Indian loumal of Chemistry Vol. 398, August 2000, pp. 638 - 642

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, 730 000. P.R.China

and

J P Kalalinic & L Blok-Tip

Westfaelische Wilhelms-Universitaet Muenster. Institut fuer Medi zin ische 0 -48 149 Muenster. Germany

Received 14 August 1997; accepted (revised) II January 2000

From Aster a/bescellS, a new 3.28-bisglycosidic triterpenoid glycoside named albesoside-A 3 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-3-0-[P-o­glucopyranosyl-( 1---) 3 )-P-o-gl ucopyranosyl]-28-0-[P-o-x ylopyra­nosyl-( l -74)-a..J..-rhamnopyranosyl-( 1-72)-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 1-3 and one diterpenoid 4, 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-3-

O-[p..o-glucopyranosyl- (1 ~3)- p..glucopyranosyIJ-

28-0-[fi-0-xylopyranosyl-( 1 ~4)- a-L-rhamnopyra­

nosyl-(l ~2)-a-L-arabinopyranosyIJ ester. The known compounds 1, 2 and 4 were reidentified as

medicagenic acid-3-0-[fi-0-glucopyranosyIJ-28-0-

[a-L-rhamnopyranosyl-( I ~2)-a-L-arabinopyranosyIJ

ester, medicagenic acid-3-0-[p..0-glucopyranosyl]-

28-0-[ p..o-xylopyranosyl-( 1-~4)-a-L-rhamnopyrano­syl-(l ~2)-a-L-arabinopyranosyl] ester and soulidiol,

1 R, = ~OH

H 0 H

OH

~OH

HO 0 H

OH

3R;e~ OH

H~O OH

Figure 1

R2=H~ OH

R1..=Hif~O\ HO~

OH

which have been isolated previously from alfalfa (Medieago sativa L.) roots 2·4 and from Aster soulie5

,

respectively. A 70% ethanolic extract of the roots of A. albes­

eens was extracted successively with petrol, ethyl acetate and n-butanol. The n-butanol-soluble fraction was further separated by repeated chromatography giving compounds 1,2,3 and 4.,

Compounds 1-3 (ef Figure ]) responded positively to the Liebermann Burchard and Moli sh test. Their IR spectra show strong hydroxyl group absorption (3450 cm" ) and carbonyl group absorption ( 1710-1745 cm·').

The "c NMR spectra of the intact tri terpenoid gly­cos ides suggested that the aglycone of 1-3 had oleanolic acid triterpene skeleton. Acid hydrolysis of ]-3 yielded medicagenic acid, identified from its spectral data6

. The sugar obtained from the hydrol y­sates were identified as glucose, arabinose and rham­nose in 1, and glucose, arabinose, xy lose and rham-

NOTES 639

Table I_I3C NMR data of aglycone and compounds 1-3 in pyridine-d5 (400 MHz)

Carbon

I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Aglycone

44.9 71.5 75.7 53.8 52.1 21.3 33.0 40.1 48 .6 36.8 23.5 122.4 144.8 42 .0 23.8 28 .0 46.5 41.8 46:2 30.7 34.0 33 .0 180.7 13 .5 17.2 16.8 26.0 179.9 33.0 23 .6

1

44.1 70.1 85 .9 52.7 52.3 21.0 32.5 40.2 48 .5 36.7 23.0 122.7 144.1 42.1 23.8 28 . 1 47 .1 41.5 46.1 30.7 34.0 32.5 180.6 14.1 17.2 16.4 25.9 176.1 33 .0 23.5

2

44.1 69.5 85.9 52.7 52.3 21.0 32.5 40.1 48.5 36.6 23.0 122.8 144.0 42.1 23 .8 28 .0 47.1 41.5 46.0 30.7 33 .9 32.5 180.1 14.1 17 .2 16.7 25 .9 176.1 33.0 23.5

nose In 2 and 3, by PC and GC companson with authentic samples.

The J3C NMR spectra of 1-3 indicated the presence

of three anomeric carbon signals (6 105.2, 101.2,

93.4); four anomeric carbon signals (6106.6, 105.1,

100.7, 93 .2), five anomeric carbon signals (6 107.0, 105.6, 104.8, 101.0, 93 .3), respectively. The signal at 8 93.4, 93 .2, 93 .3 suggested that 1-3 have a 28-G­glycosidic linkage and linked with arabinose8

• The presence of a 3-G-glycosidic linkag.e was identified

by the attendant downfield shift at 6 85 .9, 85.9, 86.0 for C-3. Thus, compounds 3 are 3,28-bisglycosidic triterpenoid glycoside as well as compounds 1 and 2.

The alkaline hydrolysis of 1-3 afforded arabinose and rhamnose in 1, arabinose, rhamnose and xylose in 2-3. The negative FAB-MS of 1-3 exhibited psendo­molecular ions at mlz 'J41 [M-Hr, 1073 [M-Hr and

1235 [M-Hr. ind icating Mr of 942 , 1074 and 1236, respectively. Fragment ions at 779 [M-H-Glcr ' 663

[M-H-Rham-AraL 617 [M-H-Rham-Ara-H2C~},

3

44.2 69.4 86.0 52.7 52.4 21.0 32.6 40.2 48 .6 37.7 23 .1 122.8 144.1 42.2 23.9 28. 1 47.2 4. 16 46.2 30.8 34.0 32.6 180. 1 14.1 17.3 16.8 26.0 176.1 33 .0 23.6

3-0-Glc I 2 3 4 5 6

Outer Glc I 2 3 4 5 6

28-0-Ara I 2 3 4 5

Rha I 2 3 4 5 6

Xyll 2 3 4 5

1

105.2 75 .0 78.2 71.4 78.2 62.5

93.4 75.0 70.2 66.2 63 .1 101.2 72.5 72.2 73.7 70.2 18.4

2

105.1 75 .2 78.1 71.7 78.1 62.4

93.2 75 .0 70.0 65 .8 62.6 100.1 72.5 71.3 83.6 68.4 18 .2

106.6 75 .7 78.4 70.8 67. 3

3

104.8 77.7 88.1 73.7 78.4 62.4

105.6 75.4 78.0 71.8 78.2 62.5 93.3 75 .3 70.2 65 .9 62.8 101.0 72.5 71.5 84.0 68.5 18.2 107.0 75 .9 78.4 70.8 67.3

501 [M-H-Rham-Ara-Glcr, 455 [M-H-Rham-Ara­

Glc- H2C0 2r for 1; 911 [M-H-Glc]" , 663 [M-H-Xyl­Rham-Arar, 617 [M-H-Rham-Ara- H2C02L 501

[M-H-Rham-Ara-Glcr for 2; 1103 [M-H-Xylr, 1073

[M-H-Glcr , 911 [M-H-Glc-GlcL 825 [M-H-Xyl­

Rham-Arar, 779 [M-H-Xyl-Rham-Ara-H2C02r for 3

correspond to the subsequent losses of sugar resi­dues. In the positive F AB-MS of the peracetates, 1 displayed fragment IOns at mlz 273 [(Rham)Ac3f , 331 [(Glc)Ac4f, 489 [(Rham-Ara)Ac5f; 2 di splayed fragment Ions at mlz 259 [(Xyl)Ac3f, 331 [(Glc)Ac4f , 489 [(Xyl-Rham)Ac5f , 705 [(Xyl -Rham­Ara)Ac7f ; 3 displayed fragment ions at mlz 259 [(Xyl)Ac3f, 33 1 [(Glc)Ac4f, 619 [(Glc-Glc)Ac7f , 489 [(Xyl-Rham)Ac5J " 705 [(Xyl-Rham-Ara)Acir (see Figure 2). The FAB-MS data demonstrated a sequence of sugars . 3-G-glucose and 28-G-arabi nose­rhamnose In 1; 3-G-glucose and 28-G-arabinose­rhamnose-xylose in 2; and 3-G-glucose-glucose and 28-G-arabinose-rhamnose-xylose in 3 . The sites of sugar linkages were establi shed by J3C NMR spec-

INDIAN J CHEM, SEC B, AUGUST 2000

HO

...••• AcO - AA~Z 705) CooH ~ (mil

Figure 2 - FAB-MS or the acetyl derivative or compound 3

troscopy (Table I) and comparison with that of methyl glycoside of glucose, rhamnose, arabinose and

xylose7• The appearance of downfield signals at 5

75.0, 75.0, 75.3 revealed the presence of a (1~2)­glycosidic linkage between rhamnose and arabinose in 1-3. The downfield chemical shifts of rhamnose at

583.6, 84.0 showed a (1~4)-glycosidic linkage be­tween rhamnose and xylose in 2-3. The appearance of

a downfield signal at 5 88.1 showed a (1 ~3)­

glycosidic linkage between two glucose units in 3. The anomeric configuration of each sugar was

fully defined by the 'H NMR spectra. In the 'H NMR,

the anomeric proton signals for 1 appeared at 56.44 (1H, d, J=2.8 Hz), 5.84 (1H, s), 5.08 (1H, d, J=7 .6 Hz); for 2 appeared at 6.51 (1H, bras), 5.73 (1H, s), 5.14 (lH, d, J=7 .1 Hz), 5.11 (1H, d, J=7.7 Hz); for 3

~ppeared at 56.47 (1H, bras), 5.72 (1H, s), 5.22 (1H, d, J=7.5 Hz), 5.09 (1H, d, J=6.6 Hz), 5.07 (1H, d, J=7.0 Hz ). 111ese data led to the assignment of the anomeric configuration of the glucose and xylose

units as /3, those of rhamnose and arabinose units as a. These assignments were supported by their 13C NMR signals (Table I). In view of the above evi­dences, compound 1 was identified as medicagenic acid-3-0-[ ,B-D-glucopyranosyl ]-28-0-[ a-L-rhamno­

pyranosyl-( I ~·2)-a-L-arabinopyranosyl] ester and

compound 2 as medicagenic acid-3-0-[p.o­glucopyranosyl]-28-0-[p.o-xylopyranosyl-(1~4)-a­

L-rhamnopyranosyl-(l ~ 2)-a-L-arabinopyranosyl] ester, which were previously identified in alfalfa (Medicago sativa L.) roots2

-4. Compound 3 was a new compound, and it was identified as medicagenic acid -3-0-[p.o-glucopyranosyl-(1 ~ 3)-,B-o-glucopyrano­

syl]-28-0-[p.o-xylopyranosyl-(1 ~ 4)-a-L-rham­nopyranosyl-( 1 ~ 2)-a-L-arabinopyranosyl] ester. The position of the glycosidic linkage in compound 3 was comfinned further by GC-MS analysis9

. '0 of the par-

tially methylated alditol acetates. The 1,2,5-triacetyl-3,4,6-trimethyl glucitol was detected from this mix­ture by GC-MS analysis (m/z 189, 161, 145, 101,99, 87, 71 and 55), which demonstrated the terminal glu­cose was linked at C-2 of the inner glucose. There­fore, the structure of compound 3 was established as mentioned above.

The IR spectrum of compound 4 exhibited bands at 3262 ,1751 and 1650 cm" indicating the presence of

hydroxyl, a,,B-unsaturated-y-lactone and double bond, respectively. 'H NMR spectru.m of 4 displayed a

mutually cOtlpled signal at 5 7.10 (I H, t, J= 1.9Hz),

which was characteristic of a proton on the ,B-carbon

of a-substituted butenolide ring, and a signal at

54.78 (2H, d, J=1.9Hz) indicative of the presence of

an a-substituted endocyclic, a ,,B-unsaturated-y­lactone moiety. The 13C NMR spectrum showed two

ethylenic carbon signals at 5129.5 and 144.8 as well

as at 5134.8 and 143.5 which were attributed to C-3/C-4 and C-13/C-14, respectively. By comparison of 'H NMR and 13C NMR spectral data of 4 with those of soulidiol, which was previously identified in Aster souliei5

, compound 4 was .identified as 18,19-

dihydroxyl-5a, I 0,B-neo-cleroda-3 , 13( 14)-dien-16, 15-olide (soulidiol) .

Experimental Section General. Melting points are uncorrected. IR spec­

tra were recorded on a Nicolct-5DX infrared spec­trometer, 'H and 13C NMR spel;;tra in pyridin-d5 on a Bruker AM-400 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-5988 GC-MS spectrometer.

Plant material. Aster a/bescem; was collected at Hubei province, P R China. The specimen was identi­fied by Wuhan Botanical Institute, 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% EtOH 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 (25 g), EtOAc extract (20 g) and n­BuOH extract (100 g). The butanolic lextract (46 g) was chromatographed on silica gel column with

•

NOTES 641

CHCh-MeOH-H20 (20: 1:0~ I: I :0.1) and finally with MeOH to yield 8 fractions in order of increasing po­larity.

Fraction I (80mg) was separated by CC with pet­rol-EtOAc (3: I) as eluant to give compound 4 (15mg). Fraction 3 (600mg) was separated by CC using CHCI3-MeOH-H20 (10: I :0.05) to afford com­pound 1 (150mg) . Fraction 4 was repeatedly chro­matographed over silica gel with CHCI3-MeOH-H20 (8: I :0.05) as eluant to give compound 2 (250mg). Fraction 5 was repeatedly chromatographed on silica gel with CHCl r MeOH-H20 (6: I :0.05) as eluant to yield compound 3 (200mg).

Compound 1: Powder, mp 250-53°C, [a ]D+0.3° (CH30H, c 0.96); IR (KEr): 3496, 1715, 1076, 1052 cm' l ; FAB-MS (negative ion mode): mJz 941 [M-Hr, 779 [M-H-Glcr, 663 [M-H-Rham-Arar, 617 [M-H­Rham-Ara-HC02r, 501 [M-H-Rham-Ara-Glcr, 455 [M-H-Rham-Ara-Glc-H2C02r , IH NMR (400MHz), aglycone moiety : 8 0 .85 , 0 .93 , 1.06, 1.19, 1.50, 1.92 (s, 6 tert .-Me), 5.40 (brs , H-12), sugar moiety: 86.44 (d, J=2.8Hz, H-I of arabinose unit), 5.84 (s, H-l of rhamnose unit), 5.08 (d, J= 7 .6Hz, H-I of glucose unit), 1.67 (d, J=6 .0Hz, methyl of rhamnose unit) ; I3C NMR data: see Table I.

Compound 2: Powder, mp 260-63°C; [a ]D+I0.3° (CH30H, c 0.41), IR (KBr): 3489,3409, 1745, 1711, 1079, 1052 cm' l; FAB-MS (negative ion mode): mJz 1073 [M-Hr, 911 [M-H-Glcr, 663 [M-H-Xyl-Rham­Arar, 617 [M-H-Xyl-Rham-Ara-HC02L 501 [M-H­Xyl-Rham-Ara-Glcr , IH NMR (400 MHz), aglycone moiety: 8 0 .86, 0 .96, 1.09, 1.20, 1.52, 1.98 (s, 6 tert.­

Me), 5.43 (brs, H-12), sugar moiety: 86.51 (brs. H-I of arabinose unit), 5.73 (s , H-I of rhamnose unit), 5.14 (d,J=7.1 Hz, H-I of xylose unit), 5.11 (d,J=7.7 Hz, H-I of glucose unit), 1.75 (d, J=5 .2 Hz, methyl of rhamnose unit) ; 13C NMR data : see Table I.

Compound 3: Powder, mp 256-58°C, [a ]0+80.5° (CH30H, c 0 .5); IR (KBr): 3445, 1715, 1647, 1075, 1052 cm' l, FAB-MS (negative ion mode): mJz 1235 [M-Hr, 1103 [M-H-Xylr, 1073 [M-H-Glcr , 911 [M-H-GIc-Glcr, 825 [M-H-Xyl-Rham-Arar , 779 [M-H-Xyl-Rham-Ara-H2C02L IH NMR (400MHz), aglycone moiety: 8 0 .86,0.96, 1.07,1.21,1.49,1.94 (s, 6 tert.-Me), 5.41 (brs, H-12), sugar moiety: 86.47 (brs, H-I of arabinose unit), 5.72 (s , H-I of rhamnose

unit), 5.22 (d, J=7.5Hz, H-I of glucose unit), 5.09 (d, J=6.6Hz, H-I of xylose unit), 5.07 (d, J=7 .0Hz, H-l

of glucose unit), 1.69 (d, J=5 .5Hz, methyl of rham­nose unit); 13C NMR data: see Table I.

Compound 4: Crystals, mp 146-47°C, [a ]D-21.4°

(CHCl3, c 1.21); IR (KEr): 3261,1753,1644 cm' l; El­MS: mJz 316 [M-H20r , 298, 285,193,175,105 ; IH

NMR (CDCl3, 400MHz): 55.75 (IH, t, J=3.6Hz, H-3), 1.46 (lH, m, H-8), 1.45 (IH, m, H-I0), 7.10 (lH, t, J=1.8Hz, H-l4), 4.78 (2H, d, J=1.9Hz, H-15), 0.85 (3H, d, J=6.6Hz, H-17), 0.81 (3H, s, H-20); 13C

NMR: 517.3 (C-l), 26.6 (C-2), 129.5 (C-3), 144.9 (C-4), 43 .0 (C-5), 31 .0 (C-6), 26.3 (C-7), 36.4 (C-8), 38.8 (C-9), 46.2 (C-I0), 35.8 (C-ll), 19.1 (C-12), 134.8 (C-13), 143.5 (C-14), 70.2 (C-15), 174.4 (C-16), 15 .8 (C-17), 64.2 (C-18), 64.9 (C-19), 18.8 (C-20).

Acid hydrolysis of glycosides. A solution of 1, 2 or 3 (10 mg ) in IN HCI-MeOH (2 mL) was refluxed for 4hr. After neutralization with 5% KOH, water layer was extracted with EtOAc and concentrated under reduced pressure to afford the aglycone, identi­fied by IR, EIMS and IH 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 2, 3. GC of their trimethylsiyl derivatives further con­firmed the sugars.

Alkaline hydrolysis of 1-3. A solution of 1, 2 or 3 (20mg) in 5% KOH -MeOH (3 mL) refluxed for 16 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 2 and 3 .

The sugars obtained after removal of water from the ag. layer was silylated in pyridine with hex­amethyldisilazane and trimethylchlorosilane for 5 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 Insti­tute, 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 IH and 13C NMR spectra. The authors are also thankful to the

National Natural Sci~nce Foundation of China for financial support (No.29272048).

642 [NOlAN J CHEM, SEC B, AUGUST 2000

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