Synthesis of 5-(1H-benzimidazol-2-yl)-1H-pyrazolo-[3,4-b]pyridines by the p-(dimethylamino)benzaldehyde modification of Hantzsch reaction

Article abstract of DOI:10.1007/s10593-007-0068-z

The reactions of p-(dimethylamino)benzaldehyde with 5-amino-3-methyl-1- phenylpyrazole and 2-RCOCH₂-1H-benzimidazoles have produced 5-(1H-benzimidazol-2-yl)-1H-pyrazolo[3,4-b]pyridines. The transformation includes the formation of compounds in accordance with a Hantzsch reaction containing a 1,4-dihydropyridine ring, and is completed by the aromatization either by the splitting off of N,N-dimethylanyline or oxidation. The splitting is produced by acetic acid and the oxidation by a nitrocompound.

Full text of DOI:10.1007/s10593-007-0068-z

Chemistry of Heterocyclic Compounds, Vol. 43, No. 4, 2007
SYNTHESIS OF 5-(1H-BENZIMIDAZOL-2-YL)-1H-PYRAZOLO-
[3,4-b]PYRIDINES BY THE p-(DIMETHYLAMINO)BENZALDEHYDE
MODIFICATION OF HANTZSCH REACTION
I. B. Dzvinchuk
The reactions of p-(dimethylamino)benzaldehyde with 5-amino-3-methyl-1-phenylpyrazole and
2-RCOCH2-1H-benzimidazoles have produced 5-(1H-benzimidazol-2-yl)-1H-pyrazolo[3,4-b]pyridines.
The transformation includes the formation of compounds in accordance with a Hantzsch reaction
containing a 1,4-dihydropyridine ring, and is completed by the aromatization either by the splitting off
of N,N-dimethylanyline or oxidation. The splitting is produced by acetic acid and the oxidation by a
nitrocompound.
Keywords: aldehydes, benzimidazoles, pyrazolo[3,4-b]pyridines, pyrazoles, Hantzsch reaction,
aromatization, selectivity.
The synthesis of substituted pyridines in the Hantzsch reaction (cyclocondensation of an aldehyde with
acetoacetic ester and ammonia followed by the oxidation of the resulting 1,4-dihydropyridines) is widely used in
various modifications in organic synthesis [1-4]. If one boils p-(dimethylamino)benzaldehyde (1) in acetic acid
with dimedone and ammonium acetate, the product containing a pyridine ring is as follows without the use of an
oxidizing agent: the reaction occurs via a derivative of acridine with 10-[p-(dimethylamino)phenyl]-
9,10-dihydropyridine moiety and is completed by the aromatization of the latter as a result of the splitting off of
N,N-dimethylaniline [5]. If there is a 4-methoxy or 4-hydroxyphenyl substituent on atom C-10 in the
1,4-dihydropyridine moiety of these compounds, then the analogous splitting off of benzene, anisole, or phenol
respectively occurs only on heating with mineral acids [6]. It is best to use the easy splitting off of
N,N-dimethylaniline to obtain new compounds containing unsubstituted pyridine rings. By comparison with the
usual synthesis methods, that approach has certain advantages: the process occurs in one stage and is simple to
operate, being accessible and stable on storage and ease in dispensing the initial aldehyde 1, together with high
solubility of the resulting N,N-dimethylaniline, which eases the isolation of the target products from the reaction
mixture.
Here I examine the scope for applying the new approach to examples of three-component reactions of
the aldehyde 1, 5-amino-3-methyl-1-phenylpyrazole (2), and 2-R-COCH2-substituted 1H-benzimidazoles 3a-g.
Other researchers have reported [7-9] the use of type 2 aminopyrazoles in the synthesis of pyrazolopyridines;
carbonyl components of type 3 have not been used in this reaction before, although it allows one to synthesize
previously unknown benzimidazolyl-substituted pyrazolopyridines.
__________________________________________________________________________________________
Institute of Organic Chemistry, Ukrainian National Academy of Sciences, Kiev 02094; e-mail:
Rostov@bpci.kiev.ua Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 578-584, April 2007.
Original article submitted November 20, 2006.
474
0009-3122/07/4304-0474©2007 Springer Science+Business Media, Inc.

NMe2
Me
H
N
N
+
+
N
O
N
HOAc,
120°C,
2 h
Ph
PhNO2
R
H2N
CHO
2
3a–g
HOAc,
120°C,
1.5 h
1
[4a–g]
[O]
Me2N
Me2N
Me
N
Me
3
[O]
7'
4'
N
H
N
H
N
N
R = 4-O2NC6H4
H
N
N
1
(4g)
Ph
5
N
Ph
N
N
R
7a–g
H
R
4a–g
+
Me2N
R = 4-O2NC6H4
(4a–f)
Me
N
+
H
H
N
H
N
H
+
– H ,
N
– PhNMe2
Ph
N
R
H
Me
7'
4'
H
4
N
2
N
6'
5'
N
1
5
Ph
N
N
6
R
6a–f
3–7 a R = Me, b R = Ph, c R = 4-MeOC6H4, d R = 3,4,5-(MeO)3C6H2,
e R = 4-BrC6H4, f R = 2-furyl, g R = 4-O2NC6H4
The interaction of reagents 1, 2, and 3a-f on boiling in acetic acid (method A) does not end with the
formation of compounds 4a-f containing the 1,4-dihydropyridine ring, but is instead accompanied by
aromatization, which probably occurs by the C-protonized forms 5a-f, which readily split off
N,N-dimethylaniline and converted to the final products: 4-unsubstituted 5-(1H-benzimidazol-2-yl)-
1H-pyrazolo[3,4-b]pyridines 6a-f with yields of 69-95% (Table 1). The process occurs smoothly even with
compound 3f, which contains a furyl substituent and may be unstable in an acid medium. The aromatization due
to oxidation (possibly by atmospheric oxygen) preserves the (dimethylamino)phenyl substituent and leads to the
pyrazolo[3,4-b]pyridines 7a-f to only a slight extent (as indicated by TLC). On the other hand, in the case of the
benzimidazole 3g (R = 4-O2NC6H4), the conditions of method A lead to the oxidation predominating and the
main product being pyrazolo[3,4-b]pyridine 7g (yield 50%)
475

TABLE 1. Characteristics of Synthesized Compounds
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
Yield, %
(method)
mp, °С
С
Н
N
6a
6b
6c
6d
6e
6f
C21H17N5
74.24
74.32
77.53
77.79
75.02
75.16
70.68
70.86
64.87
65.01
73.48
73.64
75.81
75.96
4.98
5.05
4.95
4.77
4.78
4.91
5.32
5.13
3.95
3.78
4.21
4.38
5.54
5.71
20.48
20.63
17.28
17.44
16.07
16.23
14.09
14.25
14.43
14.58
17.56
17.89
18.39
18.33
276.5-278
313-315
276-278
262.5-264
330-331.5
251-252.5
>350
95 (A)
80 (A)
89 (A)
85 (A
C26H19N5
C27H21N5O
C29H25N5O3
C26H18BrN5
C23H15N5О
C29H26N6
95 (A)
69 (A)
7a
52 (B);
46 (C)
7b
7c
C29H28N6
78.29
78.44
76.23
76.34
5.57
5.42
5.36
5.49
16.04
16.14
15.18
15.26
>350
>350
75 (B)
C35H30N6O
58 (B);
48 (C)
7d
7e
C37H34N6O3
C34H27BrN6
72.46
72.77
68.08
68.12
5.47
5.61
4.63
4.54
13.62
13.76
13.94
14.02
291-292.5
>350
66 (B)
82 (B);
76 (C)
7f
C32H26N6O
C34H27N7O2
75.11
75.27
5.22
5.13
16.37
16.46
>350
56 (B);
29 (C)
7g
72.06
72.20
4.66
4.81
17.21
17.33
328.5-330
50 (A);
71 (B);
69 (C)
The reactions of reagents 1, 2, and 3a-g on the addition of nitrobenzene (oxidizing agent) is displaced
towards the formation of products of type 7. For example, with ratios for nitrobenzene: 1:2:3 of 5:1:1:1
(method B) that conversion line becomes preferred and the yields of products 7a-g constitute 52-82%. Reduction
of the amount of oxidizing agent by a factor 5 (method C) where compounds 3a,c,e-g leads to reduction in the
yield, particularly for compound 7f (from 56 to 29%).
Increased electroacceptor capacity of the substituents R favors the formation of compounds 7a-g; probably,
the tendency of the intermediates 4 to oxidation increases with the acidity of the hydrogen atom in the γ-position of
the 1,4-dihydropyridine fragment, but the better yield of the nitrophenyl-substituted product 7g (71%), although
higher than when nitrobenzene is used (50%) is less than the yields of the bromophenyl and even phenyl derivatives
7e and 7b (82 and 75%). Consequently, the initial compound 3g is appreciably consumed in the oxidation even in
the presence of nitrobenzene possibly because of the high oxidizing capacity of the nitrogroup.
The compositions and structures of the synthesized pyrazolo[3,4-b]pyridines of types 6 and 7 are
1
confirmed by elemental analysis (Table 1) as well as H NMR spectra (Table 2). Compounds 6a-f are of the
same structure type, as is implied by the chemical shifts of the signals for identically placed protons in the
bicyclic fragments having only very small differences, which are quite regularly related to the nature of the
substituents R. The same applies to compounds 7a-g.
1
The H NMR spectra of compounds of type 6 contain a distinct singlet signal for the H-4 proton on the
weak-field side (8.63-8.76 ppm), which is absent from the spectra of compounds of type 7. The signals from the
o-, m-, and p-protons in the N-phenyl substituent appear separately in order of increasing field, with the
o-protons at a fairly weak field (8.34-8.42 ppm), which is related to the descreening effect of the nitrogen atoms
in the pyridine and pyrazole rings.
476

TABLE 2. 1H NMR Spectra of Synthesized Compounds
Com-
pound
δ, ppm (J, Hz)
6a
6b
2.66 (3H, s, 3-CH3); 2.99 (3H, s, 6-CH3); 7.26-7.30 (2H, m, H-5',6'); 7.33 (1H, t,
J = 7.5, HPh-p); 7.58 (3H, t, J = 7.5, 2HPh-m, H-7'); 7.73 (1H, d, J = 7.8, H-4');
8.34 (2H, d, J = 8.4, 2HPh-o); 8.71 (1H, s, H-4); 12.86 (1H, s, NH*)
2.68 (3H, s, 3-CH3); 7.18-7.21 (2H, m, H-5',6'); 7.32-7.36 (4H, m, HNPh-p, 3HCPh-m,p);
7.39-7.42 (1H, m, H-7'); 7.49 (2H, d, J = 7.5, 2HCPh-o); 7.59 (2H, t, J = 7.5, 2HNPh-m);
7.63-7.66 (1H, m, H-4'); 8.36 (2H, d, J = 7.2, 2HNPh-o); 8.73 (1H, s, H-4);
12.39 (1H, s, NH)
6c
6d
6e
6f
2.64 (3H, s, 3-CH3); 3.73 (3H, s, CH3O); 6.87 (2H, d, J = 8.7, 2HAr-m);
7.19-7.22 (2H, m, H-5',6'); 7.33 (1H, t, J = 7.5, HPh-o); 7.42 (2H, d, J = 8.1, 2HAr-m, H-7');
7.58 (2H, t, J = 8.1, 2HPh-m); 7.64-7.67 (1H, m, H-4'); 8.37 (2H, d, J = 8.1, 2HPh-o);
8.66 (1H, s, H-4); 12.42 (1H, s, NH)
2.68 (3H, s, 3-CH3); 3.44 (6H, s, 2CH3O-m); 3.65 (3H, s, CH3O-p); 6.81 (2H, s, 2HAr-o);
7.21-7.24 (2H, m, H-5',6'); 7.34 (1H, t, J = 7.5, HPh-p); 7.44-7.47 (1H, m, H-7');
7.60 (2H, t, J = 8.1, 2HPh-m); 7.67-7.70 (1H, m, H-4'); 8.42 (2H, d,
J = 7.5, 2HPh-o); 8.72 (1H, s, H-4); 12.47 (1H, s, NH)
2.67 (3H, s, CH3); 7.19-7.22 (2H, m, H-5',6'); 7.34 (1H, t, J = 7.5, HPh-p);
7.38 (2H, d, J = 9.0, 2HAr-m); 7.44-7.46 (1H, m, H-7'); 7.53 (2H, d, J = 9.0, 2HAr-o);
7.58 (2H, t, J = 7.5, 2HPh-m); 7.62-7.66 (1H, m, H-4'); 8.34 (2H, d, J = 7.8, 2HPh-o);
8.76 (1H, s, H-4); 12.50 (1H, s, NH)
2.64 (3H, s, CH3); 6.45 (1H, d, J = 3.3, HHet-3); 6.54-6.55 (1H, m, HHet-4);
7.25-7.27 (2H, m, H-5',6'); 7.35 (1H, t, J = 7.5, HPh-p); 7.53 (1H, d, J = 6.0,
H-7'); 7.61 (2H, t, J = 7.8, 2HPh-m); 7.71 (1H, d, J = 8.7, H-4'); 7.75 (1H, d,
J = 0.9, HHet-5); 8.39 (2H, d, J = 8.4, 2HPh-o); 8.63 (1H, s, H-4); 12.75 (1H, s, NH)
7a
7b
2.06 (3H, s, 3-CH3); 2.37 (3H, s, 6-CH3); 2.84 [6H, s, N(CH3)2];
6.58 and 7.14 (2×2H, two d, J = 9.0, 2HAr-o, 2HAr-m); 7.10-7.13 (2H, m, H-5',6');
7.34 (1H, t, J = 7.5, HPh-p); 7.39-7.42 (1H, m, H-4'); 7.55-7.61 (3H, m, 2HPh-m, H-4');
8.31 (2H, d, J = 7.8, 2HPh-o); 12.45 (1H, s, NH*)
2.08 (3H, s, 3-CH3); 2.85 [6H, s, N(CH3)2]; 6.60 (2H, d, J = 8.7, 2HAr-m);
7.05-7.11 (2H, m, H-5',6'); 7.17 (2H, d, J = 8.4, 2HAr-o);
7.19-7.26 (3H, m, 2HCPh-m, HCPh-p); 7.26-7.29 (1H, m, H-7'); 7.34 (1H, t, J = 7.5, HNPh-p);
7.44-7.49 (3H, m, 2HCPh-o, H-4'); 7.58 (2H, t, J = 7.8, 2HNPh-m); 8.34 (2H, d,
J = 7.8, 2HNPh-o); 12.31 (1H, s, NH)
7c
2.05 (3H, s, 3-CH3); 2.84 [6H, s, N(CH3)2]; 3.67 (3H, s, CH3O), 6.58 (2H, d,
J = 8.7, 2H4-Ar-m); 6.75 (2H, d, J = 8.7, 2H6-Ar-m); 7.07-7.09 (2H, m, H-5',6');
7.13 (2H, d, J = 8.7, 2H4-Ar-o); 7.27-7.31 (1H, m, H-7'); 7.33 (1H, t, J = 7.5, HPh-p);
7.40 (2H, d, J = 8.7, 2H6-Ar-o); 7.48-7.50 (1H, m, H-4'); 7.58 (2H, t, J = 7.8, 2HPh-m);
8.34 (2H, d, J = 7.8, 2HPh-2,6); 12.35 (1H, s, NH)
7d
7e
7f
2.04 (3H, s, 3-CH3); 2.80 [6H, s, N(CH3)2]; 3.33 (6H, s, 2CH3O-m); 3.57 (3H, s, CH3O-p);
6.56 (2H, d, J = 8.7, H4-Ar-m); 6.73 (2H, s, C6H2); 7.05-7.09 (4H, m, H-5',6', 2H4-Ar-o);
7.22-7.25 (1H, m, H-7'); 7.33 (1H, t, J = 7.8, HPh-p); 7.40-7.43 (1H, m, H-4');
7.59 (2H, t, J = 7.8, 2HPh-m); 8.38 (2H, d, J = 7.8, 2HPh-o); 12.60 (1H, s, NH)
2.08 (3H, s, 3-CH3); 2.84 [6H, s, N(CH3)2]; 6.58 (2H, d, J = 8.7, 2H4-Ar-m);
7.08-7.10 (2H, m, H-5',6'); 7.15 (2H, d, J = 9.0, 2H4-Ar-o); 7.27-7.30 (1H, m, H-7');
7.35-7.44 (5H, m, HPh-p, 4H6-Ar); 7.47-7.50 (1H, m, H-4'); 7.58 (2H, t, J = 7.8, 2HPh-m);
8.31 (2H, d, J = 7.8, 2HPh-o); 12.38 (1H, s, NH)
2.02 (3H, s, 3-CH3); 2.84 [6H, s, N(CH3)2]; 5.75 (1H, d, J = 3.6, HHet-3);
6.37-6.38 (1H, m, HHet-4); 6.55 и 7.13 (2 × 2H, two d, J = 9.0, 4H4-Ar);
7.14-7.18 (2H, m, H-5',6'); 7.36 (1H, t, J = 7.5, HPh-p); 7.39-7.41 (1H, m, H-7');
7.57-7.58 (1H, m, H-4'); 7.61 (2H, t, J = 7.8, 2HPh-m); 7.67 1H, d, J = 0.9, HHet-5);
8.39 (2H, d, J = 7.8, 2HPh-m); 12.69 (1H, s, NH)
7g
2.11 (3H, s, 3-CH3); 2.84 [6H, s, N(CH3)2]; 6.60 (2H, d, J = 8.7, 2H4-Ar-m);
7.07-7.10 (2H, m, H-5',6'); 7.16 (2H, d, J = 9.0, 2H4-Ar-o); 7.28-7.30 (1H, m, H-7');
7.36 (1H, t, J = 7.5, HPh-p); 7.47-7.50 (1H, m, H-4'); 7.58 (2H, t, J = 7.8, 2HPh-m);
7.66 and 8.05 (2×2H, two d, J = 8.7, 4H6-Ar); 8.30 (2H, d, J = 8.1, 2HPh-o);
12.46 (1H, s, NH)
_______
* Undergoes deuterium exchange.
477

The signals from the 3-Me group in compounds of type 6 lie in the region of 2.64-2.68 ppm, while for
compounds 7, they lie at stronger fields (2.02-2.11 ppm), which is evidently due to the screening action of the
adjacent p-(dimethylamino)phenyl substituent, which is deflected from the plane of the pyrazolopyridine ring
because of steric factors related to the 5-benzimidazolyl substituent and the 3-Me group.
The 6-Me group in compound 6a resonates at 2.99, while that of compound 7a does so at 2.37 m.p., i.e.,
in a stronger field. The descreening action of the benzimidazole fragment in position 5 as regards this group is
reduced by the steric influence from position 4.
In compounds 6 and even more in compounds 7, steric interactions between the aromatic substituents in
the pyridine ring of the pyrazolopyridine system prevent their having a coplanar position with respect to the
plane of the latter. Consequently, the hydrogen atoms in substituent R located near the junction with the
α-position of the position ring resonate at a stronger field than might be expected. Evidently, they to a
considerable extent experience not descreening by the pyridine nitrogen atom but screening and steric
displacement of the benzimidazole fragment. This is particularly evident in the case of the 2-furyl substituent:
the signal from НHet-3 is in an extremely strong field at 6.45 (compound 6f or 5.75 ppm (compound 7f), whereas
the НHet-4 signal is displaced to a weaker field: 6.54 (6f) or 6.37 ppm (7f).
The signals from protons Н-4',7' in the benzimidazole ring appear separately, which indicates hindered
migration of the protons between the nitrogen atoms. That is typical of benzimidazoles containing bulky
heterocyclic substituents in position 2 [10].
Important information was obtained from the Overhauser effect of compounds 7g and 6b. The
suppression of the signal from the 3-Me group in the first is not reflected in any of the other signals, including
those from the 4-(dimethylamino)phenyl substituent (probably on account of its deviation from the plane of the
pyrazolopyridine fragment). On the other hand, suppression of the 3-Me group signal of compound 6b increases
the integral intensity of the H-4 signal, while suppression of the latter signal is reflected in the first. These results
confirm the spatially close position of the interacting nuclei, i.e., they indicate that reagents 1-3 react on the
above scheme. This rules out the second possible mode of cyclocondensation, in which the aldehyde reacts on
the aminogroup of the pyrazole 2 and the active methylene group of compound 3, while the ring closure occurs
as a result of the condensation of the C=O group in position 4 of the pyrazole ring. That transformation path
would give rise to isomeric products differing in the arrangement of the substituents in the
pyrazolo[3,4-b]pyridine fragment, and in particular having an α-unsubstituted position in the pyridine ring,
which is spatially remote from the methyl group in the pyrazole ring.
These are examples of effective synthesis of previously unknown 4-unsubstituted 5-(1H-benzimidazol-2-
yl)-1H-pyrazolo[3,4-b]pyridines of type 6 and show that the p-(dimethylamino)benzaldehyde modification of the
reaction is promising.
EXPERIMENTAL
The reaction was monitored and the purity of the synthesized compounds was checked by TLC on
Silufol UV-254 plates in a benzene-ethanol solvent system, 9:1, development in UV light. The 1H NMR spectra
were recorded with a Varian VXR-300 spectrometer (300 MHz) and a Varian Unity Plus-400 spectrometer (400
MHz, NOE experiment) in DMSO-d6, standard TMS. All the compounds were dried for 5 h at 145°C before
elemental analysis and the spectral studies.
5-(1H-benzimidazol-2-yl)-3,6-dimethyl-1-phenyl-1H-pyrazolo[3,4-b]pyridine (6a). A. A mixture of
aldehyde 1 (0.149 g, 1 mmol), of aminopyrazole 2 (0.173 g, 1 mmol), of 2-acetonylbenzimidazole (3a) (0.174 g,
1 mmol), and 2 ml of glacial acetic acid was kept at 120°C for 2 h. Then 1 ml of water was added and the
478

mixture was heated with mixing to the start of crystallization. The solidified part was filtered off and the
sediment of product 6a was washed with a mixture of 2-propanol and water, 1:1. Yield 0.324 g (95%). The
product was recrystallized from a pyridine–water mixture, 2:1.
Compounds 6b-f, 7g were made similarly from compounds 1, 2, and 3b-g respectively. The 6f product
was recrystallized from a 1:1 pyridine–water mixture, while 7g was recrystallized from a 5:1
dimethylformamide–water mixture.
5-(1H-benzimidazol-2-yl)-4-[4-(dimethylamino)phenyl]-3,6-dimethyl-1-phenyl-1H-pyrazolo[3,4-b]-
pyridine (7a). B. A mixture of aldehyde 1 (0.149 g, 1 mmol), aminopyrazole 2 (0.173 g, 1 mmol),
2-acetonylbenzimidazole (3a) (0.174 g, 1 mmol), nitrobenzene (0.5 ml, 5 mmol), and 1 ml of glacial acetic acid
was kept at 120°C for 1.5 h, and then we added 1 ml of water, 1 ml of 2-propanol, and boiled the mixture with
stirring to the start of crystallization. The solidified mixture was filtered, and the deposit of product 7a was
washed with 2-propanol. Yield 0.238 g (52%). The product was recrystallized from dimethylformamide.
Compounds 7b-g were made similarly respectively from reagents 1, 2, and 3b-g with nitrobenzene. To
isolate the 7d product, the reaction mixture was diluted with water (5 ml), evaporated to dryness, the residue was
then boiled with 4 ml of acetone, and after cooling compound 7d was filtered off, which was recrystallized from
a 2:1 pyridine–water mixture. Compound 7e was purified by crystallization from a 3:1 dimethylformamide–
acetic acid mixture.
C. Compounds 7a, c, and e-g were synthesized by the above method but with a nitrobenzene:1:2:3 ratio
of 1:1:1:1.
I am indebted to V. V. Polovinko, an expert on NMR at the UkrOrgSintez firm in Kiev, for performing
the NOE experiments.
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