Synthesis and tautomerism study of 7-substituted pyrazolo[3,4-c]pyridines

Article abstract of DOI:10.1016/j.tet.2006.09.081

A number of 7-substituted pyrazolo[3,4-c]pyridine derivatives have been synthesized in order to investigate the N1-N2 tautomerism within this class of biologically interesting compounds. Tautomeric equilibrium has been studied using NMR ¹³C,

Full text of DOI:10.1016/j.tet.2006.09.081

Tetrahedron 62 (2006) 11987–11993
Synthesis and tautomerism study of 7-substituted
pyrazolo[3,4-c]pyridines
Vassilios N. Kourafalos,a Panagiotis Marakos,a Emmanuel Mikros,a,
*
b
c
Nicole Pouli,a, Jaromır Marek and Radek Marek
*
´
aDepartment of Pharmacy, Division of Pharmaceutical Chemistry, University of Athens, Panepistimiopolis, 15771 Zografou, Greece
bLaboratory of Functional Genomics and Proteomics, Faculty of Science, Masaryk University,
Kamenice 5/A2 Brno, CZ-62500, Czech Republic
cNational Center for Biomolecular Research, Faculty of Science, Masaryk University, Kamenice 5/A4 Brno, CZ-62500, Czech Republic
Received 20 July 2006; revised 1 September 2006; accepted 21 September 2006
Available online 20 October 2006
Abstract—A number of 7-substituted pyrazolo[3,4-c]pyridine derivatives have been synthesized in order to investigate the N1–N2 tauto-
merism within this class of biologically interesting compounds. Tautomeric equilibrium has been studied using NMR 13C, 15N chemical shifts
1
1
and heteronuclear H–15N and H–13C spin–spin couplings, in conjunction with X-ray crystallography. The N1 tautomer predominates in
DMF solution in all the compounds tested.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
antiviral, immunodepressant, antitumor, and antimetabolic
activities.12
Biomolecules are often characterized by the presence of
more than one basic atom and can exist in multiple tauto-
meric and protonated states, which should be considered in
the study of their mode of interaction with their substrates,
where hydrogen bonding between donor and acceptor motifs
are usually involved. Within this context, the structure and
the 7H–9H prototropic tautomeric equilibrium of isolated
nucleic acid bases and several purine derivatives have been
extensively investigated, both theoretically and experimen-
tally.1–5 Among a large number of purine-related hetero-
aromatic compounds that have also been studied, all five
isomers of pyrazolopyridines have received much attention,
as concerns their pharmacological activity. Some pyrazolo-
pyridines are well-known non-sedative anxiolytic agents,6
while some others can act as human enzyme inhibitors, for
example, a number of differently substituted pyrazolopyri-
dines have been recently shown to be potent inhibitors of
phosphodiesterases,7 matrix metalloproteinases,8 glycogen
synthase kinase-3,9 and of cyclin-dependent kinases.10
The presence of the pyrazole ring in the formycins results in
a N1–H/N2–H prototropic tautomerism and consequently at
physiological pH they exist as mixture of tautomers. Con-
cerning formycin A, it has been shown that in aqueous solu-
tion the N1–H tautomer is the predominant one (>94%),
although protonation at N-4 was accompanied with the
migration of the hydrogen from N-1 to N-2.13 A shift in
the tautomeric equilibrium for both formycins towards the
O
N
NH2
H
N
H
N
HN
HO
N
N
H
N
H
N
O
HO
O
HO
OH
HO
OH
Formycin A
Formycin B
O
H
N
NH2
H
We are involved in the synthesis and structure–activity rela-
tionship studies of some pyrazolo[3,4-c]pyridine nucleo-
sides (Fig. 1, I and II, respectively), which can be viewed
as singly modified (4-deaza) formycins.11 Formycin A and
formycin B (Fig. 1) are potent antibiotics with proven
N
N
HN
N
N
HO
HO
O
O
H
OH
H
OH
HO
HO
I
II
* Corresponding authors. Tel.: +30 2107274813; fax: +30 2107274747;
e-mail addresses: mikros@pharm.uoa.gr; pouli@pharm.uoa.gr
Figure 1. Structures of purine-like C-nucleosides.
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.09.081

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V. N. Kourafalos et al. / Tetrahedron 62 (2006) 11987–11993
H3
C
C
H3
H3CO
N
NH2
H3C(O)CHN
N
O
N
H
N
H
N
H
N
H
H
N
N
N
N
HN
N
N
N
N
N
1
2
3
4
5
H3CO
N
H3CO
H3CO
N
H3CO
N
COCH3
N
N
N
N
N
N
N
COCH3
N
N
6
7
8
9
Figure 2. Structures of the examined pyrazolo[3,4-c]pyridines.
minor N2–H form was also observed upon complex forma-
tion with purine nucleoside phosphorylase (PNP), suggest-
ing the preferential binding of a specific tautomer to this
biologically important enzyme.14
Compound 3 was prepared from 7-chloropyrazolo[3,4-c]-
pyridine (10)18b upon treatment with an ethanolic solution
of dimethylamine (Scheme 2).
CH3
H3C
1
Cl
N
During initial studies, a peak broadening in the H NMR
H
H
N
spectra of compounds I and II was observed, which was
attributed in the coexistence of tautomers, in accordance
with the findings reported for the formycins.15 In order to
further investigate this phenomenon, we have selected a
number of pyrazolo[3,4-c]pyridine derivatives (Fig. 2) and
we have decided to study their tautomerism by low-temper-
ature NMR spectroscopy.16
N
N
a
N
N
N
10
3
Scheme 2. Reagents and conditions: [a] (CH3)2NH/CH3CH2OH, reflux,
12 h, 75%.
Compounds 6 and 7 were prepared by direct acetylation of
4,18a whereas the benzyl substituted regio isomers 8 and 9,
resulted upon treatment of compound 4 with NaH, followed
by addition of benzyl bromide to the generated anion. The
substitution occurs readily at position 2, when the reaction
is carried out at room temperature and this is probably due
to the steric hindrance, exerted by the 7-substituent. On the
other hand, analogous treatment of a DMF solution of 4
with acetic anhydride or benzyl bromide at reflux provided
both 1- and 2-isomers, and from this mixture the 1-substituted
analogue was isolated by column chromatography.
Relative populations of individual tautomers are generally
influenced by temperature, solvent, and substitution pattern,
which modifies the electronic distribution within the mole-
cule. We have thus prepared compounds 1–5, which bear
7-substituents with various electronic and steric properties.
Moreover, since the 13C and 15N NMR chemical shifts
were utilized to specify the protonation site of purines16,17
we have considered the syntheses of N-1 and N-2 substituted
analogues 6–9 as models for the determination of the spec-
tral data. In addition to chemical shift analysis, hetero-
nuclear coupling constants were revealed to be a useful
tool for tautomeric equillibria studies, consequently all com-
pounds lack a 3-substituent, in order to utilize the JH3–Cx and
JH3–Ny couplings to accomplish the study.
During the preparation of the 1-benzyl isomer 8, a minor
amount of a third product, apart from the 2-regio isomer 9,
was also isolated. This derivative was identified as the 2,6-
dibenzylpyrazolo[3,4-c]pyridine-7-one (11, Fig. 3a), based
on NMR analysis. Furthermore, compound 11 furnished
crystals from methanol suitable for an X-ray diffraction
2. Results and discussion
2.1. Synthesis
O
The syntheses of compounds 1, 4, and 5 (Fig. 2) has already
been reported starting from 2-amino-3-nitro-4-picoline,
which was suitably substituted and ring-closed in the pres-
ence of nitrosyl chloride or isoamylnitrite.18 Compound 2
was prepared by acetylation of 1 and treatment of the result-
ing mixture of regio-isomers with methanolic ammonia
(Scheme 1).
N
N
N
a
11
NH2
H3C(O)CHN
H3C(O)CHN
N
H
H
N
N
N
COCH3
N
b
N
a
N
N
N
b
1
2
Scheme 1. Reagents and conditions: [a] acetic anhydride, rt, 12 h; [b] NH3/
CH3OH, rt, 1 h, 92% (overall).
Figure 3. (a) Structure of compound 11 and (b) perspective view of the
X-ray structure of compound 11.

V. N. Kourafalos et al. / Tetrahedron 62 (2006) 11987–11993
11989
analysis, which provided the unambiguous assigment of its
structure (Fig. 3b).
The assignment of 13C and 15N spectra was performed using
the corresponding 2D gHMBC and gHSQC23 spectra and
chemical shifts are summarized in Table 1.
Taking into account the regioselective formation of the N-2
substituted isomers, we have also prepared the 3-benzyl
analogue 13 (Scheme 3), in order to study the influence of
a bulky 3-substituent in the N-alkylation. Thus, reaction
of 3-benzyl-7-methoxypyrazolo[3,4-c]pyridine (12)19 with
benzyl bromide in the presence of sodium hydride, provided
exclusively the 1-benzyl analogue 13. This regio-isomer was
identified by the use of the corresponding HMBC spectrum,
where a cross-peak between the NCH2Ph and C-7a is obvi-
ous and additionally, this methylene presented NOE with the
7-methoxy group.
Heteronuclear coupling constants were determined using
the GSQMBC spectra24 from the anti-phase doublets of the
corresponding cross-peaks and the most important are
summarized in Table 2.
The spectroscopic data concerning the pair of compounds 6–
7 and 8–9 show that C-7a, C-3a, C-3, N-1, and N-2 chemical
shifts and the JH–C and JH–N coupling constants can be a use-
ful tool for the determination of the substitution site and/or
the tautomeric form, as previously seen in the case of formy-
cin and purine derivatives.16,25 When N-1 is substituted, N-2
is deshielded (at w326 ppm), as well as C-3 and C-3a
(w135 ppm). When N-2 is substituted, N1 and C-7a are
deshielded (atw291 ppm and 138 ppm, respectively). More-
over, indirect spin–spin coupling constants of H-3 with N-1,
N-2, and C-7a exhibit characteristic differences between
N-1 and N-2 substitution and can be additionally used for
tautomer discrimination.
H3CO
N
H3CO
N
H
N
N
a
N
N
12
13
Scheme 3. Reagents and conditions: [a] NaH, benzyl chloride, DMF, rt, 1 h,
68%.
Considering the above results the qualitative inspection of
the spectroscopic data concerning compounds 1–5 and
mainly N-1 and N-2 chemical shifts, suggests that in all cases
the N1–H tautomer should be predominant in DMF solution.
2.2. NMR spectroscopy
Low-temperature NMR represents an important tool for ex-
amining tautomeric equilibrium.16 Fast chemical exchange
among the individual components usually occurs at labora-
tory temperatures. The resulting NMR spectra correspond
to a time-averaged contribution, which reflects Boltzmann
populations of the individual tautomers.20 Decreasing the
temperature slows down the chemical exchange process.
At low temperatures, separated signals of individual tauto-
mers could be detected, however, this methodology has
rarely been used in the field of purine chemistry since ex-
change studies of purine derivatives have been performed
primarily in water.21 Separation of the NMR signals of indi-
vidual tautomers of purine derivatives at low temperatures
has been recently described in DMF and DMSO/acetonitrile
solutions.16,22
In order to study the tautomeric equilibrium concerning
compounds 1–5, variable temperature studies have been per-
formed, in a temperature range of 213–303 K. In the case of
compound 5, on lowering the temperature to 213 K, a second
set of signals appeared in the 1H NMR spectrum with rela-
tive intensities 100 and 7, which corresponds to the ratio
of major and minor components 93.5:6.5.
1
1
2D H–13C and H–15N gHSQC, gHMBC, and GSQMBC
spectra recorded at this temperature, allowed for the com-
plete resonance assignment and determination of coupling
1
constants. H–15N GSQMBC spectrum of compound 5 is
shown in Figure 4. All characteristic NMR spectroscopic
data are summarized in Table 3, along with the correspond-
ing data for compounds 6 and 7, in order to facilitate the
comparison. All data suggest that the major component cor-
responds to the N1–H tautomer, while the minor component
corresponds to N2–H.
The 1H NMR spectra of compounds 1–9 in DMF solution are
characterized by four main signals: the AX system of H-4 and
H-5 at 7.11–7.57 ppm and 7.50–7.93 ppm, respectively, a
singlet at 8.20–8.65 ppm attributed to H-3 and a broad signal
at w14 ppm attributed to the proton suspected for tauto-
merism exchanging between N-1 and N-2. In all cases, except
pyrazolopyridinone 5 and 7-dimethylamino derivative 3, the
H-3, H-4, and H-5 signals were sharp at room temperature.
1
No significant changes in the H NMR patterns were ob-
served on decreasing the temperature in all other derivatives
1–4. One set of signals was detected, even if signals were
Table 1. 13C and 15N NMR chemical shifts (ppm) for compounds 1–9 and 11 in DMF-d7 at 303 K
Compounds
C3
C3a
C4
C5
C7
C7a
N1
N2
N6
1
2
4
5
6
7
8
9
11
133.65
134.30
134.68
134.0
139.71
123.46
133.58
125.49
125.74
127.54
130.42
129.79
125.5
135.46
126.73
130.34
127.25
124.13
104.79
112.64
109.82
98.6
109.87
109.78
109.64
109.42
99.25
138.25
137.54
136.49
126.1
141.30
137.94
135.98
136.08
130.71
147.73
139.03
151.66
155.1
152.51
157.93
150.89
156.65
157.90
128.77
130.16
128.23
133.1
125.52
140.44
126.43
137.97
142.29
190.5
188.0
184.4
not obsd
222.5a
291.5a
191.1
292.2
307.1
309.6
322.1
322.1
not obsd
325.7a
252.5a
327.4
233.9
230.5
248.9
276.6
252.7
155.2
263.1a
252.3a
252.1
247.9
160.5
a
Chemical shifts determined at 273 K.

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V. N. Kourafalos et al. / Tetrahedron 62 (2006) 11987–11993
Table 2. Characteristic JH–C and JH–N scalar coupling constants (Hz) of
compounds 1–9 and 11 in DMF-d7 at 303 K
Table 3. Characteristic 15N and 13C chemical shifts (d in ppm) and indirect
spin–spin coupling constants (J in Hz) of tautomers 5 major and 5 minor
in DMF-d7 at 213 K along with the corresponding spectroscopic data for
derivatives 6 and 7
1JH3–C3
2JH3–C3a
3JH3–C7a
3JH3–N1
2JH3–N2
1
2
4
5
6
7
8
9
11
188.9
190.2
190.2
189.0
196.6
198.8
192.1
194.2
192.9
11.3
11.7
11.5
10.7
11.1
8.2
11.0
8.8
8.5
4.4
3.9
4.5
3.7
3.2
6.8
3.7
7.6
7.2
7.2
7.6
12.3
12.9
6
7
5 major
5 minor
8.2
13.2
Chemical shifts
222.5
325.7
139.32
134.97
124.92
a
a
N1
N2
C3
C3a
C7a
291.5
252.5
124.34
123.67
142.29
196.7
319.1
134.98
126.06
132.82
298.7
221.7
124.34
123.67
142.29
8.0
2.3
7.5
0.9
—
14.0
4.4
12.9
4.5
4.7
Coupling constants
a
a
JH3–N1
JH3–N2
JH3–C3a
JH3–C7a
8.0
14.0
11.1
3.2
2.3
4.4
8.2
6.8
6.9
13.2
11.0
5.5
Not observed.
6.4
8.2
6.4
sharpened. In the case of compound 3 the two N-7 methyl
group signals were clearly observed at 213 K, while a broad
signal exists at room temperature (thus not considered in
the tables), showing that an additional conformational equi-
librium exists around the C7–N7 bond, due to the steric
hindrance imposed by the bulk of the two methyl groups.
a
Not observed.
experimental 15N chemical shifts existing for the acetyl-
and benzyl- derivatives 6–9 permit the theoretical calcula-
tion of the population ratio of the two tautomers at room
temperature, according to equations similar to those
described for the purine tautomerism.5 The ratio was calcu-
lated to be 97/3 and, as the reliability of the approaches for
determining the tautomeric ratios using averaged chemical
shifts is approximately 10% and substitution at N-1 and
N-2 is not the same as in compounds 6–9 and 4, we conclude
that the N2–H tautomer population is very low and undetect-
able by NMR at low temperature.
As mentioned above, from a qualitative point of view, the
spectroscopic data for compounds 1–4 are in good agree-
ment with the N1–H tautomer predominance. Simple AM1
semi-empirical calculations showed that the energy differ-
ence between N1–H and N2–H tautomers is w3 kcal/mol
in the case of the pyridinone 5, while this difference is
>5 kcal/mol for all other derivatives. These theoretical cal-
culations are in agreement with the experimental observa-
tions described above, predicting that the population of
N2–H tautomer should be undetectable by NMR. The calcu-
lated energy differences present an analogy to those per-
formed on adenine and guanine26 predicting that in both
cases the N9–H was the most stable tautomer, followed
by N7–H, however, concerning adenine, the N7–H was
considerably less stable. In the case of derivative 4, the
7-Methoxy-1H-pyrazolo[3,4-c]pyridine (4) furnished crys-
tals from methanol suitable for a single-crystal X-ray ana-
lysis, which provided the unambiguous assignment of its
structure (Fig. 5). The crystallographic analysis reveals
that, in the solid state the proton is also localized at the nitro-
gen atom N1, confirming the NMR analysis.
DMF
H3
N-H
H6
ppm
160
180
200
220
240
260
280
300
320
N6/H6
N1/H3
N1/H1
N2/H2(minor)
DMF
N2/H1
N2/H3
15
14
13
12
11
10
9
8
7
ppm
Figure 4. 1H–15N GSQMBC spectrum of compound 5 in DMF-d7 at 213 K.

V. N. Kourafalos et al. / Tetrahedron 62 (2006) 11987–11993
11991
residue was purified by column chromatography (silica gel)
using a mixture of cyclohexane/ethylacetate 20/80 (v/v) as
the eluent to give 2 (240 mg, 92%) as white crystals. Mp
1
263 ꢁC (CH2Cl2). H NMR (DMF-d7, 300 MHz) d 2.32 (s,
3H, CH3), 7.57 (d, 1H, H-4, J4–5¼5.38 Hz), 7.93 (d, 1H,
H-5, J5–4¼5.38 Hz), 8.22 (s, 1H, H-3). 13C NMR (DMF-d7,
50 MHz) d 24.02 (CH3), 112.64 (C-4), 130.16 (C-7a),
130.42 (C-3a), 134.30 (C-3), 137.54 (C-5), 139.03 (C-7),
171.10 (CO). Anal. Calcd for C8H8N4O: C, 54.54; H, 4.58;
N, 31.80. Found: C, 54.62; H, 4.43; N, 31.98.
4.1.2. 7-Dimethylamino-1H-pyrazolo[3,4-c]pyridine (3).
7-Chloro-1H-pyrazolo[3,4-c]pyridine18b
(10,
150 mg,
0.98 mmol) was added to a saturated solution of dimethyl-
amine in ethanol (15 ml) and the resulting solution was
refluxed for 12 h. The solvent was evaporated and the residue
was purified by column chromatography (silica gel) using
a mixture of methylenechloride/methanol 90/10 (v/v) as the
eluent, to give 3 (120 mg, 75%) as white crystals. Mp
Figure 5. Perspective view of the X-ray structure of compound 4.
1
260 ꢁC (dec) (Et2O/MeOH). H NMR (DMF-d7, 300 MHz)
d 3.65 [br s, 6H, (CH3)2N], 7.11 (br d, 1H, H-4), 7.50 (br,
1H, H-5), 8.46 (br s, 1H, H-3). Anal. Calcd for C8H10N4: C,
59.24; H, 6.21; N, 34.54. Found: C, 59.51; H, 6.16; N, 34.32.
3. Conclusions
In conclusion, we have shown that the N1–H tautomer of the
7-substituted pyrazolo[3,4-c]pyridines is predominant in
solution. Only in the case of the pyrazolopyridinone 5 could
the N2–H tautomer be clearly detected at appropriate
NMR spectra, probably due to the rather small energy differ-
ence of the N1–H and N2–H pair of tautomers, which could
be attributed to the relatively reduced aromatic character of
the pyridine ring in both tautomers. On the other hand, the
reduced aromaticity of the pyridine ring of the N2–H tauto-
mer of compounds 1–4, when compared to the correspond-
ing N1–H tautomer, results in a high energy difference
between them and this is in favor of the existence of the
N1–H species. N-substitution of the 7-substituted pyra-
zolo[3,4-c]pyridine ring system occurs predominantly at
the less hindered N-2. However, the presence of a 3-bulky
substituent favors the regioselective formation of the N-1
isomer.
4.1.3. 1-Benzyl-7-methoxypyrazolo[3,4-c]pyridine (8). 7-
Methoxy-1H-pyrazolo[3,4-c]pyridine18a (50 mg, 0.34 mmol)
was dissolved in anhydrous DMF (7 ml) under argon. A sus-
pension of NaH (60% in paraffin oil, 20 mg, 0.51 mmol),
which had previously been washed with pentane, was added
to the solution, under cooling and the mixture was stirred at
room temperature for 30 min. Then, a solution of benzyl bro-
mide (0.05 ml, 0.37 mmol) in anhydrous DMF (1 ml) was
added and the reaction mixture was heated at reflux for 1 h.
The solvent was vacuum evaporated and the product was pu-
rified by column chromatography (silica gel) using a mixture
of cyclohexane/ethylacetate 80/20 (v/v) as the eluent, to give
compounds 8, 9, and 11.
4.1.4. Data for 1-benzyl-7-methoxypyrazolo[3,4-c]pyr-
idine (8). Yield: 28%. Mp 147 ꢁC (white crystals).
1H NMR (DMF-d7, 300 MHz) d 4.13 (s, 3H, CH3), 5.85
(s, 2H, CH2), 7.26–7.36 (m, 5H, Ph), 7.37 (d, 1H, H-4,
J4–5¼5.85 Hz), 7.75 (d, 1H, H-5, J5–4¼5.85 Hz), 8.20 (s,
1H, H-3). 13C NMR (DMF-d7, 50 MHz) d 53.30 (CH3),
54.73 (CH2), 109.64 (C-4), 126.43 (C-7a), 127.79
[CH(Ph)], 128.04 [CH(Ph)], 128.82 [CH(Ph)], 130.34
(C-3a), 133.58 (C-3), 135.98 (C-5), 138.29 [C(Ph)],
150.89 (C-7). Anal. Calcd for C14H13N3O: C, 70.28; H,
5.48; N, 17.56. Found: C, 70.35; H, 5.62; N, 17.38.
4. Experimental
4.1. Chemistry
All chemicals were purchased from Aldrich Chemical Co.
€
Melting points were determined on a Buchi apparatus and
are uncorrected. Flash chromatography was performed on
Merck silica gel 60 (0.040–0.063 mm). Analytical thin layer
chromatography (TLC) was carried out on precoated
(0.25 mm) Merck silica gel F-254 plates. Elemental analyses
were performed on a Perkin–Elmer PE 240C Elemental
Analyzer (Norwalk, CT, USA) and were within ꢀ0.4% of
the theoretical values.
4.1.5. Data for 2-benzyl-7-methoxypyrazolo[3,4-c]pyr-
idine (9). Yield: 32%. Oil. H NMR (DMF-d7, 300 MHz)
1
d 4.06 (s, 3H, CH3), 5.80 (s, 2H, CH2), 7.25 (d, 1H, H-4,
J4–5¼6.06 Hz), 7.36–7.49 (m, 5H, Ph), 7.62 (d, 1H, H-5,
J5–4¼6.06 Hz), 8.65 (s, 1H, H-3). 13C NMR (DMF-d7,
50 MHz) d 53.61 (CH3), 58.09 (CH2), 109.42 (C-4),
125.49 (C-3), 127.25 (C-3a), 129.25 [CH(Ph)], 129.79
[CH(Ph)], 136.08 (C-5), 137.97 (C-7a), 138.02 [C(Ph)],
156.65 (C-7). Anal. Calcd for C14H13N3O: C, 70.28; H,
5.48; N, 17.56. Found: C, 70.43; H, 5.38; N, 17.32.
4.1.1. 7-Acetamido-1H-pyrazolo[3,4-c]pyridine (2). 7-
Aminopyrazolo[3,4-c]pyridine18b (200 mg, 1.49 mmol) was
stirred overnight with 5 ml of acetic anhydride, at room tem-
perature. Acetic anhydride was removed under reduced pres-
sure and the residue was dissolved in a saturated solution of
ammonia in methanol. The solution was stirred at room tem-
perature for 1 h, the solvent was vacuum evaporated and the
4.1.6. Data for 2,6-dibenzylpyrazolo[3,4-c]pyridin-7-one
1
(11). Yield: 15%. Mp 148 ꢁC (white crystals). H NMR

11992
V. N. Kourafalos et al. / Tetrahedron 62 (2006) 11987–11993
(DMF-d7, 300 MHz) d 5.23 (s, 2H, N6–CH2), 5.65 (s, 2H,
N2–CH2), 6.53 (d, 1H, H-4, J4–5¼7.31 Hz), 7.23 (d, 1H,
H-5, J5–4¼7.31 Hz), 7.25–7.44 (m, 10H, 2ꢂPh), 8.31
(s, 1H, H-3). 13C NMR (DMF-d7, 50 MHz) d 50.74
(N6–CH2), 57.26 (N2–CH2), 99.25 (C-4), 124.13 (C-3a),
125.74 (C-3), 128.00 [CH(Ph)], 128.35 [CH(Ph)], 128.67
[CH(Ph)], 128.76 [CH(Ph)], 129.17 [CH(Ph)], 129.30
[CH(Ph)], 130.71 (C-5), 137.51 [C(Ph)], 139.12 [C(Ph)],
142.29 (C-7a), 157.90 (C-7). Anal. Calcd for C20H17N3O:
C, 76.17; H, 5.43; N, 13.32. Found: C, 76.38; H, 5.54; N,
13.07.
measurements. We performed the u-scan technique with dif-
ferent k and 4 offsets in order to cover the entire independent
part of reflections in the 3–25ꢁ q range. Cell parameters were
refined from all the strong reflections. Data reduction was
carried out using the program CrysAlis RED (Oxford Dif-
fraction, UK). Direct methods in SHELXS-97,28 were used
to solve the structures, and the structures were refined by
using SHELXL-97.29 The tables were prepared for publica-
tion by using SHELXL and PARST,30 and the figures were
generated with ORTEP-III.31 Crystallographic data for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Center (CCDC).
Copies of the data can be obtained, free of charge, on appli-
cation to the Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK.
4.1.7. 1,3-Dibenzyl-7-methoxypyrazolo[3,4-c]pyridine
(13). 3-Benzyl-7-methoxypyrazolo[3,4-c]pyridine19 (12,
40 mg, 0.17 mmol) was dissolved in anhydrous DMF
(7 ml) under argon. A suspension of NaH (60% in paraffin
oil, 10 mg, 0.26 mmol) was added to the solution, under
cooling and the mixture was stirred at room temperature
for 30 min. A solution of benzyl bromide (0.03 ml,
0.19 mmol) in anhydrous DMF (1 ml) was then added and
the reaction mixture was stirred at room temperature for
1 h. The solvent was vacuum evaporated and the product
was purified by column chromatography (silica gel) using
a mixture of cyclohexane/ethylacetate 85/15 (v/v) as the
eluent, to give 13 (38 mg, 68%) as white crystals. Mp
4.3.1. Crystal data for 4. CCDC Ref. No. 608944. Crystal-
lized from methanol, C7H7N3O, Mrel¼149.16, T¼120(2) K,
˚
Orthorhombic, l¼0.71073 A, space group Pbac
,
˚
˚
˚
a¼11.3022(8) A,
b¼7.4180(6) A,
c¼16.2981(12) A,
V¼1366.43(18) A , Z¼8, Dcalcd¼1.450 Mg/m3, crystal
size 0.40ꢂ0.30ꢂ0.20 mm, GOF¼1.140, R¼0.0402/0.0453
(I>2s(I)/all data).
3
˚
4.3.2. Crystal data for 11. CCDC Ref. No. 608945. Crystal-
lized from methanol, C20H17N3O, Mrel¼315.37, T¼
1
89 ꢁC. H NMR (CDCl3, 400 MHz) d 4.08 (s, 3H, CH3),
4.29 (s, 2H, C3–CH2), 5.78 (s, 2H, C3–CH2), 6.87 (d, 1H,
H-4, J4–5¼5.85 Hz), 7.16–7.30 (m, 10H, 2ꢂPh), 7.60 (d,
1H, H-5, J5–4¼5.86 Hz). 13C NMR (CDCl3, 50 MHz)
d 33.77 (C3–CH3), 53.50 (CH3), 54.84 (N1–CH2), 108.89
(C-4), 126.52 [CH(Ph)], 127.55 [CH(Ph)], 127.69 (C-7a),
128.63 [CH(Ph)], 128.64 (C-3a), 128.80 [CH(Ph)], 135.27
(C-5), 138.05 [C(Ph)], 138.89 [C(Ph)], 144.60 (C-3),
150.93 (C-7). Anal. Calcd for C21H19N3O: C, 76.57; H,
5.81; N, 12.76. Found: C, 76.29; H, 5.76; N, 12.88.
120(2) K, Orthorhombic, l¼0.71073 A, space group
˚
˚
˚
˚
P212121, a¼5.8111(6) A, b¼10.0362(10) A, c¼26.568(3) A,
V¼1549.5(3) A , Z¼4, Dcalcd¼1.352 Mg/m3, crystal size
3
˚
0.50ꢂ0.25ꢂ0.20 mm,
(I>2s(I)/all data).
GOF¼0.948,
R¼0.0343/0.0480
Acknowledgements
The financial support of this work by a grant from the
National Scholarship Foundation of Greece (IKY), from
the Special Account for Research Grants Committee of the
University of Athens (Program Kapodistrias) and by the Min-
istry of Education of the Czech Republic (MSM0021622413
to R.M., MSM0021622415 to J.M.) is gratefully acknowl-
edged.
4.2. NMR spectroscopy
NMR spectra were recorded using a Bruker Avance DRX
500 spectrometer operating at frequencies of 500.13 MHz
(1H), 125.77 MHz (13C), and 50.68 MHz (15N), a Bruker
Avance DRX 400 spectrometer operating at frequencies of
400.13 MHz (1H), 100.61 MHz (13C), and 40.54 MHz
(15N) and a Bruker Avance 300 spectrometer operating at
frequencies of 300.13 MHz (1H), 75.48 MHz (13C), and
30.41 MHz (15N). NMR spectra were measured at various
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