Some reactions with Ketene dithioacetals. Part I: Synthesis of antimicrobial pyrazolo[1,5-a]pyrimidines via the reaction of ketene dithioacetals and 5-aminopyrazoles

Article abstract of DOI:10.1016/S0014-827X(01)01042-4

Pyrazolo[1,5-a]pyrimidines were synthesized via the reaction of ketene dithioacetals and 5-aminopyrazoles. The antibacterial and antifungal activities of some selected compounds are also reported. Copyright

Full text of DOI:10.1016/S0014-827X(01)01042-4

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Il Farmaco 56 (2001) 277–283
Some reactions with ketene dithioacetals
Part I: Synthesis of antimicrobial pyrazolo[1,5-a]pyrimidines via
the reaction of ketene dithioacetals and 5-aminopyrazoles
Medhat A. Zaharan a, Ahmed M.Sh. El-Sharief a, Mohamed S.A. El-Gaby b,*,
Yousry A. Ammar a, Usama H. El-Said a
a Chemistry Department, Faculty of Science, Al-Azhar Uni6ersity, Nasr City, P.O.Box 11884, Cairo, Egypt
b Chemistry Department, Faculty of Science, Al-Azhar Uni6ersity at Assiut, P.O.Box 71524, Assiut, Egypt
Received 1 June 2000; accepted 14 November 2000
Abstract
Pyrazolo[1,5-a]pyrimidines were synthesized via the reaction of ketene dithioacetals and 5-aminopyrazoles. The antibacterial
and antifungal activities of some selected compounds are also reported. © 2001 Elsevier Science S.A. All rights reserved.
Keywords: Pyrazolo[1,5-a]pyrimidines; Ketene dithioacetals; Antimicrobial activity
1. Introduction
2. Chemistry
Pyrazolo[1,5-a]pyrimidines are of considerable chem-
ical and pharmacological importance as purine ana-
logues. The purine analogue 4-hydroxypyrazolo-
pyrimidine (allopurin), used in the treatment of hyper-
uricemia and gout, inhibits de novo purine biosynthesis
and xanthine oxidase. Azathioprine which is catabo-
lized to 6-mercaptopurine is employed in organ trans-
plantation to suppress events involved in immunologic
rejection [1]. It has been reported that ketene dithioac-
etals are important and versatile reagents, which have
been especially used for the synthesis of polyfunctional-
ized heterocycles [2–4]. These ketene dithioacetals are
extensively used for the synthesis of pyrazole and
pyrimidine derivatives by the displacement of the
methylthio group with bifunctionalized amine as hy-
drazines [5–7]. We report in this paper a novel synthe-
sis of functionalized pyrazolo[1,5-a]pyrimidines by the
reaction of ketene dithioacetals with aminopyrazoles.
The antibacterial and antifungal testing of some se-
lected compounds is also included.
Compounds 2a,b were obtained via the reaction of
[bis(methylthio)methylene]malononitrile (1) and an ap-
propriate aromatic amine in refluxing ethanol [8]. The
starting aminopyrazoles 3a,b were synthesized by the
reaction of hydrazine with [(arylamino)(methylthio)-
methylene]malononitriles (2a,b). Both the elemental and
spectral data of 3 are consistent with the assigned
structures (Scheme 1).
The reaction of compound 1 with aminopyrazoles
(3a,b) in refluxing DMF containing a catalytic amount
of triethylamine afforded the corresponding pyra-
zolo[1,5-a]pyrimidines (5a,b). The structures of com-
pounds 5a,b were established on the basis of analytical
and spectral data. Thus, the IR spectra of 5a,b revealed
characteristic bands for NH2 and CꢀN functional
1
groups. Also, the H NMR spectrum of compound 5a
showed a signal l=2.97 assignable to the SCH3 group.
The formation of 5 from the reaction of 3 with
dithioacetals (1) proceeds via initial alkylation of the
ring nitrogen [9] in 3 to give 4 as intermediate which
underwent cyclization to the final products 5 (Scheme
2). Fusion of 5 with aromatic amines at 140°C fur-
nished the corresponding anilino derivatives 6a,b.
Confirmation for structures of 6a and 6b was obtained
* Corresponding author.
E-mail address: m elagaby@hotmail.com (M.S.A. El-Gaby).
–
0014-827X/01/$ - see front matter © 2001 Elsevier Science S.A. All rights reserved.
PII: S0014-827X(01)01042-4

278
M.A. Zaharan et al. / Il Farmaco 56 (2001) 277–283
Scheme 1.
through their synthesis via another reaction route.
Thus, the reaction of aminopyrazoles (3a,b) with 2 in
refluxing DMF containing a catalytic amount of tri-
ethylamine gave the same pyrazolopyrimidines (6a,b)
(identical IR, m.p. and mixed m.p.).
The synthetic approach demonstrated here was ex-
tended to enable synthesis of other functionally substi-
tuted pyrazolo[1,5-a]pyrimidines for their biological
evalution. Thus, when compound 1 was reacted with
1,2-phenylenediamine derivatives (7) in refluxing abso-
lute ethanol for 24 h, the (2,3-dihydrobenzimidazol-2-
yl) malononitriles (8a–d) were obtained. The structures
of 8a–d were established on the basis of their elemental
analysis, spectral data (MS, 1H NMR and IR) and
analogy with previous work [7]. The reaction is as-
sumed to proceed via a nucleophilic attack of the NH2
group to the ethylenic bond in 1 with elimination 2
mole of methyl mercaptan. The behaviour of com-
pounds 8a–d towards hydrazine was investigated.
Thus, when compounds 8a–d were treated with hydra-
zine the novel aminopyrazoles (9a–d) were produced
(Scheme 3). Elemental and spectral data were consistent
with the assigned structure. Thus, structure 9a is sup-
ported by its mass spectrum which showed a molecular
ion corresponding to the formula C10H10N6 (M+=214;
100%) (Scheme 4).
Aminopyrazoles (9a,c,d) were reacted with dithioac-
etals (1) under fusion conditions to yield the novel
pyrazolopyrimidines (10a–c) on the basis of elemental
and spectral data. The mass spectrum of compound 10a
exhibited a molecular ion peak at m/z 336 which is the
base peak in the spectrum (Scheme 5). The reaction
pathway was thus assumed to involve alkylation of the
NH group (pyrazole ring) followed by cyclization. In a
similar manner ketene (2a) reacted with compound 9a
to give the pyrazolopyrimidine (11). Also, the pyra-
zolopyrimidines (14a–c) were obtained when the
aminopyrazole (9a) was reacted with arylidene-
malononitriles (12) in refluxing ethanol catalysed by
piperidine. The formation of 14 is assumed via initial
Michael addition of the endocyclic NH in 9 to the
double bond of 12 to yield the Michael adduct [10] 13,
which then cyclizes and loses H2 to afford 14. Finally,
the aminopyrazole (9a) was reacted with acetic anhy-
dride to afford the polyheterocyclic system (15)
(Scheme 6).
Scheme 2.
3. Experimental
All melting points are uncorrected. IR spectra were
recorded on a Shimadzu-440 IR spectrophotometer us-
Scheme 3.

M.A. Zaharan et al. / Il Farmaco 56 (2001) 277–283
279
Scheme 4.
1
ing the KBr technique (Shimadzu, Japan). H NMR
3.3. 5-Amino-2-arylamino-7-methylthiopyrazolo-
[1,5-a]pyrimidin-3,6-dicarbonitriles (5a,b)
spectra were measured on a Varian EM-360 90 MHz
spectrophotometer (Varian, UK) using TMS as an in-
ternal standard. The mass spectra were recorded by a
Shimadzu-GC-MS-QP 100 EX (Shimadzu, Japan). Ele-
mental analyses were carried out by the Microanalytical
Research Centre, Faculty of Science, Cairo University.
The characteristic data for prepared compounds are
given in Table 1, analytical results for C, H, N were
within 90.1% of the calculated values. The spectral
data are collected in Table 2.
To a suspension of compounds 3 (0.01 mol) and 1
(0.01 mol) in absolute ethanol (30 ml), three drops of
triethylamine were added. The mixture was refluxed for
3 h and then allowed to cool. The solid precipitate was
isolated by suction and crystallized from ethanol to give
5a,b.
3.4. 5-Amino-2,7-diarylamino-pyrazolo[1,5-a]-
pyrimidin-3,6-dicarbonitriles (6a,b)
3.1. [(Arylamino)(methylthio)methylene]malononitriles
(2a,b)
Method A: a mixture of 5 (0.01 mol) and the requi-
site aromatic amine (0.01 mol) was fused at 160°C for 1
h, then triturated with ethanol, poured into water and
then acidified with hydrochloric acid. The solid product
so formed was collected by filtration and crystallized
from ethanol to give 6a,b.
Method B: the experimental procedure used for the
synthesis of 5 was carried out except for the use of 2
instead of 1.
A mixture of 1 (0.01 mol) and the requisite amino
compound (0.01 mol) in absolute ethanol (30 ml) was
heated under reflux for 0.5 h. The reaction mixture was
concentrated and the obtained product was recrystal-
lized from ethanol to give 2a,b.
3.2. 3-Arylamino-5-aminopyrazole-4-carbonitriles
(3a,b)
3.5. (2,3-Dihydrobenzimidazol-2-yl)malononitriles
(8a–d)
A mixture of 2 (0.01 mol) and hydrazine hydrate
(0.012 mol) was heated at 100°C for 1 h. The obtained
product was collected and recrystallized from ethanol
to give 3a,b.
These were prepared according to the literature pro-
cedure [7] as follows. A mixture of 1 (0.01 mol) and

280
M.A. Zaharan et al. / Il Farmaco 56 (2001) 277–283
3.8. 2,5-Diamino-3-(benzimidazol-2-yl)-7-
(2-ethoxyphenyl)-pyrazole[1,5-a]pyrimidin-6-carbonitrile
(11)
1,2-phenylenediamine or its derivatives 7 (0.01 mol) in
absolute ethanol (30 ml) was heated under reflux for 24
h. The reaction mixture was concentrated and the ob-
tained products were recrystallized from dioxane to
give (8a–d).
To a mixture of the aminopyrazole 9a (0.01 mol) and
compound 2a (0.01 mol) in DMF (10 ml), three drops
of triethylamine were added. The resulting mixture was
refluxed for 2 h, and then allowed to cool at room
temperature and diluted with water (100 ml). The solid
product so formed was collected by filtration and re-
crystallized from ethanol to give 11.
3.6. 3,5-Diamino-4-(benzimidazol-2-yl)pyrazoles (9a–d)
A mixture of 8 (0.01 mol) and hydrazine hydrate
(0.03 mol) was heated at 100°C for 0.5 h. The obtained
product was collected and recrystallized from ethanol
to give 9a–d.
3.9. 2,5-Diamino-3-(benzimidazol-2-yl)-7-
aryl-pyrazolo[1,5-a]pyrimidin-6-carbonitriles (14a–c)
3.7. 2,5-Diamino-3-(benzimidazol-2-yl)-7-
methylthiopyrazolo[1,5-a]pyrimidin-6-carbonitriles
(10a–c)
A mixture of aminopyrazoles (9a,c,d) and compound
1 (0.01 mol) was fused at 100°C for 0.4 h. The obtained
product was collected and recrystallized from ethanol
to give 10a–c.
A mixture of compound 9 (0.01 mol), nitrile 12 (0.01
mol) and piperidine (0.5 ml) in absolute ethanol (30 ml)
was heated under reflux for 1 h. The solid obtained was
collected by filtration and recrystallized from ethanol to
give 14a–c.
3.10. Pyrazolo[4%,5%:4,5]pyrimido[1,6-a]benzimidazole
(15)
A solution of 9a (0.01 mol) in acetic anhydride (10
ml) was heated under reflux for 0.5 h. The solid ob-
tained after filtration was recrystallized from benzene to
give 15.
4. Antimicrobial activity
Most of the synthesized compounds were evaluated
for their antimicrobial activity using the agar diffusion
technique [11]. A 1 mg ml−1 solution in dimethylfor-
mamide was used. The tested organisms were gram
positive bacteria [Staphylcoccus aureus (NCTC-7447)
and Bacillus cereus (ATCC-14579)], gram negative bac-
teria [Serratia marcescens (IMRU-70) and Proteus
mirabilis (NTC-289)] and fungi [Aspergillus ochraceus
Wilhelm (AUCC-230) and Penicillium chrysogenum
Thom (AUCC-530)]. The bacteria and fungi were main-
tained on nutrient agar and Czapek’s–Dox agar media,
respectively. DMF showed no inhibition zones. The
agar media were inoculated with different microorgan-
isms culture tested. After 24 h of incubation at 30°C for
bacteria and 48 h of incubation at 28°C for fungi, the
diameter of the inhibition zone (mm) was measured
(Table 3). Ampicillin in a concentration of 25 mg ml−1
and mycostatine (30 mg ml−1) were used as a references
for antibacterial and antifungal activities, respectively.
The minimal inhibitory concentration (MIC) of some of
the tested compounds was measured by a twofold serial
dilution method [12].
Scheme 5.

M.A. Zaharan et al. / Il Farmaco 56 (2001) 277–283
281
Scheme 6.
Table 1
Characterization data for the synthesized compounds
Compound
Melting point T (°C)
Yield (%)
Molecular formula (molar weight)
2a
2b
3a
3b
5a
5b
6a
6b
8b
8c
8d
9a
9b
9c
9d
10a
10b
10c
11
14a
14b
14c
15
97
215
130
160
175
210
206
180
\300
\300
\300
\300
275
\300
\300
\300
\300
\300
110
72
70
68
75
70
67
57
68
90
82
85
72
65
62
67
50
57
48
50
60
65
62
65
C
C
13H13N3OS (259)
13H13N3OS (259)
C12H13N5O (243)
C
C
C17H15N7OS (365)
C24H23N8O2 (455)
C24H23N8O2 (455)
C17H10N4O (286)
C10H5N4Cl (216.5)
C12H10N4 (210)
C10H10N6 (214)
C17H14N6O (318)
C10H9N6Cl (248.5)
12H13N5O (243)
17H15N7OS (365)
C12H14N6 (242)
C15H12N8S (336)
C15H11N8SCl (370.5)
C17H16N8S (364)
C
22H19N9O (416)
\300
\300
\300
225
C20H14N8 (366)
C20H13N8Cl (400.5)
C27H18N8O (470)
C18H16N6O3 (364)

282
M.A. Zaharan et al. / Il Farmaco 56 (2001) 277–283
Table 2
Spectral data for the synthesized compounds
Compound IR wmax (cm−1
)
1H NMR l (ppm)
m/z
2a
3274 (NH), 2979 (CH-aliph), 2206
(CꢀN)
1.34 (t, 3H, CH3), 2.32 (s, 3H, SCH3),
4.17 (q, 2H, OCH2), 7.27 (q, 4H,
AB-system), 11.12 (s, 1H, NH; cancelled
with D2O)
2b
3a
5a
3278 (NH), 2980 (CH-aliph), 2198
(CꢀN)
3200, 3166 (NH2), 2976 (CH-aliph),
2210 (CꢀN)
3404, 3307, 3342 (NH, NH2), 2981
(CH-aliph), 2214 (CꢀN)
259 (75%), 211 (51%), 187 (100%), 158
(25%), 93 (26%), 76 (9.3%)
1.37 (t, 3H, CH3), 2.97 (s, 3H, SCH3),
4.16 (q, 2H, OCH2), 6.41 (s, 2H, NH2;
cancelled with D2O), 7.06 (m, 4H, Ar–H),
11.41 (s, 1H, NH; cancelled with D2O)
1.30 (t, 3H, CH3), 2.70 (s, 3H, SCH3),
2.91 (s, 2H, NH2; cancelled with D2O),
4.01 (q, 2H, OCH2), 6.70–8.0 (m, 4H,
Ar–H), 9.20 (s, 1H, NH; cancelled with
D2O)
1.39 (t, 6H, 2CH3), 4.14 (q, 4H, 2OCH2),
6.43 (s, 2H, NH2; cancelled with D2O),
6.80–6.99 (m, 8H, Ar–H), 7.91 (s, 1H,
NH; cancelled with D2O), 11.28 (s, 1H,
NH; cancelled with D2O).
5b
6a
6b
3433, 3336, 3246 (NH, NH2), 2979
(CH-aliph), 2221(CꢀN)
365 (100%), 367 (25%), 339 (10%), 337
(83%), 199 (10%), 92 (11%)
3409, 3342, 3278 (NH, NH2), 2983
(CH-aliph), 2219 (CꢀN)
1.31 (t, 6H, 2CH3), 4.01 (q, 4H, 2OCH2), 455 (100%), 454 (95%), 425 (66.5%), 398
6.51–7.50 (q, 8H, 2AB-system), 9.01 (s,
1H, NH; cancelled with D2O), 9.8 (s, 1H,
NH; cancelled with D2O), 10.41 (s, 2H,
NH2; cancelled with D2O)
(22.8%), 263 92.8%), 159 (3.6%)
8d
9a
9d
3190, 3111 (NH), 2922 (CH-aliph),
2208, 2171(CꢀN)
2.28 (s, 6H, 2CH3), 7.09 (s, 2H, Ar–H),
12.7 (s, 2H, 2NH; cancelled with D2O)
214 (100%), 183 (24.5%), 157 (11.6%),
118 (10.8%), 65 (12.2%)
3354, 3300, 3222 (NH, NH2), 2852
(CH-aliph)
2.29 (s, 6H, 2CH3), 5.60 (s, 4H, 2NH2;
cancelled with D2O), 7.12 (s, 2H, Ar–H),
11.0 (s, 2H, 2NH; cancelled with D2O)
10a
3313, 3174 (NH2), 2923 (CH-aliph),
2212 (CꢀN)
336 (100%), 290 (10.8%), 265 (16.4%),
225 (20%), 168 (15.2%), 135 (15.3%),
118 (19.2%)
10b
10c
3400, 3250 (NH2), 2924 (CH-aliph),
2203 (CꢀN)
2.47 (s, 6H, 2CH3), 2.81(s, 3H, SCH3),
6.74 (s, 2H, Ar–H), 7.19, 7.60 (s, 4H,
2NH2; cancelled with D2O), 11.2 (s, 1H,
NH; cancelled with D2O)
11
3400, 3272 (NH2), 2980 (CH-aliph),
2200 (CꢀN)
1.35 (t, 3H, CH3), 2.61 (s, 2H, NH2;
cancelled with D2O), 4.13 (q, 2H, OCH2),
6.76–7.35 (m, 8H, Ar–H), 9.21 (s, 1H,
NH; cancelled with D2O), 10.16 (s, 2H,
NH2; cancelled with D2O), 12.87 (s, 1H,
NH; cancelled with D2O)
14a
3438, 3315, 3217 (NH2), 2210 (CꢀN)
366 (100%), 301 (16%), 223 (1%), 168
(2%), 131 (1.1%), 90 (2%), 77 (4.9%)
14b
14c
3321, 3209 (NH2), 2212 (CꢀN)
3305, 3163 (NH2), 2209 (CꢀN), 1623
(CꢁO)
2950 (CH-aliph), 1755, 1732, 1714
(CꢁO)
15
2.40, 2.87, 3.26 (35, 12H, 3COCH3+CH3) 364 (6.2%), 322 (21.9%), 281 (31%), 265
7.50–8.33 (s, 4H, Ar–H) (37%), 238 (100%)

M.A. Zaharan et al. / Il Farmaco 56 (2001) 277–283
283
Table 3
Antimicrobial activity of some prepared compounds (diameter zones in mm) a
Compound
Staphylcoccus
aureus (NCTC
7447)
Bacillus cereus
(ATCC-14579)
Serratia
marcescens
(IMRU-70)
Proteus mirabilis Aspergillus ochraceus
Penicillium
(NTC-289)
Wilhelm (AUCC-230) chrysogenum Thom
(AUCC-530)
5a
5b
6a
6b
9a
9c
10a
10b
14a
14c
15
Ampicillin
Mycostatin
16
15
28
26
25
18
16
26
18
28
16
40
26
28
20
20
30
30
20
20
28
18
18
42
20
18
16
18
30
30
30
28
18
30
20
40
18
16
30
30
30
16
18
20
30
20
18
44
16
10
20
18
14
16
10
18
20
10
20
10
18
20
16
18
16
10
10
20
10
20
40
42
a Less active 1–1.5 cm; moderately active 1.5–2 cm; highly active 2–3 cm; very highly active 3–4.5 cm.
[6] G.H. Elgemeie, A.H. Elghandour, A.M. Elzanate, S.A. Ahmed,
Novel synthesis of thioguanine and sulfanylpurine analogues:
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Chem. Res. (S) (1998) 164–165.
[7] A.K. El-Shafei, A.M. Soliman, A.A. Sultan, A.M.M. El-Saghier,
Synthesis of some new fused heterocycles containing a bridge-
head nitrogen atom under PTC conditions. A new application of
cyanoketene s,s-acetals, Gazz. Chim. Ital. 125 (1995) 115–118.
[8] Y.A. Ammar, A.M.Sh. El-Sharief, M.A. Zahran, M.Z. El-Said,
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The results were represented in Table 3. Most of the
synthesized compounds were found to possess various
antimicrobial activities with minimal inhibitory concen-
tration (MIC) values 100–250 mg ml−1. However, none
of the tested compounds showed superior activity than
the reference drugs.
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