A convenient synthesis of novel 5-aryl-pyrido[2,3-d]pyrimidines and screening of their preliminary antibacterial properties

Article abstract of DOI:10.1016/j.tetlet.2014.01.128

Exploiting the diene nature of 6-[1-aza-2-(dimethylamino)prop-l-enyl]-1,3- dimethyluracil (2), novel 5-aryl-pyrido[2,3-d]pyrimidines (5a-h) have been synthesized. The reaction proceeds through a triene intermediate whose structure has been conclusively established by single crystal X-ray analysis. Role of water is intriguing as the reaction can be stopped at the intermediate stage. Synthesized compounds have been screened for possible antibacterial properties and results showed modest activity.

Full text of DOI:10.1016/j.tetlet.2014.01.128

Tetrahedron Letters 55 (2014) 1796–1801
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A convenient synthesis of novel 5-aryl-pyrido[2,3-d]pyrimidines
and screening of their preliminary antibacterial properties
Lakhinath Saikia a,b, Babulal Das c, Pranjal Bharali d, Ashim Jyoti Thakur a,
⇑
a Department of Chemical Sciences, Tezpur University (A Central University), Tezpur, Napaam 784028, Assam, India
b Department of Chemistry, Rajiv Gandhi University (A Central University), Rono Hills, Doimukh 791112, Arunachal Pradesh, India
c Department of Chemistry, Indian Institute of Technology, Guwahati 781039, Assam, India
d Department of Molecular Biology and Biotechnology, Tezpur University (A Central University), Tezpur, Napaam 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Exploiting the diene nature of 6-[1-aza-2-(dimethylamino)prop-l-enyl]-1,3-dimethyluracil (2), novel
5-aryl-pyrido[2,3-d]pyrimidines (5a–h) have been synthesized. The reaction proceeds through a triene
intermediate whose structure has been conclusively established by single crystal X-ray analysis. Role
of water is intriguing as the reaction can be stopped at the intermediate stage. Synthesized compounds
have been screened for possible antibacterial properties and results showed modest activity.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 26 October 2013
Revised 26 January 2014
Accepted 27 January 2014
Available online 3 February 2014
Keywords:
Uracil
Pyrido[2,3-d]pyrimidine
Amidine
Antibacterial
Diels–Alder
Nature has been more perspicacious than the organic chemists
in designing effective bioactive molecules. Nevertheless, mimick-
ing nature, synthetic organic chemists also put their best efforts
to find out new potent molecules. The nucleo base pyrimidine
and its derivatives1 are not only powerful biological and medicinal
entities,2 but their annulated derivatives, pyrido[2,3-d]pyrimidines
are also associated with interesting pharmacological properties.3
Recently, in Nature Reviews Drug Discovery,4 M. H. Flight high-
lighted the work of Stover et al.5 where screening of 1.6 million
compounds from Pfizer compound library was carried out for anti-
bacterial activity. This screening resulted in the discovery of three
pyrido[2,3-d]pyrimidine derivatives as potent synthetic antibacte-
rials that targeted the bacterial biotin carboxylase selectively. In a
very recent report,6 A. Zega et al. demonstrated 6-aryl-pyrido[2,3-
O
N
O
N
Nucleophilic
N
Nucleophilic
N
N
N
O
N
O
N
1
2
Figure 1. Methylated (2) and non methylated amidine (1).
amino)methylene-amino]-1,3-dimethyluracil (1, Fig. 1) via [4+2]
cycloaddition reaction with various electron deficient dienophiles.
Recently, our group has reported the synthesis of bis-pyrimido[4,5-
d]pyrimidines from 1 via aza Diels–Alder methodology followed by
their preliminary bioactivity.9 However, the simple acetamidine
counterpart of 1, that is 6-[1-aza-2-(dimethylamino)prop-l-enyl]-
1,3-dimethyluracil10 (2, Fig. 1) and its diene behaviour are not well
explored so far. In fact, no report of cycloaddition reaction using 2
has been found in the literature and to the best of our knowledge,
only a single report11 of coupling reaction using the 5-iodinated 2
in the presence of palladium catalyst has been found. Therefore, we
report here the exploitation of diene behaviour of 2 for the conve-
nient synthesis of 5-aryl-7-methylpyrido[2,3-d]pyrimidines and
their in vitro antibacterial activity. Formation of 5-aryl-7-methyl-
pyrido[2,3-d]pyrimidines is not quite expected as 1 has been found
to furnish 5-aryl-5,8-dihydropyrido[2,3-d]pyrimidines with nitro-
styrenes under similar condition as reported8f very recently.
d]pyrimidines as novel ATP-competitive inhibitors of bacterial
D-
alanine: -alanine ligase. The need for new and novel antibiotics
D
to combat bacterial drug resistance has led the researchers to give
enormous efforts in antibiotic research. Hence, there has been
flurry of synthetic and biological activities3,7 centered on pyr-
ido[2,3-d]pyrimidine that attract considerable attentions from
chemists and biologists. A plethora of methods7g–i,8 have been
reported for the synthesis of pyrido[2,3-d]pyrimidines and related
compounds by exploiting the diene character of 6-[(dimethyl-
⇑
Corresponding author. Tel.: +91 275059; fax: +91 (3712) 267005/6.
E-mail address: ashim@tezu.ernet.in (A.J. Thakur).
http://dx.doi.org/10.1016/j.tetlet.2014.01.128
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

L. Saikia et al. / Tetrahedron Letters 55 (2014) 1796–1801
1797
O
N
O
N
Ar
O
N
O
N
Ph
NO2
NO2
NO2
NO2
DMSO, reflux
N
N
DMF, reflux, 3 hrs
-NHMe2
N
N
+
+
O
N
N
O
N
Ph
3a
O
N
N
O
N
Ar
3a-h
82%
5a
5a-h
2
2
DMF, rt
overnight stirring
Scheme 2. Synthesis of pyrido[2,3-d]pyrimidines (5a–h) in one step.
-H2
30% conversion
O
N
Ph
O
Ph
N
the final product. The intermediate compound responsible for this
new spot was isolated using column chromatography and its struc-
ture was established as 4a, a Michael type product by single crystal
X-ray analysis. A similar reaction condition was imposed (i.e.
refluxed in DMF) to 4a and to our delight, it offered the same prod-
uct 5a within shorter period of time and thus supported the
involvement of 4a as an intermediate.
NO2
NO2
N
DMF, reflux, 45 min
-NHMe2
N
O
N
N
O
N
92%
5a
4a
Scheme 1. Synthesis of pyrido[2,3-d]pyrimidine 5a.
To study the effect of solvents in the formation of 4a as well as
5a, the reaction was carried out in different solvents (both polar
and non-polar) by stirring the reaction mixture at room tempera-
ture for 12 h followed by refluxing it for 3 h. The results are sum-
marized in Table 1. Table 1 reveals that high boiling aprotic polar
solvents like DMF and DMSO (Table 1, entries 1 & 2) are suitable
for the formation of the final product 5a, but the rate of conversion
towards the intermediate 4a is quite low. In contrast, water is
found to be the best solvent for the formation of the intermediate
4a, but not so satisfactory for furnishing the final product 5a. To
examine the effect of H2O, the reaction was then carried out in a
1:1 mixture of 4a, but not good enough to give 5a in satisfactory
yield. On the basis of all these observations, it can be concluded
that H2O favours the formation of 4a whereas it disfavours the con-
version of 4a to the product (5a). Other solvents like chloroform,
acetonitrile, benzene and methanol were not found to be suitable.
Among all these solvents checked, DMSO was found to be the best
solvent for the synthesis of pyrido[2,3-d]pyrimidine (5a) in a single
step process. Generalization of the reaction was carried out using
various b-nitrostyrenes (3a–h) as shown in Scheme 2 and summa-
rized in Table 2. The reaction did not proceed with aliphatic nitro-
alkenes. The synthesis of 5 was also attempted via a two-step route
(Scheme 3) considering the observation that H2O was the best
solvent for the intermediate (4) formation step whereas DMF and
DMSO were the excellent solvents for the formation of intermedi-
ate (4) to final product (5). It was found that this route was slightly
more beneficial than the single step route when the overall iso-
lated yields of 5 were considered. The 2nd step was carried out
in refluxing DMF although both DMF & DMSO were found to give
almost comparable yield. However, DMF is preferred over DMSO,
as DMSO has an extremely unpleasant smell when refluxed.
Chloro, nitro, hydroxyl, methoxy and heterocyclic moieties
In our initial attempt to investigate the interesting diene behav-
iour of the methylated amidine (2) in [4+2] cycloaddition reaction,
we treated 2 with b-nitrostyrene (3a) (Scheme 1), an electron defi-
cient dienophile. During the process, we observed lower reactivity
of 2 towards 3a than 1. The percentage conversion of 2 in chloro-
form was found to be 35% only (stirring at room temperature for
12 h, (entry 5, Table 1) whereas 1 underwent complete conversion
within 5 minutes (stiring at 0 °C) as demonstrated in our earlier
report.7e This lower reactivity of 2 towards b-nitrostyrene, in spite
of having one extra electron releasing methyl group is contradic-
tory to the general rule of Diels–Alder reaction which suggests en-
hanced reactivity of 2 than 1. This might be due to the reason that
the presence of this extra methyl group hinders the extent of delo-
calization starting from the dimethylamino group and hinders the
planarity. However, the reaction occurs smoothly in refluxing DMF
with complete conversion of 2 within 3 h furnishing 1,3,7-tri-
methyl-6-nitro-5-phenylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-
dione (5a) in 82% yield. The structure of 5a has been established
using FTIR spectroscopy, MS (ESI), 1H & 13C NMR spectroscopy
and elemental analyses. To gain some insight into the mechanism
of the reaction, the reaction was monitored at different time inter-
vals using TLC. As the reaction progressed, we noticed an extra spot
in TLC with distinctly lower Rf value than that of 5a, whose inten-
sity decreased gradually over time. The same spot was observed in
TLC as the single spot other than the starting compounds when we
carried out the reaction at room temperature for 12 h (Scheme 1),
although only 30% conversion of the starting materials was ob-
served here. These observations suggest that this extra spot in
TLC might be due to the formation of an intermediate which
underwent further reaction under the reaction condition to furnish
Table 1
Screening of solvents for Scheme 1
Entry
Solvent
Reaction condition
Refluxed for 3 ha
Stirred at rt for overnight (12 h)
Conversion of 2 (%)
Yield of 4a (%)
Yield of 5a (%)
Conversion of 2 (%)
Yield of 4a (%)
Yield of 5a (%)
1
2
3
4
5
7
8
9
DMF
DMSO
H2O
DMF–H2O (1:1)
CHCl3
CH3CN
C6H6
30
32
100
100
35
38
15
75
100
100
100
100
100
100
100
100
No product formation
No product formation
No product formation
No product formation
No product formation
No product formation
No product formation
No product formation
100
100
100
100
50
68
60
92
<10
<10
8
82
85
22b
28b
NF
12
100
>90
45
<10
18b
<10b
CH3OH
65
a
Reaction mixture was stirred at room temperature for 12 h followed by reflux for 3 h.
Some other unidentified products were formed.
b

1798
L. Saikia et al. / Tetrahedron Letters 55 (2014) 1796–1801
Table 2
(thiophene and furan) remained undisturbed during the reaction.
Synthesis of pyrido[2,3-d]pyrimidines (5a–h)12 via Scheme 2
The results are summarized in Table 3. All the intermediates (4a–
h) were obtained by simple filtration and used for the 2nd step
without any further purification. The structure of all the final prod-
ucts was confirmed by IR spectroscopy, 1H and 13C NMR spectros-
copy (see ESI), elemental analyses and mass spectral data.
Entry
Ar– in (3)
Product (5)
Time (h)
Yielda (%)
O
N
NO2
NO2
NO2
Conclusive evidence for the structures of 4a and 5g were ob-
1
C6H5-3a
3
85
tained from single crystal X-ray analyses (Figs.
respectively).
2 and 3,
O
N
N
As illustrated in Table 2, in all the reactions with unsymmetrical
nitrostyrenes only one regioisomer was obtained. Reactions of 2
with other comparatively less electron deficient dienophiles such
as acryl amide, acrylic acid, cinnamaldehyde, cinnamic acid, cinn-
amamide, benzylideneacetophenone (not shown in Table 2) were
also examined under similar reaction conditions, but failed to give
product. Although the detailed mechanistic studies were not
performed, a plausible mechanism is suggested to rationalize the
formation of the product 5 via aza-Diels–Alder pathway followed
by elimination of hydrogen molecule, rearrangement and loss of
dimethylamine (Scheme 4).
Cl
O
N
2
p-ClC6H4-3b
3
82
N
O
N
NO2
O
N
3
4
5
p-NO2C6H4-3c
o-HOC6H4-3d
p-MeOC6H4-3e
3.5
3.5
2.5
72
75
92
We became enthusiastic by the success of our methodology and
this prompted us to extend the methodology to di(nitrostyrene) 6
obtained from terephthaldehyde also (Scheme 5). The reaction was
found to be effective, although the yield was low (58%).
N
N
O
O
N
The synthesized pyrido[2,3-d]pyrimidines (5a–h and 7) were
screened for their in vitro antibacterial property against both the
Gram positive (Bacillus subtilis and Staphylococcus aureus) and
Gram negative (Klebsilla pneumonia and Escherichia coli) bacterial
strains. The minimum inhibitory concentrations (MIC) of the
synthesized compounds were determined (Table 4) by the micro
dilution method taking streptomycin sulfate (1 mg/mL) as positive
control while sterilized DMSO as negative control. Among the com-
pounds, 5a exhibited maximum activity against all the tested
microorganisms although compound 5h showed identical activity
against S. aureus whereas compound 7 possessed identical activity
against K. pneumonia and E. coli. Among all the compounds, 5a was
found to be the best antibacterial agent against a broad spectrum
of microorganisms followed by 7 and 5h. When the activity against
gram positive and gram negative bacteria was considered, 5a re-
tained its best position against gram positive bacteria while 7
shared the top position with it against the gram negative bacteria.
The results indicate that introduction of both electron withdrawing
(Table 2, entries 2 and 3) and electron releasing (Table 2, entries 4
and 5) groups to the aryl ring at position 5 of the pyrido[2,3-
d]pyrimidine decreases the activity of the compounds although
the effect of electron releasing groups is much higher. Replacement
of aryl rings by heteroaryl rings (Table 2, entries 6 and 7) also leads
to an increase in MIC values. Compounds with unsubstituted aro-
matic ring (phenyl, naphthyl in 5a, 5h and 7) at positions 5 offer
the best activity. To examine the effect of the –NO2 group at posi-
tion 6 of pyrido[2,3-d]pyrimidines towards the antibacterial activ-
ity, the –NO2 group of 5e (compound with the highest MIC value)
was reduced to –NH2 functionality (Scheme 6) and the resulting
amine (8e) was screened for possible antibacterial activity and to
our delight, it showed excellent activity as compared to 5e (Table 4,
entry 11), that is upon conversion of –NO2 to –NH2, the activity
O
N
OH
NO2
N
OMe
O
NO2
NO2
NO2
N
N
N
O
O
O
N
O
N
O
O
3f
6
7
2.5
2.5
87
90
N
O
N
S
S
3g
N
N
O
N
NO2
8
2.5
85
N
3h
O
N
a
Yields refer to the isolated pure products.
O
N
Ar
O
N
Ar
O
N
NO2
NO2
N
NO2
Step 2
Step 1
H2O, rt
N
N
+
DMF, reflux
O
N
O
N
O
N
N
N
Ar
4a-h
3a-h
5a-h
2
Scheme 3. Synthesis of pyrido[2,3-d]pyrimidines (5a–h) in two steps.

L. Saikia et al. / Tetrahedron Letters 55 (2014) 1796–1801
1799
Table 3
Synthesis of pyrido[2,3-d]pyrimidines 5a–h13 via Scheme 3 in two steps
Entry
Ar–
Step 1
Product
mp (°C)
Step 2
Overall yielda (%)
Reaction time (h)
Yielda (%)
Reaction time
Product
Yielda (%)
1
2
3
4
5
C6H5–
8
8
12
12
7
4a
4b
4c
4d
4e
100
100
96
98
100
145
140
172
118
139
45 min
50 min
1.5 h
1.5 h
40 min
5a
5b
5c
5d
5e
92
90
85
85
95
92
90
82
83
95
p-ClC6H4–
p-NO2C6H4–
o-HOC6H4–
p-MeOC6H4–
O
6
7
7.5
7.5
4f
100
100
157
152
45 min
45 min
5f
92
90
92
90
S
4g
5 g
8
8.5
4h
100
161
1 h
5 h
88
88
a
Yields refer to the isolated pure products.
N
N
O
O
NO2
N
O2N
2
DMSO, reflux,
O
N
7hrs
NO2
N
NO2
O
N
6
(58%)
7
Scheme 5. Synthesis of 7.
Figure 2. ORTEP diagram of 4a.
increases enormously. Functionally transformed products (8a, 8h
and 9) of the most active compounds (5a, 5h and 7) were also
screened against bacterial strains and an increase in activity was
observed (Table 4, entries 10, 12 and 13). However, the magnitude
of increase in activity upon functional transformation here is very
less in comparison to that observed in the functional transforma-
tion of the least active compound.
In conclusion, a convenient catalyst-free regioselective synthe-
sis of 5-aryl-pyrido[2,3-d]pyrimidines has been accomplished via
hetero annulation, where pyridine component carries a methyl
substituent at 7-position. The generality of the protocol has been
demonstrated by the successful conversion of eight substrates into
pyrido[2,3-d]pyrimidines 5a–h. This method allows for the intro-
duction of a high degree of chemical and structural diversity onto
the pyrimidine scaffold with tolerability to several groups. Easy
access of the starting materials, good yields of the products and
Figure 3. ORTEP diagram of 5g.
O
N
Ph
N
O
N
Ph
N
O
N
Ph
N
3a
NO2
[4+2]
NO2
N
NO2
N
-H2
N
cycloaddition
N
O
N
O
N
O
2
O
Ph
O
N
Ph
O
N
Ph
H
Ring
opening
Ring
NO2
NO2
NO2
N
-Me2NH
N
N
closure
O
N
N
N
O
N
O
N
N
5a
4a
Scheme 4. Plausible mechanism for the formation of 5a.

1800
L. Saikia et al. / Tetrahedron Letters 55 (2014) 1796–1801
Table 4
References and notes
MIC (in mg mLÀ1) values of novel 5-Aryl-pyrido[2,3-d]pyrimidines (5a–h, 7, 8 and 9)
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E. coli
1
2
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5
6
7
8
5a
5b
5c
5d
5e
5f
5g
5h
7
8a
8e
8h
9
0.375
1.8
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4
3.75
27
7.5
27
7.5
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1.25
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O
N
R
N
NH2
NO2
Na2S2O4/OH-
(CH3)2O
N
O
N
O
N
8
5
ˇ
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5a
5e
5h
72
74
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3537–3540; (b) Prajapati, D.; Thakur, A. J. Tetrahedron Lett. 2005, 46, 1433–
1436; (c) Saikia, P.; Thakur, A. J.; Prajapati, D.; Sandhu, J. S. Indian J. Chem. 2002,
41B, 804–807; (d) Gohain, M.; Prajapati, D.; Gogoi, B. J.; Sandhu, J. S. Synlett
2004, 1179–1182; (e) Sarma, R.; Sarmah, M. M.; Prajapati, D. J. Org. Chem. 2012,
77, 2018–2023; (f) Sarmah, M. M.; Prajapati, D.; Hu, W. Synlett 2013, 471–474.
9. Das, S.; Thakur, A. J.; Medhi, T.; Das, B. RSC Adv. 2013, 3, 3407–3413.
10. Procedure for the synthesis of compound 6-[1-aza-2-(dimethylamino)prop-l-enyl]-
p-MeOC6H4-
7
9
42
O
NO2
N
O
N
N
Scheme 6. Reduction of –NO2 to –NH2.14
the simplicity of the experimental procedure make it a useful one.
Synthesized compounds possess modest activity towards gram
positive and gram negative bacteria. Noticeably, functional conver-
sion of the –NO2 group to –NH2 leads to enhanced antibacterial
activity.
1,3-dimethyluracil
2:
6-amino-1,3-dimethyl-pyrimidine-2,4(1H,3H)-dione
(1.55 g, 0.01 mol) and N,N-Dimethylacetamide dimethyl acetal (DMA-DMA)
(1.33 g, 0.01 mol) were refluxed in a round bottomed flask for 1 h and then the
reaction mixture was allowed to cool to room temperature. The solid mass thus
obtained was recrystallized from ethanol to get 2 as pure product. Yield (2.03 g,
90.62%).
Solid; mp. 152 °C; IR (KBr) (m
max/cmÀ1) 3025, 2926, 1716, 1673, 1662, 1483; dH
Acknowledgments
(400 MHz, CDCl3, TMS) 4.83 (s, 1H), 3.34 (s, 3H), 3.29 (s, 3H), 3.11 (d, J=6.8 Hz,
6H), 2.12 (s, 3H) ppm; dC (100 MHz, CDCl3, TMS) 163.7, 159.7, 157.7, 152.9,
86.9, 38.6, 37.8, 30.2, 27.7, 15.9 ppm; MS (ESI): m/z 224 (M)+; Anal. Calcd (%)
for C10H16N4O2: C, 53.56; H, 7.19; N, 24.98. Found: C, 53.57; H, 7.23; N, 24.95%.
11. Roh, Y. H.; Bae, J. W.; Nam, G. S.; Kim, J. H.; Kim, S. H.; Yoon, C. M. Synth.
Commun. 2000, 30, 81–86.
The authors are indebted to the Council of Scientific and Indus-
trial Research (CSIR), New Delhi, India, for financial support to the
project CSIR (01(2147)/07/EMR-II).
12. General procedure for the synthesis of 5a–h via Scheme 2: 6-[1-aza-2-
(dimethylamino)prop-l-enyl]-1,3-dimethyl-uracil
2
(0.5 mmol) and the
Supplementary data
nitrostyrene derivative 3a–h (0.5 mmol) were taken in DMSO (5 mL) and
refluxed. After the completion of the reaction as monitored by TLC, DMSO was
removed by vacuum distillation. The crude residue was purified to get the pure
products 5a–h by column chromatography using silica gel (100–200 mesh) as
adsorbent and Ethyl acetate–Hexane (1:4) as eluent.
Crystallographic information: Crystallographic data (excluding
structure factors) for compounds 4a & 5g have been deposited with
the Cambridge Crystallographic Data Centre as deposition Nos.
CCDC 875794 & CCDC 875795 respectively. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (Fax: +44 (0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk).
Supplementary data (experimental section, screening for anti-
bacterial activity of the synthesized compounds, 1H and 13C NMR
spectra of compounds) associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
128.
1,3,7-Trimethyl-6-nitro-5-phenylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione (5a)
(Table 2, entry 1): Solid; mp. 180 °C; IR (KBr) (m
max/cmÀ1) 3455, 2926, 2370,
1716, 1673, 1561, 1483, 1346; dH (400 MHz, CDCl3, TMS) 7.16–7.43 (m, 5H),
3.75 (s, 3H), 3.31 (s, 3H), 2.62 (s, 3H) ppm; dC (100 MHz, CDCl3, TMS) 153.5,
149.9, 149.8, 145.2, 131.1, 128.1, 127.1, 126.2, 29.3, 27.6, 20.3 ppm; MS (ESI)
m/z 326 (M)+; Anal. Calcd (%) for C16H14N4O4: C, 58.89; H, 4.32; N, 17.17%.
Found: C, 58.91; H, 4.37; N, 17.19%.
13. General procedure for the synthesis of 5a–h via Scheme 3:
Step
dimethyluracil
1
(Synthesis of 4a–h): 6-[1-aza-2-(dimethylamino) prop-l-enyl]-1,3-
(0.5 mmol) and the nitrostyrene 3a–h (0.5 mmol) were
2
taken in H2O (10 mL) and allowed to stir at room temperature. The reaction
was monitored using TLC. After the completion of the reaction, the solid

L. Saikia et al. / Tetrahedron Letters 55 (2014) 1796–1801
1801
products 4a–h were collected by filtration. The crude products 4a–h were used
for the 2nd step without further purification.
acetate and washed with water, brine and dried over Na2SO4. Ethyl acetate was
evaporated under vacuum and the residue was purified by column
chromatography using silica gel (100–200 mesh) as adsorbent and Ethyl
acetate–Hexane (1:3) as eluent.
Step 2 (Synthesis of 5a–h): The crude product of the 1st step (0.5 mmol) was
refluxed in DMF (5 mL). After the completion of the reaction as monitored by
TLC, DMF was evaporated out under vacuum. The crude residue was purified
by column chromatography using silica gel (100–200 mesh) as adsorbent and
Ethyl acetate–Hexane (1:4) as eluent.
6-Amino-1,3,7-trimethyl-5-phenylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione
(8a): Solid; mp 134 °C; IR (KBr) (m
max/cmÀ1) 3467, 3369, 2928, 2374, 1702,
1647, 1570, 1455, 131417; dH (400 MHz, DMSO-d6, TMS) 7.05–7.41 (m, 5H),
4.16 (s, b, 2H), 3.50 (s, 3H), 3.05 (s, 3H), 2.41 (s, 3H) ppm; dC (100 MHz, CDCl3,
TMS) 161.1, 151.2, 149.6, 143.8, 135.9, 129.1, 129.0, 128.1, 127.6, 127.1, 106.5,
29.6, 28.3, 21.7 ppm; MS (ESI) m/z 296 (M)+; Anal. Calcd (%) for C16H14N4O4: C,
64.85; H, 5.44; N, 18.91%. Found: C, 64.93; H, 5.52; N, 18.83%.
14. General procedure for the reduction of –NO2 to –NH2 via Scheme 6: The nitro
compound was dissolved in acetone (3 mL/mmol) and aqueous NaOH (0.5 N,
5 equiv). Excess sodium hydrosulfite (5 equiv) was added and the reaction was
refluxed for 1.5 h. Acetone was evaporated, residue was taken up in ethyl