NMR determination of the structure of azolopyrimidines produced from reaction of bidentate electrophiles and aminoazoles

Article abstract of DOI:10.1515/znb-2008-0216

A variety of aminoazoles were reacted with bidentate electrophiles producing azolopyrimidines. The regioselectivity of the nucleophilic attack could be defined from the ¹³C chemical shift of the pyrimidine carbons and through NOE experiments.

Full text of DOI:10.1515/znb-2008-0216

NMR Determination of the Structure of Azolopyrimidines Produced from
Reaction of Bidentate Electrophiles and Aminoazoles
Huwaida M. E. Hassaneena, Hamdi M. Hassaneena, Sherif F. M. Khirya,
and Richard M. Pagnib
a Chemistry Department, Faculty of Science, Cairo University, Giza, A. R. Egypt
b Department of Chemistry, University of Tennessee, Knoxville, TN, USA
Reprint requests to Dr. H. M. E. Hassaneen. E-mail: huwaidah@hotmail.com
Z. Naturforsch. 2008, 63b, 217 – 222; received September 20, 2007
A variety of aminoazoles were reacted with bidentate electrophiles producing azolopyrimidines.
The regioselectivity of the nucleophilic attack could be defined from the 13C chemical shift of the
pyrimidine carbons and through NOE experiments.
Key words: Aminoazoles, NOE Experiment, Azolopyrimidines, Enaminonitrile
Introduction
Azolopyrimidines are biologically interesting
molecules as indicated by the large number of recent
papers dealing with synthesis, chemistry, and phar-
macology of these molecules [1 – 9]. For example,
zaleplon (1), a pyrazolopyrimidine, is a hypnotic agent
licensed for treatment of insomnia [10], and allopurine
(2), a 4-hydroxypyrazolopyrimidine used in the
treatment of hyperuricemia and gout, inhibits de novo
purine biosynthesis and xanthine oxidase [11] (Fig. 1).
The triazolopyrimidine trapidil and its derivatives are
coronary vasodilating and potential antiatherosclerotic
drugs [12 – 14]. Also, pyrimidobenzimidazoles have
been found to be of pharmacological interest; for
example, pyrimido[1,2-a]benzimidazoles have been
described as anti-hypertensive [15], anti-diabetic [16]
and anti-inflammatory [17].
Fig. 1. Structures of zaleplon (1) and allopurine (2).
trogen atom, as it was generally accepted [22], is the
prime site of attack.
We now report on the reaction of 3(5)-amino-
pyrazoles 3a, b, 2-aminobenzimidazole (4), and 3-
amino-1,2,4-triazole (5) with arylidenemalononitrile
6, enaminonitrile 7, and β-ketoester 8 to show that
the 13C chemical shifts of pyrimidine carbons and
NOE experiments can be utilized to ascertain the re-
gioselectivity of addition or condensation reactions
(Fig. 2).
Compounds 3a, b reacted with arylidenemalononi-
trile 6 to yield aminopyrazolo-[1,5-a]pyrimidines that
may be formulated as 10a, b or isomeric 12a, b. Thus,
if the initial addition involves ring nitrogen atom N-2,
as has been assumed earlier by Elnagdi et al. [22],
Michael adduct 11 would be formed, which then on
cyclization would yield 12. On the other hand, if the
exocyclic amino function reacts with the electrophilic
carbon atom of 6, 9 would be formed. The latter com-
Results and Discussion
Azolopyrimidines with bridgehead nitrogen atoms
are usually obtained from the reaction of bidentate
electrophiles with cyclic amidines [18 – 20]. Because
of the aromaticity of the reaction products in a vari-
ety of cases, acyclic intermediates for such reactions
could not be isolated and structures assigned for reac-
tion products were based mainly on ambiguous con-
siderations. Recently, for example, Elnagdi et al. [21]
could show through X-ray crystallography that, in the
reaction of 3-aminopyrazole with benzylidenemalono-
nitrile, the exocyclic amino group and not the ring ni-
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218
H. M. E. Hassaneen et al. · Azolopyrimidines Produced from Reaction of Bidentate Electrophiles and Aminoazoles
Fig. 2. Structures of compounds
3 – 8.
Scheme 1.
pounds would undergo cyclization and autoxidation to
afford 10 (Scheme 1).
The 13C NMR spectra of the reaction products re-
vealed the pyrimidine carbon atom C-6 at δ ∼ 71 ppm.
This can be assigned only to C-6 in compound 10, as
in 12 this carbon would be shielded only by the cyano
group, while in 10 shielding by both the cyano group
and the lone pair of electrons of the amino group leads
to such high-field chemical shift. This conclusion is
based on analogy to 13C NMR data of 10 whose struc-
ture could be supported by X-ray crystallography [21].
Similarly, 2-aminobenzimidazole 4 reacted with 6
to yield a pyrimido[1,2-a]benzimidazole that may be
formulated as 13 or isomeric 14 (Scheme 2). Struc-
ture 13 could be readily established for this product
based on its 13C NMR spectrum, as well as on results
of NOE difference experiments. Thus, the 13C NMR
signal of the pyrimidine carbon atom C-3 appeared
Scheme 2.
Moreover, irradiation at the amino group resonance has
enhanced the 6-H signal in the pyrimido[1,2-a]benz-
imidazole ring.
The reaction of 3-amino-1,2,4-triazole 5 with 6
at δ = 72 ppm, which is consistent with structure 13 could afford the isomeric products 15 – 18 (Scheme 3).
in which both amino and cyano groups are shielding. The 13C chemical shift indentified the pyrimidine car-
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H. M. E. Hassaneen et al. · Azolopyrimidines Produced from Reaction of Bidentate Electrophiles and Aminoazoles
219
Scheme 3.
Fig. 3. Structures of compounds 19 – 21.
bon atom C-6 at δ = 75 ppm, which clearly excludes 7-Amino-1,2,4-triazolo[1,5-a]pyrimidine 21 was fur-
possible formation of 16 or 18, and we are left with ther established based on NOE difference experiments
15 and 17. NOE difference experiments indicated that which indicated that the triazole proton and the amino
the triazole proton and the amino protons do not show protons do not show significant mutual signal enhance-
mutual signal enhancement, thus establishing structure ment.
15 as the reaction product. Thus, it is believed that 3-
aminotriazole 5 behaves analogously to 3 and 4; the
exocyclic amino function is also the prime site of at-
tack by the electrophilic α,β-unsaturated moiety.
Similar to the behavior of 6 toward 3a, 4, and 5, 3-
(piperidin-1-yl)acrylonitrile 7 reacted with 3a, 4, and 5
Forty years ago Hori reported that 3-amino-5-
methyl-pyrazole (22) reacts with ethyl acetoacetate
to yield 2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-one
(23) but not the isomeric 2,7-dimethyl-pyrazolo[1,5-a]
pyrimidin-5-one (24) (Fig. 4) [23].
In the course of our investigations, we found that the
to yield aminoazolopyrimidines 19, 20 and 21, respec- reaction of 3a and ethyl acetoacetate 8 in acetic acid
tively (Fig. 3). The structure of the latter products was solution afforded 25 as indicated from NOE experi-
established by 13C NMR spectroscopy which revealed ments showing that the methyl and the 6-H protons are
chemical shift values of δ ∼ 90 ppm for the pyrimi- proximal. Similarly, reaction of 4 and 5 with ethyl ace-
dine carbons of compounds 19, 20 and 21, respectively. toacetate led to formation of 4-methyl-pyrimido[1,2-a]
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220
H. M. E. Hassaneen et al. · Azolopyrimidines Produced from Reaction of Bidentate Electrophiles and Aminoazoles
Fig. 4. Structures of compounds 22 – 24.
Fig. 5. Structures of compounds 25 – 27 and
selected NOE correlations.
benzimidazol-2-one (26) and 7-methyl-1,2,4-triazolo 2H, Ar-H), 8.09 (d, J = 8.2 Hz, 2H, Ar-H), 8.83 (br., 2H,
NH2). – 13C NMR (75 MHz, [D6]-DMSO): δ = 72.13 (C-6),
[1,5-a]pyrimidin-5-one (27), respectively. The struc-
94.77, 116.34 (CN), 126.38, 128.31, 128.73, 129.41, 130.37,
tures of products 26 and 27 were also derived on the
131.84, 134.86, 136.21, 148.71, 150.63, 156.29, 157.43. –
basis of NOE experiments (Fig. 5). Thus, the pres-
MS (EI, 70 eV): m/z (%) = 345 (100) [M]+. – C19H12ClN5
ence or absence of an NOE between specific protons
allowed the structures for these compounds to be es-
(345.79): calcd. C 66.00, H 3.50, Cl 10.25, N 20.25; found
C 66.20, H 3.75, Cl 10.10, N 20.12.
tablished.
7-Amino-6-cyano-5-(4-chlorophenyl)-2-(4-methylphenyl)-
pyrazolo[1,5-a]pyrimidine (10b)
Experimental Section
General procedures
This compound was obtained as yellow crystals, (2.88 g,
All melting points were determined on a GallenKamp
electrothermal apparatus and are uncorrected. IR spectra
80 %), m. p. 310 ◦C. – IR: ν = 3304, 3233 (NH2), 2211
(CN) cm−1. – 1H NMR (300 MHz, [D6]-DMSO): δ = 2.32
(s, 3H, CH3), 7.02 (s, 1H, 3-H), 7.25 (d, J = 8.2 Hz, 2H,
Ar-H), 7.56 (d, J = 8.2 Hz, 2H, Ar-H), 7.82 (d, J = 8.2 Hz,
were recorded as KBr pellets with a Pye Unicam SP 3000
1
infrared spectrophotometer. H and 13C NMR spectra were
recorded in deuterated dimethylsulfoxide [D6]-DMSO solu-
tion at 300 MHz on a Varian Gemini NMR spectrometer us-
ing tetramethylsilane (TMS) as internal reference, and the re-
sults are expressed as δ values. Mass spectra were performed
on a Shimadzu GCMS-QP 1000 Ex mass spectrometer at
70 eV. Elemental analyses were carried out at the Micro-
analytical Center of Cairo University.
2H, Ar-H), 7.95 (d, J = 8.2 Hz, 2H, Ar-H), 8.83 (br., 2H,
NH2). – 13C NMR (75 MHz, [D6]-DMSO): δ = 20.94 (CH3),
71.98 (C-6), 94.51, 116.39 (CN), 126.27, 128.27, 129.06,
129.25, 130.35, 134.83, 136.22, 138.98, 148.65, 150.55,
156.35, 157.27. – MS (EI, 70 eV): m/z (%) = 359 (100)
[M]+. – C20H14ClN5 (359.82): calcd. C 66.76, H 3.92,
Cl 9.85, N 19.46; found C 66.60, H 3.99, Cl 9.66, N 19.53.
General procedure for the preparation of compounds 10a,b,
4-Amino-3-cyano-2-(4-chlorophenyl)-pyrimido[1,2-a]
13 and 15
benzimidazole (13)
A mixture of aminoazoles 3a, b, 4 or 5 (0.01 mol) in pyr-
This compound was obtained as yellow crystals, (2.46 g,
idine (10 mL), and arylidenemalononitrile 6 (0.01 mol) was
77 %), m. p. 330 ◦C. – IR: ν = 3421, 3326 (NH2), 2186
refluxed for 6 h. The solid product was filtered off and crys-
tallized from dimethylformamide to give compounds 10a,b,
(CN) cm−1. – 1H NMR (300 MHz, [D6]-DMSO): δ = 7.02
(m, 1H, 7-H), 7.34 (m, 1H, 8-H), 7.53 (d, J = 9 Hz, 1H, 9-H),
13 and 15, respectively.
7.66 (d, J = 8.2 Hz, 2H, Ar-H), 7.82 (d, J = 8.2 Hz, 2H,
Ar-H), 8.22 (d, J = 9 Hz, 1H, 6-H), 8.62 (br., 2H, NH2). –
7-Amino-6-cyano-5-(4-chlorophenyl)-2-phenyl-pyrazolo
13C NMR (75 MHz, [D6]-DMSO): δ = 72.26, 112.68, 12.14,
[1,5-a]pyrimidine (10a)
121.28, 121.78, 125.05, 126.28, 130.85, 131.45, 136.72,
This compound was obtained as yellow crystals, (2.9 g, 136.24, 143.38, 152.58, 153.15, 163.07. – MS (EI, 70 eV):
84 %), m. p. 330 ◦C. – IR: ν = 3304, 3161 (NH2), 2212 m/z (%) = 319 (38.2) [M]+. – C17H10ClN5 (319.76): calcd.
1
(CN) cm−1. – H NMR (300 MHz, [D6]-DMSO): δ = 7.09 C 63.86, H 3.15, Cl 11.09, N 21.90; found C 63.66, H 3.02,
(s, 1H, 3-H), 7.45 – 7.59 (m, 5H, Ar-H), 7.83 (d, J = 8.2 Hz, Cl 11.00, N 21.87.
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H. M. E. Hassaneen et al. · Azolopyrimidines Produced from Reaction of Bidentate Electrophiles and Aminoazoles
221
7-Amino-6-cyano-5-(4-chlorophenyl)-triazolo[1,5-a]
pyrimidine (15)
(NH2) cm−1. – 1H NMR (300 MHz, [D6]-DMSO): δ = 6.31
(d, J = 8 Hz, 1H, 6-H), 8.12 (br., 2H, NH2), 8.25 (d, J = 8 Hz,
1H, 5-H), 8.42 (s, 1H, 2-H). – 13C NMR (75 MHz, [D6]-
DMSO): δ = 90.79, 149.23, 153.47, 154.36, 155.86. – MS
(EI, 70 eV): m/z (%) = 135 (100) [M]+. – C5H5N5 (135.13):
calcd. C 44.44, H 3.73, N 51.83; found C 44.40, H 3.55,
N 51.79.
This compound was obtained as colorless crystals,
◦
(2.08 g, 77 %), m. p. 310 C. – IR: ν = 3304, 3233 (NH2),
2211 (CN) cm−1. – 1H NMR (300 MHz, [D6]-DMSO): δ =
7.56 (d, J = 8.2 Hz, 2H, Ar-H), 7.83 (d, J = 8.2 Hz, 2H,
Ar-H), 8.54 (s, 1H, 2-H), 9.20 (br., 2H, NH2). – 13C NMR
(75 MHz, [D6]-DMSO): δ = 75.54 (C-6), 115.43 (CN),
120.71, 128.37, 130.53, 135.47, 151.47, 154.94, 155.93,
163.06. – MS (EI, 70 eV): m/z (%) = 270 (100) [M]+. –
C12H7ClN6 (270.68): calcd. C 53.25, H 2.61, Cl 13.10,
N 31.05; found C 53.08, H 2.44, Cl 13.25, N 31.02.
General procedure for the preparation of compounds 25, 26
and 27
A mixture of aminoazoles 3a, 4 or 5 (0.01 mol) and ethyl
acetoacetate 8 (1.3 g, 0.01 mol) was refluxed in acetic acid
solution (10 mL) for 3 h. The solid product formed was col-
lected by filtration and crystallized from dimethylformamide
to give compounds 25, 26 and 27, respectively.
General procedure for the preparation of compounds 19, 20
and 21
A solution of 3a, 4 and 5 (0.01 mol) and 7 (1.36 g,
0.01 mol) in pyridine (10 mL) was refluxed for 5 h, then left
to cool at r. t. The solid product, formed was collected by
filtration and crystallized from dimethylformamide to give
compounds 19, 20 and 21, respectively.
7-Methyl-2-phenyl-pyrazolo[1,5-a]pyrimidin-5-one (25)
This compound was obtained as yellow crystals, (1.4 g,
62 %), m. p. 320 ◦C. – IR: ν = 3319 (NH), 1676 (CO) cm−1. –
1H NMR (300 MHz, [D6]-DMSO): δ = 2.27 (s, 3H, CH3),
5.57 (s, 1H, 6-H), 6.54 (s, 1H, 3-H), 7.35 – 7.45 (m, 3H,
Ar-H), 7.94 – 7.97 (m, 2H, Ar-H), 12.43 (br., 1H, NH). –
13C NMR (75 MHz, [D6]-DMSO): δ = 18.59 (CH3), 85.47,
95.24, 120.76, 126.10, 128.62, 132.48, 142.73, 150.12,
152.90, 156.17 (CO). – MS (EI, 70 eV): m/z (%) = 225
(100) [M]+. – C13H11N3O (225.25): calcd. C 69.32, H 4.92,
N 18.65; found C 69.19, H 4.55, N 18.42.
7-Amino-2-phenyl-pyrazolo[1,5-a]pyrimidine (19)
This compound was obtained as yellow crystals, (1.62 g,
◦
77 %), m. p. 200 C. – IR: ν = 3374, 3297 (NH2) cm−1. –
1H NMR (300 MHz, [D6]-DMSO): δ = 6.14 (d, J = 8 Hz,
1H, 6-H), 6.85 (s, 1H, 3-H), 7.35 – 7.58 (m, 5H, Ar-H), 7.81
(br., 2H, NH2), 8.05 (d, J = 8 Hz, 1H, 5-H). – 13C NMR
(75 MHz, [D6]-DMSO): δ = 87.95, 91.08, 126.14, 128.62,
128.99, 133.0, 147.85, 149.49, 150.46, 153.85. – MS (EI,
70 eV): m/z (%) = 210 (100) [M]+. – C12H10N4 (210.24):
calcd. C 68.56, H 4.79, N 26.65; found C 68.60, H 4.55,
N 26.46.
4-Methyl-pyrimido[1,2-a]benzimidazol-2-one (26)
This compound was obtained as yellow crystals, (1.27 g,
64 %), m. p. 200 ◦C. – IR: ν = 3031 (NH), 1690 (CO) cm−1. –
1H NMR (300 MHz, [D6]-DMSO): δ = 2.27 (s, 3H, CH3),
5.79 (s, 1H, 3-H), 7.23 (m, 1H, 7-H), 7.37 (m, 1H, 8-H), 7.46
(d, J = 9 Hz, 1H, 9-H), 8.39 (d, J = 9 Hz, 1H, 6-H), 12.17
(br., 1H, NH). – 13C NMR (75 MHz, [D6]-DMSO): δ =
21.71 (CH3), 98.42, 111.43, 113.55, 119.61, 120.74, 125.37,
126.81, 135.17, 148.33, 159.13 (CO). – MS (EI, 70 eV):
m/z (%) = 199 (100) [M]+. – C11H9N3O (199.21): calcd.
C 66.32, H 4.55, N 21.09; found C 66.30, H 4.55, N 21.22.
4-Amino-pyrimido[1,2-a]benzimidazole (20)
This compound was obtained as yellow crystals, (1.23 g,
◦
67 %), m. p. 272 C. – IR: ν = 3262, 3069 (NH2) cm−1. –
1H NMR (300 MHz, [D6]-DMSO): δ = 6.21 (d, J = 8 Hz,
1H, 3-H), 7.20 (m, 1H, 7-H), 7.44 (m, 1H, 8-H), 7.69 (d,
J = 9 Hz, 1H, 9-H), 8.22 (d, J = 8 Hz, 1H, 2-H), 8.39
(d, J = 9 Hz, 1H, 6-H), 12.54 (br., 2H, NH2). – 13C NMR
(75 MHz, [D6]-DMSO): δ = 89.16, 114.42, 117.59, 119.20,
121.17, 125.10, 125.91, 144.06, 152.30, 153.99. – MS (EI,
70 eV): m/z (%) = 184 (100) [M]+. – C10H8N4 (184.20):
calcd. C 65.21, H 4.38, N 30.42; found C 65.35, H 4.33,
N 30.32.
7-Methyl-triazolo[1,5-a]pyrimidin-5-one (27)
This compound was obtained as colorless crystals,
◦
(0.98 g, 65 %), m. p. 280 C. – IR: ν = 3106 (NH), 1666
(CO) cm−1. – 1H NMR (300 MHz, [D6]-DMSO): δ = 2.24
(s, 3H, CH3), 5.67 (s, 1H, 6-H), 8.07 (s, 1H, 2-H), 12.52 (br.,
1H, NH). – 13C NMR (75 MHz, [D6]-DMSO): δ = 18.50
(CH3), 98.0, 121.20, 150.46, 151.46, 155.68 (CO). – MS (EI,
70 eV): m/z (%) = 150 (100) [M]+. – C6H6N4O (150.14):
calcd. C 48.00, H 4.03, N 37.32; found C 47.19, H 3.29,
N 37.05.
7-Amino-triazolo[1,5-a]pyrimidine (21)
This compound was obtained as colorless crystals,
(0.93 g, 69 %), m. p. 280 ◦C. – IR: ν = 3334, 3264
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H. M. E. Hassaneen et al. · Azolopyrimidines Produced from Reaction of Bidentate Electrophiles and Aminoazoles
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