Synthesis of novel 2,3-substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-diones

Article abstract of DOI:10.1016/j.tetlet.2009.08.098

Novel 2,3-substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-diones were successfully synthesized with moderate to good yields using a new synthetic approach. The structures of the regio-isomers in this series were determined by single crystal X-ray an

Full text of DOI:10.1016/j.tetlet.2009.08.098

Tetrahedron Letters 50 (2009) 6223–6227
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of novel 2,3-substituted-2,4-dihydro-pyrazolo[4,3-d]-
pyrimidine-5,7-diones
*
Thomas Brady, Khang Vu, Jack R. Barber, Shi Chung Ng, Yuefen Zhou
CytRx Corporation, 3030 Bunker Hill Street, Suite 101, San Diego, CA 92109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 May 2009
Revised 27 August 2009
Accepted 27 August 2009
Available online 2 September 2009
Novel 2,3-substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-diones were successfully synthesized
with moderate to good yields using a new synthetic approach. The structures of the regio-isomers in this
series were determined by single crystal X-ray analysis and NMR spectra.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
2,3-Substituted-2,4-dihydro-pyrazolo-
[4,3-d]pyrimidine-5,7-diones
2,4-Dihydro-pyrazolo[4,3-d]pyrimidine-
5,7-dione derivatives
1,4-Dihydro-pyrazolo[4,3-d]pyrimidine-5,7-
dione derivatives
Assignment of regio-isomer structures
Single crystal X-ray structure analysis
Pyrazolo[4,3,d]pyrimidine-5,7-dione derivatives constitute an
important class of pharmacologically active compounds with
known activities as factor Xa inhibitors for treatment of thrombo-
embolic disorders,1 with desirable affinity at A1 adenosine recep-
tors for the treatment of bronchoconstriction and cardiac
insufficiency,2 and as a potential purine antagonist for chemother-
apy.3 Therefore, synthesis of novel pyrazolo[4,3-d]pyrimidine-5,7-
dione derivatives is of interest to scientists working in the drug
discovery research field.
The majority of the pyrazolo[4,3-d]pyrimidine-5,7-dione deriv-
atives that have been synthesized or studied so far are either those
without substitution at C-3 (1 and 2 where R1 = H)4–7 or those with
substitution at N-1 but not at N-2 (2) (Fig. 1)1 or no substitution at
either N-1 or N-2.8 There are only few examples reported in the lit-
erature for the analogs 1 which have substitutions at both N-2 and
C-3 (e.g., 2-methyloxoformycin B) as shown in Figure 1.9
In the course of our research to develop novel small molecule
chaperone amplifiers aiming to treat neurodegenerative diseases,
we are highly interested in the design and synthesis of novel
analogs of 2,3-substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimi-
dine-5,7-diones (1) for biological testing.
shown in Schemes 1–3.1,4–9 In the method shown in Scheme 1,
the synthesis of the pyrazolo[4,3-d]pyrimidine-5,7-diones (V and
VII) were accomplished by diazotization of 5-amino-6-methyl-
1H-pyrimidine-2,4-dione (Ia and Ib) followed by the cyclization
and subsequent methylation to give the N-1 alkylated compound
as the major product (V).4–6 The two isomers of V and VII were
separated by flash chromatography purification on silica gel
(V:VII = 71%:29%).
The method described in Scheme 2 applied 4-amino-5-carbox-
ylate ester pyrazole (VIII) as starting material. The synthesis
involved converting the ortho-aminoester pyrazole (VIII) to the
corresponding ortho-amino-amide (IX) via a 4-step synthesis
followed by the cyclization via the treatment of carbonyldiimidaz-
ole or other phosgene equivalent reagents.1 This method generates
O
N
R2
N
NH
O
O
7
H3C N
O
R3
O
1
R3
6
N
N
N
H
OH
O
N
2
R2
N
N
5
O
4
N
R4
Previously reported literature methods for the synthesis of this
class of pyrazolo[4,3-d]pyrimidine-5,7-dione compounds are
N
R1
3
R1
R4
OH
OH
2
1
2-Methyloxoformycin B
* Corresponding author. Tel.: +1 858 273 2501x221; fax: +1 858 273 2697.
E-mail addresses: yuefen.zhou@gmail.com, yzhou@cytrx.com (Y. Zhou).
Figure 1. 2,4- and 1,4-Dihydro-pyrazolo[4,3-d]pyrimidine -5,7-dione derivatives.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.098

6224
T. Brady et al. / Tetrahedron Letters 50 (2009) 6223–6227
O
NH2
N
NH2
N
H
H
N
H3C
O
N
N
N
N
N
O
H3C N
O
Na / EtOH
CH3I
N
N
N
H
CH3
Me2SO4
OH
OH
IV
NaOH
OH
Formycin
OH
R = Me
OH
OH
O
CH3
N
N
Cl
N
2-Methylformycin
O
O
N+
H3C
O
R
N
N
N
NH
H3C N
N
1. Br2 / H2O
O
O
N
R
CH3
H
N
H
O
CH3
R
O
NH2
CH3
NaNO2
III
N
O
V
2. NOCl, DMF
OH
+
+
N
R
O
O
OH
R
N
OH
2-Methyloxoformycin B
H3C
N
N
N
N
N+
Ia, R = CH3
N
CH3
O
N
R
Ib, R = H
O
N
O
H
Scheme 3. Synthesis of 2-methyloxoformycin B.
CH3
IIa, R = CH3
IIb, R = H
VII
SnCl2
R = H
we designed a new and general synthetic approach to make C-3
and N-2 substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-
diones as shown in Scheme 4.
Me2SO4
O
H
With this design, we started with ketones 3 which were easily
transferred to the intermediates 4 by the treatment of diethyl
oxalate in the presence of a base. The intermediates 4 were then
reacted with N2O3 gas generated in situ to form oximine deriva-
tives 5 according to literature procedures.10,11 The oximine deriv-
atives were then treated with substituted hydrazine at 0 °C to
generate the key intermediates of ortho-nitroso-esters 6 and 7
as regio-isomers that were reduced by sodium dithionite to their
corresponding ortho-amino-esters 8 and 9 according to the liter-
ature method.12 The isomers of 8 and 9 were easily separated
by flash column chromatography purification on silica gel. The
key in this method was the ratio of the regio-isomers of 8 and
9. Only when isomers 8 were the major products, the synthesis
of the N-2 substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimi-
dine-5,7-diones would be practical. It turned out that intermedi-
ates 8 were always the major components (67–100%) based on
their isolated yields.
N
HN
O
N
N
H
H
VI
Scheme 1. Synthesis of 1,4,6-trimethyl-1,4-dihydro-pyrazolo[4,3-d]pyrimidine-
5,7-dione (V) and 2,4,6-trimethyl-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-dione
(VII).
1) N-protection
2) Ester deprotection
3) H2N-R''', coupling agent
4) N-deprotection
NH2
H
NH2
R'
N R'''
R'
N
CO2R
N
R''
O
N
N
R''
VIII
IX
According to the mechanism proposed by Corey and co-work-
ers,13 when a diketo oximine reacts with an un-substituted hydra-
zine, one of the hydrazine N-atoms first undergoes nucleophilic
attack at one of the more reactive ketones. In the case of alkyl
mono-substituted hydrazine like ours (R2-NHNH2), the N-atom
directly connected to the R2 alkyl group should be more nucleo-
philic. Meantime the carbonyl carbon directly connected to R1
group in the diketo oximine 5 is likely more reactive (because it
is more positively charged) than the other carbonyl carbon directly
connected to the ester group according to the extended Hückel
charge calculation using Chem3D Pro 11.0. Therefore when the
diketo oximine 5 reacts with R2-NHNH2, the N-atom adjacent to
R2 alkyl group will likely first react with the ketone adjacent to
R1 to form 6 as a major product, which could explain why com-
pound 8 was the major component.
H
O
R'
N
CO(imid)2
N
R'''
N
or phosgene
equivalent
N
O
R''
X
Scheme 2. Synthesis of N-1 and C-3 substituted-1,4-dihydro-pyrazolo[4,3-d]pyrim-
idine-5,7-dione derivatives.
exclusively the N-1 substituted-1,4-dihydro-pyrazolo[4,3-d]pyrimi-
dine-5,7-dione derivatives, not the N-2 substituted analogs that
we are particularly interested in.
The method shown in Scheme 3 describes the preparation of
both the C-3 and N-2 substituted-2,4-dihydro-pyrazolo[4,3-
d]pyrimidine-5,7-diones,9 which was our desired substitution
pattern for the analogs that we wished to make and submit for
biological testing. In this method, 2-methyloxoformycin B was pre-
pared from the methylation of formycin followed by oxidation and
then deamination with nitrosyl chloride. However, this method
does not provide a general application to meet our needs since
we are not interested in adding the sugar moiety at C-3. Therefore
The above pure 4-amino-5-carboxylated ester pyrazoles (8)
were then reacted with isocyanate to form ureas (10), which were
subsequently cyclized in the presence of KOtBu at room tempera-
ture to construct the pyrimidine rings to form compounds 12.
Finally, alkylation occurring at N-4 furnished the synthesis giving
the desired products of N-2 and C-3 substituted-2,4-dihydro-pyr-
azolo[4,3-d]pyrimidine-5,7-diones (1) in good yields. Some of the
minor products of N-1 and C-3 substituted-1,4-dihydro-pyrazol-
o[4,3-d]pyrimidine-5,7-diones (13 and 2) were also made follow-

T. Brady et al. / Tetrahedron Letters 50 (2009) 6223–6227
6225
H
O
O
4.60
N
O
O
O
O
a
OEt
N
O
R1
R2
N
+
N
R1
N
EtO
N
4.19
O
H
N
O
O
N
H
3
N
H
O
4
O
N
Cl
12b
O
Cl
O
O
R1
NO
6
13a
O
b
c
R1
+
R2
N
Figure 2. The chemical shift difference between the CH2 adjacent to N-1 (13a) and
adjacent to N-2 (12b).
N
O
5
OH
O
N
O
In order to further confirm the absolute structure of the regio-
isomers of 12 and 13, compound 12a (Fig. 3)15 was re-crystallized
in EtOAc/hexanes in a dilute solution to get single crystals. The
single crystal thus formed was evaluated by X-ray analysis to
determine the absolute structure of 12a, the major isomer
(Fig. 4).17 As shown in Figure 4, the major product 12a was clearly
the desired N-2 (not as labeled in Fig. 4, but as labeled in Fig. 1)
substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-dione. This
single crystal X-ray structure analysis result confirmed the struc-
ture assignment based on the 1H NMR spectra. Therefore, the
structures for the rest of the novel analogs 12 we made (Table 1)
were assigned based on their 1H NMR spectra.
R1
R2
R1
NO
7
O
O
N
R2
R1
N
N
O
N
O
NH
10
R3
8
NH2
O
d
e
O
+
R2
N
N
H
R2
N
O
N
O
In conclusion, we have designed a new synthetic approach and
used it successfully to synthesize a number of novel N-2 and C-3
substituted-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-diones in
fairly good yields. The structures of their regio-isomers were
N
R1
O
R1
NH2
9
NH
11
O
R3
O
N
H
O
O
N
R2
R1
N
R2
N
N
R3
N
N
N
R3
N
N
N
H
O
N
R1
N
H
12
O
R4
O
1
f
g
R2
N
R2
N
O
O
F
N
R3
F
N
12a
R3
F
N
N
R1
R1
13
N
Figure 3. 2-Butyl-6-ethyl-3-(4-trifluoromethyl-phenyl)-2,4-dihydro-pyrazolo[4,3-
d]pyrimidine-5,7-dione (12a).
N
O
H
R4
O
2
Scheme 4. Reagents and conditions: (a) NaOEt, EtOH, 0 °C, 4 h; (b) N2O3 gas, EtOH,
rt; (c) R2-NHNH2Áoxalate salt, EtOH/H2O (1:1), 0 °C, 40 min; (d) sodium dithionite,
H2O; (e) R3NCO, DCM, rt, o.n.; (f) KOtBu (2.0 M in THF), THF, rt, 30 min; (g) R4I,
Cs2CO3, DMF, 80 °C, 1 h.
ing the same synthetic sequence for the comparison of NMR spec-
tra to help assignment of the regio-isomers.
The assignment of the regio-isomers of 12 and 13:
The structures of the N-1 and N-2 substituted isomers (V and
VII, Scheme 1) were assigned by Dodson’s group based on the com-
parison of their infrared spectra, ultraviolet spectra, and NMR spec-
tra. They assigned the compounds with the N-CH3 absorption at
lower field in the NMR spectrum as the N-1 substituted analogs
(e.g., V). In order to determine the structures of the isomers (12
and 13) we made, we specifically prepared the minor isomer 13a
to compare with the major isomer 12b in their 1H NMR spectra
(Fig. 2).14 Consistent with Dodson’s result, the minor isomer 13a
had the chemical shift of 4.60 ppm (lower field) for the CH2 adja-
cent to N-1 in its 1H NMR spectrum; while the major isomer of
12b had the chemical shift of 4.19 ppm (higher field) for the CH2
adjacent to N-2.
Figure 4. Single crystal structure of 12a determined by X-ray analysis.

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T. Brady et al. / Tetrahedron Letters 50 (2009) 6223–6227
Table 1
Synthesis of the C-3 substituted-2,4- and 1,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-dione derivatives (1, 12, 2 and 13)
R2
N
O
O
R3
O
R3
O
N
N
N
R2
N
N
N
R4
N
R4
R1
R1
13: R4 = H
12: R4 = H
2: R4 = not H
R4
1:
= not H
Compd
R1
R2
R3
R4
Substitution position
Yielda (%)
12a
1a
1b
12b
1c
13a
2a
12c
1d
12d
1e
12e
1f
4-CF3-Ph
4-CF3-Ph
4-CF3-Ph
3-Cl-Ph
3-Cl-Ph
3-Cl-Ph
3-Cl-Ph
3,4-Di-Cl-Ph
3,4-Di-Cl-Ph
Ph
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
Me
Et
Et
H
Me
H
H
Me
H
Me
H
Me
H
Me
H
Me
N-2
N-2
N-2
N-2
N-2
N-1
N-1
N-2
N-2
N-2
N-2
N-2
N-2
54
85
71
54
86
50
86
24
81
66
88
73
50
(CH3)2CHCH2
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Me
n-Bu
n-Bu
n-Bu
n-Bu
Ph
Me
Me
12f
n-Bu
Et
H
N-2
57
a
The isolated yield for the last two steps (steps e and f) for 12 or 13, and for the last step (step g) for 1 or 2.
determined by single crystal X-ray structure analysis and 1H NMR
spectra. We believe our synthetic method has general application
based on the readily available starting materials and reasonably
good yields. More importantly, our method provides future access
to this class of novel N-2 and C-3 substituted-2,4-dihydro-pyrazol-
o[4,3-d]pyrimidine-5,7-diones for potential pharmaceutical
applications.
J = 6 Hz), 1.16 (t, 3H, J = 5 .6 Hz), 1.25 (m, 2H, J = 5 Hz), 1.82 (m, 2H, J = 6 Hz),
3.99 (q, 2H, J = 5.6 Hz), 4.19 (t, 2H, J = 5.6 Hz), 7.28–7.26 (m, 1H), 7.38 (s, 1H),
7.51–7.49 (m, 2H), 9.54 (br s, 1H).
The minor isomer: 1-butyl-3-(3-chloro-phenyl)-6-ethyl-1,4-dihydro-pyrazolo-
[4,3-d]pyrimidine-5,7-dione (13a): 1H NMR (CDCl3, 400 MHz): d 0.96 (t, 3H,
J = 6 Hz), 1.23 (t, 3H, J = 5.6 Hz), 1.38 (m, 2H, J = 6 Hz), 1.90 (m, 2H, J = 6 Hz),
4.09 (q, 2H, J = 5.6 Hz), 4.60 (t, 2H, J = 6 Hz), 7.42–7.36 (m, 2H), 7.63 (dd, 1H,
J = 6, 1.2 Hz), 7.75 (m, 1H), 10.11 (br s, 1H).
15. The typical procedure for the synthesis of compound 12a and 1a (R1 = 4-CF3-Ph,
R
2 = n-Bu, R3 = Et, R4 = H (12a); R4 = Me (1a)): Sodium ethoxide solution (21% by
weight, 20 mL, 54 mmol) was added to a stirring solution of diethyl oxalate
(9 g, 61 mmol) in ethanol (70 mL) cooled to 0 °C by an ice bath. The solution
was stirred for 20 min at 0 °C and then 1-(4-trifluoromethyl-phenyl)-ethanone
(3a) (5 g, 27 mmol) was added drop wise over 5 min. The reaction was stirred
for 4 hours and then concentrated on a rotary evaporator. HCl (1.0 N, 110 mL)
was added to the residue and the mixture was extracted into ethyl acetate
(3 Â 60 mL). The combined organic extracts were dried over sodium sulfate,
filtered, and concentrated on a rotary evaporator. The crude oil was purified by
silica gel chromatography to afford 2,4-dioxo-4-(4-trifluoromethyl-phenyl)-
butyric acid ethyl ester (4a) (3 g, 38%) as an orange solid. 1H NMR (CDCl3,
400 MHz): d 1.41 (t, 3H, J = 5.6 Hz), 4.40 (q, 2H, J = 5.6 Hz), 7.07 (s, 1H, enolate
C@CH), 7.75 (d, 2H, J = 6.8 Hz), 8.08 (d, 2H, J = 6.4 Hz), 15.2 (br s, 1H, enolate
OH).
Acknowledgments
The authors would like to thank Professor Arnold L. Rheingold
and Dr. Curtis Moore at University of California, San Diego for their
single crystal X-ray structure analysis.
References and notes
1. Pinto, D. J. P.; Quan, M. L.; Woerner, F. J.; Li, R., International Patent WO
02000655 A1, 2002.
2. Hamilton, H. W. U.S. Patent 4,663,326, May 5, 1987.
Into a solution of 2,4-dioxo-4-(4-trifluoromethyl-phenyl)-butyric acid ethyl
ester (4a) (4 g, 15.6 mmol) in ethanol (150 mL), N2O3 gas16 was bubbled in
at ambient temperature until the starting material was completely
consumed (by LC/MS). The solvent was removed and the residue was
taken up in ethyl acetate (100 mL) and washed with water (2 Â 50 mL). The
3. Robins, R. K. J. Am. Chem. Soc. 1956, 78, 784.
4. Papesch, V.; Dodson, R. M. J. Org. Chem. 1965, 30, 199.
5. Robins, R. K.; Furcht, F. W.; Drauer, A. D.; Jones, J. W. J. Am. Chem. Soc. 1956, 78,
2418.
organic phase was dried over sodium sulfate, filtered, concentrated on
a
6. Papesch, V.; Dodson, R. M. J. Org. Chem. 1963, 28, 1329.
7. Gilbert, A. M.; Caltabiano, S.; Koehn, F. E.; Chen, Z.-J.; Francisco, G. D.; Ellingboe,
J. W.; Kharode, Y.; Mangine, A.; Francis, R.; TrailSmith, M.; Gralnick, D. J. Med.
Chem. 2002, 45, 2342.
rotary evaporator, and the residue was purified by silica gel chromatography
(0–100% ethyl acetate in hexanes) to afford 3 g of a clear viscous oil (5a)
which was directly taken up in ethanol (150 mL) and cooled to 0 °C. n-Butyl
hydrazine oxalate (1.8 g, 10 mmol) dissolved in a mixture of ethanol/H2O
(1:1) (20 mL) was added drop-wise to the above cooled stirring solution.
After 40 min the solution had turned an intense electric blue color to form
the cyclized pyrazole nitroso moieties (6a and 7a). Sodium dithionite
(saturated in H2O) was then added until the color faded, upon which time
LC–MS analysis showed complete conversion to the corresponding amino-
pyrazole product. The solids were filtered off and the filtrate was
concentrated on a rotary evaporator and the residue was purified by silica
gel chromatography (0–100% ethyl acetate in hexanes) to afford the key
intermediate 4-amino-1-butyl-5-(4-trifluoromethyl-phenyl)-1H-pyrazole-3-
carboxylic acid ethyl ester (8a) (1.4 g, 22%, three steps) as a light yellow
oil and 0.7 g of the minor isomer 4-amino-2-butyl-5-(4-trifluoromethyl-
phenyl)-2H-pyrazole-3-carboxylic acid ethyl ester (9a) for a total yield of
37.8% for three steps (8a:9a = 67%:33%). LC–MS (ESI+) m/z = 356.1 [M+H]+.
The above 4-step synthesis was based on the literature procedures as
described in the above Refs. 10–12.
8. (a) Robins, R. K.; Holum, L. B.; Furcht, F. W. J. Org. Chem. 1956, 21, 833; (b) Long,
R. A.; Gerster, J. F.; Townsend, L. B. J. Heterocycl. Chem. 1970, 7, 863.
9. (a) Townsend, L. B. et al J. Org. Chem. 1974, 39, 2023; (b) Ugarker, B. G.;
Revankar, G. R.; Robins, R. K. J. Heterocycl. Chem. 1984, 21, 1865.
10. Albrecht, W.; Greim, C.; Striegel, H.-G.; Tollmann, K.; Merckle, P.; Laufer, S.
International Patent WO 2006/089798 A1, Aug. 31, 2006.
11. Takei, H.; Yasuda, N.; Takagaki, H. Bull. Chem. Soc. Jpn. 1979, 52, 208.
12. Yuan, J.; Gulianello, M.; Lombaert, S. D.; Brodbeck, R.; Kieltyka, A.; Hodgetts, K.
*
J. Bioorg. Med. Chem. Lett. 2002, 12, 2133. Note: This paper only briefly
mentioned reagents for the synthesis of the intermediates 8 and 9, and it did
not give the ratio of intermediates 8 to 9 or their characterization.
13. Majid, T.; Hopkins, C. R.; Pedgrift, B.; Collar, N. Tetrahedron Lett. 2004, 45, 2137.
14. Comparison of the 1H NMR data between 12b (major) and 13a (minor):
The major isomer: 2-butyl-3-(3-chloro-phenyl)-6-ethyl-2,4-dihydro-pyrazolo-
[4,3-d]pyrimidine-5,7-dione (12b): 1H NMR (CDCl3, 400 MHz): d 0.84 (t, 3H,

T. Brady et al. / Tetrahedron Letters 50 (2009) 6223–6227
6227
Ethyl isocyanate (106 mg, 1,5 mmol) was added to a solution of 4-amino-1-
butyl-5-(4-trifluoromethyl-phenyl)-1H-pyrazole-3-carboxylic acid ethyl ester
(8a) (300 mg, 0.84 mmol) in DCM (5 mL). The reaction was stirred at
ambient temperature for 18 h until complete conversion was confirmed by
LC–MS analysis ((ESI+) m/z = 427.0 [M+H]+) to afford the pyrazole urea
intermediate (10a). The solvent was removed under reduced pressure and
the residue was taken up in THF (10 mL), potassium tert-butoxide (2.0 M in
THF, 0.5 mL, 1 mmol) was added and the reaction was stirred for 30 min at
ambient temperature. The solution was then diluted with aqueous 1.0 N HCl
(100 mL) and extracted with ethyl acetate (3 Â 75 mL). The combined
organic extracts were dried over sodium sulfate, filtered, and concentrated.
The crude product was purified by silica gel chromatography (0–100% ethyl
acetate in hexanes) to afford 2-butyl-6-ethyl-3-(4-trifluoromethyl-phenyl)-
2,4-dihydro-pyrazolo*[4,3-d]pyrimidine-5,7-dione (12a) (175 mg, 54%) as
a white solid. 1H NMR (CDCl3, 400 MHz): d 0.83 (t, 3H, J = 6 Hz), 1.12 (t, 3H,
J = 5.6 Hz), 1.23 (m, 2H, J = 6 Hz), 1.82 (m, 2H, J = 6 Hz), 3.90 (q, 2H,
J = 5.6 Hz), 4.21 (t, 2H, J = 5.6 Hz), 7.56 (d, 2H, J = 6.8 Hz), 7.82 (d, 2H,
Methyl iodide (30 mg, 0.2 mmol) was added to a mixture of 2-butyl-6-ethyl-
3-(4-trifluoromethyl-phenyl)-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-5,7-dione
(12a) (50 mg, 0.13 mmol) and cesium carbonate (43 mg, 0.13 mmol) in DMF
(4 mL), and the reaction mixture was stirred at 80 °C for 1 h. The mixture
was diluted with aqueous 1.0 N HCl (50 mL) and extracted with ethyl
acetate (3 Â 20 mL). The combined organic extracts were dried over sodium
sulfate, filtered, concentrated, and applied to silica gel chromatography
purification (0–100% ethyl acetate in hexanes) to afford 2-butyl-6-ethyl-4-
methyl-3-(4-trifluoromethyl-phenyl)-2,4-dihydro-pyrazolo[4,3-d]pyrimidine-
5,7-dione (1a) (42 mg, 85%) as a clear oil. 1H NMR (CDCl3, 400 MHz): d 0.81
(t, 3H, J = 5.6 Hz), 1.19 (m, 2H, J = 6 Hz), 1.25 (t, 3H, J = 5.6 Hz), 1.75 (m, 2H,
J = 6 Hz), 3.01 (s, 3H), 3.97 (q, 2H, J = 5.6 Hz), 4.14 (t, 2H, J = 5.6 Hz), 7.54 (d,
2H, J = 6.4 Hz), 7.82 (d, 2H, J = 6.8 Hz). LC–MS (ESI+) m/z = 395.2 [M+H]+. The
other analogs in Table 1 were synthesized in similar way as that described
above for the synthesis of 12a and 1a.
16. N2O3 gas was generated by slowly adding concentrated HCl to aqueous slurry
of sodium nitrite in a 3-necked flask via an additional funnel. One of the necks
was covered with a stopper and the other one was directed, via Tygon tubing,
to a syringe immersed in the solvent in the reaction flask.
17. Crystallographic data (excluding structure factors) for the structures in this
Letter have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publications nos. The deposition number is CCDC 730905.
J = 6.4 Hz), 10.45 (br s, 1H); 13C NMR (CDCl3, 125 MHz):
d 13.29, 13.65,
19.87, 32.39, 36.22, 51.28, 122.83, 124.34, 124.99, 126.43, 126.63, 130.30,
130.73, 131.82 (1C, q, J = 32.9 Hz, CF3), 153.03, 157.56; LC–MS (ESI+) m/
z = 381.1 [M+H]+; Elemental Anal. Calcd for C18H19F3N4O2: C, 56.84; H, 5.03;
N, 14.73. Found: C, 56.89; H, 5.38; N, 15.07.