Synthesis and high-resolution NMR spectra of A-nor-derivatives of 11-deoxyglycyrrhetic acid

Article abstract of DOI:10.1007/s10600-006-0212-5

Several skeletal and oxidative transformations of 11-deoxyglycyrrhetic acid were carried out to produce A-nor-derivatives. Signals for protons and C atoms in the PMR and ¹³C NMR spectra of the A-nor-derivatives were assigned using high-resolution ¹H (400 MHz) and ¹³C (125 MHz) NMR spectroscopy.

Full text of DOI:10.1007/s10600-006-0212-5

Chemistry of Natural Compounds, Vol. 42, No. 5, 2006
SYNTHESIS AND HIGH-RESOLUTION NMR SPECTRA OF
A- -DERIVATIVES OF 11-DEOXYGLYCYRRHETIC ACID
nor
L. R. Mikhailova,1 L. A. Baltina,1,2 R. M. Kondratenko,2 O. Kunert,3
L. V. Spirikhin,1 F. Z. Galin,1 and G. A. Tolstikov4
UDC 547.598.458.22+
543.422.25
Several skeletal and oxidative transformations of 11-deoxyglycyrrhetic acid were carried out to produce
13
nor
nor
-
A- -derivatives. Signals for protons and C atoms in the PMR and C NMR spectra of the A-
1
13
derivatives were assigned using high-resolution H (400 MHz) and C (125 MHz) NMR spectroscopy.
Key words:
11-deoxyglycyrrhetic acid, A- -derivatives, high-resolution NMR spectra.
nor
Transformations of biologically active natural triterpenoids that alter the pentacyclic carbon skeleton are interesting
for preparing analogs with new structures and different biological activity [1-5].
1
Syntheses of A- -derivatives of glycyrrhetic acid (GA, ) [6], the principal triterpenoid of licorice root (
nor Glycyrrhiza
L. and
Fisher), oleanolic acid [1, 7], and betulin and lupeol [5, 8, 9] have been published.
glabra
G. uralensis
The structural similarityof GA and corticosteroids (the 11-on-12-ene group) is known to lead to the appearance of side
effects(hypokaliemia, increasedarterial pressure, retention ofsodium, etc.)with prolongeduseofGApreparations, in particular,
2
carbenoxolone [10]. Therefore, the synthesis of derivatives of 11-deoxy-GA ( ), one of the minor triterpenoids of licorice root,
2
is interesting. We prepared by reduction of GA with Zn powder and HCl in dioxane at 0-10°C according to the literature [11]
2
3
in 70% yield. Treatment of in CH OH with an ether solution of diazomethane isolated the methyl ester of 11-deoxo-GA ( )
3
[12], which was used as a substrate for preparing A- -derivatives (Scheme 1).
nor
COOR
1. O , -60°C
3
°
PCl , 5 - 10 C
5
2. Zn / AcOH
5
3
5
3
O
4
HO
5
2, 3
7
2:
3:
5:
R = H
R = Me
R = Me
30
29
17
PCl , -10 - 0°C
5
COOMe
19
20
12
13
11
9
1. O , -60°C
2. Zn / AcOH
3
25
26
28
5
15
3
1
3
5
7
27
H
4
H
H
6a:
6b:
R =
R =
R
RuO / CCl
4
4
4
O
6a, b
Scheme 1
1) Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences, 450054, Ufa, pr. Oktyabrya,
71, fax (3472) 35 60 66, e-mail: baltina@anrb.ru; 2) Bashkir State Medical University, 450000, Ufa, ul. Lenina, 3; 3) Institut
fur PharmazeutischeChemnieundPharmazeutischeTechnologie, Kapl-Franzens-UniversitatGraz Universitatsplatz1, A-8010
Graz, Austria, fax: +433163809846, e-mail: olaf.kunert@kfunigraz.ac.at; 4) N. N. VorozhtsovNovosibirsk InstituteofOrganic
Chemistry, Siberian Division, Russian Academy of Sciences, 630090, Novosibirsk, pr. Akad. Lavrent′eva, 9. Translated from
Khimiya Prirodnykh Soedinenii, No. 5, pp. 445-448, September-October, 2006. Original article submitted September 7, 2006.
0009-3130/06/4205-0553 ©2006 Springer Science+Business Media, Inc.
553

TABLE 1. Chemical Shifts in PMR and 13C NMR Spectra (δ, ppm) of 11-Deoxyglycyrrhetic Acid Derivatives with a
Cyclopentane Ring A (Pyridine-d5, 310 K, TMS Internal Standard)
4
5
7
Atom C
δC
δH
δC
δH
δC
δH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Me
39.6
29.0
135.8
120.9
56.0
23.4
32.9
40.0
45.6
44.2
25.9
123.1
145.3
42.0
26.8
27.4
32.4
49.1
43.3
44.5
31.6
38.9
23.0
19.7
15.0
17.2
26.2
28.5
28.6
177.4
51.5
1.52; 1.02
2.10; 2.23
41.9
28.4
136.1
26.8
140.9
20.0
32.4
40.7
46.6
49.8
25.6
123.5
144.8
42.2
26.7
27.4
32.4
49.3
43.2
44.5
31.6
38.6
21.5
22.1
19.0
16.0
25.8
28.5
28.5
177.3
51.5
1.72; 1.47
2.28; 2.21
35.3
23.9
75.1
28.9
75.9
20.2
29.4
40.4
41.3
41.3
25.3
123.0
145.4
42.5
26.7
27.4
32.5
49.2
43.1
44.6
31.5
38.8
18.7
19.1
18.0
16.6
26.2
28.5
28.5
177.3
51.1
1.17; 1.17
1.73; 1.61
-
-
-
-
2.72
-
2.37; 1.97
1.45; 1.37
1.77
-
2.02; 1.33
2.02; 1.33
1.73
1.60; 2.19
1.31; 1.50
-
-
-
1.79
1.87
2.25
-
2.03; 1.86
5.42
-
2.03; 1.84
5.45
-
2.03; 1.79
5.44
-
-
-
-
-
-
1.82; 0.99
2.02; 0.88
-
2.14
2.08; 1.76
-
2.07; 1.39
1.49; 1.40
1.62
1.82; 1.02
1.99; 0.97
-
2.14
2.06; 1.74
-
2.06; 1.37
1.49; 1.37
0.97
1.83; 1.03
2.00; 0.88
-
2.14
2.07; 1.75
-
2.08; 1.38
1.49; 1.38
1.08
1.81
0.75
1.02
1.22
0.87
1.22
-
1.01
1.00
1.09
1.14
0.87
1.16
-
0.95
0.91
1.11
1.30
0.88
1.18
-
3.71
3.68
3.70
Ring A in 3 was contracted using PCl in benzene:toluene (1:1) through the classical Wagner—Meerwein
5
rearrangement, which occurred in two directions depending on the reaction temperature. At -10 to 0°C, the methyl ester of
3-isopropylidene-A-neo-5αH-olean-12-en-30-oic acid (4) was formed in 79.2% yield; at 5-10°C, the endo-isomer of A-nor-
olean-3(5)-ene (5) in 82.5% yield.
Oxidation of 4 in CCl by an excess of RuO , prepared from RuCl and HIO , produced a mixture of isomeric 5βH-
4
4
3
4
and 5αH-A-nor-ketones 6a and 6b, which were isolated in 62% overall yield by column chromatography (CC) over Al O as
2
3
a 2:1 mixture according to NMR spectroscopy. CC of the reaction products over Al O also afforded starting 4 (32% yield).
2
3
13
The C NMR spectrum (75.5 MHz) of 4,23,24-tri-nor-derivatives 6a and 6b gave signals for C-3 and C-5 at 221.4
and 57.8 ppm (5βH-isomer) and 219.8 and 57.2 ppm (5αH-isomer), respectively. Analogous chemical shifts (CS) were
previously reported for the 5βH-3-A-nor-ketone of allobetulin and the 5αH-3-A-nor-ketone of oleanolic acid [1, 5].
The PMR spectra of 6a and 6b showed two singlets for carbomethoxy groups at 3.64 and 3.68 ppm with a 1:2 ratio
of intensities. These belonged to the 5αH and 5βH isomers, respectively. The signals for the cyclopentane protons and its
closest neighbors (H-1, H-2, H-5, H-6) in the PMR spectra of 6a and 6b that belong to C-1, C-2, C-5, and C-6 with CS 32.0
(32.2), 38.0 (38.2), 57.2 (57.8), and 17.0 (17.1) ppm, respectively, appeared at 1.80-2.05 ppm as a multiplet.
Recrystallization of the mixture of isomers 6a and 6b from ethanol afforded pure single crystals. 5βH-A-nor-ketone
6a was characterized by an x-ray structure analysis (Fig. 1).
554

O 2
C 3 1
C 2 9
C 3 0
C 2 0
C 2 1
O 3
C 1 2
C 1 3
C 1 9
C 11
C 1
C 1 8
C 1 7
C 2 7
C 8
C 2
C 2 2
C 1 0
C 9
C 1 4
C 1 5
C 2 5
C 3
O 1
C 2 8
C 5
C 1 6
C 2 6
C 7
C 6
Fig. 1. X-ray structure of the methyl ester of A-nor-3-oxo-5βH-olean-12-en-30-oic acid.
Partial ozonolysis of 4 at -60°C in CH Cl with subsequent reduction of the peroxide products by Zn/AcOH formed
2
2
only the 5βH-A-nor-ketone 6a, which was isolated by CC in 58.8% yield. The reaction mixture also contained starting
13
triterpenoid 4 (30%). The C NMR spectrum of pure 6a exhibited onlyone signal for C-3 (C=O) at 221.4 ppm. The resonance
for the 5βH-proton had CS 1.82 ppm (singlet).
Partial ozonolysis of 5 at -60°C produced the 3α,5α-epoxide of 7 in 56% yield. CC of the reaction mixture also
produced starting endo-isomer 5, which was identified by TLC with a marker.
The NMR spectra of 11-deoxy-GA derivatives 4, 5, and 7 were studied using a high-resolution NMR spectrometer
1
13
(Varian Unity Inova, working frequency 400 MHz for H and 125 MHz for C). Signals for protons were assigned using
two-dimensional (2D) homonuclear H—H COSY and heteronuclear HSQC and HMBC CH correlation spectroscopy as before
[13, 14].
13
Table 1 gives the CS in the high-resolution PMR and C NMR spectra for the studied compounds. It can be seen that
the C NMR spectrum of 4 contains characteristic signals for olefinic C-3 and C-4 at 135.8 and 120.9 ppm. Two singlets for
13
the isopropylidene methyls in the PMR spectrum have CS 1.62 and 1.81 ppm. The CS for the cyclopentane C atoms of ring
A confirm the 5αH-configuration and the trans-fusion ofrings A/Bin 4. The 5αH signal is observed at 1.73 ppm and correlates
with C-5, which has CS 56.0 ppm.
13
The PMR spectrum of isomeric 5 contains a septet for H-4 at 2.72 ppm. The C NMR spectrum has CS 136.1 and
140.9 ppm for C-3 and C-5, respectively.
13
The C NMR spectrum of epoxide 7 shows signals for C–O bonds at 75.1 and 75.9 ppm. Signals of olefinic C atoms
C-3 and C-5 are absent. The double bond in ring C is not ozonated by partial ozonolysis, which is consistent with retention of
13
signals for C-12 and C-13 in C NMR spectra of 6a and 7.
13
The signals for cyclopentane C-1 and C-2 in the C NMR spectrum of 7 are shifted to strong field by 4-5 ppm and
correlate with protons at 1.17 and 1.73/1.61 ppm, respectively.
EXPERIMENTAL
13
PMR and C NMR spectra were recorded on a Varian Unity Inova spectrometer at working frequency 400 and
125 MHz and a Bruker AM 300 at working frequency 300 and 75.5 MHz in Py-d or CDCl with TMS internal standard. IR
5
3
spectra were recorded on a Specord M80 spectrometer. Specific rotations were measured on a Perkin—Elmer 341 MC
polarimeter in a 1-dm cuvette.
The x-ray structure analysis (XSA) was performed on a diffractometer using a Bruker SMART APEX2 CCD area
*
detector at 100 K (Mo-Kα-radiation, 2qmax = 52.16°) with a single crystal of dimensions 0.35 × 0.25 × 0.20 mm.
______
*
The XSA was performed at the Center for x-ray structure analysis of the RAS INEOS, for which we thank the staff of the
center.
555

TLC was performed on Silufol (Czech Rep.) and Sorbfil (Sorbopolimer) plates. Spots were developed using
phosphotungstic acid (20%) in ethanol with subsequent heating at110-120°Cfor 2-3min. CC used Al O (Brockmann neutral).
2
3
11-Deoxy-GA (2) was prepared by reduction of 18β-GA with Zn powder in dioxane with an excess of HCl as before
[11] (70% yield), mp 322-325°C, lit. [11] mp 323-325°C. The methyl ester of 11-deoxy-GA (3) was isolated by treatment of
a solution of 2 in CH OH with an ether solution of diazomethane (90% yield) and recrystallization from EtOH:CHCl , mp 232-
3
3
20
20
234°C, [α]
+110° (c 0.1, CHCl ), lit. [12] mp 230-232°C, [α]
+110° (c 0.02, CHCl ).
3
D
3
D
Solvents were purified by literature methods [15] and evaporated in vacuo at <50°C.
Methyl Ester of 3-Isopropylidene-A-neo-5αH-18βH-olean-12-en-30-oic Acid (4). Compound 3 (2 g, 4.25 mmol)
was dissolved in a mixture of dry benzene and toluene (1:1, 400 mL), cooled to -10 to 0°C, treated in one portion with PCl
5
(2.1 g), stirred for 1 h with TLC monitoring (until the spot of the starting substrate disappeared), treated with saturated aqueous
Na CO solution (120 mL), stirred for another 30 min, and warmed to room temperature. The organic layer was separated,
2
3
washed with water, dried, and evaporated. The solid was recrystallized from CHCl :EtOH. The mother liquor was
3
chromatographed over a column of Al O with elution by benzene to afford 4 (1.52 g, 79.2%) that was homogeneous by TLC,
2
3
20
R 0.62 (hexane:ethylacetate 10:1), mp 176-178°C, [α]
+117.5° (c 0.008, CHCl ). C H O , MW = 452.72. Table 1 gives
3 31 48 2
f
D
13
the PMR and C NMR spectra.
Methyl Ester of A-nor-Olean-3,12-dien-3-(2-isopropyl)-30-oic Acid (5). Compound 3 (2.1 g, 4.6 mmol) was
dissolved in a mixture of dry benzene and toluene (1:1, 300 mL), cooled to +10°C, treated in one portion with PCl (2.2 g),
5
stirred for 1 h, treated with saturated aqueous Na CO solution (150 mL), stirred for another 30 min, and warmed to room
2
3
temperature. The organic layer was separated, washed with water, dried, and evaporated. The solid was recrystallized from
EtOH:CHCl . The mother liquor was chromatographed over a column of Al O with elution by benzene to afford 5 (1.63 g,
3
2 3
20
82.5%) that was homogeneous by TLC, R 0.57 (benzene), mp 174-176°C, [α]
MW = 452.72. Table 1 gives the PMR and C NMR spectra.
+153.3° (c 0.04, CHCl ). C H O .
3 31 48 2
f
D
13
Methyl Ester of A-nor-3-Oxo-5βH-olean-12-en-30-oic Acid (6a). 1. A mixture of CCl (40 mL), CH CN (40 mL),
4
3
and distilled water (60 mL) was vigorouslystirred. The two-phase system was treated first with 4 (0.9 g, 2 mmol) and then with
NaIO (2.7 g, 2.6 mmol) and RuCl ·H O (30 mg, 13.2 mol% based on RuCl ·H O) in three portions over 5 h. The mixture was
4
3
2
3
2
vigorously stirred at 20-22°C for 3 d with TLC monitoring and diluted with CH Cl (40 mL). The aqueous layer was separated
2
2
and extracted with CH Cl (3 × 20 mL). The combined organic extracts were washed with water, dried over MgSO , and
2
2
4
evaporated. The solid was dissolved in ethylacetate (40 mL) and filtered through a thin layer of cellite. The filtrate was
evaporated. The dry solid containing two spots on TLC was chromatographed over a column of Al O with elution by
2
3
CHCl :CH OH (300:1, 200:1, 100:1, v/v). Fractions that were homogeneous by TLC were combined, evaporated, and
3
3
-1
recrystallized from aqueous ethanol to afford a mixture of 6a and 6b (0.52 g, 62%, 2:1 ratio). IR spectrum (ν, cm ): 1700
(C=O). PMR spectrum (300 MHz, CDCl , δ, ppm): 0.77, 1.03, 1.12, 1.20, 1.26 (15H, all s, 5CH ), 3.64, 3.68 (6H, both s,
3
3
13
OCH ), 1.80-2.05 (m, H1 , H-2, H-5, H-6), 5.30 (1H, t, H-12). C NMR spectrum (75.5 MHz, pyridine-d , δ, ppm): 32.0, 32.2
3
b
5
(C-1), 38.05, 38.21 (C-2), 219.8, 221.4 (C-3), 57.2, 57.8 (C-5), 17.1, 17.0 (C-6), 29.6 (C-7), 41.8 (C-8), 35.3 (C-9), 40.9 (C-10),
24.7 (C-11), 122.3 (C-12), 144.4 (C-13), 42.5 (C-14), 26.3 (C-15), 26.5 (C-16), 34.9 (C-17), 48.6 (C-18), 39.9 (C-19), 44.2
(C-20), 31.2 (C-21), 35.9 (C-22), 25.1, 25.15, 25.2, 25.25 (C-25, C-29), 16.05, 16.1 (C-26), 28.1, 28.15 (C-27), 28.5 (C-28),
177.6, 177.65 (C-30), 51.6, 51.65 (C-31).
The second compound (upper spot) was isolated by CC over Al O and identified by TLC with markers as starting 4
2
3
(32%), R 0.62.
f
A second recrystallization of6aand6bfrom CHCl :EtOHat20-22°Cproducedsingle crystals of6a (colorless platelike
3
monoclinic crystals). C H O . MW = 426.64, characterized by XSA (Fig. 1). The structure was solved by direct methods.
28 42
3
2
All nonhydrogen atoms were located in difference electron-density syntheses and refined anisotropically over F hkl. All
calculations were performed using the SHELXTL PLUS5 programs [16].
2. Ozone (~1 mmol) was bubbled through a solution of 4 (0.45 g, 1 mmol) in dry CH Cl (50 mL) at -60°C. The
2
2
reaction mixture was warmed to room temperature. The ozonide was reduced with Zn powder (1 g) in the presence of glacial
acetic acid (GAA, 20 mL) for 1 h. The mixture was filtered and neutralized with saturated Na CO solution. The organic layer
2
3
was washed with water, dried over MgSO , and evaporated. The solid was chromatographed over a column of Al O with
4
2 3
elution byhexane:ethylacetate (25:1, 10:1, 5:1, v/v). Fractions that were homogeneous byTLC were combined and evaporated
20
to afford 6a (0.30 g, 58.8%) that was recrystallized from EtOH. 6a, mp 176-178°C, [α]
+82.5° (c 0.04, CH OH). PMR
3
D
spectrum (300 MHz, CDCl , δ, ppm): 0.72, 0.98, 1.10, 1.16, 1.23, 1.25 (15H, all s, 5CH ), 0.88-1.75 (12H, 2H-1, H-6 , 2H-7,
3
3
a
556

2H-15, H-16 , H-19 , H-21 , 2H-22), 1.78 (1H, H-9), 1.82 (1H, s, 5βH), 1.89-2.08 (9H, m, 2H-2, H-6 , 2H-11, H-16 , H-18,
a
a
a
b
b
13
H-19 , H-21 ), 3.69 (3H, s, OCH ), 5.33 (1H, t, H-12, J = 13.6). C NMR spectrum (75.5 MHz, CDCl , δ, ppm): 31.97 (C-1),
b
b
3
3
38.21 (C-2), 221.39 (C-3), 57.82 (C-5), 17.07 (C-6), 29.6 (C-7), 41.85 (C-8), 35.29 (C-9), 40.95 (C-10), 24.69 (C-11), 122.87
(C-12), 144.42 (C-13), 42.54 (C-14), 26.29 (C-15), 26.54 (C-16), 34.91 (C-17), 48.65 (C-18), 38.96 (C-19), 44.19 (C-20), 31.16
(C-21), 35.30 (C-22), 16.05 (C-26), 28.13, 28.45 (C-28, C-27), 25.07, 25.18 (C-25, C-29), 177.61 (C-30), 51.58 (C-31).
Methyl Ester of 3,5-Epoxy-A-nor-olean-12-en-3-(2-isopropyl)-30-oic Acid (7). Ozone (1 mmol) was bubbled
through a solution of 5 (0.9 g, 2 mmol) in CH Cl (90 mL) at -60°C. The reaction mixture was warmed to room temperature,
2
2
treated with GAA (20 mL) and Zn powder (2 g), and stirred for 1.5 h. The mixture was filtered. The excess of GAA was
neutralized with saturated Na CO solution. The organic layer was washed with water, dried over MgSO and evaporated. The
2
3
4
drysolid was chromatographed over a column ofAl O with elution bybenzene toafford 7 (0.51 g, 56%) that was recrystallized
2
3
20
from aqueous ethanol, mp 177°C, [α]
C NMR spectra.
+35° (c 0.02, CHCl ). C H O . MW = 440.66. Table 1 gives the PMR and
3 29 44 3
D
13
ACKNOWLEDGMENT
The work was supported financially by Rosnauki (2005-RI-12.0/004/088, State Contract 02.434.11.7060), (2006-RI-
112.0/001/409, State Contract 02.445.11.7430), the RFBR, and Austria (03-03-20004 BNTSa).
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