Triterpene glycosides of Astragalus and their genins. LXVII. Structure of cycloexoside B

Article abstract of DOI:10.1023/A:1022647006958

Another seven components from the roots of Astragalus exilis A.Kor. (Leguminosae) were identified using spectral data and chemical transformations. A triterpenoid of genin nature was identical to cyclosiversigenin. One compound of glycosidic nature turned out to be a new cycloartane glycoside called by us cycloexoside B of structure 20R,24S-epoxycycloartan-3β,6α,16β ,25-tetraol 3-O-β-D-(2-O-acetyl)xylopyranoside. Five glycosides were identified as cyclosiversigenin 3-O-β-D-xylopyranoside and cyclosiversiosides B, C, D, and G.

Full text of DOI:10.1023/A:1022647006958

Chemistry of Natural Compounds, Vol. 38, No. 6, 2002
TRITERPENE GLYCOSIDES OF Astragalus AND THEIR GENINS.
LXVII. STRUCTURE OF CYCLOEXOSIDE B
R. P. Mamedova, M. A. Agzamova, and M. I. Isaev
UDC 547.918:547.926
Astragalus exilis
Another seven components from the roots of
A.Kor. (Leguminosae) were identified using
spectral data and chemical transformations. A triterpenoid of genin nature was identical to
cyclosiversigenin. One compound of glycosidic nature turned out to be a new cycloartane glycoside called
β α
β
β D
by us cycloexoside B of structure 20R,24S-epoxycycloartan-3 ,6 ,16 ,25-tetraol 3-O- - -(2-O-
β D
acetyl)xylopyranoside. Five glycosides were identified as cyclosiversigenin 3-O- - -xylopyranoside and
cyclosiversiosides B, C, D, and G.
Key words: triterpenoids, cycloartanes, cyclosiversigenin, cycloexosideB, cyclosiversiosides,
, Leguminosae,
Astragalus
PMR, 13C NMR, DEPT, J-modulation.
We continued chemical investigations of isoprenoids from plants of the
genus (Leguminosae) [1] by
Astragalus
isolating and identifying from the roots of
A.Kor. four cycloartane glycosides: cycloexoside (4) and cyclosiversiosides
A. exilis
A (5), E (9), and F (10) [2]. We also isolated another seven substances in continuing the study of this plant. Six of the
compounds turned out to be known and were identified as cyclosiversigenin (2), cyclosiversigenin 3-O-β-D-xylopyranoside (3),
and cyclosiversiosides B (6), C (7), D (8), and G (11) [3]. One compound was a new glycoside called by us cycloexoside B (1).
OH
OH
This work examines its structure.
H
H
O
O
OH
OH
+
H
HO
O
1
2
O
OH
OH
OH
OH
HO
OH
CH COO
3
H
H
−
HO
O
O
OH
OH
R = R = Ac, R = H;
4:
1
2
5: R = R = Ac, R = β-D-Xylp;
1
2
6: R = R = Ac, R = β - D-Glcp;
1
2
O
O
7: R = Ac, R = H, R = β-D-Xylp;
1
2
O
O
OH
8: R = Ac, R = H, R = β-D-Glcp;
OR
1
2
2
OH
OR
1
9: R = R = H, R = β-D-Xylp;
1
2
HO
10: R = R = H, R = β-D-Glcp;
HO
1
2
3
4 - 11
OH
OR
11: R = α-L-Rhap, R = H, R = β-D-Xylp
1
2
S. Yu. Yunusov Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan,
Tashkent, fax (99871) 120 64 75. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 460-463, November-December,
2002. Original article submitted December 9, 2002.
0009-3130/02/3806-0579$27.00 ©2002 Plenum Publishing Corporation
579

TABLE 1. Chemical Shifts of C Atoms of compounds 1-3 (δ, ppm, C5D5N, 0 = TMS)
Compound
C atom
1
2
3
1
2
3
4
5
6
7
8
32.25
30.08
89.01
42.24
53.80
68.09
38.66
47.14
20.95
29.43
26.10
33.30
44.96
46.05
46.61
73.40
58.32
21.54
30.70
87.18
28.63
34.86
26.35
81.67
71.22
27.03
28.08
20.12
28.47
16.39
32.81
31.47
78.32
42.46
54.00
68.38
38.85
47.30
20.99
29.92
26.32
33.47
45.09
46.21
46.81
73.48
58.44
21.66
31.02
87.27
28.59
34.97
26.17
81.75
71.27
27.17
28.21
20.27
29.44
16.14
32.50
30.35
88.72
42.72
54.12
68.02
38.67
47.07
21.05
29.55
26.25
33.43
45.07
46.16
46.73
73.44
58.40
21.53
30.63
87.25
28.94
34.95
26.44
81.73
71.26*
27.15
28.19
20.20
28.55
16.68
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
β-D-Xylp
1
2
3
4
5
104.79 (-2.84)
76.15 (+0.52)
75.63 (-2.87)
71.27
107.63
75.63
78.50
71.26*
67.05
67.02
CH3COO
CH3COO
21.21
170.09
______
*Signals overlap.
The PMR spectrum of 1 contains 1H doublets of an AX system at 0.16 and 0.42 ppm with spin—spin coupling
2
constants (SSCC) J = 4.5 Hz in addition to signals of seven methyls in the range 0.88-1.68 ppm. This indicates that the
observed compound is a cycloartane triterpenoid [3, 4]. Therefore, cyclosiversigenin was isolated from the acid hydrolysate of
1 as the genin. Paper chromatography (PC) of the carbohydrate part of the acid hydrolysate detected D-xylose. PMR and
13C NMR spectra of 1 (Table 1), which contain signals of one pentose, indicate that the studied glycoside is a
monoxylopyranoside.
580

The PMR spectrum of glycoside 1 has a 3H singlet at 2.05 ppm, which indicates that this glycoside contains one acetic
acid unit. As expected, signals of one acetyl are found in the 13C NMR spectrum of a new glycoside 1 at 21.21 and 170.09 ppm.
Alkaline hydrolysis of 1 produces glycoside 3, identified as cyclosiversigenin 3-O-β-D-xylopyranoside [3]. Therefore,
cycloexoside B is cyclosiversigenin 3-O-β-D-xylopyranoside monoacetate. The site of attachment of the acetyl group was
revealed by comparing the PMR and 13C NMR spectra of compounds 1-3.
Atoms C-6, C-16, and C-25 of 1 resonate in the 13C NMR spectrum at 68.09, 73.40, and 71.22 ppm, respectively.
These values are practicallyidentical tothose in the13C NMR spectra of cyclosiversigenin and glycoside 3. Therefore, the acetyl
unit is not located in the genin part of 1. In fact, the PMR spectrum of 1 has at 5.46 ppm a 1H doublet of doublets with SSCCs
3
3
J = 9 and J = 8 Hz that belongs to a proton geminal to the acetoxy unit. These parameters are similar to those for H-2 and
1
2
H-3 of a β-D-xylopyranoside unit. The magnitude of the chemical shift does not allow unambiguous assignment of this signal.
Sometimes H-2 and H-3 of a β-D-xylopyranoside unit resonate as triplets of identical SSCC, which complicates the assignment
[2]. In this instance, the signals can be assigned by analyzing the fine multiplicities and the SSCCs of H-1, H-2, and H-3 of
the β-D-xylopyranoside unit, which forms a system of vicinally coupled protons. The anomeric proton resonates as a doublet
with J = 8 Hz. As already mentioned, the doublet of doublets from the proton geminal to the acetoxygroup has the same
1,2
SSCC (J1,2 = 8 Hz) and therefore belongs to H-2 of the pentose. The third proton resonates as a triplet with J2,3 = J3,4 = 9
Hz at 4.07 ppm. Therefore, it is H-3 of the monosaccharide unit.
This means that the acetyl group is located on C-2 of β-D-xylose. The positive reaction to periodate oxidation of 1,
which indicates the presence of an α-diol in the molecule, confirms the location of the acetyl group.
Additional confirmation of the acetyl position comes from a comparison of the 13C NMR spectra of 1 and 3. Table 1
shows that the chemical shifts of C-1 (-2.84 ppm), C-2 (+0.52), and C-3 (-2.87) ofthe monosaccharide unit change considerably
on going from 3 to 1. The signs and magnitudes of these changes agree well with the α- and β-effects of an acetyl located on
C-2 of the β-D-xylopyranoside ring [5] and unambigously determine the location of the acyl group.
Thus, 1 has the structure 20R,24S-epoxycycloartan-3β,6α,16β,25-tetraol 3-O-β-D-(2-O-acetyl)-xylopyranoside.
EXPERIMENTAL
For general comments, see the literature [6]. The followingsolventsystemswereused:CHCl —CH OH(25:1, 1; 15:1,
3
3
2), CHCl —CH OH—H O (140:14:1, 3; 70:12:1, 4; 70:23:4, 5), n-BuOH—C H N—H O (6:4:3, 6).
3
3
2
5
5
2
PMR and 13C NMR spectra were recorded on Bruker DRX-500, Bruker AM-400, and UNITYplus 400 spectrometers
in C D N (δ, ppm, 0 = TMS). 13C NMR spectra were obtained with complete C–H decoupling and under J-modulation and
5
5
DEPT conditions.
Isolation and Separation of Triterpene Glycosides of Astragalus exilis. Another batch of air-dried roots (750 g) of
A. exilis collected during flowering (July) from a gorge near the Gornaya Khanaki river (southern slopes of the Gissarsk crest,
Tadzhikistan) was extracted with CH OH (2.5 L ×3). The methanol extracts were evaporated to dryness to afford dry extract
3
(120 g). Chromatography over a silica-gel column with successive elution by CHCl and systems 1-5 and rechromatography
3
of the individual fractions isolated 11 pure compounds that are presented in order of increasing polarity.
Known compounds were identified bycomparison with authentic samples. PMRand 13C NMR spectral data are given
for the new compound 1 and for other compounds to prove their structures.
Cyclosiversigenin (2), 15 mg (0.002%, yields here and hereafter are based on air-dried raw material), C30H50O5,
mp 239-241°C (MeOH) [3, 7, 8].
2
PMR spectrum (400 MHz, C D N, 0 = TMS, δ, ppm, J/Hz): 0.36 and 0.63 (2H-19, d, J = 4), 1.03, 1.31, 1.34, 1.38,
5
5
3
2
3
3
3
1.46, 1.59, 1.90 (7×CH , s), 2.55 (H-17, d, J = 7.8), 3.12 (H-22, td, J = J = 11.3, J = 9), 3.66 (H-3, dd, J = 11.5,
3
3
1
2
1
3
3
3
3
3
3
3
3
J = 4.6), 3.80 (H-6, td, J = J = 9.6, J = 3.6), 3.90 (H-24, dd, J = 8.8, J = 5.7), 5.03 (H-16, q, J = J = J = 7.6).
2
1
2
3
1
2
1
2
3
Table 1 lists the 13C NMR spectrum.
Cycloexoside (4), 200 mg (0.27%), C39H62O11, mp 193-196°C (MeOH) [2].
Cycloexoside B (1), 70 mg (0.009%), C37H60O10. PMR spectrum (500 MHz, C D N, 0 = TMS, δ, ppm, J/Hz): 0.16
5
5
2
3
and 0.42 (2H-19, d, J = 4.5), 0.88, 1.16, 1.21, 1.23, 1.32, 1.48, 1.68 (7×CH , s), 2.05 (CH COO, s), 2.43 (H-17, d, J = 8), 2.97
(H-22, q, J = J = J = 10), 3.38 (H-3, dd, J = 12, J = 4), 3.59 (H-5a of D-xylose, dd, J = 11, J = 10), 3.63 (H-6, td,
J = J = 9, J = 3), 3.79 (H-24, dd, J = 9, J = 5), 4.07 (H-3 of D-xylose, t, J = J = 9), 4.11 (H-4 of D-xylose, m),
3
3
2
3
3
3
3
2
3
1
2
1
3
2
3
3
3
3
3
3
1
2
3
1
2
1
2
581

2
3
3
4.22 (H-5e of D-xylose, dd, J = 11, J = 5), 4.74 (H-1 of D-xylose, d, J = 8), 4.90 (H-16, m), 5.46 (H-2 of D-xylose, dd,
3
3
J = 9, J = 8).
1
2
Table 1 lists the 13C NMR spectrum.
Cyclosiversioside A (5), 180 mg (0.024%), C44H70O15, mp 229-230°C (MeOH) [3, 9].
Cyclosiversigenin 3-O-β-D-xylopyranoside (3), 35 mg (0.0047%), C35H58O9, mp 263-264 C (MeOH) [3, 10]. PMR
2
spectrum (400 MHz, C D N, 0 = TMS, δ, ppm, J/Hz): 0.27 and 0.56 (2H-19, d, J = 4), 0.99, 1.28, 1.30, 1.31, 1.41, 1.56, 1.96
5
5
3
2
3
3
3
3
2
(7×CH , s), 2.52 (H-17, d, J = 8), 3.08 (H-22, q, J = J = J = 10), 3.61 (H-3, dd, J = 12, J = 4), 3.71 (H-5a, dd, J = 11,
3
1
2
1
2
3
3
3
3
3
3
3
3
J = 9), 3.73 (H-6, td, J = J = 9, J = 3), 3.87 (H-24, dd, J = 9, J = 5.5), 4.03 (H-2 of D-xylose, dd, J = 9, J = 7.4),
1
3
2
3
3
1
2
1
2
2
3
3
3
4.12 (H-3 of D-xylose, t, J = J = 9), 4.21 (H-4 of D-xylose, td, J = J = 9, J = 5.5), 4.34 (H-5e of D-xylose, dd, J = 11,
1
2
1
2
3
3
3
3
3
3
J = 5.5), 4.89 (H-1 of D-xylose, d, J = 7.4), 5.00 (H-16, q, J = J = J = 8).
1
2
3
Table 1 lists the 13C NMR spectrum.
Cyclosiversioside B (6), 55 mg (0.007%), C45H72O16, mp 185-188°C (MeOH) [3, 11].
Cyclosiversioside C (7), 40 mg (0.005%), C42H68O14, mp 253-255°C (MeOH) [3, 9].
Cyclosiversioside D (8), 60 mg (0.008%), C43H70O15, mp 254-257°C (MeOH) [3,11].
Cyclosiversioside E (9), 890 mg (0.12%), C40H66O13, mp 216-218°C (MeOH) [3, 12].
Cyclosiversioside F (10), 3.5 g (0.46%), C41H68O14, mp 284-286°C (MeOH) [3, 13].
Cyclosiversioside G (11), 27 mg (0.0036%), C46H76O17, mp 237-240°C (MeOH) [3, 14].
Cyclosiversigenin (2) from1. Cycloexoside B (40 mg) was hydrolyzed bymethanolic H SO (10 mL, 0.25%) at 60°C
2
4
for 2.5 h. The reaction mixture was diluted with water. The methanol was evaporated. The resulting solid was filtered off and
dried. The solid was chromatographed over a column with elution by system 2. Yield 16 mg of cyclosiversigenin, mp 239-
241°C (MeOH).
The aqueous filtrate was condensed to 3 mL, boiled for 1 h, and neutralized by anion exchanger ARA-8p. The solid
was removed. The solution was evaporated to dryness. PC in system 6 of the resulting solid detected D-xylose. The PMR and
13
C NMR spectra of 1 indicate that this glycoside includes one D-xylose.
Cyclosiversigenin 3-O-β-D-xylopyranoside (3) from 1. Glycoside 1 (20 mg) was hydrolyzed by methanolic NaOH
(8 mL, 0.1%) at room temperature for 3 h. Workup of the products and chromatographyover a column using system 4 afforded
glycoside 3 (11 mg), mp 263-264°C (MeOH), identified as cyclosiversigenin 3-O-β-D-xylopyranoside.
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7.
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