Microwave-assisted cyclocondensation: A rapid and solvent-free synthesis of 3-benzyl-2H-pyrido[1,2-a]pyrimidin-2-one derivatives

Article abstract of DOI:10.1016/j.tetlet.2013.06.138

Microwave-induced cyclocondensation of methyl 2-(acetoxy(phenyl)methyl) acrylate with 2-aminopyridines under catalyst and solvent-free conditions has been achieved for a rapid synthesis of 3-substituted-2H-pyrido[1,2-a]pyrimidin- 2-ones in good yields with high selectivity in short reaction times.

Full text of DOI:10.1016/j.tetlet.2013.06.138

Accepted Manuscript
Microwave-assisted cyclocondensation: a rapid and solvent-free synthesis of 3-
benzyl-2H-pyrido[1,2-a]pyrimidin-2-one derivatives
S. Satyanarayana, K. Praveen Kumar, P. Lakshmi Reddy, R. Narender, G.
Narasimhulu, B.V. Subba Reddy
PII:
S0040-4039(13)01123-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.06.138
TETL 43191
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
22 February 2013
26 June 2013
27 June 2013
Please cite this article as: Satyanarayana, S., Praveen Kumar, K., Lakshmi Reddy, P., Narender, R., Narasimhulu,
G., Subba Reddy, B.V., Microwave-assisted cyclocondensation: a rapid and solvent-free synthesis of 3-
benzyl-2H-pyrido[1,2-a]pyrimidin-2-one derivatives, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/
j.tetlet.2013.06.138
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Microwave-assisted cyclocondensation: a
rapid and solvent-free synthesis of 3-benzyl-
2H-pyrido[1,2-a]pyrimidin-2-one derivatives
S. Satyanarayana, K. Praveen Kumar, P. Lakshmi Reddy, R. Narender*
G. Narasimhulu, B. V. Subba Reddy*

Tetrahedron Letters
1
TETRAHEDRON
LETTERS
Pergamon
Microwave-assisted cyclocondensation: a rapid and solvent-free synthesis of 3-
benzyl-2H-pyrido[1,2-a]pyrimidin-2-one derivatives
S. Satyanarayana,a K. Praveen Kumar,a P. Lakshmi Reddy,a R. Narender*a
G. Narasimhulu,b B. V. Subba Reddy*b
aCrop Protection Chemicals, bNatural Product Chemistry, Indian Institute of Chemical Technology, Hyderabad –500 007,
India.
E-mail: raviralanarender@gmail.com; basireddy@iict.res.in; Fax: 91-40-27160512.
Abstract: Microwave-induced cyclocondensation of methyl 2-(acetoxy(phenyl)methyl)acrylate with 2-
aminopyridines under catalyst and solvent-free conditions has been achieved for a rapid synthesis of 3-
substituted-2H-pyrido[1,2-a]pyrimidin-2-ones in good yields with high selectivity in short reaction times.
Keywords: Microwave, cyclocondensation, methyl 2-(acetoxy(phenyl)methyl)acrylates, 2-aminopyridines,
solvent-free, 2H-pyrido[1,2-a]pyrimidin-2-ones
Pyrimidine core is often present in various natural
products and synthetic molecules such as cytosine,
uracil and thymine.1 In particular, pyridopyrimidines
display a diverse range of biological activities and are
well-known pharmacophores2 such as PDE-
Figure 1. Examples of biologically active pyrido[1,2-
a]pyrimidin-4-ones
The densely functionalized Baylis–Hillman adducts
allow numerous transformations and have made these
adducts a valuable synthetic intermediates.6,7 2-
Aminopyridine is a versatile synthetic intermediate in
pharmaceutical chemistry.
inhibitors,3a
analgesic,3b
antiinflammatory,3c
antiallergic or antipsychotic agents3d and inhibitors of
tyrosine kinase activity in the epidermal growth factor
receptor.4 The pyrido[1,2-a]pyrimidine derivatives are
also most useful building blocks for the synthesis of
Consequently, several methods have been reported for
the synthesis of substituted pyrido[1,2-a]pyrimidin-4-
one derivatives.8 A few processes have been reported
for the synthesis of 3-substituted-2H-pyrido[1,2-
a]pyrimidin-2-ones using different starting materials
(Fig. 2). 9,10
biologically
active
molecules
like
fused
azinopyrimidinone derivative (I),5a antiasthmatic
agent (pemirolast (II)),5b an antiulcerative agent
(III),5c tranquilizer (pirenperone (IV)),5d and the
antiallergic agent (barmastine (V)) (Fig. 1). 5e
Figure 2. Biologically active pyrido[1,2-a]pyrimidin-
2-ones and 5-phenylthiazolo[3,2-a]pyrimidin-7-one
However, many of these methods often suffer from
long reaction time, drastic reaction conditions as well
as a narrow scope of the substrates. Thus, there is
need to develop a rapid process for the synthesis of
2H-pyrido[1,2-a]pyrimidin-2-one derivatives. In the

2
Tetrahedron Letters
past decades, microwave (MW) irradiation has
become powerful tool for rapid synthesis of several
organic molecules because it provides enhanced
reaction rates, good selectivity, improved yields, ease
of manipulation and rapid optimization of the
reaction.11,12 However, there have been no reports on
the effect of microwave irradiation for the synthesis
of 3-substituted-2H-pyrido[1,2-a]pyrimidin-2-ones.
Following our interest on the microwave irradiation,13
we herein report for the first time, a simple and
efficient method for the synthesis of 3-substituted-2H-
pyrido[1,2-a]pyrimidin-2-ones from 2-heteroaryl
amines and alkyl-2-(acetoxy(phenyl)methyl)acrylates
using microwave (MW, 150 W). Initially, we
attempted
(acetoxy(phenyl)methyl)acrylate
the
reaction
of
methyl
with
2-
2-
(1)
aminopyridine (2) under MW without any catalyst.
Interestingly, the reaction proceeded smoothly at 95
˚C under solvent-free conditions and the desired
product 3a was obtained in 90% yield (Scheme 1,
Table 1).
Scheme 1. Reaction of Baylis-Hillman acetate with 2-
aminopyridine
Interestingly, substituted 2-aminopyridines such as 3-
methyl-, and 3-hydroxy- derivatives participated well
in this reaction (entries b, c, k and l, Table 1). Other
substrates
such
as
2-aminopyrimidine,
2-
aminobenzothiazole and 3H-imidazo[4,5-c]pyridin-4-
amine also underwent smooth coupling with different
2-(acetoxy(aryl)methyl)acrylates to afford the
corresponding
3-substituted-2H-pyrido[1,2-
a]pyrimidin-2-one derivatives in good yields (entries
d, e, f and m, Table 1). The structure of the products
was assigned by 1H, C NMR and mass spectral data
13
and also by comparison with authentic samples.10 The
scope and generality of this process is illustrated with
respect to various 2-aminopyridines and 2-
(acetoxy(aryl)methyl)acrylates and the results are
presented in Table 1.14

Tetrahedron Letters
3
As depicted in Table 1, there was no significant effect
of the substituent present on aromatic ring of the 2-
(acetoxy(aryl)methyl)acrylates. ortho-Substituted 2-
(acetoxy(aryl)methyl)acrylates also gave the 3-
substituted-2H-pyrido[1,2-a]pyrimidin-2-one
method in relation to other methods. The products
prepared by this method are the isomers to the
biologically active molecules which are depicted in
Figure 1.
In summary, we have demonstrated a rapid synthesis
derivatives in good yields (Table 1). This method
works not only with 2-aminopyridines but also with
of
derivatives by means of cyclocondensation of 2-
(acetoxy(aryl)methyl)acrylates with 2-
3-substituted-2H-pyrido[1,2-a]pyrimidin-2-one
3H-imidazo[4,5-c]pyridin-4-amine,
2-
aminopyrimidine and 2-aminobenzothiazoles. Indeed,
2-aminothiazoles are more effective than 2-
aminopyrimidine and 3H-imidazo[4,5-c]pyridin-4-
amine in terms of yields (entries d, e, f and m, Table
1).
aminopyridines using microwave irradiation under
neat conditions. The major advantages of the present
method are short reaction times, high conversions,
catalyst and solvent-free conditions and operational
simplicity, which makes this method more attractive.
Mechanistically, the cyclocondensation was assumed
to proceed through the Michael addition of pyridine
onto acrylate moiety followed by elimination of acetic
acid giving the iminopyridine acrylate. A subsequent
amidation of acrylate would give the desired product
as shown in Scheme 2.
Acknowledgements
S.S., K.P. and G. N. are thankful to the Council of
Scientific and Industrial Research, New Delhi, India,
for the award of fellowships. P.L.R. is thankful to
University Grants Commission, for the award of a
fellowship.
References and Notes
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Scheme 2. A plausible reaction pathway
In order to know the effect of microwave irradiation,
two parallel experiments were conducted using
microwave as well as conventional heating
conditions. For instance, treatment of 1 with 2e under
microwave and conventional heating conditions (oil
bath) at 95 C gave the 90% and 75% respectively.
Low conversion and long reaction time (3 h) were
observed under conventional conditions. High
conversion and short reaction time were achieved
using microwave irradiation. The use of microwave
makes this method quite simple and more convenient
to prepare a wide range of 3-substituted-2H-
pyrido[1,2-a]pyrimidin-2-one derivatives in a single
step. Enhanced reaction rate is the main ‘goal’ of this
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Tetrahedron Letters
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14. General procedure (conventional heating): A mixture
of methyl 2-(acetoxy(phenyl)methyl)acrylate (1, 1
mmol), 2-aminobenzothiazole (2, 1 mmol) was mixed
in a 10 mL R.B. flask. The resulting mixture was
stirred at 95 ˚C for the appropriate time (3h). After
complete conversion, as indicated by TLC, the mixture
was diluted with water and the resulting precipitate
was then collected by filtration. The precipitate was
purified by column chromatography on silica gel
(Merck, 60-120 mesh, EtOAc:MeOH, 8:2) to afford
the pure 3-substituted-2H-pyrido[1,2-a]pyrimidin-2-
one derivative.
General procedure (microwave irradiation): A mixture
of methyl 2-(acetoxy(phenyl)methyl)acrylate (1, 1
mmol), 2-heteroaryl amine (2, 1 mmol) was placed in a
10 mL sealed glass tube. The resulting mixture was
then irradiated for a specified time as required to
complete the reaction (see Table 1) at 95 ˚C
(temperature was monitored by inbuilt infrared sensor)
at 150 W. After complete conversion, as indicated by
TLC, the reaction mixture was cooled to room
temperature by air-flow. The mixture was diluted with
water and the resulting precipitate was then collected
by filtration. The precipitate was purified by column
chromatography on silica gel (Merck, 60-120 mesh,
EtOAc:MeOH, 8:2) to afford the pure 3-substituted-
2H-pyrido[1,2-a]pyrimidin-2-one derivative. The
products 3a, 3f-3i are reported in literature.10a-10b
3a: Solid: m.p. 216-218 °C; IR (neat): max 3066, 2978,
2921, 1951, 1651, 1604, 1548, 1491, 1425, 1306,
10. (a) Basavaiah, D.; Satyanarayana, T. Tetrahedron Lett.
2002, 43, 4301. (b) Sreevani, R.; Manjula, A.; Rao, B.
V. J. Heterocyclic Chem. 2000, 48, 586. (c) Shahrisa,
A.; Ghasemi, Z. Chemistry of Heterocyclic
Compounds, 2010, 46, 1.
1
1155, 1071, 970, 766 cm-1; H NMR (300 MHz,

Tetrahedron Letters
5
CDCl3+DMSO-d6): δ 3.81 (s, 2H), 6.73-6.89 (m, 1H),
7.14-7.35 (m, 5H), 7.48-7.62 (m, 1H), 7.76-8.15 (m,
3H); 13C NMR (75 MHz, DMSO-d6): δ 33.6, 112.5,
122.4, 126.1, 127.4, 128.2, 128.8, 133.9, 136.2, 136.4,
138.6, 148.8, 150.0, 150.6, 169.4; ESI-MS: m/z 237
(M+H); HRMS calcd for C15H13N2O: 237.1027, found:
237.1020.
6.98-7.62 (m, 7H), 7.64-7.83 (m, 1H); 13C NMR (75
MHz, CDCl3+DMSO-d6): δ 33.8, 112.2, 122.9, 123.9,
126.8, 127.1, 127.9, 130.9, 132.2, 132.4, 134.8, 134.9,
136.2, 150.3, 167.3; ESI-MS: m/z 317 (M+2); HRMS
calcd for C15H12ON2Br: 315.0127, found: 315.0120.
3k: Solid: m.p. 183-185 °C; IR (neat): max 2987, 1660,
1593, 1469, 1384, 1260, 1227, 1152, 1021, 971, 781,
1
3b: Solid: m.p. 241-242 °C; IR (neat): max 3076, 3024,
2915, 1651, 1602, 1469, 1385, 1345, 1230, 1158,
1071, 975, 896, 784 cm-1; 1H NMR (300 MHz,
CDCl3+DMSO-d6): δ 2.41 (s, 3H), 3.83 (s, 2H), 6.69
(t, 1H, J = 6.8 Hz), 7.13-7.33 (m, 5H), 7.38 (d, 1H, J =
6.8 Hz), 7.78-7.89 (m, 2H); 13C NMR (75 MHz,
DMSO-d6): δ 17.6, 33.5, 112.1, 126.1, 127.2, 128.2,
128.7, 130.5, 131.8, 134.1, 136.6, 140.3, 144.6, 149.5,
150.2, 171.1; ESI-MS: m/z 251 (M+H); HRMS calcd
for C16H15N2O: 251.1184, found: 251.1180.
742 cm-1; H NMR (300 MHz, CDCl3+DMSO-d6): δ
2.37 (s, 3H), 3.91 (s, 2H), 6.81 (t, 1H, J = 6.9 Hz), 7.18
(t, 1H, J = 6.9 Hz), 7.26-7.41 (m, 2H), 7.51 (d, 1H, J =
6.9 Hz), 7.60 (d, 1H, J = 6.9 Hz), 7.94 (s, 1H), 8.08 (d,
1H, J = 6.8 Hz); 13C NMR (75 MHz, CDCl3+DMSO-
d6): δ 16.5, 32.8, 111.6, 124.9, 126.4, 127.2, 129.9,
130.2, 131.3, 133.1, 135.3, 136.3, 142.7, 149.1, 154.2,
165.1; ESI-MS: m/z 331 (M+2); HRMS calcd for
C16H14ON2Br: 329.0284, found: 329.0278.
3l: Solid: m.p. 192-194 °C; IR (neat): max 3460, 3087,
3c: Solid: m.p. 195-197 °C; IR (neat): max 3419, 3044,
1662, 1550, 1462, 1333, 1212, 1143, 1025, 916, 891,
1
2923, 2855, 1731, 1643, 1566, 1458, 1394, 1286,
780, 726 cm-1; H NMR (300 MHz, CDCl3+DMSO-
1
1220, 1138, 1026, 880 cm-1; H NMR (300 MHz,
d6): δ 3.91 (s, 2H), 6.37 (t, 1H, J = 7.5 Hz), 6.71 (t, 1H,
J = 7.3 Hz ), 6.83 (d, 2H, J = 7.3 Hz), 7.11 (t, 1H, J =
7.5 Hz), 7.33-7.46 (m, 2H), 7.47-7.59 (m, 1H), 7.69 (s,
1H); 13C NMR (75 MHz, CDCl3+DMSO-d6): δ 32.7,
111.5, 117.8, 122.6, 126.3, 127.2, 129.8, 131.2, 134.3,
135.4, 138.4, 143.6, 145.5, 148.4, 165.9; ESI-MS: m/z
333 (M+2); HRMS calcd for C15H12O2N2Br: 331.0076,
found: 331.0064.
CDCl3+DMSO-d6): δ 3.82 (s, 2H), 6.77 (t, 1H, J = 7.3
Hz), 6.92 (d, 1H, J = 7.3 Hz), 7.13-7.35 (m, 5H), 7.56-
7.71 (m, 1H), 7.97 (d, 1H, J = 7.3 Hz), 8.05-8.16 (brs,
1H); 13C NMR (75 MHz, DMSO-d6): δ 33.6, 113.9,
118.9, 119.6, 126.5, 128.5, 129.0, 129.8, 130.8, 136.1,
137.1, 141.7, 142.8, 149.7, 167.5; ESI-MS: m/z 253
(M+H); HRMS calcd for C15H13N2O2: 253.0977,
found: 253.0986.
3m: Solid: m.p. 196-198 °C; IR (neat): max 3057,
1707, 1637, 1581, 1506, 1436, 1351, 1245, 1124,
1021, 922, 749 cm-1; 1H NMR (300 MHz,
CDCl3+DMSO-d6): δ 3.92 (s, 2H), 7.16 (t, 1H, J = 8.3
Hz), 7.21-7.33 (m, 2H), 7.45 (t, 1H, J = 7.3 Hz), 7.58
(t, 2H, J = 8.3 Hz), 7.84-7.95 (m, 2H), 8.65 (s, 1H);
13C NMR (125 MHz, CDCl3+DMSO-d6): δ 32.6,
110.7, 119.8, 121.8, 124.5, 125.6, 125.8, 126.6, 128.9,
129.4, 130.5, 130.8, 132.9, 135.8, 148.2, 161.2, 165.5;
ESI-MS: m/z 373 (M+2); HRMS calcd for
C17H12ON2BrS: 370.98482, found: 370.98447.
3d: Solid: m.p. 128-130 °C; IR (neat): max 3026, 2948,
1633, 1583, 1520, 1448, 1224, 1026, 802, 771, 699,
641 cm-1; 1H NMR (300 MHz, CDCl3+DMSO-d6): δ
3.70 (s, 2H), 6.31-6.46 (m, 2H), 6.58 (t, 1H, J = 6.9
Hz), 6.92-7.53 (m, 5H), 8.19 (d, 1H, J = 6.9 Hz); 13C
NMR (75 MHz, CDCl3+DMSO-d6): δ 32.9, 108.2,
126.1, 126.3, 126.5, 126.58, 126.6, 126.8, 126.9,
127.1, 127.6, 139.7, 155.9, 161.7; ESI-MS: m/z 238
(M+H).
3e: Solid: m.p. 220-222 °C; IR (neat): max 2911, 1707,
1661, 1605, 1535, 1472, 1433, 1370, 1261, 1179, 989,
1
748 cm-1; H NMR (300 MHz, CDCl3+DMSO-d6): δ
3.82 (s, 2H), 7.18 (t, 1H, J = 6.9 Hz), 7.26 (t, 2H, J =
6.9 Hz), 7.34 (d, 2H, J = 7.1 Hz), 7.40 (t, 1H, J = 7.3
Hz), 7.51 (t, 1H, J = 8.1 Hz), 7.77 (d, 1H, J = 7.3 Hz),
7.83 (d, 1H, J = 8.1 Hz), 8.56 (s, 1H); 13C NMR (75
MHz, CDCl3+DMSO-d6): δ 33.3, 110.4, 119.8, 120.3,
121.7, 122.4, 123.5, 124.8, 126.4, 127.5, 128.0, 128.4,
129.2, 150.1, 153.5, 165.7, 171.6; ESI-MS: m/z 293
(M+H); HRMS calcd for C17H13N2OS: 293.0748,
found: 293.0737.
3j: Solid: m.p. 245-247 °C; IR (neat): max 3069,
3025,1949,1654,1596, 1479, 1425, 1341, 1234, 1140,
1022, 941, 778 cm-1; 1H NMR (300 MHz,
CDCl3+DMSO-d6): δ 3.95 (s, 2H), 6.65-6.85 (m, 1H),