3R;e~ OH H~O. Albesoside-A, a new triterpenoid glycoside from the roots of Aster albescens R2=H~ Note

Transcription

1 ndian loumal of Chemistry Vol. 98, August 000, pp. 8 - Note Albesoside-A, a new triterpenoid glycoside from the roots of Aster albescens lie-kai Cheng, Ce-Bin Yu & Dong-Liang Cheng* Oepartment of Chemistry, National Laboratory of Applied Organic Chemistry. Lanzhou University, Lanzhou. Gansu, P.R.China and J P Kalalinic & L Blok-Tip Westfaelische Wilhelms-Universitaet Muenster. nstitut fuer Medi zin ische Muenster. Germany Received 1 August 1997; accepted (revised) January 000 From Aster a/bescells, a new.8-bisglycosidic triterpenoid glycoside named albesoside-a has been isolated along with two known triterpenoid glycosides and one known diterpenoid. On the basis of chemical and spectral evidence. the structure of albesoside-a has been elucidated as medi cagenic acid--0-[p-oglucopyranosyl-( 1---) )-P-o-glucopyranosyl]-8-0-[P-o-x ylopyranosyl-( l -7)-a..J..-rhamnopyranosyl-( 1-7)-a-L-arabinopyranosyl] ester. Aster a/beseens, Chinese name "Xiaosheziwan", has been used as a folk medicine in Tibet for the treatment of many diseases', such as fever,. intoxication, expectoration and tussis. However, as yet no detailed study has been undertaken on its constituents. Chemical studies on the butanol-soluble fraction of dilute ethanolic extract of A. albeseen led to the isolation of three triterpenoid glycosides 1- and one diterpenoid, among these was a new triterpenoid glycoside. This paper deals with the isolation and structural elucidation of the new triterpenoid glycoside, named albesoside-a, together with two known triterpenoid glycosides and one known diterpenoid. The structure of albesoside-a was deduced as medicagenic acid [fi-0-xylopyranosyl-( 1 ~)- O-[p..o-glucopyranosyl- (1 ~)- p..glucopyranosyj- a-l-rhamnopyranosyl-(l ~)-a-l-arabinopyranosyj ester. The known compounds 1, and were reidentified as medicagenic acid--0-[fi-0-glucopyranosyj-8-0- [a-l-rhamnopyranosyl-( ~)-a-l-arabinopyranosyj ester, medicagenic acid--0-[p..0-glucopyranosyl]- 8-0-[ p..o-xylopyranosyl-( 1-~)-a-L-rhamnopyranosyl-(l ~)-a-l-arabinopyranosyl] ester and soulidiol, 1 R, = H 0 H ~ HO 0 H ~ R;e~ H~O Figure 1 R=H~ R1..=Hif~O\ HO~ which have been isolated previously from alfalfa (Medieago sativa L.) roots and from Aster soulie, respectively. A 70% ethanolic extract of the roots of A. albeseens was extracted successively with petrol, ethyl acetate and n-butanol. The n-butanol-soluble fraction was further separated by repeated chromatography giving compounds 1,, and., Compounds 1- (ef Figure ]) responded positively to the Liebermann Burchard and Molish test. Their R spectra show strong hydroxyl group absorption (0 cm" ) and carbonyl group absorption ( cm '). The "c NMR spectra of the intact triterpenoid glycos ides suggested that the aglycone of 1- had oleanolic acid triterpene skeleton. Acid hydrolysis of ]- yielded medicagenic acid, identified from its spectral data. The sugar obtained from the hydrolysates were identified as glucose, arabinose and rhamnose in 1, and glucose, arabinose, xy lose and rham-

2 NOTES 9 Table _C NMR data of aglycone and compounds 1- in pyridine-d (00 MHz) Carbon Aglycone : Glc Outer Glc 8-0-Ara Rha Xyll nose n and, by PC and GC companson with authentic samples. The JC NMR spectra of 1- indicated the presence of three anomeric carbon signals ( 10., 101., 9.); four anomeric carbon signals (10., 10.1, 100.7, 9.), five anomeric carbon signals ( 107.0, 10., 10.8, 101.0, 9.), respectively. The signal at 8 9., 9., 9. suggested that 1- have a 8-Gglycosidic linkage and linked with arabinose 8 The presence of a -G-glycosidic linkag.e was identified by the attendant downfield shift at 8.9, 8.9, 8.0 for C-. Thus, compounds are,8-bisglycosidic triterpenoid glycoside as well as compounds 1 and. The alkaline hydrolysis of 1- afforded arabinose and rhamnose in 1, arabinose, rhamnose and xylose in -. The negative FAB-MS of 1- exhibited psendomolecular ions at mlz 'J1 [M-Hr, 107 [M-Hr and 1 [M-Hr. ind icating Mr of 9, 107 and 1, respectively. Fragment ions at 779 [M-H-Glcr ' [M-H-Rham-AraL 17 [M-H-Rh am-ara-h C~}, 01 [M-H-Rham-Ara-Glcr, [M-H-Rham-Ara Glc- H C0 r for 1; 911 [M-H-Glc]", [M-H-Xyl Rham-Arar, 17 [M-H-Rham-Ara- H C0 L 01 [M-H-Rham-Ara-Glcr for ; 110 [M-H-Xylr, 107 [M-H-Glcr, 911 [M-H-Glc-GlcL 8 [M-H-Xyl Rham-Arar, 779 [M-H-Xyl-Rham-Ara-H C0 r for correspond to the subsequent losses of sugar residues. n the positive F AB-MS of the peracetates, 1 displayed fragment Ons at mlz 7 [(Rham)Ac f, 1 [(Glc)Ac f, 89 [(Rham-Ara)Ac f; di splayed fragment ons at mlz 9 [(Xyl)Ac f, 1 [(Glc)Ac f, 89 [(Xyl-Rham)Ac f, 70 [(Xyl-Rham Ara)Ac 7 f ; displayed fragment ions at mlz 9 [(Xyl)Ac f, 1 [(Glc)Ac f, 19 [(Glc-Glc)Ac 7 f, 89 [(Xyl-Rham)Ac J " 70 [(Xyl-Rham-Ara)Acir (see Figure ). The FAB-MS data demonstrated a sequence of sugars. -G-glucose and 8-G-arabi noserhamnose n 1; -G-glucose and 8-G-arabinoserhamnose-xylose in ; and -G-glucose-glucose and 8-G-arabinose-rhamnose-xylose in. The sites of sugar linkages were establi shed by JC NMR spec-

3 NDAN J CHEM, SEC B, AUGUST 000 HO... AcO - AA~Z 70) CooH ~ (mil Figure - FAB-MS or the acetyl derivative or compound troscopy (Table ) and comparison with that of methyl glycoside of glucose, rhamnose, arabinose and xylose 7 The appearance of downfield signals at 7.0, 7.0, 7. revealed the presence of a (1~) glycosidic linkage between rhamnose and arabinose in 1-. The downfield chemical shifts of rhamnose at 8., 8.0 showed a (1~)-glycosidic linkage between rhamnose and xylose in -. The appearance of a downfield signal at 88.1 showed a (1 ~ ) glycosidic linkage between two glucose units in. The anomeric configuration of each sugar was fully defined by the 'H NMR spectra. n the 'H NMR, the anomeric proton signals for 1 appeared at. (1H, d, J=.8 Hz),.8 (1H, s),.08 (1H, d, J=7. Hz); for appeared at.1 (1H, bras),.7 (1H, s),.1 (lh, d, J=7.1 Hz),.11 (1H, d, J=7.7 Hz); for ~ppeared at.7 (1H, bras),.7 (1H, s),. (1H, d, J=7. Hz),.09 (1H, d, J=. Hz),.07 (1H, d, J=7.0 Hz ). 111ese data led to the assignment of the anomeric configuration of the glucose and xylose units as /, those of rhamnose and arabinose units as a. These assignments were supported by their 1C NMR signals (Table ). n view of the above evidences, compound 1 was identified as medicagenic acid--0-[,b-d-glucopyranosyl ]-8-0-[ a-l-rhamnopyranosyl-( ~ )-a-l-arabinopyranosyl] ester and compound as medicagenic acid--0-[p.o glucopyranosyl]-8-0-[p.o-xylopyranosyl-(1~)-a L-rhamnopyranosyl-(l ~ )-a-l-arabinopyranosyl] ester, which were previously identified in alfalfa (Medicago sativa L.) roots -. Compound was a new compound, and it was identified as medicagenic acid --0-[p.o-glucopyranosyl-(1 ~ )-,B-o-glucopyranosyl]-8-0-[p.o-xylopyranosyl-(1 ~ )-a-l-rhamnopyranosyl-( 1 ~ )-a-l-arabinopyranosyl] ester. The position of the glycosidic linkage in compound was comfinned further by GC-MS analysis 9. '0 of the partially methylated alditol acetates. The 1,,-triacetyl-,,-trimethyl glucitol was detected from this mixture by GC-MS analysis (m/z 189, 11, 1, 101,99, 87, 71 and ), which demonstrated the terminal glucose was linked at C- of the inner glucose. Therefore, the structure of compound was established as mentioned above. The R spectrum of compound exhibited bands at,171 and 10 cm" indicating the presence of hydroxyl, a,,b-unsaturated-y-lactone and double bond, respectively. 'H NMR spectru.m of displayed a mutually cotlpled signal at 7.10 ( H, t, J= 1.9Hz), which was characteristic of a proton on the,b-carbon of a-substituted butenolide ring, and a signal at.78 (H, d, J=1.9Hz) indicative of the presence of an a-substituted endocyclic, a,,b-unsaturated-ylactone moiety. The 1C NMR spectrum showed two ethylenic carbon signals at 19. and 1.8 as well as at 1.8 and 1. which were attributed to C- /C- and C-1/C-1, respectively. By comparison of 'H NMR and 1C NMR spectral data of with those of soulidiol, which was previously identified in Aster souliei, compound was.identified as 18,19- dihydroxyl-a, 0,B-neo-cleroda-, 1( 1)-dien-1, 1-olide (soulidiol). Experimental Section General. Melting points are uncorrected. R spectra were recorded on a Nicolct-DX infrared spectrometer, 'H and 1C NMR spel;;tra in pyridin-d on a Bruker AM-00 instrument, using TMS as internal standard. FAB-MS were recorded on a ZAB-HS mass spectrometer. Gas chromatography was carried out on a Shimadzu Model GC-9AM. GC-MS was taken on HP-988 GC-MS spectrometer. Plant material. Aster a/bescem; was collected at Hubei province, P R China. The specimen was identified by Wuhan Botanical nstitute, Academia Sinica, and deposited in the Herbarium of this institute and in Laboratory of Natural Organic Chemistry, Lanzhou University, Lanzhou. Extraction and isolation. The plant material was extracted with 70% Et at 70 C, and then concentrated in vacuo. The residue was suspended in water, extracted with petrol, EtOAc and n-butanol to give petrol extract ( g), EtOAc extract (0 g) and n Bu extract (100 g). The butanolic lextract ( g) was chromatographed on silica gel column with

4 NOTES 1 CHCh-Me-H 0 (0: 1:0~ : :0.1) and finally with Me to yield 8 fractions in order of increasing polarity. Fraction (80mg) was separated by CC with petrol-etoac (: ) as eluant to give compound (1mg). Fraction (00mg) was separated by CC using CHC-Me-H 0 (10: :0.0) to afford compound 1 (10mg). Fraction was repeatedly chromatographed over silica gel with CHC -Me-H 0 (8: :0.0) as eluant to give compound (0mg). Fraction was repeatedly chromatographed on silica gel with CHCl r Me-H 0 (: :0.0) as eluant to yield compound (00mg). Compound 1: Powder, mp 0- C, [a ]D+0. (CH0H, c 0.9); R (KEr): 9, 171, 107, 10 cm' l ; FAB-MS (negative ion mode): mjz 91 [M-Hr, 779 [M-H-Glcr, [M-H-Rham-Arar, 17 [M-H Rham-Ara-HC0 r, 01 [M-H-Rham-Ara-Glcr, [M-H-Rham-Ara-Glc-H C0 r, H NMR (00MHz), aglycone moiety: 8 0.8, 0.9, 1.0, 1.19, 1.0, 1.9 (s, tert.-me),.0 (brs, H-1), sugar moiety: 8. (d, J=.8Hz, H- of arabinose unit),.8 (s, H-l of rhamnose unit),.08 (d, J= 7.Hz, H- of glucose unit), 1.7 (d, J=.0Hz, methyl of rhamnose unit); C NMR data: see Table. Compound : Powder, mp 0- C; [a ]D+0. (CH0H, c 0.1), R (KBr): 89,09, 17, 1711, 1079, 10 cm' l; FAB-MS (negative ion mode): mjz 107 [M-Hr, 911 [M-H-Glcr, [M-H-Xyl-Rham Arar, 17 [M-H-Xyl-Rham-Ara-HC0 L 01 [M-H Xyl-Rham-Ara-Glcr, H NMR (00 MHz), aglycone moiety: 8 0.8, 0.9, 1.09, 1.0, 1., 1.98 (s, tert. Me),. (brs, H-1), sugar moiety: 8.1 (brs. H- of arabinose unit),.7 (s, H- of rhamnose unit),.1 (d,j=7.1 Hz, H- of xylose unit),.11 (d,j=7.7 Hz, H- of glucose unit), 1.7 (d, J=. Hz, methyl of rhamnose unit); 1C NMR data: see Table. Compound : Powder, mp -8 C, [a ]0+80. (CH0H, c 0.); R (KBr):, 171, 17, 107, 10 cm' l, FAB-MS (negative ion mode): mjz 1 [M-Hr, 110 [M-H-Xylr, 107 [M-H-Glcr, 911 [M-H-Gc-Glcr, 8 [M-H-Xyl-Rham-Arar, 779 [M-H-Xyl-Rham-Ara-H C0 L H NMR (00MHz), aglycone moiety: 8 0.8,0.9, 1.07,1.1,1.9,1.9 (s, tert.-me),.1 (brs, H-1), sugar moiety: 8.7 (brs, H- of arabinose unit),.7 (s, H- of rhamnose unit),. (d, J=7.Hz, H- of glucose unit),.09 (d, J=.Hz, H- of xylose unit),.07 (d, J=7.0Hz, H-l of glucose unit), 1.9 (d, J=.Hz, methyl of rhamnose unit); 1C NMR data: see Table. Compound : Crystals, mp 1-7 C, [a ]D-1. (CHCl, c 1.1); R (KEr): 1,17,1 cm' l; El MS: mjz 1 [M-H 0r, 98, 8,19,17,10; H NMR (CDCl, 00MHz):.7 (H, t, J=.Hz, H- ), 1. (lh, m, H-8), 1. (H, m, H-0), 7.10 (lh, t, J=1.8Hz, H-l),.78 (H, d, J=1.9Hz, H-1), 0.8 (H, d, J=.Hz, H-17), 0.81 (H, s, H-0); 1C NMR: 17. (C-l),. (C-), 19. (C-), 1.9 (C-),.0 (C-), 1.0 (C-),. (C-7),. (C-8), 8.8 (C-9),. (C-0),.8 (C-ll), 19.1 (C-1), 1.8 (C-1), 1. (C-1), 70. (C-1), 17. (C- 1), 1.8 (C-17),. (C-18),.9 (C-19), 18.8 (C- 0). Acid hydrolysis of glycosides. A solution of 1, or (10 mg ) in N HC-Me ( ml) was refluxed for hr. After neutralization with % K, water layer was extracted with EtOAc and concentrated under reduced pressure to afford the aglycone, identified by R, EMS and H NMR as medicagenic acid. PC of the water layer compared with the authentic sugars which showed glucose, rhamnose and arabinose in 1, glucose, rhamnose, arabinose and xylose in,. GC of their trimethylsiyl derivatives further confirmed the sugars. Alkaline hydrolysis of 1-. A solution of 1, or (0mg) in % K -Me ( ml) refluxed for 1 hr. The reaction mixture was cooled to room temperature and neutralized with 1% HC!. The solution was repeatedly extracted with EtOAc and PC of water layer showed rhamnose and arabinose in 1, xylose, rhamnose and arabinose in and. The sugars obtained after removal of water from the ag. layer was silylated in pyridine with hexamethyldisilazane and trimethylchlorosilane for min. The gas chromatography of trimethylsilyl deri vatives compared with that of authentic samples. Acknowledgement The authors are grateful to Prof. Zhi-Ben Tu and senior engineer Zi ~ En Zhao, Wuhan Botanical nstitute, Academia Sinica for his help in obtaining and identification of the plant material, and to the National Laboratory of Applied Organic Chemistry, Lanzhou University, P R China, for recording H and 1C NMR spectra. The authors are also thankful to the National Natural Sci~nce Foundation of China for financial support (No.9708).

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