Tandem Schiff-Base Formation/Heterocyclization: An Approach to the Synthesis of Fused Pyrazolo-Pyrimidine/Isoxazolo-Pyrimidine Hybrids

Article abstract of DOI:10.1055/s-0037-1612081

A new synthesis of pyrazolo[4,3-d]pyrimidines and isoxazolo[4,5-d]pyrimidines is described. Key steps in the synthesis involve Stille coupling of 4,6-dichloro-2-phenyl-pyrimidine with tributyl(1-ethoxyvinyl)stannane and tandem Schiff-base formation/heterocyclization of 2,6-di-aryl-5-fluoro-4-acetylpyrimidine with hydrazines or hydroxylamine to give pyrazolo[4,3-d]pyrimidines and isoxazolo[4,5-d]pyrimidines, respectively. The position of the fluoro group in the A-pyrimidine ring is important for the success of heterocylization reaction.

Full text of DOI:10.1055/s-0037-1612081

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–G
letter
A
en
M. Sambaiah et al.
Letter
Synlett
Tandem Schiff-Base Formation/Heterocyclization: An Approach to
the Synthesis of Fused Pyrazolo–Pyrimidine/Isoxazolo-Pyrimidine
Hybrids
M. Sambaiaha,b
Poosa Malleshama
K. Shiva Kumarb
R1
R1
Cl
F
R2
N
Tandem
Heterocyclization
Tandem
Heterocyclization
Yamini Bobdec
O
N
O
N
N
N
N
N
Prasanta Kumar Hotad
Satyanarayana Yennama
Balaram Ghoshc
N
N
R
10
14
16
R
R
*
Flexible method for synthesis of pyrazolo–pyrimidine
as well as isoxazolo–pyrimidine
Stille coupling on pyrimidine using EVTBT
Tandem heterocyclization reaction
Manoranjan Behera*a
*
*
*
Pyrazolo[4,3-d]pyrimidines
Isoxazolo[4,5-d]pyrimidines
14 Examples
10 Examples
a Chemistry services, GVK Biosciences Pvt. Ltd., Survey
Nos:125 (part) & 126, IDA Mallapur, Hyderabad-500076,
Telangana State, India
16b [IC50 = 11.51 µM]
16h [IC50 = 11.48 µM]
14f [IC50 = 13.69 µM]
B16F10 cell line
manoranjan.behera@gvkbio.com
b Department of Chemistry, GITAM, Hyderabad Campus,
Rudraram Village, Patancheru Mandal, Sangareddy Dist.,
502329, Telangana State, India
c Department of Pharmacy, Birla Institute of Technology and
Science, Pilani, Hyderabad Campus, Shameerpet, Hyder-
abad, 500078, Telangana, India
d Department of Chemistry, School of Sciences, HNBG Uni-
versity, Srinagar (Garhwal), Uttarakhand 246174, India
OCH3
Received: 06.12.2018
Accepted after revision: 02.01.2019
Published online: 05.02.2019
OCH3
F
OH
DOI: 10.1055/s-0037-1612081; Art ID: st-2018-d0620-l
N
N
N
N
N
N
N
OCH3
Abstract A new synthesis of pyrazolo[4,3-d]pyrimidines and isoxazo-
lo[4,5-d]pyrimidines is described. Key steps in the synthesis involve
Stille coupling of 4,6-dichloro-2-phenyl-pyrimidine with tributyl(1-
ethoxyvinyl)stannane and tandem Schiff-base formation/heterocycliza-
tion of 2,6-di-aryl-5-fluoro-4-acetylpyrimidine with hydrazines or
hydroxylamine to give pyrazolo[4,3-d]pyrimidines and isoxazolo[4,5-
d]pyrimidines, respectively. The position of the fluoro group in the
pyrimidine ring is important for the success of heterocylization reac-
tion.
N
N
N
H
N
N
allopurinol
H3CO
6-arylpurines
diarylpurines
Cl
NH
N
H3CO
NH
O
N
N
N
O
N
N
N
N
O
HN
N
N
H
H3CO
H2N
pyrrolo[2,3-d-]pyrimidine
ACK1 inhibitors
Key words pyrazolo[4,3-d]pyrimidine,
isoxazolo[4,5-d]pyrimidine,
5,7-diphenylisoxazolo[4,5-d]-
pyrimidine-3-carboxamide
myoseverin
Stille coupling, Schiff base
Figure 1 Representative biologically active azolo fused pyrimidines
and isoxazolopyrimidine derivatives
Pyrimidine derivatives are privileged heterocyclic struc-
tures in medicinal chemistry.1 Along with the varied biolog-
ical activities of pyrimidines, other heterocycles fused with
pyrimidines play essential roles in several biological pro-
cesses and have considerable chemical and pharmacologi-
cal importance.2 Fused pyrimidines exhibit a broad spec-
trum of biological properties3 such as anticancer,4 anti-
viral,5 antibacterial,6 antioxidant,7 anxiolytic,8 and
antidepressant activities.9 Studies indicate that fusion of
the pyrimidine moiety with different heterocycle scaffolds
gives rise to new classes of hybrid heterocycles with im-
proved biological activity (Figure 1).10
Many simple fused pyrimidines such as purines and
pteridines are biologically active by themselves or are
essential components of important naturally occurring sub-
stances, such as nucleic acids.11 Pyrrolo[2,3-d]pyrimidines,
also known as deazapurines because of their structural
similarity to natural purines, are interesting templates for
drug discovery programs in medicinal chemistry12 and have
proved to be valuable scaffolds in organic chemistry.13 Sem-
inal work on the synthesis of 7-deazapurines derivatives,
carried out by Hocek, highlights the applications of this
class of biologically active pyrimidine derivatives.14
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

B
M. Sambaiah et al.
Letter
Synlett
1H-Pyrazolo[4,3-d]pyrimidines are a class of fused
heterocycles with potent biological activities.15 They are
selective second-generation phosphodiesterase 5 (PDE-5)
inhibitors,16 and others are known to function as central
nervous system depressants,17 neuroleptic agents,18 tuber-
culostatic,19 and to be active on adenosine receptors.20 The
application of pyrazolopyrimidines as antipyretic and anal-
gesic agents has also been reported.21,22 A recent report on
the synthesis and chemistry of these compounds has ap-
peared (Scheme 1).23
The most common method for the preparation of pyra-
zolo[4,3-d]pyrimidines is the stepwise condensation of a
hydrazine with a 2-alkoxy-1,1-dicyanoalkene to obtain a
pyrazole derivative, onto which the pyrimidine is fused.
This sequence requires multiple discrete steps and the con-
ditions involved are incompatible with a number of func-
tional groups, in particular the nitrile functionality. There is
a report for the synthesis of pyrazolo[4,3-d]pyrimidine
derivatives by treatment of hydrazine hydrate with 2,4
bis(trichloromethyl)-5-cyano-6-phenyl pyrimidine. A more
efficient method for achieving such a transformation would
be to condense a substituted hydrazine with 2,6-diaryl-5-
fluoro-4-acetyl-pyrimidine allowing formation of the
desired pyrazole ring system in a tandem manner. Such a
transformation will give 3-methyl-5,7-diaryl-pyrazolo[4,3-
d]pyrimidine derivatives selectively, which are otherwise
difficult to prepare using existing methodology.
In the course of our research to develop novel hybrid
molecules, we have been interested in the synthesis of nov-
el analogues of 1,3- and 5,7-substituted 1H-pyrazolo[4,3-
d]pyrimidines 14 as well as 5,7-disubstituted isoxazolo[4,5-
d]pyrimidines 16. Initially our aim was to synthesize the
key compound, 5-bromo-4,6-dichloro-2-phenylpyrimidine
5a (Scheme 2), so that a variety of structural and functional
moieties could be introduced by palladium-assisted meth-
odology.
Initially, compound 3 was prepared by condensation of
benzamidine with dimethyl malonate in the presence of so-
dium methoxide in methanol. Subsequent chlorination
with POCl3using a catalytic amount of N,N-diethyl aniline at
130 °C gave the 3,6-dichloro-2-phenyl-pyrimidine deriva-
tive 4 in good yield. The key bromination reaction was
attempted using a variety of brominating reagents as sum-
marized in Table 1 without success. Although bromination
at C-5 has been reported with unsubstituted pyrimidines
using a variety of methods, the position of two chlorine
substituents at adjacent positions may be detrimental for
this bromination. After realizing the difficulties in halo-
genation at the C-5 position of pyrimidine, we decided to
construct the pyrimidine ring with suitably substituted
halogen derivatives of dimethyl malonate. The synthesis
started with the reaction of commercially available 2-fluoro-
dimethyl malonate 6 with benzamidine in the presence of
sodium methoxide in methanol. Gratifyingly, we were able
to obtain 4,6-dihydroxy-2-phenyl-5-fluoro-pyrimidine (7a)
in good yield. Compound 7a was characterized by 1H NMR,
Cl
CH3
O
Previous Work:
CH3
N
O
R1
N
CH3
O
N
N
HN
POCl3
N
N
N
N
+
chlorination
CH3
N
H2N
CH3
annulation
H2N
R = H,OH,Cl,OMe,
R1
Present Work:
R1
F
Cl
N
R2
HN
NH2
F
N
N
N
O
N
N
O
N
N
R
R
R
R
Scheme 1 Synthetic approaches for pyrazoleo[4,3-d]pyrimidines
The pyrazolo[4,3-d]pyrimidine ring system has received
less attention.24 As a result the chemical as well as biologi-
cal properties of this system are less known compared to its
isomeric form. Besides, pyrazolopyrimidines, other hetero-
cycle-fused pyrimidine derivatives, such as isoxazolopyrim-
idine derivatives have shown analgesic, anti-inflammatory,
anti-bactericidal, circulatory, and anxiolytic activities.25 Of
the four structural isomers of isoxazolopyrimidines, only
the [4,5-d] isomer has not been thoroughly investigated.26
To date there have been only a few reports concerning the
synthesis of derivatives of this isomer.27–30 Therefore there
is a need to develop synthetic methodology for the prepara-
tion of isoxazolo[4,5-d]pyrimidine and pyrazolo[4,3-d]py-
rimidine derivatives.
Synthesis of hybrid molecules wherein two naturally
occurring structures are linked by a C–C bond has received
much attention recently.31–33 Taking into consideration the
biological significance of pyrazoles, isoxazole, and pyrimi-
dines, we planned to develop a general methodology for the
synthesis of pyrazolopyrimidine and isoxazolopyrimidine
hybrids and herein we report our initial results.
Cl
Cl
Br
Cl
OH
N
O
O
HN
NH2
N
n-BuLi, BrCCl2CCl2Br,
THF, –78 °C, 8 h
POCl3, N,N-diethylaniline,
130 °C, 4 h
N
N
O
O
N
2
X
N
Cl
OH
25% NaOMe in MeOH,
RT,16 h
5a
Step 2
Step 3
4
3
1a
Step 1
Scheme 2
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

C
M. Sambaiah et al.
Letter
Synlett
19F NMR spectroscopy and MS analysis. Compound 7a was
then treated with POCl3using a catalytic amount of N,N-di-
ethyl aniline at 130 °C to provide the 4,6-dichloro-2-
phenyl-5-fluoro-pyrimidine derivative 8a. Inspired by the
statistical Stille coupling34 on 3,5-dichloropyrimidine by
Lehn’s group,35 we went on to explore the Stille coupling on
4,6-dichloro-2-phenyl-5-fluoro-pyrimidine derivative 8a.
Table 2 Screening Optimal Conditions for the Stille Coupling Reaction
Entry Reaction conditions
Resulta
1
2
Pd(PPh3)4, THF, 65 °C, 12 h
25% of compound 9a
33% of compound 9a
6% of compound 9a
15% of compound 9a
no product
Pd(PPh3)4, dioxane, 90 °C, 14 h
Pd(PPh3)4, toluene, 95 °C, 16 h
PdCl2dppf, THF, 65 °C, 12 h
3
4
Table 1 Screening the Optimal Conditions for the Bromination
Reaction of Compound 4
5
PdCl2dppf, dioxane, 90 °C,14 h
PdCl2dppf, toluene, 95 °C, 16 h
Pd(dba)2, THF, 65 °C, 12 h
6
no product
7
8% of compound 9a
5% of compound 9a
2% of compound 9a
no product
Entry
Reaction conditions
Resulta
8
Pd(dba)2, dioxane, 90 °C,14 h
Pd(dba)2, toluene, 95 °C, 16 h
Pd(OAc)2, TPP, THF, 65 °C, 12 h
Pd(OAc)2, TPP, dioxane, 90 °C, 14 h
Pd(OAc)2, TPP, toluene, 95 °C, 16 h
PdCl2(PPh3)2, toluene, 100 °C, 16 h
1
2
3
4
5
6
7
8
NBS, DMF, RT, 16 h
SM intact
no product
SM intact
SM intact
no product
no product
SM intact
no product
9
HBr, AcOH, RT, 16 h
10
11
12
13
Br2, CHCl3, RT, 16 h
16% of compound 9a
46% of compound 9a
Br2, AcOH, RT, 16 h
C2Br2Cl4, LDA, THF, –78 °C to RT, 16 h
C2Br2Cl4, n-BuLi, THF, –78 °C to RT, 16 h
POBr3, DCM, RT, 16 h
61% of compound 9a and
10% of compound 10a
aReaction conditions: compound 8a (0.41 mmol) and 1-ethoxyvinyl tri-n-bu-
tyltin (0.44 mmol), results based on LC–MS analysis of crude reaction mix-
tures.
PBr3, DCM, RT, 16 h
aReaction conditions: compound 4 (0.44 mmol) and brominating agents
(0.53 mmol), results based on LC–MS analysis of crude product mixtures.
C-5 of the pyrimidine ring (Scheme 3). Compound 10a was
then desymmetrized by Suzuki coupling with various
phenylboronic acids to give 2,6-diphenyl-substituted py-
rimidines 12. Finally, we explored the tandem Schiff-base
formation/heterocyclization reaction of compound 12a
with hydrazine to obtained fused pyrazolopyrimidine
derivatives.36 Compound 12a was treated with hydrazine
hydrate under refluxing EtOH conditions to give the inter-
mediate hydrazone 13a, which was subjected to hetero-
cyclization using K2CO3/DMF with heating37 to give the final
compound 14a. Compound 14a was characterized by 1H
NMR, 13C NMR spectroscopy and MS analysis.
To prove the generality of this method various highly
substituted aryl, hetero aryl, alkyl benzamidines, as well as
polysubstituted hydrazines were examined, and the results
are summarized in Figure 2. Similar yields were observed
for substrates having electron-withdrawing substituents
The Stille reaction of compound 8a was tried with
ethoxyvinyltributyltin in the presence of Pd(0)/THF under
reflux, which gave 25% product formation as observed by
LC–MS analysis. Various palladium catalysts as well as sol-
vents and temperature were screened for the Stille coupling
reaction (Table 2).
The best result was obtained using 1.1 equivalents of 1-
ethoxyvinyltri-n-butyltin and PdCl2(PPh3)2 as catalyst in
toluene (Table 2, entry 13), giving 9a in 61% yield that was
pure enough to proceed to the next step. Compound 10a
was obtained by hydrolysis of compound 9a using HCl in
THF and was purified by flash column chromatography on
1
silica and characterized by H NMR spectroscopy and LC–
MS analysis. The characteristic peak at δ = 2.8 ppm in the 1H
NMR spectrum confirms the presence of an acetyl group at
Cl
O
O
Cl
OH
N
Ethoxyvinyl-tributyl tin,
PdCl2(PPh3)2, toluene,
110 °C, 16 h
F
HN
NH2
N
F
POCl3, N,N-diethylaniline,
130 °C, 4 h
F
O
O
N
N
6
N
F
N
Cl
OH
O
25% NaOMe in MeOH,
MeOH, RT, 16 h
Step 2
Step 3
9a
8a
7a
1a
Step 1
HO
OH
B
Cl
N
F
H
K2CO3, DMF,
100 °C, 2 h
NH2-NH2•HCl, EtOH,
110 °C, 6 h
F
N
F
1N HCl, THF,
RT, 16 h
N
N
11
N
N
O
N
O
N
N
Step 6
Na2CO3, Pd(PPh3)4,
toluene:THF:H2O,
100 °C, 16 h
N
N
Step 4
Step 6
NH2
10a
14a
13a
12a
Step 5
Scheme 3
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

D
M. Sambaiah et al.
Letter
Synlett
(compounds 14f, g,i,j) as well as electron-donating substit-
uents (compounds 14j–n). Alkyl hydrazines (methyl, iso-
propyl, and benzyl) were also converted into the corre-
sponding novel fused biphenyl-substituted pyrazolo[4,3-
d]pyrimidines in good yields.
F
NH2OH•HCl, NaOH,
MeOH, RT, 16 h
NaOH, DMF,
RT, 6 h
N
F
O
N
N
OH
N
N
O
N
Step 6
Step 7
N
N
12a
15a
16j
Scheme 4
H
N
N
N
N
N
N
N
N
N
CF3
N
N
N
14c
90%
F3C
14b
87%
14a
70%
CF3
O
N
O
O
N
N
N
N
N
N
N
N
H
N
16b
41%
16c
53%
N
N
16a
40%
N
N
N
N
N
N
N
N
14e
62%
N
14f
74%
Cl
O
Cl
14d
59%
NO2
CF3
Cl
Cl
O
O2N
O
Cl
N
N
O
O
N
N
H
N
N
N
N
N
N
N
N
N
N
N
16f
29%
N
N
N
16e
N
N
16d
48%
F
30%
14h
N
N
14g
Cl
14i
O
67%
78%
O
NO2
O
Cl
O
Cl
Cl
N
Cl
O
O
N
N
O
N
N
O
O
N
N
N
N
16g
N
N
N
N
N
N
N
N
16i
N
N
16h
35%
N
20%
34%
N
14l
N
O
14m
14j
77%
F
98%
N
O
N
N
N
N
N
N
N
N
16j
N
N
14n
52%
40%
N
14k
N
46%
Figure 3 List of fused isoxazolo–pyrimidine hybrids (% isolated yield)
Figure 2 List of fused pyrazolo–pyrimidine hybrids (% isolated yield)
In summary, we have synthesized novel pyrazolopyrim-
idine as well as isoxazolopyrimidine hybrids using a tan-
dem Schiff-base formation/heterocyclization reaction of
2,6-diaryl-5-fluoro-4-acetyl-pyrimidines. This method has
several advantages over previously reported methods as it
starts with simple benzamidine; whereas previous meth-
ods use 4-amino-5-carboxamide pyrazole derivatives,
which are difficult to prepare.
In order to expand further our synthetic approaches to-
wards fused heterocyclic pyrimidine hybrids, we turned
our attention to the prepare 3-methyl-5,7-disubstituted
isoxazolo[4,5-d]pyrimidines using the same key intermedi-
ate 12a. Thus, 12a was treated with hydroxylamine hydro-
chloride in the presence of NaOH/MeOH at room tempera-
ture to give the hydrazone 15a that was subsequently sub-
jected to heterocyclization using K2CO3/DMF with heating38
to give the isoxazolo[4,5-d]pyrimidine 16a in good yield.
This is the first report of diphenylisoxazolo[4,5-d]pyrimi-
dine preparation by a tandem Schiff-base/heterocyclization
sequence. Using a similar sequence (Scheme 4) the target
compounds 16a–j (Figure 3) were prepared and fully char-
acterized.39
Acknowledgment
The authors are grateful to GVK Biosciences Pvt. Ltd. for financial sup-
port. Help from the analytical department for provision of analytical
data is appreciated. We thank Dr. Sudhir Kumar Singh for his invalu-
able support and motivation. We thank Dr. Hemant Joshi for the use-
ful discussions on medicinal chemistry aspects of the synthesized
compounds.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

E
M. Sambaiah et al.
Letter
Synlett
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Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1612081.
S
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p
p
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n
gInformati
o
n
S
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Soc. 2010, 54, 74.
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11, 6883.
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S. J. Am. Chem. Soc. 2011, 133, 676.
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22.
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4451.
(14) (a) Anna, T.; Barbora, L.; Sabina, S.; Taborska, N.; Michal, T.;
Sona, G.; Pavla, P.; Ivo, F.; Eva, T.; Pawel, Z.; Kaiserova, H. M.;
Slavetinska, P. S.; Pohl, R.; Klepetarova, B.; Khalid, N.; Wenren,
Y.; Laposa, R. R.; Dzubak, P.; Hajduch, M.; Hecek, M. J. Med.
Chem. 2018, 61, 9347. (b) Milisavljevic, N.; Perlikova, P.; Pohl, R.;
Hocek, M. Org. Biomol. Chem. 2018, 16, 5800. (c) Perlikova, P.;
Hocek, M. Med. Res. Rev. 2017, 37, 1429. (d) Sabat, N.; Naus, P.;
Matyasovsky, J.; Dziuba, D.; Slavetinska, L. P.; Hocek, M. Synthe-
sis 2016, 48, 1029. (e) Hocek, M.; Holy, A.; Dvorakova, H. Collect.
Czech. Chem. Commun. 2002, 67, 325.
(37) Typical Experimental Procedures
3-Methyl-5,7-diphenyl-1H-pyrazolo [4,3-d]pyrimidine (14a)
To a stirred solution of compound 12a (0.25 g, 0.85 mmol) in
ethanol (10 mL) was added hydrazine hydrochloride (0.055 g,
1.71 mmol), and the resulting reaction mixture was heated to
110 °C for 6 h. The progress of the reaction was monitored by
TLC (20% ethyl acetate in petroleum ether). The ethanol was
evaporated, and the crude the 13a was dissolved in DMF (10
mL), K2CO3 (0.177 g, 1.28 mmol) was added and the reaction
mixture stirred at 100 °C for 2 h. After completion of reaction;
water was added, and the reaction mixture was extracted with
ethyl acetate (X × Y mL). The combined organic extracts were
washed with water (X × Y mL), brine (X × Y mL), dried over
Na2SO4, filtered and evaporated to give the crude product. The
crude product was purified by silica gel column chromatogra-
phy, eluting with 10% ethyl acetate in petroleum ether to afford
pure 14a (180 mg, 73%) as a pale yellow solid; mp 268–272 °C.
1H NMR (400 MHz, CDCl3): δ = 10.24 (br s, 1 H), 8.67 (dd, J = 8
Hz, 2 H), 8.23 (d, J = 8 Hz, 2 H), 7.63 (m, 6 H), 2.78 (s, 3 H). IR
(KBr): 3247, 2920, 1954, 1554, 1465, 1375, 1292, 1203, 990,
(15) Marjani, A. P.; Khalafy, J.; Salami, F.; Ezzati, M. ARKIVOC 2015,
(v), 277.
(16) Tollefson, M. B.; Acker, B. A.; Jacobsen, E. J.; Hughes, R. O.;
Walker, J. K.; Fox, D. N. A.; Palmer, M. J.; Freeman, S. K.; Yu, Y.;
Bond, B. R. Bioorg. Med. Chem. Lett. 2010, 20, 3120.
(17) Julino, M.; Stevens, M. F. G. J. Chem. Soc, Perkin Trans 1 1998, 10,
1677.
(18) Filler, R. Chem. Technol. 1974, 4, 752.
(19) Ghorab, M. M.; Ismail, Z. H.; Abdel, G. S. M.; Abdel, A. A. Het-
eroat. Chem. 2004, 15, 57.
(20) Davies, L. P.; Brown, D. J.; Chow, S. C.; Johnston, G. A. R. Neurosci
Lett. 1983, 41, 189.
(21) (a) Zacharie, B.; Connolly, T. P.; Rej, R.; Attardo, G.; Penney, C. L.
Tetrahedron. 1996, 52, 2271. (b) Taylor, E. C.; Mc Killop, A. Adv.
Org. Chem. 1970, 7, 415.
930, 755, 685, 542 cm–1 13C NMR (100 MHz, DMSO-d6) = 156.2,
.
148.9, 145.3, 142.3, 137.9, 135.3, 131.2, 129.7, 129.7, 129.6,
129.0, 128.7, 128.5, 128.5, 128.5, 127.5, 127.5, 10.7. MS (EI): m/z
= 286 (100) [M + 1]. HRMS (ESI): m/z calcd for C18H14N4 [M + H]:
287.1218; found: 287.1298.
(22) (a) Robins, R. K. J. Am. Chem. Soc. 1956, 78, 784. (b) Cheng, C. C.;
Robins, R. K. J. Org. Chem. 1956, 21, 1240.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

F
M. Sambaiah et al.
Letter
Synlett
1-Isopropyl-3-methyl-5,7-diphenyl-1H-pyrazolo[4,3-
d]pyrimidine (14c)
MeOH (8 mL) were added hydroxylamine hydrochloride (0.052
g, 0.75 mmol) and powdered NaOH (0.033 g, 0.82 mmol) at RT,
then the reaction mixture was stirred under N2 atmosphere for
16 h. Progress of the reaction was monitored by TLC (5% ethyl
acetate in petroleum ether). After completion of the reaction,
the methanol was evaporated to give crude 15a as a solid. This
was dissolved in DMF under N2 and cooled to 0 °C. To this, NaOH
(0.033 g, 0.82 mmol) powder was added and the mixture stirred
at RT for 6 h. The progress of the reaction was monitored by TLC
(5% ethyl acetate in petroleum ether). After completion of reac-
tion, water was added, and the mixture was extracted with
ethyl acetate (X × Y mL). The combined extracts were washed
with water (X mL), brine (Y mL), dried over Na2SO4, filtered, and
evaporated to afford the crude product, which was purified by
silica column chromatography, eluting with 3% ethyl acetate in
petroleum ether to give the pure 16j (80 mg, 40%) as a pale
yellow solid; mp 153–157 °C. 1H NMR (400 MHz, CDCl3): δ =
8.73 (m, 2 H), 8.71 (m, 2 H), 7.65 (m, 3 H), 7.56 (m, 3 H), 2.77 (s,
3 H). IR (KBr): 3414, 2921, 1589, 1457, 1376, 1274, 1164, 1067,
Pale yellow solid (0.2 g, 90%); mp 109–113 °C. 1H NMR (400
MHz, CDCl3): δ = 8.59 (d, J = 8 Hz, 2 H), 7.78 (d, J = 4 Hz, 2 H),
7.75 (m, 3 H), 7.43 (m, 3 H), 4.53 (m 1 H), 2.75 (s, 3 H), 1.37 (d,
J = 4 Hz, 6 H). IR (KBr): 3423, 2974, 1996, 1550, 1414, 1369,
1230, 1123, 1070, 926, 767, 603, 544 cm–1 13C NMR (100 MHz,
.
CDCl3) = 157.1, 151.5, 145.9, 143.1, 138.3, 137.3, 130.0, 130.0,
129.5, 129.5, 129.5, 129.5, 128.8, 128.8, 128.5, 128.5, 128.4,
128.0, 51.4, 22.2, 10.9. MS (EI): m/z = 328 (100) [M + 1]. HRMS
(ESI): m/z calcd for
C21H20N4 [M + H]: 329.1688; found:
329.1770.
7-[3-(Trifluoromethyl)phenyl]-3-methyl-5-phenyl-1H-
pyrazolo [4,3-d]pyrimidine (14f)
Yellow solid (0.18 g 74%); mp 217–221 °C. 1H NMR (400 MHz,
CDCl3): δ = 10.40 (br s, 1 H), 8.66 (m, 2 H), 8.53 (m, 1 H), 8.45 (d,
J = 8 Hz, 1 H), 7.88 (m, 1 H), 7.81 (t, J = 8, 8 Hz, 1 H), 7.54 (m, 3
H), 2.80 (s, 3 H). IR (KBr): 3243, 3069, 1556, 1469, 1425, 1328,
1250, 1166, 1119, 1070, 993, 772, 544 cm–1 13C NMR (100 MHz,
.
CDCl3): δ =158.4, 148.3, 138.0, 137.1, 132.1, 131.8, 131.5, 130.0,
129.9, 129.9, 129.8, 128.5, 128.5, 128.1, 127.6, 127.6, 125.2,
124.8, 10.8. MS (EI): m/z = 354 (100) [M + 1]. HRMS (ESI): m/z
calcd for C19H13N4 [M + H]: 355.1092; found: 355.1173.
920, 853, 797, 750, 688 cm–1 13C NMR (100 MHz, CDCl3): δ = 161.2,
.
156.5, 150.3, 149.0, 147.7, 137.3, 133.5, 132.1, 130.6, 130.5,
129.7, 129.4, 129.0, 128.6, 128.5, 128.3, 127.5, 9.2. MS (EI): m/z
= 287 (100) [M + 1]. HRMS (ESI): m/z calcd for C18H13N3O [M +
H]: 288.3153; found: 288.1143.
3-Methyl-5-phenyl-7-[3-(trifluoromethyl)phenyl]isoxaz-
olo[4,5-d]pyrimidine (16b)
1-(3,5-Dichlorophenyl)-7-[4-(trifluoromethyl)phenyl]-3-
methyl-5-phenyl-1H-pyrazolo[4,3-d]pyrimidine (14g)
Off-white solid (0.28 g, 67%); mp 216–220 °C. 1H NMR (400
MHz, CDCl3): δ = 8.65 (m, 2 H), 7.61 (m, 4 H), 7.51 (m, 3 H), 7.21
(m, 1 H), 7.00 (d, J = 1.6 Hz, 2 H), 2.80 (s, 3 H). IR (KBr): 3086,
2323, 1585, 1455, 1326, 1167, 1124, 1069, 1018, 848, 704, 628,
Pale yellow solid (0.12 g, 41%); mp 161–165 °C. 1H NMR (400
MHz, CDCl3): δ = 8.96 (m, 1 H), 8.91 (d, J = 8 Hz, 1 H), 8.64 (m, 2
H), 7.88 (m, 1 H), 7.79 (t, J = 8, 4 Hz, 1 H), 7.53 (m, 3 H), 2.79 (s, 3
H). IR (KBr): 3073, 2329, 1655, 1594, 1412, 1326, 1231, 1161,
600 cm–1 13C NMR (100 MHz, CDCl3): δ = 158.7, 149.8, 148.7,
.
146.8, 140.3, 139.1, 137.4, 134.9, 132.4, 132.1, 130.4, 129.7,
129.7, 128.6, 128.6, 128.2, 127.1, 127.0, 127.0, 125.0, 125.0,
124.7, 123.1, 122.5, 10.9. MS (EI): m/z = 498 (100) [M + 1].
HRMS (ESI): m/z calcd for C25H15Cl2N4 [M + H]: 499.0626; found:
499.0708.
1069, 920, 769, 690, 594 cm–1 13C NMR (100 MHz, CDCl3): δ =
.
161.4, 161.4, 156.6, 150.0, 149.6, 145.9, 136.9, 134.9, 134.3,
132.8, 132.6, 131.8, 131.5, 130.7, 130.2, 129.6, 128.7, 128.6, 9.2.
MS (EI): m/z = 355 (100) [M + 1]. HRMS (ESI): m/z calcd for
C
19H12F3N3O [M + H]: 356.0932; found: 356.1014.
1-(3,5-Dichlorophenyl)-7-(2,4-dimethoxyphenyl)-5-(furan-
3-yl)-3-methyl-1H-pyrazolo[4,3-d]pyrimidine (14m)
Off-white solid (0.19 g, 90%); mp 208–217 °C. 1H NMR (400
MHz, CDCl3): δ = 8.33 (m, 1 H), 7.73 (d, J = 8 Hz, 1 H), 7.52 (m, 1
H), 7.22 (m, 1 H), 7.16 (m, 1 H), 7.04 (m, 2 H), 6.72 (d, J = 8 Hz, 1
H), 6.08 (s, 1 H), 4.85 (s, 3 H), 3.19 (s, 3 H), 2.81 (s, 3 H). IR (KBr):
3438, 2926, 1736, 1610, 1584, 1504, 1461, 1277, 1111, 1027,
7-(4-Chloro-3-nitrophenyl)-3-methyl-5phenylisoxazolo[4,5-
d]pyrimidine (16d)
Pale yellow solid (0.07 g, 48%); mp 187–191 °C. 1H NMR (400
MHz, CDCl3): δ = 9.20 (d, J = 4 Hz, 1 H), 8.86 (d, J = 8 Hz, 1 H),
8.62 (m, 2 H), 7.84 (d, J = 8 Hz, 1 H), 7.58 (m, 3 H), 2.79 (s, 3 H).
IR (KBr): 3092, 2921, 2850, 2324, 1742, 1600, 1542, 1355, 1272,
1038, 844, 758, 696 cm–1 13C NMR (100 MHz, CDCl3): δ = 162.4,
.
934, 795, 596 cm–1
.
13C NMR (100 MHz, CDCl3): δ =163.4, 157.9,
162.4, 156.7, 154.8, 150.2, 149.6, 143.9, 138.1, 136.6, 133.5,
133.2, 132.6, 131.0, 130.3, 128.8, 128.3, 125.7, 9.26. MS (EI): m/z
= 366 (100) [M + 1]. HRMS (ESI): m/z calcd for C18H11ClN4O [M +
H]: 367.0520; found: 366.9997.
7-(2,4-Dimethylphenyl)-3-methyl-5-phenylisoxazolo[4,5-
d]pyrimidine (16g)
157.9, 155.0, 149.4, 146.7, 146.6, 145.3, 144.2, 143.5, 141.0,
134.2, 131.6, 127.8, 127.2, 126.1, 122.2, 118.6, 109.8, 105.7,
97.3, 55.6, 54.5, 10.8. MS (EI): m/z = 480 (100) [M + 1]. HRMS
(ESI): m/z calcd for C24H18Cl2N4O3 [M + H]: 480.0756; found:
481.0845.
3-Methyl-1,7-diphenyl-5-(pyridin-3-yl)-1H-pyrazolo
d]pyrimidine (14n)
[4,3-
Pale yellow solid (0.07 g, 20%); mp 99–103 °C. 1H NMR (400
MHz, CDCl3): δ = 8.59 (m, 2 H), 7.86 (d, J = 4 Hz, 1 H), 7.54 (m, 3
H), 7.24 (m, 2 H), 2.81 (s, 3 H), 2.61 (s, 3 H), 2.40 (s, 3 H). IR
(KBr): 3418, 2919, 2626, 2525, 1591, 1533, 1392, 1354, 1276,
Light brown solid (0.13 g, 52%); mp 181–185 °C. 1H NMR (400
MHz, CDCl3): δ = 9.86 (s, 1 H), 8.93 (d, J = 4 Hz, 1 H), 8.72 (d, J = 4
Hz, 1 H), 7.48 (m, 3 H), 7.39 (m, 1 H), 7.23 (m, 5 H), 7.12 (m, 2
H), 2.85 (s, 3 H). IR (KBr): 3481, 3036, 2922, 1558, 1498, 1417,
1219, 841, 769, 695 cm–1 13C NMR (100 MHz, CDCl3): δ =161.0,
.
161.0, 156.6, 150.9, 150.8, 148.0, 141.0, 138.0, 137.4, 132.6,
131.1, 130.4, 129.9, 129.9, 128.6, 128.3, 126.9, 21.4, 21.0, 9.3.
MS (EI): m/z = 315 (100) [M + 1]. HRMS (ESI): m/z calcd for
1376, 1227, 1118, 1015, 759, 696, 686 cm–1 13C NMR (100 MHz,
.
CDCl3): δ =156.1, 152.1, 152.1, 150.7, 150.1, 147.8, 147.8, 145.5,
139.6, 135.6, 133.8, 130.1, 129.6, 129.6, 128.7, 128.7, 127.9,
127.9, 127.7, 127.5, 125.1, 123.4, 11.0. MS (EI): m/z = 363 [M + 1].
HRMS (ESI): m/z calcd for C23H17N5 [M + H]: 363.1484; found:
364.1571.
C
20H17N3O [M + H]: 316.1372; found: 316.1458.
(E)-7-(4-Chlorostyryl)-3-methyl-5-phenylisoxazolo[4,5-
d]pyrimidine (16h)
Pale yellow solid (0.07g, 35%); mp 89–93 °C. 1H NMR (400 MHz,
CDCl3): δ = 8.57 (m, 2 H), 8.30 (d, J = 20 Hz, 1 H), 7.68 (d, J = 8 Hz,
1 H), 7.53 (m, 4 H), 7.45 (m, 3 H), 2.75 (s, 3 H). IR (KBr): 3443,
3044, 2920, 2299, 1978, 1586, 1502, 1409, 1176, 1083, 964, 809,
(38) Preparation of 3-Methyl-5,7-diphenylisoxazolo[4,5-d]pyrim-
idine (16j)
To a stirred solution of compound 12a (0.2 g, 0.68 mmol) in
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

G
M. Sambaiah et al.
Letter
Synlett
705 cm–1
.
13C NMR (100 MHz, CDCl3): δ = 161.5, 161.5, 156.5,
H), 6.73 (m, 1 H), 6.65 (d, J = 4 Hz, 1 H), 3.95 (s, 3 H), 3.92 (s, 3
H), 2.75 (s, 3 H). IR (KBr): 3445, 2923, 2841, 1600, 1503, 1374,
150.1, 148.1, 147.1, 137.3, 136.0, 134.1, 130.4, 129.2, 129.1,
129.1, 129.1, 128.6, 128.3, 128.3, 128.3, 123.1, 9.26. MS (EI):
m/z = 347 (100) [M + 1]. HRMS (ESI): m/z calcd for C20H14ClN3O
[M + H]: 347.0825; found: 348.0908.
1283, 1147, 1025, 922, 821, 770, 696 cm–1 13C NMR (100 MHz,
.
CDCl3): δ =163.8, 161.0, 161.0, 159.8, 156.2, 151.1, 147.8, 147.5,
137.6, 133.0, 130.2, 130.2, 128.5, 128.3, 128.3, 105.9, 98.7, 55.8,
55.6, 9.3. MS (EI): m/z 347 (100) [M + 1]. HRMS (ESI): m/z calcd
for C20H17N3O3 [M + H]: 347.1270; found: 348.1352.
7-(2,4-Dimethoxyphenyl)-3-methyl-5-phenylisoxazolo[4,5-
d]pyrimidine (16i)
Pale brown solid (0.05 g, 34%); mp 170–174 °C. 1H NMR (400
MHz, CDCl3): δ = 8.60 (m, 2 H), 7.92 (d, J = 8 Hz, 1 H), 7.51 (m, 3
(39) See Supporting Information for biological activity graphs, inter-
mediates preparation, and spectroscopic data.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G