Synthesis of the first acetylene derivatives of betulonic acid

Full text of DOI:10.1134/S0012500809020050

ISSN 0012-5008, Doklady Chemistry, 2009, Vol. 424, Part 2, pp. 39–42. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © S.F. Vasilevskii, A.I. Govdi, E.E. Shul’ts, M.M. Shakirov, I.V. Alabugin, G.A. Tolstikov, 2009, published in Doklady Akademii Nauk, 2009, Vol. 424, No. 5,
pp. 631–634.
CHEMISTRY
Synthesis of the First Acetylene Derivatives
of Betulonic Acid
S. F. Vasilevskiia, A. I. Govdia, E. E. Shul’tsa, M. M. Shakirovb,
I. V. Alabuginc, and Academician G. A. Tolstikovb
Received September 4, 2008
DOI: 10.1134/S0012500809020050
Modification of natural compounds with the aim of
Taking into account that the results of practical
searching for efficient therapeutic agents is one of the importance in the search for antitumor and antiviral
main trends of the promising research direction agents [2] were obtained by the transformation of the
appeared at the interface between fine organic synthesis carbon atom at the 28-position of betulonic acid, we
and medicinal chemistry.
have attempted to introduce acetylenic fragments just
in the 28-position of betulonic acid.
In recent years, there has been growing interest in
plant triterpenoids, which provide a basis for the prep-
aration of highly efficient antitumor agents [1]. Lupane
triterpenoids, in particular, betulinic acid and its deriv-
atives, are of special interest [1, 2]. Amides and pep-
tides of betulonic acid showing antitumor activity are
known [3, 4].
Studies within the last two decades have shown that
acetylenic compounds are a rather representative group
of natural metabolites produced mainly by high plants,
It should be emphasized that the use of the currently
most common method for the preparation of alkynes by
the Sonogashira cross-coupling of terminal acetylenes
with halogenated derivatives in the CuI–PdCl2(PPh3)2
system is not trivial. Indeed, the initial halobetulonic
acid contains the terminal vinyl group and a disubsti-
tuted olefin can form under the copper–palladium catal-
ysis conditions due to the Heck reaction [7].
Therefore, we used acid chloride function of mole-
as well as by fungi and microorganisms. In spite of the cule 2 to introduce acetylene residues into the betulonic
high anticancer activity of many natural acetylenic acid molecule (Scheme 1). The use of p-aminopheny-
metabolites [5, 6], betulonic acid derivatives with triple lacetylene as the amine component allowed us to pre-
bonds are still unknown; therefore, the synthesis of pare, in one step, a highly reactive synthon, a betulonic
such compounds is an independent synthetic task. acid with terminal acetylene group 1. The synthetic
Moreover, alkyne residue is one of the most convenient value of compounds with the ethynyl group is deter-
fragments in designing new medicinal agents owing to mined by the high acidity of the çë≡ë fragment,
the high reactivity and facile functionalization of the which provides the facile functionalization of a mole-
triple bond.
cule and the formation of new C–C bonds.
29
20
30
19 21
22
H
32
31
N
12
18
25
33
34
28
11 26 13
Et3N, 80°C
17
16
O
+
14
H2N
15
37
O
36
1
2
9
6
Cl
10
8
7
38
35
3
5
27
4
O
O
2
1
24
23
Scheme 1.
a Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences,
Institutskaya ul. 3, Novosibirsk, 630090 Russia
b Vorozhtsov Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Lavrent’eva 9, Novosibirsk, 630090 Russia
c Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida, 32306 USA
39

40
VASILEVSKII et al.
The use of high reactivity of ethynyl derivatives for yldialkylamines of general formula Är-C≡C–CH2-
the modification of betulonic acid was demonstrated by
the examples of typical reactions for 1-alkynes.
N(Alk)2 [8]. The aminoalkylation of acetylene was carried
out under typical conditions of the Mannich reaction. The
Aminopropargyl derivatives are of special interest in reaction of amidoacetylene 1 with diethylamine and
the search for biologically active compounds in the series paraformaldehyde in the presence of cuprous chloride on
of acetylenes. Thus, inhibitors of enzyme, mammalian heating in dioxane at 80°ë led to the formation of target
squalene epoxidase, were found among 3-arylpropyn-2- propargylamine 3 in 68% yield (Scheme 2).
H
32
N
31
36
33
34
40
CH2
41
CH3
O
37
38
35
39
N
O
3
CH2 CH3
a
40'
41'
(a) Et2NH, (CH2O)x, CuCl, 80°C, 3 h
(b) Dimethyl-3-bromopropynol, Et2NH, CuCl, 35°C, 4 h
(c) 2-Bromopyridine, PdCl2(PPh3)2, CuI, PPh3, Et3N, 80°C, 18 h
1
c
b
H
H
N
32
35
32
31
36
N 31
33
34
33
34
36
O
O
37
37
38
38
3'
6'
35
39
2'
42
4'
5'
40
41 Me
O
O
42'
N
Me
Me
HO
4
5
Scheme 2.
The Cadiot–Chodkiewicz cross-coupling is a con- acetylene 5 in 62% yield. No products of the Heck
cross-coupling reaction were detected.
ventional important method for the synthesis of diacet-
ylene derivatives because high anticancer activity was
revealed among diacetylene compounds [5]. Many nat-
ural conjugated butanediynylcarbinols were obtained
by this method [9]. We synthesized also α-diacetylene
tertiary alcohol 4. The reaction of alkyne 1 with a bro-
Thus, we have obtained first betulonic acid deriva-
tives containing acetylene fragments. The possibility of
the selective course of the Sonogashira reaction for the
preparation of the acetylene derivatives of betulonic
acid (1) in spite of the presence of vinyl group in the ini-
mocarbinol in methanol in the presence of CuCl, tial compound has been shown.
Et2NH, NH2OH · HCl proceeded smoothly at 30–35°ë
to form butane-1,3-diynylcarbinol 4 in 73% yield.
The highly reactive derivatives of betulonic acid
containing ethynyl fragments (ë≡ëH) in addition to
triterpene derivatives open wide opportunities for pre-
paring a new group of compounds promising for the
search for biologically active compounds for medicinal
purposes.
Taking into consideration the presence of the vinyl
group in the molecule of betulonic acid (and, therefore,
the possibility of the Heck reaction), it is of fundamen-
tal importance to check the selectivity of cross-cou-
pling of terminal acetylene 1 with haloarene under the
copper–palladium catalysis conditions. The selective
reaction would open a way to the synthesis of a wide
variety of betulonic acid derivatives with ethynylaryl
and ethynylhetaryl substituents. We used α-bromopyri-
dine as a haloarene component. The reaction of ethynyl
derivative 1 with 2-bromopyridine (55°C, 18 h) under
standard conditions of the Sonogashira reaction
(PdCl2(PPh3)2, CuI, Et3N) gave rise to disubstituted
EXPERIMENTAL
Commercial PdCl2(PPh3)2, 2-methyl-3-butyn-2-ol
from Aldrich were used in the work. Melting points
were determined on a Kofler hot-stage apparatus. IR
spectra were recorded on a Vector 22 spectrophotome-
ter as KBr pellets. High-resolution mass spectra were
obtained on a Finnigan MAT model 8200 spectrometer
(EI, 70 eV). Elemental analysis was performed on a
DOKLADY CHEMISTRY Vol. 424 Part 2 2009

SYNTHESIS OF THE FIRST ACETYLENE DERIVATIVES OF BETULONIC ACID
41
1
N-(3-Oxo-20(29)-lupen-28-oyl)-4-(N,N-diethy-
Carlo Erba model 1106 CHN analyzer (Italy). H and
13C NMR spectra were recorded on a Bruker AV-300
spectrometer (operating at 300.13 and 75.47 MHz,
respectively) and a BrukerAM-400 spectrometer (oper-
ating at 400.13 and 100.61 MHz, respectively). Signal
laminomethylethynyl)aniline 3. A mixture of 60 mg
(2.0 mmol) of paraformaldehyde and 146 mg
(2.00 mmol) of diethylamine in 5 mL of dioxane was
kept in an argon flow for 30 min at 45°C. Cuprous chlo-
ride (I) (14 mg, 0.14 mmol) was added, and the mixture
was stirred until blue color appeared (10 min); com-
pound 1 (550 mg, 0.99 mmol) was added, and the mix-
ture was heated to 80°C for 3 h. After completion of the
synthesis, the reaction mixture was washed with aque-
ous ammonia. The organic layer was dried with
Na2SO4, filtered through a plug of Al2O3 (1 × 1.5 cm).
The solvent was removed in a vacuum, the residue was
triturated with hexane. The precipitate was filtered off
to give 410 mg (68%) of the Mannich base, mp 131–
133°C (benzene). High-resolution mass spectrum,
found (m/z): 638.4717 [M]+. For C43H62N2O2 calcu-
lated: M = 638.4811. 1H NMR (300.13 MHz, CDCl3, δ,
ppm, J, Hz): 0.94 (s, 3H, 25-CH3), 0.99 (s, 3H, 24-
CH3), 1.02 (s, 6H, 26,27-CH3), 1.07 (s, 3H, 23-CH3),
1.67 (s, 3H, 30-CH3), 2.61 (m, 4H, N(CH2)2), 3.13 (dt,
1H, 19-H, J1 = 4 Hz, J2 = 11 Hz), 3.61 (s, 2H, 39-CH2),
4.60 (s, 1H, 29-H), 4.73 (s, 1H, 29-H), 7.40 (m, 4H,
32,33,35,36-H). IR (KBr, ν, cm–1): 1704 (C=O), 2212
(ë≡ë).
1
assignment in H and 13ë NMR spectra was accom-
plished with the use of different types of proton–proton
and carbon–proton shift correlation spectroscopy
(COSY, COLOC) (the spectra of compounds 4 and 5
were obtained on a Bruker DRX-500 spectrometer
(operating at 500.13 (1H) and 125.76 MHz (13ë)); sig-
nal assignment for the polycylic cage of compounds 1–
5 was made taking into account data on the carbon
chemical shifts for betulonic acid as a key compound
[10]. Chemical shifts were determined relative to the
residual signals of the solvent, ëçël3 (δH 7.24 ppm and
δC 76.90 ppm). The multiplicity of signals in 13ë NMR
spectra was determined in a J-modulation mode
(JMOD). The data of characteristic signals are pre-
1
sented for the noted compounds in H NMR spectra
because of the complexity of the assignment of all sig-
nals. The main portion of the protons of the triterpenoid
skeleton show resonances within the range 2.7–
0.8 ppm. The assignment of chemical shifts for the car-
bon atoms of triple bonds in compounds 1–5 is made by
comparison with corresponding chemical shifts of acet-
ylenic compounds in 13ë NMR spectra described in
[11, 12].
N-(3-Oxo-20(29)-lupen-28-oyl)-4-(2-hydroxy-5-
methylhexadiyn-1,3-yl)aniline 4. A mixture of 188 mg
(0.339 mmol) of compound 1, 0.2 mL of diethylamine,
3.4 mg (0.05 mmol) of hydroxylamine hydrochloride,
and 1 mg (0.01 mmol) of cuprous chloride in 2 mL
of methanol was stirred in an argon flow. After forma-
tion of a bright yellow precipitate of copper acety-
lenide, 62 mg (0.38 mmol) of 1-bromo-3-methylbutyn-
1-ol was added. The mixture was heated at 30–35°C for
4 h. After completion of the reaction, 20 mL of toluene
was added, and the reaction mixture was washed with
aqueous ammonia and dried with Na2SO4. The organic
layer was filtered to remove the drying agent, and the
solvent was removed in a vacuum. Hexane was added
to the residue and the precipitate was filtered off to give
156 mg (73%) of compound 4, mp 183–184°C (ben-
zene). High-resolution mass spectrum, found (m/z):
635.4318 [M]+. For C43H57NO3 calculated: M =
N-(3-Oxo-20(29)-lupen-28-oyl)-4-ethynylaniline 1.
A mixture of 1.6 g (3.4 mmol) of acid chloride 2, 0.39 g
(3.4 mmol) of p-aminophenylacetylene, and 3 mL of
triethylamine in 15 mL of dry benzene was heated in an
argon flow at 70–75°C for 18 h. After cooling, the pre-
cipitate of Et3N · HCl was filtered off and washed with
benzene (3 × 10 mL), and the solvent was removed in a
vacuum. The residue was dissolved in 20 mL of ben-
zene, washed with diluted hydrochloric acid (1 : 4), and
dried with anhydrous Na2SO4; the organic layer was fil-
tered through Al2O3 (1.0 × 2.5 cm); the solution was
concentrated to 5 mL, loaded onto aluminum oxide col-
umn (2.0 × 2.5 cm), and eluted with benzene. The sol-
vent was removed in a vacuum to give 1 g (55%) of
compound 1, mp 159–160°ë. High-resolution mass
spectrum, found (m/z): 553.3918 [M]+. For C38H51NO2
1
635.4333. H NMR (300.13 MHz, CDCl3, δ, ppm, J,
Hz): 0.89 (s, 3H, 25-CH3), 0.94 (s, 6H, 26,27-CH3),
0.98 (s, 3H, 24-CH3), 1.03 (s, 3H, 23-CH3), 1.66 (s, 3H,
30-CH3), 1.54 (s, 6H, 42,42'-H), 3.12 (dt, 1H, 19-H,
J1 = 4 Hz, J2 = 11 Hz), 4.58 (s, 1H, 29-H), 4.72 (s, 1H,
1
calculated: M = 553.3919. H NMR (300.13 MHz,
CDCl3, δ, ppm, J, Hz): 0.90 (s, 3H, 25-CH3), 0.95 (s,
3H, 24-CH3), 0.98 (s, 6H, 26,27-CH3), 1.03 (s, 3H,
23-CH3), 1.67 (s, 3H, 30-CH3), 3.14 (dt, 1H, 19-H, J1 =
4 Hz, J2 = 11 Hz), 4.59 (s, 1H, 29-H), 4.73 (s, 1H,
29-H), 3.01 (s, 1H, 38-H), 7.39 (m, 4H, 32,33,35,36-H).
IR (KBr, ν, cm–1): 1696 (C=O), 2108 (C≡C), 3387
(C≡C–H).
29-H), 7.42 (m, 4H, 32,33,35,36-H). IR (KBr, ν, cm–1):
1692 (C=O), 2145 and 2232 (C≡C).
For C43H57NO3 anal. calcd. (%): C, 81.21; H, 9.03;
N, 2.20.
Found (%): C, 81.17; H, 10.10; N, 2.01.
For C38H51NO2 anal. calcd. (%): C, 82.41; H, 9.28;
N, 2.53.
N-(3-Oxo-20(29)-lupen-28-oyl)-4-(2-ethynylpyri-
dine)aniline 5. A mixture of 78 mg (0.49 mmol) of
α-bromopyridine, 7 mg (0.04 mmol) of CuI, 7 mg
Found (%): C, 82.32; H, 8.72; N, 2.38.
DOKLADY CHEMISTRY Vol. 424 Part 2 2009

42
VASILEVSKII et al.
(0.01 mmol) of PdCl2(PPh3)2, 4 mg (0.02 mmol) of
PPh3, 300 mg (0.542 mmol) of acetylene 1, and 3 mL
of triethylamine in 10 mL of toluene was stirred in an
argon atmosphere at 55°C for 18 h. After cooling of the
reaction mixture, the precipitate of Et3N · HBr was sep-
arated by filtration, washed with toluene (3 × 10 mL),
and the solvent was removed in a vacuum. The residue
was triturated with hexane, and the precipitate was fil-
tered off to give 193 mg (62%) of compound 5, mp
181–182°C. 1H NMR (300.13 MHz, CDCl3, δ, ppm, J,
Hz): 0.89 (s, 3H, 25-CH3), 0.94 (s, 3H, 24-CH3), 0.97
(s, 6H, 26,27-CH3), 1.06 (s, 3H, 23-CH3), 1.66 (s, 3H,
30-CH3), 3.13 (dt, 1H, 19-H, J1 = 4 Hz, J2 = 11 Hz),
4.62 (s, 1H, 29-H), 4.76 (s, 1H, 29-H), 7.50 (m, 4H,
32,33,35,36-H), 7.46 (d, 1H, 3'-H, J = 3 Hz), 7.64 (t,
1H, 4'-H, J1 = 1.7 Hz, J2 = 7.7 Hz), 7.20 (m, 1H, 5'-H),
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ACKNOWLEDGMENTS
This work was supported by the Siberian Branch,
Russian Academy of Sciences (Integration Grant no.
54), the Russian Foundation for Basic Research
(project no. 07–03–00048a), CRDF RUXO 008–NO–
06, and Chemical Service Center, Siberian Branch,
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DOKLADY CHEMISTRY Vol. 424 Part 2 2009