Synthesis and biological activity of 3,4-dihydropyrimidin-2-thione aminomethylene derivatives based on the alkaloid cytisine

Article abstract of DOI:10.1007/s10600-011-0004-4

3,4-Dihydropyrimidin-2-thiones derivatized by the alkaloid cytisine were synthesized by Mannich aminomethylation. The compositions and structures of the products were confirmed by mass spectrometry and PMR and ¹H- ¹H NOESY spectrosco

Full text of DOI:10.1007/s10600-011-0004-4

Chemistry of Natural Compounds, Vol. 47, No. 4, September, 2011 [Russian original No. 4, July-August, 2011]
SYNTHESIS AND BIOLOGICAL ACTIVITY OF
3,4-DIHYDROPYRIMIDIN-2-THIONE AMINOMETHYLENE
DERIVATIVES BASED ON THE ALKALOID CYTISINE
I. V. Kulakov
UDC 547.94: 547.853
3,4-Dihydropyrimidin-2-thiones derivatized by the alkaloid cytisine were synthesized by Mannich
aminomethylation. The compositions and structures of the products were confirmed by mass spectrometry
1
1
and PMR and H– H NOESY spectroscopy. One of the products showed pronounced antimicrobial activity.
1
1
Keywords: alkaloid cytisine, PMR and H– H NOESY spectroscopy, antimicrobial activity.
Incorporation into the structure of plant alkaloids of other pharmacophores, including physiologically active heterocyclic
compounds, many of which are analogs of natural nucleotides, is known to be a basic approach to the chemical design of new
biologically active compounds. The alkaloid cytisine, which exhibits analeptic and antismoking activity, occupies a special
place among a multitude of natural alkaloids [1]. Furthermore, the presence of a free secondary amine in cytisine facilitates
the incorporation into its structure of various functional groups and heterocycles. The number of publications on the synthesis
of heterocyclic N-derivatives of cytisine has increased in the last decade. Among these, compounds with types of biological
activity that are uncharacteristic of cytisine itself are constantly being discovered. Until now many cytisine derivatives with
various heterocylic groups such as coumarin [2, 3]; 1,2,3-triazole [4]; 1,2,4-thiadiazole [5]; 1,3-thiazoline [6]; 2,5-dimercapto-
1,3,4-thiadiazole [7]; barbituric acid [8]; pyridine [9]; 1,4-dihydropyridine [10]; and phenothiazine [11] were prepared.
The Mannich reaction, which is widely used in organic chemistry to synthesize a variety of practically important
compounds, is a convenient method for preparing new N-derivatives of cytisine in addition to the broadly used nucleophilic
substitution and addition reactions of cytisine [2–7, 9–11].
The starting synthons for preparing new N-heterocyclic cytisine derivatives were 3,4-dihydropyrimidin-(1H)-2-thione
derivatives, which were obtained via three-component condensation using a Biginelli reaction. The number of publications on
the chemistry of 4-aryl-3,4-dihydropyrimidin-2-ones and 4-aryl-3,4-dihydropyrimidin-2-thiones has recently increased
significantly in the scientific literature. This is due to not only their preparative availability but also the manifestation by them
of a broad spectrum of pharmacological activity such as analgesic, antibacterial, antihypertensive, etc. [12–14].
OCH
OCH
3
3
R
R
2
1
1
N
H
O
13'
9'
O
DMF, 120ꢀC
3
N
CH O
5'
7'
2
+
+
HN
4
N
11'
CH
N
4'
3'
OEt
OEt
8'
10'
1
N
H
S
N
H
CH
3
S
CH
N
3
O
O
1, 2
3, 4
R = H (1, 3), OCH (2, 4)
1
3
Because the starting 3,4-dihydropyrimidin-(1H)-2-thiones (1 and 2) had two reaction centers with nucleophilic
N atoms (in the ring) and the S atom, which also exhibited a certain nucleophilicity and participated in possible thione–thiol
tautomerism, it seemed interesting to us to study the possible involvement of these thiones in the Mannich synthesis as an
Institute of Organic Synthesis and Coal Chemistry of the Republic of Kazakhstan, 100008, Karaganda,
fax: (87212) 41 38 66, e-mail: kulakov_iv@mail.ru. Translated from Khimiya Prirodnykh Soedinenii, No. 4, July–August,
2011, pp. 529–531. Original article submitted December 26, 2010.
©
0009-3130/11/4704-0597 2011 Springer Science+Business Media, Inc.
597

N–H or S–H acidic component and the possibility of preparing from them Mannich mono- or bis-bases incorporating the
pharmacologically important alkaloid. For this, the 3,4-dihydropyrimidin-(1H)-2-thiones (1 and 2) were derivatized by cytisine
and aqueous formaldehyde solution (40%) using Mannich aminomethylation.
The reaction was carried out with heating of the starting materials in DMF solution at 120°C for 15–20 h with an
excess of formalin and various thione:cytisine ratios (1:1, 1:1.5, 1:2). In every instance, TLC analysis showed the formation
of a single product, acceptable yields of which after isolation from the reaction mixture were greatest for thione:cytisine ratios
of 1:1.5 and 1:2.
It was supposed that both N- and S-aminomethylene derivatives of the Mannich bases formed. Furthermore, the
possible formation of bis-bases with aminomethyl groups bonded to N(1)-, N(3)-, or an N-, S-dihydropyrimidine ring was also
considered. It was also possible that the 3,4-dihydropyrimidin-(1H)-2-thione (1 and 2) did not undergo the aminomethylation
reaction but a simple linking of two cytisine molecules to form dicytisinomethane, as reported previously [15].
+
Mass spectral analysis of 3 showed a weak molecular ion [M] 508 (2%) that corresponded to the molecular weight
of supposed structure 3 in addition to fragment ions at 306 (22), 277 (32), 233 (39), 203 (76), 58 (100), and 42 (59). Among
+
these, the fragment ion with m/z 203 (76%) corresponded to the >N–CH fragment of the cytisine framework.
2
PMR spectroscopy of the isolated products also showed the presence of protons from the starting 3,4-dihydropyrimidin-
(1H)-2-thiones (1 and 2) and cytisine. Furthermore, an analysis of the PMR spectrum of 3 revealed a characteristic singlet for
the C(4) –H proton of the dihydropyrimidine ring at 5.00 ppm, indicating a lack of coupling with the neighboring N(3)–H
proton, in the presence of which, for example in the starting 3,4-dihydropyrimidin-(1H)-2-thiones (1 and 2), the C(4)–H
resonance would be split into a doublet.
The free N(1) –H proton was observed as a singlet at 10.35 ppm. The aminomethylene protons of the >NCH N<
2
group were non-equivalent and appeared as two characteristic doublets in different spectral regions at 5.33 and 3.27 ppm with
a difference of 2.06 ppm and the same SSCC J = 11.7 Hz. This assignment of the aminomethylene protons was confirmed
a,b
also by recording the 2D NOESY spectrum, which showed signals for coupled non-equivalent aminomethylene protons and a
signal for coupling of the N(1)–H proton with the neighboring C(6)–CH methyl.
3
1
1
Therefore, the PMR and H– H NOESY spectra were consistent with N(3)-aminomethylation of the starting
3,4-dihydropyrimidin-(1H)-2-thiones (1 and 2).
Bioscreening of 3 for antimicrobial activity against strains of Gram-positive bacteria Staphylococcus aureus and
Bacillus subtilis, Gram-negative strains Pseudomonas aeruginosa and Escherichia coli, and yeast fungus Candida albicans
was performed using the agar diffusion method (wells) in order to establish the biological activity of the synthesized derivatives
(3 and 4). The reference drugs were gentamicin for bacteria and nystatin for the yeast fungus C. albicans. The bioscreening
of 3 revealed its pronounced antibacterial activity only against the Gram-positive strains S. aureus and B. subtilis and weak
activity against Gram-negative strains P. aeruginosa and E. coli in addition to the yeast fungus C. albicans.
EXPERIMENTAL
PMR spectra were recorded in DMSO-d solution on a Bruker DRX500 spectrometer relative to TMS internal standard.
6
Mass spectra were recorded on a Finnigan Mat.Incos 50 instrument at ionization energy 70 eV by direct sample introduction.
Melting points were determined on a Boetius apparatus. TLC was performed on Sorbfil plates with detection by iodine vapor.
4-(4-Methoxyphenyl)-6-methyl-3-(N-cytisinomethyl)-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylicAcid
Ethyl Ester (3). A solution of formalin (2.5 g, 40%) was treated with 1 (1.23 g, 4 mmol), diluted with DMF (10 mL), refluxed
with stirring on a magnetic stirrer for 2 min, treated with cytisine (1.52 g, 8 mmol), heated for 25 h until 1 disappeared (TLC),
cooled, and poured into a beaker with icewater (100 mL). The resulting gray precipitate was filtered off and washed several
times with H O to afford a crude product (1.85 g, 90%). Two recrystallizations from benzene and benzene:hexane produced
2
white needle-like crystals, mp 214–215°C. Elemental analysis of 3 agreed with the calculated formula C H N O S. Mass
27 32
4 4
+
spectru (EI, 70 eV, m/z, I , %): 508 (2) [M] , 306 (22), 277 (32), 233 (39), 203 (76), 58 (100), 42 (59).
rel
PMR spectrum (500 MHz, DMSO-d , ꢀ, ppm, J/Hz): 1.02 (3H, t, J = 7.1, CH CH ), 1.73, 1.83 (2H, 2 br.d, J = 11.9,
6
2
3
H-8ꢁ), 2.21 (3H, s, CH ), 2.40 (1H, br.d, H-11ꢁa), 2.46 (1H, br.s, H-9ꢁ), 2.54 (1H, br.d, H-13ꢁa), 2.83 (2H, br.t, H-13ꢁe, 11ꢁe),
3
3.02 (1H, br.s, H-7ꢁ), 3.27 (1H, d, J = 11.7, N–CH ), 3.71 (3H, s, OCH ), 3.80 (1H, dd, J = 6.83, 14.91, H-10ꢁa), 3.94 (1H, d,
b
3
J
= 15.0, H-10ꢁe), 3.97 (2H, q, CH CH ), 4.99 (1H, s, H-4), 5.33 (1H, d, J = 11.7, N–CH ), 6.04 (1H, d, J
= 6.90,
10ꢁe,10ꢁa
2
3
a
5ꢁ,4ꢁ
598

H-5ꢁ), 6.15 (1H, d, J
= 8.90, H-3ꢁ), 6.86 (2H, d, J = 8.67, 2H–Ar), 7.00 (2H, d, J = 8.66, 2H–Ar), 7.23 (1H, dd, J
= 6.90,
4ꢁ,5ꢁ
3ꢁ,4ꢁ
J
= 8.90, H-4ꢁ), 10.35 [1H, s, N(1) –H].
4ꢁ,3ꢁ
4-(2,4-Dimethoxyphenyl)-6-methyl-3-(N-cytisinomethyl)-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic
acid ethyl ester (4) was prepared by an analogous method in 70% yield, mp 227-228°C (benzene). Elemental analysis of 4
agreed with the calculated formula C H N O S.
28 34
4 5
PMR spectrum (500 MHz, DMSO-d , ꢀ, ppm, J/Hz): 0.96 (3H, t, J = 7.08, CH CH ), 1.73, 1.84 (2H, 2 br.d, J = 12.50,
6
2
3
H-8ꢁ), 2.22 (3H, s, CH ), 2.41 (1H, br.d, H-11ꢁa), 2.47 (1H, br.s, H-9ꢁ), 2.57 (1H, br.d, H-13ꢁa), 2.84 (2H, br.t, H-13ꢁe, 11ꢁe),
3
3.01 (1H, br.s, H-7ꢁ), 3.24 (1H, d, J = 11.48, N–CH ), 3.72 (3H, s, OCH ), 3.78 (3H, s, OCH ), 3.82 (1H, m, H-10ꢁa), 3.86 (2H,
b
3
3
q, CH CH ), 3.96 (1H, d, J
= 15.59, H-10ꢁe), 5.29 (1H, d, J = 11.44, N–CH ), 5.44 (1H, s, H-4), 6.00 (1H, d,
2
3
10ꢁe,10ꢁa
a
J
= 6.88, H-5ꢁ), 6.09 (1H, d, J
= 9.00, H-3ꢁ), 6.44 (1H, d, J = 8.40, 1H–Ar), 6.49 (1H, s, 1H–Ar), 6.92 (1H, d, J = 8.47,
5ꢁ,4ꢁ
3ꢁ,4ꢁ
1H–Ar), 7.19 (1H, dd, J
= 6.88, J
= 9.00, H-4ꢁ), 10.19 [1H, s, N(1) –H].
4ꢁ,5ꢁ
4ꢁ,3ꢁ
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