Biochemical Systematics and Ecology 38 (2010) 460e462
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Cycloastragenol glycosides from Astragalus illyricus Matej Barbi c a, *, Allan Patrick G. Macabeo b, Samo Kreft c, Jörg Heilmann a a b c
Institut für Pharmazeutische Biologie, Universität Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany Institut für Organische Chemie, Universität Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany Faculty of Pharmacy, University of Ljubljana, Askerceva cesta 7, 1000 Ljubljana, Slovenia
a r t i c l e i n f o Article history: Received 19 February 2010 Accepted 27 March 2010 Keywords: Astragalus illyricus Cycloastragenol glycosides astragalosides astraverrucin I astragaloside III cyclounifolioside B Fabaceae
1. Subject and source Astragalus, a genus of the leguminous family Fabaceae, comprises about 2500 species of small shrubs and herbs, making it the largest genus of vascular plants on earth. A notable species thriving in southwest Slovenia is Astragalus illyricus Bernh., otherwise known as the Illirian milk-vetch (Jogan et al., 2001). This plant is described as a small flowering-shrub growing in flysch soil and it is usually abundant during the months of September and October. Fresh roots were collected in Gracisce, Slovenia (September, 2005) and a voucher specimen is deposited at the Herbarium of the Faculty of Pharmacy, University of Ljubljana, Slovenia (Nr.: 09/2005-045).
2. Previous work The accumulation of cycloartane- and oleanane-type saponins is a characteristic feature of the secondary metabolite spectrum of the genus Astragalus (Hirotani et al., 1994a). A number of species are well-known to produce cycloastragenol saponins that contain a cycloartane aglycone core chemically known as the epoxy-9b,19-cyclolanostan-3b,6a,16b,25-tetrol structure with a defined stereochemistry at C-20(R) and C-24(S). This cycloartan-3b-ol skeleton is usually oxygenated and glycosylated at various sites i.e. at C-6, C-16, C-20, C-23 and C-24 in a number of Astragalus saponins (Isaev et al., 1989a; Verotta and El-Sebakhy, 2001; Mamedova and Isaev, 2004). Hitherto, no previous phytochemical studies have been done on the title plant.
* Corresponding author. Tel.: þ49 (0) 941 943 4798; fax: þ49 (0) 941 943 4990. E-mail address:
[email protected] (M. Barbi c). 0305-1978/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.bse.2010.03.016
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3. Present study 3.1. Extraction and isolation Dried-powdered roots of A. illyricus(180 g) were extracted with 2 L MeOH, the extract was dried to give a reddish-brown syrupy extract (29 g) and this was subjected to further purification as described below. Isolation of 1e3. A portion of the MeOH extract (15 g) was chromatographed on a reversed-phase column (RP-18 silica) using step-gradient elution with water/MeOH (10% increment) to give 16 fractions. The fraction eluting with 90% MeOH (1.9 g) was purified twice on silica gel with DCM/MeOH (9:1 to 7:3) to give compounds 1 (15.0 mg) and 2 (48.5 mg) and a subfraction, which was again chromatographed on the same solvent system to afford 3 (4.1 mg) Fig. 1. 23 22 21
20
18 19
17 16
24
25
OH
O OH
8
HOH 2C HO HO
30
3
O O 2'
1'
29
OR
OH
28
R = H (1) HOH 2 C HO HO
O
1''
(3)
OH OH O OH
O O
HO HO HOH2 C HO HO
2'
O
1''
O
OH
OH
1'
(2)
Fig. 1. Cycloastragenol glycosides 1e3 from Astralagus illyricus.
3.2. Identification of purified compounds Cycloastragenol glycosides 1e3 were identified unambiguously on the basis of their spectral data acquired from low resolution ESI-MS and NMR experiments. Comparison of experimental data with those reported in the literature ascertained the identity of the A illyricus saponins as astraverrucin I (1) (Pistelli et al., 1997), astragaloside III (2) (Kitagawa et al., 1983; Hirotani et al., 1994a, 1994b; Zhou et al., 1995) and cyclounifolioside B (3) (Kucherbaev et al., 2002). 4. Chemotaxonomic significance Several phytochemical studies dealing with saponins from the genus Astragalus have been reported and a large number of cycloartane-type triterpenoid glycosides were identified although the occurrence of oleanane-based saponins was also accounted in this genus (Hirotani et al., 1994a). Apart from Astragalus, the genera Abrus and Oxytropis in the family Fabaceae also contain these glycosylated metabolites (Verotta and El-Sebakhy, 2001). Our phytochemical efforts on the methanol extract of A. illyricus have resulted in the isolation and identification of three cycloastragenol glycosides (astragalosides) namely, astraverrucin I or cycloaraloside A (1), astragaloside III (2) and cyclounifolioside B (3). Compound 1 has been identified from A. amarus (Isaev et al., 1989b), A. membranaceus, A. peregrines (Verotta and El-Sebakhy, 2001) and A. verrucosus (Pistelli et al., 1997, 1998) while 2 has been found only in A. membranaceus
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and its sub-species, A. membranaceus sub.mongholicus (Kitagawa et al., 1983; Hirotani et al., 1994a, 1994b; Zhou et al., 1995; Kim et al., 2008). Furthermore, saponin diglucoside 3 has been isolated in A. membranaceus (Kim et al., 2008) and A. unifoliolatus (Kucherbaev et al., 2002). Based on this finding, A. illyricus looks chemotaxonomically related to the aforementioned Astragalus species, most especially to A. membranaceus, since both plants contain similar cycloartane saponins. Interestingly, compound 2 has been obtained also from Mirabilis jalapa (Nyctaginaceae) (Lai et al., 2008). In the light of lacking knowledge on the molecular phylogeny of this plant species, this phytochemical study may provide support to future researches regarding the proper sub-genus classification of A. illyricus in the genus Astragalus. Appendix. Supporting information Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.bse.2010.03.016. References Hirotani, M., Zhou, Y., Rui, H., Furuya, T., 1994a. Phytochemistry 36, 665. Hirotani, M., Zhou, Y., Rui, H., Furuya, T., 1994b. Phytochemistry 37, 1403. Isaev, M.I., Gorovits, M.B., Abubakirov, N.K., 1989a. Khimiya Prirodnykh Soedinenii 6, 156. Isaev, M.I., Gorovits, M.B., Abubakirov, N.K., 1989b. Khimiya Prirodnykh Soedinenii 6, 806. Jogan, N., Ba ci c, T., Frajman, B., Leskovar, I., Nagli c, D., Podobnik, A., Rozman, B., Strgulc-Krajsek, S., Tr cak, B., 2001. Materials for the Atlas of Flora of Slovenia. Center za kartografijo Favne in Flore. Miklav z na Dravskem polju. Kim, J.S., Yean, M.H., Lee, E.-J., Kang, S.S., 2008. Nat. Prod. Sci. 14, 37. Kitagawa, I., Wang, H.K., Saito, M., Yoshikawa, M., 1983. Chem. Pharm. Bull. 31, 709. Kucherbaev, K.D., Uteniyazov, K.K., Kachala, V.V., Saatov, Z., Shashkov, A.S., Uteniyazov, K.U., Khalmuratov, P., 2002. Chem. Nat. Compd. 38, 62. Lai, G., Luo, S., Cao, J., Wang, Y., 2008. Zhongguo Zhongyao Zazhi 33, 42. Mamedova, R.P., Isaev, M.I., 2004. Chem. Nat. Compd. 40, 303. Pistelli, L., Pardossi, S., Flamini, G., Bertoli, A., Manunta, A., 1997. Phytochemistry 45, 585. Pistelli, L., Pardossi, S., Flamini, G., Bertoli, A., Potenza, D., 1998. Phytochemistry 49, 2467. Verotta, L., El-Sebakhy, N.A., 2001. Stud. Nat. Prod. Chem. 25, 179e234 (Bioactive Natural Products (Part F)). Zhou, Y., Hirotani, M., Rui, H., Furuya, T., 1995. Phytochemistry 38, 1407.
