigation on C. foetida collected from prefecture of Dali county in Yunnan province, Southern China, two novel glycosides, designated cimicifoetisides A (which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Background
Black cohosh (Cimicifuga racemosa (L.) Nutt.), [1] also known as bugbane, is widely used in the United States and the European Union as a herbal dietary supplement for the relief of symptoms related to menopause, [2,3] with a clinical history spanning over the last forty years. [4] Due to the large demand of the plant material to meet the everincreasing American and European market, the alcoholic extract made from the rhizomes of several species of the same genus native to China, particularly C. heracleifolia, C. dahurica, C.simplex, and C. foetida, which are used as anti-pyretic and analgesic agents in traditional Chinese medicines, have been imported into Western markets. [5] In our continuing search for novel anti-cancer agents from natural products, we found that a methanol extract from the rhizomes of C. foetida exhibited considerable cytotoxicity to human cancer cell lines. To date, more than 30 triterpene glycosides have been isolated from C. foetida collected from different geographic regions. [6][7][8][9] In the present investigation on C. foetida collected from prefecture of Dali county in Yunnan province, Southern China, two novel glycosides, designated cimicifoetisides A (1) and B (2), containing a relatively uncommon acetyl-monosaccharide, along with seven known compounds, were isolated and were shown to exhibit potent cytotoxicity against two human cancer cell lines. This report describes the isolation, structure elucidation, and cytotoxicity evaluation of these isolates.

Results and discussions
Cimicifoetiside A (1) (see Figure 1) exhibited a molecular formula of C 37 H 58 O 10 based on its 13 C-NMR DEPT spectrum and negative HRFABMS in which a fragment ion was observed at m/z 619.3854 [M-H-OAc] -(calcd for C 35 H 55 O 9 , 619.3846) due to the facile loss of an acetyl group. The overall physical properties and NMR spectral profile revealed its identity as a member of the cycloartane group of triterpene glycosides, a characteristic and distinguishable chemical marker of Cimicifuga plants. [6] In the 1 H-NMR spectrum (  (Table 1), showed a total of 37 carbon signals, among which, 30 were ascribable to the triterpene aglycone. A characteristic ketalic quaternary carbon signal was observed at δ C 112.0 (s, C-16) together with two oxygen-bearing methine signals at δ C 80.3 (d, C-15) and 90.2 (d, C-24). Two carbons were assigned to an acetyl group [δ C 170.2 and 21.4], and these spectra also showed a set of five oxygenated carbon signals assignable to a pentose moiety [δ C 104.5 (C-1'), 74.4 (C-2'), 72.5 (C-3'), 69.8 (C-4'), and 67.3 (C-5')]. From the above information it was concluded that 1 was a cyclolanostane triterpene linked to a five carbon sugar unit with an acetyl group attached to either the triterpene aglycone or the sugar moiety. But the identities of the triterpene and the monosacchride, the sugar linkage position, and the acetyl group substitution position awaited determination.
Mild acidic hydrolysis of 1 afforded an aglycone which was shown to be cimigenol, a non-glycosylated cycloartane triterpene previously isolated from the same source, [9] by direct co-HPTLC comparison with a reference sample we isolated in the current study, indicating that the aglycone structure of 1 is cimigenol. This conclusion is further supported by comparison of the corresponding 1 H and 13 C-NMR spectral data of the aglycone portion of 1 with those of cimigenol from the literature, [10] after taking the so-called 'glycosylation effect' [11] into account. Consistently, on glycosylation, a 10.7 ppm downfield shift was observed at C-3 accompanied by up-field shifts for the neighboring carbons C-2 (1.4 ppm) and C-4 (0.1 ppm), thereby indicating the sugar moiety to be attached to the C-3 position of the aglycone cimigenol.
To confirm the structure of the aglycone and the glycosidic connection, and to further elucidate the identity of the sugar moiety, a complete 1 H and 13 C-NMR spectral assignment was carried out using a combination of DEPT, COSY, HMQC, and HMBC experiments. The 1 H-1 H COSY, combined with the HMQC spectrum revealed that 1 has the following partial structure: -CH 2 CH 2 CH-(corresponding to C 1 to C 3 ); -CHCH 2 CH 2 CH-(due to C 5 -C 8 ); -CH 2 CH-(for C 11 to C 12 ); -CHCHCH 2 CHCH-(due to C 17 -C 20 -C 22 -C 23 -C 24 -); a geminal proton pair for CH 2 -19; and a set of signals for a pentose, -CHCHCHCHCH 2 -(C-1' to C-5'). All of these segments were compatible for rings A, B, C, D, and E of a 9, 19-cycloartane-type triterpene linked to a five carbon glycosyl unit.
In the HMBC spectrum (See Figure 2), an informative correlation was also observed between the anomeric proton signal at δ H 4.73 (H-1', 1H, J = 6.7 Hz) and a methine carbon signal at δ C 88.7 (C-3), implying that the sugar moiety was linked at the C-3 position. The typical large coupling constants between H-1' and H-2' (J H1'-H2' = 6.7 Hz), and between H-2' and H-3' (J H2'-H3' = 6.7 Hz), as well as the small coupling constant between H-3' and H-4' (J H3'-H4' = 2.7 Hz) indicated the sugar moiety is a pentapyranose with the protons at C-1', C-2', and C-3' axially-oriented, while the proton at the C-4' position is equatorially disposed. Thus, the sugar moiety must be either a-L-arabinopyranosyl-[ 4 C 1 chair conformation) or β-D-arabinopyranosyl [ 1 C 4 chair conformation], with the former being more favorable, as it is a common component of the triterpene glycoside isolated from Cimicifuga plants, whereas the isolation of the latter has not been reported.
Furthermore, the location of the acetyl group could be unambiguously assigned to C-2' of the arabinose unit by HMBC, as a correlation was observed between H-2' (δ H 5.90, t, J = 6.7 Hz) and the carbonyl signal at δ 170.2. On mild alkali hydrolysis with saturated Na 2 CO 3 -MeOH solution, 1 afforded a deacetyl derivative which was shown to be cimigenol 3-α-L-arabinopyranoside, which is also a component isolated previously from CimicifugaI, [12] and was also isolated in the current investigation, by direct comparison from co-HPTLC and 1 H-NMR spectroscopy with an authentic sample. Therefore, the structure of cimicifoetiside A (1) was unambiguously elucidated as cimigenol 3-O-α-L-(2'-O-acetyl) arabinopyranoside. Further evidence supporting this conclusion was derived by direct comparison of its 13 C-NMR spectra with those of cimigenol 3-α-L-arabinopyranoside. It was found that all of the carbon signals were shown to be superimposable, except for the signals arising from C-1' (3.2 ppm up-field shifted), C-2' (1.2 ppm downfield shifted) and C-3' (2.4 ppm up-field shifted) in 1 compared to cimigenol 3-α-Larabinopyranoside. This could be satisfactorily accounted for by the established 'acylation effect' [13] due to the introduction of an acetyl group at C-2' of cimigenol 3-α-L-arabinopyranoside.  C-25, as a significant downfield shift (12.1 ppm) of C-25, and up-field shifts of C-24 (3.2 ppm), C-26 (3.2 ppm), and C-27 (3.9 ppm) were observed in its 13 C-NMR spectrum compared with those of 1, obeying the 'acylation effect' [13] with respect to acetylation occurring at the C-25 position, while the remaining carbon signals were almost identical. Following the same methodology as described for 1, all of the 1 H and 13 C-NMR spectral data of 2 were completely assigned. As shown in Figure 3, the HMBC experiment provided direct and conclusive evidence to assign one acetyl group to the C-2' position of arabinose; while indirect, but compelling, evidence for the assignment of the second acetyl group to C-25 was noted through the unambiguous assignment of the shifted signals of C-24, C-25, C-26, and C-27. The quaternary nature of C-25 prevented the linking of any proton signals to the carbonyl signal from HMBC (See Figure 3), except for the acetyl methyl group. Taken together, the Key long-range 13 C-1 H correlations of 1 observed by HMBC   [15,17] and cimilactone A, [18] by comparing their MS and NMR data to those of the literatures.
It has been reported previously that cimigenol and relative compounds possess cancer chemopreventive activity. [19,20] These results inspire us to explore other activities of this compound family may have. The cytotoxicity of all of the nine compounds was evaluated against the rat tumor EAC cell, and the human SGC7901 and A231 cancer cell lines using an established protocol [21] with a minor modification [see Additional file 1]. As shown in Table 2, both of the two new compounds, especially 2, exerted significant cytotoxicity against the rat EAC tumor cell line with IC 50 values of 0.35 and 0.14 μg/ml for 1 and 2, respectively. Both 1 and 2 also demonstrated moderate inhibition to human MDA-MB-A231 breast cancer cell. While other compounds were shown to be devoid of significant cytotoxicity, implying the higher lipophilic nature of triterpene glycosides could be critical to the cytotoxicity as observed for 1 and 2 whose cytotoxicity are proportional to their lipophilicity (vs. cimigenol 3-O-β-Dxylopyranoside, which is devoid of appreciable activity) with the introduction of acetyl groups. Interestingly, the aglycones without sugar chains are also devoid of noticeable activity, implying the glycosyl linkage is a prerequisite for optimal cytotoxicity although the SAR and the molecular mechanism underlying the cytotoxicity of these compounds have yet been defined.
The results reported herein may suggest the potential for further examination of the cycloartane triterpene glycosides from Cimicifuga for the prevention or treatment of human cancers, especially for breast cancer. It should be noted that the extracts of Cimicifuga are currently widely available for sale as a dietary supplements used for the