Method for Preparing Betulonic Acid from Betula pendula Birch Bark

Full text of DOI:10.1007/s10600-016-1770-9

DOI 10.1007/s10600-016-1770-9
Chemistry of Natural Compounds, Vol. 52, No. 4, July, 2016
METHOD FOR PREPARING BETULONIC ACID FROM
Betula pendula BIRCH BARK
V. A. Levdanskii,1,2 A. V. Levdanskii, and B. N. Kuznetsov1,2*
1
Betulinic acid [3ꢀ-hydroxylup-20(29)-en-28-oic acid] is an especially interesting betulin derivative because it is a
selective growth inhibitor of human malignant melanoma cells and possesses antibacterial, antimalarial, anti-inflammatory,
cholagogic, and other types of activity [1–4]. Betulinic acid is produced via reduction of betulonic acid [5, 6], which is
synthesized via oxidation of betulin (1) isolated from birch bark. Thus, existing methods of preparing betulonic acid are multi-
step and labor-intensive.
The goal of the study was to develop a method of preparing betulonic acid (2) from birch bark.
Known methods of preparing 2 are oxidation of 1 by CrO in AcOH [5] or by a solution of CrO in H SO in Me CO
3
3
2
4
2
(
Jones reagent) [6]. The yield of 2 in both instances is ~60%. These methods were used to produce 2 from betulin-containing
raw material, i.e., birch bark.
It was found that the reaction mixture polymerized in 5–7 min during oxidation of birch bark by CrO in AcOH and
3
that 2 could not be isolated from it. Treatment of birch bark in Me CO with Jones reagent for 3 h produced a yellow solution
2
that was readily filtered to remove inorganic compounds and residual bark. The solvent was distilled off to afford 2 (15.6% of
the absolute dry bark mass, a.d.b.). Thus, 1 in the bark was oxidized to 2 according to Scheme 1.
2
9
29
3
0
2
0
21
21
2
5
5
26
1
7
a
CH
2
OH
COOH
1
2
8
28
1
1
0
15
3
7
27
3
HO
O
1
2
2
4
23
a. CrO , H SO , (CH ) CO
3
2
4
3 2
Scheme 1
A large group of ballast compounds consisting mainly of phenolic compounds is known to be extracted together with
1
from birch bark by polar solvents such as EtOH, i-PrOH, and Me CO [7].
2
Oxidation by Jones reagent of bark that was extracted beforehand with H O and aqueous NaOH solutions was studied
2
in order to determine the influence on the yield of 2 of side products, i.e., phenolic compounds, in the bark. Table 1 presents
the yields of 2 and phenolic compounds.
The results showed that the yields of 2 from the starting and extracted barks differed insignificantly and were
1
5.5–17.5% of the a.d.b. mass. IR and NMR spectra of 2 agreed with those in the literature [5, 6].
The possibility of preparing 2 directly from B. pendula bark by treatment with Jones reagent in Me CO was
2
demonstrated for the first time. Phenolic and other constituents in the bark did not noticeably affect the yield and purity of 2.
The composition and structure of 2 from birch bark were established using elemental analysis and IR and NMR spectroscopy.
1
) Institute of Chemistry and Chemical Technology, Siberian Branch, Russian Academy of Sciences, 50-24,
Akademgorodok, Krasnoyarsk, 660036, fax: +7 (391) 249 48 94, e-mail: inm@icct.ru; 2) Siberian Federal University, 79
Svobodnyi Prosp., Krasnoyarsk, 660041. Translated from Khimiya Prirodnykh Soedinenii, No. 4, July–August, 2016,
pp. 656–657. Original article submitted December 16, 2015.
766
0009-3130/16/5204-0766 ©2016 Springer Science+Business Media New York

TABLE 1. Yields of Betulonic Acid and Phenolic Compounds Extracted from Betula pendula by H O and Aqueous NaOH
2
Solutions, wt%
Concentration, %
Betulonic acid
Phenolic compounds
Concentration, %
Betulonic acid
Phenolic compounds
0
15.5 ꢂ 0.3
15.7 ꢂ 0.3
15.6 ±0.3
15.8 ꢂ 0.3
15.8 ꢂ 0.3
1.9 ꢂ 0.1
6.2 ꢂ 0.2
7.5 ꢂ 0.2
9.1 ꢂ 0.2
10.3 ꢂ 0.2
0.5
0.7
1.0
15.8 ꢂ 0.3
16.3 ꢂ 0.3
17.5 ꢂ0.3
15.6 ꢂ 0.3
11.4 ꢂ 0.3
14.3 ꢂ 0.3
20.8 ꢂ0.5
–
0
0
0
0
.1
.2
.3
.4
Starting bark (50 g)
IR spectra were recorded in KBr pellets (3 mg sample/300 mg KBr) on a Tensor-27 FTIR spectrometer (Bruker,
–
1
Germany) in the range 400–4000 cm . NMR spectra were recorded in CD OD vs. TMS (0 ppm) on a Bruker Avance III
3
spectrometer (600 MHz).
®
Elemental analysis was performed on a FlashEA 1112 analyzer (ThermoQuest Italia) with simultaneous determination
(
(
%) of C, H, N, S, and O. Melting points were measured on an Electrothermal A9100 apparatus. TLC used Silufol plates
Chemapol, Czech Rep.) and CHCl –MeOH (20:1). Compounds were detected by phosphotungstic acid solution (20%) in
3
EtOH followed by heating at 100–200°C for 2–3 min and also by I vapor. Starting raw material was B. pendula bark that was
2
collected in June 2014 in the vicinity of Krasnoyarsk. Bark was ground to particle size 1–3 mm and dried at 105°C to <1%
moisture. The chemical composition of the bark (mass%) was triterpenes 34.7 (30.0% betulin), suberin 40.3, lignin 13.5,
cellulose 3.8, and ash 2.3.
3
-Oxolup-20(29)-en-28-oicAcid (2). A2-L four-necked flask equipped with a stirrer, thermometer, reflux condenser,
and dropping funnel was charged with bark (50 g) ground to particle size 1–3 mm and Me CO (1 L). The mixture was stirred
2
vigorously, treated slowly with freshly prepared Jones reagent [25 mL, CrO (6.7 g) dissolved in H SO (5.8 mL, 98%) and
3
2
4
then diluted with H O to 25 mL], stirred for 3 h at 20°C, and filtered of Cr salts and residual bark. The filtrate was concentrated
2
in vacuo. The residue was diluted with H O (300 mL). The resulting precipitate was filtered off, rinsed on the filter with H O,
2
2
and dried. The dry precipitate was purified by dissolving in C H (150 mL), adding activated charcoal, filtering through a
6
6
layer (7 mm) of Al O , and treating with KOH solution (10%) until the potassium salt of 2 was fully precipitated. The solid
2
3
was filtered off, rinsed on the filter with C H (30–40 mL), dried at room temperature to constant weight, and dissolved in
6
6
EtOH (60 mL). The solution was poured into a beaker containing HCl solution (200 mL, 10%). The resulting precipitate of
was filtered off, rinsed with H O, dried, and purified by recrystallization from MeOH. R 0.56, mp 245–248°C, lit. [8]
2
2
f
mp 247–249°C, C H O . Elemental analyses agreed with those calculated.
3
0 46 3
Bark that was extracted beforehand with H O and aqueous base solutions in order to remove phenolic compounds
2
was used in several experiments to prepare 2. For this, a 1-L three-necked flask equipped with a stirrer and reflux condenser
was charged with bark (50.0 g), treated with H O or aqueous NaOH solution of a given concentration (500 mL), and refluxed
2
with vigorous stirring for 1 h. The mixture was cooled to room temperature. Bark was separated from the solution by
filtration, rinsed on the filter with H O until the rinsings were neutral, and dried at 105°C to constant weight. The yield of
2
phenolic compounds was determined from the bark weight loss.
ACKNOWLEDGMENT
The work was sponsored by the Russian Ministry of Education and Science (Project RFMEF160714X0031).
Instruments of the Krasnoyarsk Regional Center for Collective Use, SB, RAS, were used.
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