INTRODUCTION

Pyrazolo[3,4-b]pyridine ring system is an important structural fragment of many biologically active com­pounds, including those exhibiting antiproliferative [1], antimicrobial [2, 3], anticancer [4–6], and antiparasitic activities [7], as well as inhibitory activity against acetylcholinesterase [8], casein kinase 1 (CK1), check­point kinase 1 (CHK1) [9], Aurora A kinase [10], and fibroblast growth factor (FGFR) [11]. Optical prop­erties of pyrazolo[3,4-b]pyridine derivatives are no less interesting. In this regard, the synthesis of efficient fluorophores [12–14] and chemosensors based thereon [15, 16] was reported.

The most widely used method for the synthesis of 3-amino-1H-pyrazolo[3,4-b]pyridines is based on nu­cleophilic substitution of halogens [17] or, more rarely, oxygen- [3, 5, 18–19] or sulfur-containing fragments [20–22]. Herein we report the synthesis of new 3,4-di­amino-6-aryl-1H-pyrazolo[3,4-b]pyridine-5-carboni­triles 1 and their optical properties.

RESULTS AND DISCUSSION

We previously found that the bromine atom in 4-amino-6-aryl-2-bromopyridine-3,5-dicarbonitriles 2 [23, 24] can be readily replaced by an amino group in reactions with primary and secondary amines to produce the corresponding 2-[(di)alkylamino]pyridines [25]. In continuation of these studies, compounds 2 were reacted with hydrazine hydrate. According to published data [26–28], replacement of the halogen atom in 2 by hydrazine leads to the formation of 2-hydrazinylpyridine-3,5-dicarbonitriles which then undergo intramolecular heterocyclization to 3-amino-1H-pyrazolo[3,4-b]pyridines. As expected, the reaction of 2 with hydrazine hydrate also afforded 3,4-diamino-6-aryl-1H-pyrazolo[3,4-b]pyridine-5-carbonitriles 1 in 83–92% yield (Scheme 1). It should be noted that, despite all attempts, we failed to isolate intermediate 2-hydrazinylpyridines.

Scheme
scheme 1

1.

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The 1H NMR spectra of 1a–1f showed signals from protons of the aryl substituent. The two amino and one NH groups gave rise to broadened singlets or exchanged with water. In the IR spectra of 1a–1f, stretching vibrations of the conjugated cyano groups were observed at 2206–2218 cm–1, and N–H stretching bands were observed in the region 3161–3466 cm–1. In the mass spectra of 1a–1f, the base peak was that of the molecular ion.

Compounds 1a–1f were isolated as yellow to orange crystalline solids poorly soluble in organic solvents (except for DMSO). Using compound 1b as an ex­ample, we found that the position of the electronic absorption maximum almost does not depend on the solvent nature. Moreover, in many cases we failed to determine its exact position because of overlap by the solvent band. The fluorescence maximum of 1a–1f ranges from λ 452 to 514 nm, and it shifts to longer wavelengths as the solvent polarity increases (Table 1). Depending on the substituent in the benzene ring, the fluorescence maximum of 1a–1f in DMSO appears in the range λ 484–548 nm (Table 2). Electron-donating groups generally increase the fluorescence intensity and shift its maximum to shorter wavelengths (Fig. 1). Compounds 1a–1f in the solid state showed almost no fluorescence.

Table 1. Optical properties of compound 1b in different solvents
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Table 2. Optical properties of compounds 1a–1f
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Fig. 1.
figure 1

Photoluminescence spectra of compounds 1a–1f.

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EXPERIMENTAL

The IR spectra were recorded on an FSM-1202 FTIR spectrometer (Russia) from samples dispersed in mineral oil. The 1H NMR spectra were recorded on a Bruker DRX-500 spectrometer (USA) using DMSO-d6 as solvent and tetramethylsilane as internal standard. The mass spectra (electron impact, 70 eV) were run on a Finnigan MAT INCOS-50 mass spec­trometer (USA). Elemental analysis was performed with a Vario Micro cube CHN analyzer (Germany). The fluorescence spectra were measured on an Agilent Cary Eclipse spectrofluorometer (USA). The melting points were determined with an OptiMelt MPA100 automated melting point apparatus (USA). The prog­ress of reactions and the purity of the isolated com­pounds were monitored by TLC on Sorbfil PTSKh-AF-A-UF plates using ethyl acetate as eluent; visualization was done under UV light, by treatment with iodine vapor, and by thermal decomposition. Compounds 2a–2f were synthesized as described in [23]. Hydrazine hydrate (100%) was commercial product (Germany).

3,4-Diamino-6-phenyl-1H-pyrazolo[3,4-b]pyri­dine-5-carbonitrile (1a). A mixture of 0.299 g (1 mmol) of 4-amino-2-bromo-6-phenylpyridine-3,5-dicarbonitrile (2a) and 0.15 g (3 mmol) of hydrazine hydrate in 5 mL of 1,4-dioxane was refluxed for 4 h. After completion of the reaction (TLC), the mixture was diluted with 30 mL of distilled water, and the precipitate was filtered off, washed with small portions of distilled water, and recrystallized from 1,4-dioxane. Yield 0.228 g (91%), mp 291–292°C (decomp.). IR spectrum, ν, cm–1: 3454, 3358, 3177 (NH, NH2), 2206 (CN), 1654 (C=C). 1H NMR spectrum (DMSO-d6), δ, ppm: 5.72 s (2H, NH2), 7.27 s (2H, NH2), 7.48–7.51 m (3H, C6H5), 7.71–7.74 m (2H, C6H5), 12.20 s (1H, NH). Mass spectrum: m/z 250 (Irel 100%). Found, %: C 62.45; H 4.09; N 33.46. C13H10N6. Calculated, %: C 62.39; H 4.03; N 33.58. M 250.27.

Compounds 1b–1f were synthesized in a similar way.

3,4-Diamino-6-(4-methylphenyl)-1H-pyrazolo­[3,4-b]pyridine-5-carbonitrile (1b). Yield 0.219 g (83%), mp 306–307°C (decomp.). IR spectrum, ν, cm–1: 3467, 3395 (NH, NH2), 2218 (C≡N), 1696 (C=C). 1H NMR spectrum (DMSO-d6), δ, ppm: 2.39 s (3H, CH3), 7.38 d (2H, C6H4, J = 7.7 Hz), 7.62 d (2H, C6H4, J = 7.7 Hz), 8.78 br.s (3H, NH, NH2). 13C NMR spectrum (DMSO-d6), δC, ppm: 21.51, 81.85, 89.97, 116.15, 129.53, 129.57, 142.14, 146.87, 148.41, 155.41, 155.51, 159.70. Mass spectrum: m/z 264 (Irel 100%). Found, %: C 63.52; H 4.63; N 31.85. C14H12N6. Calculated, %: C 63.62; H 4.58; N 31.80. M 264.29.

3,4-Diamino-6-(3,4-dimethoxyphenyl)-1H-pyra­zolo­[3,4-b]pyridine-5-carbonitrile (1c). Yield 0.276 g (89%), mp 246–247°C (decomp.). IR spectrum, ν, cm–1: 3439, 3379, 3300 (NH, NH2), 2208 (C≡N), 1662 (C=C). 1H NMR spectrum (DMSO-d6), δ, ppm: 3.82 s (3H, OCH3), 3.83 s (3H, OCH3), 5.70 s (2H, NH2), 7.07 d (1H, C6H3, J = 8.3 Hz), 7.22 s (2H, NH2), 7.32–7.38 m (2H, C6H3), 12.17 br.s (1H, NH). 13C NMR spectrum (DMSO-d6), δC, ppm: 56.14, 56.20, 80.72, 91.48, 111.63, 112.91, 119.35, 122.16, 131.95, 148.74, 148.80, 150.44, 153.98, 154.15, 161.25. Mass spec­trum: m/z 310 (Irel 100%). Found, %: C 58.18; H 4.61; N 27.00. C15H14N6O2. Calculated, %: C 58.06; H 4.55; N 27.08. M 310.32.

3,4-Diamino-6-(2-chlorophenyl)-1H-pyrazolo­[3,4-b]pyridine-5-carbonitrile (1d). Yield 0.262 g (92%), mp 308–309°C (decomp.). IR spectrum, ν, cm–1: 3448, 3177 (NH, NH2), 2218 (C≡N), 1687 (C=C). 1H NMR spectrum (DMSO-d6), δ, ppm: 7.57 t (1H, C6H4, J = 7.6 Hz), 7.64 t (1H, C6H4, J = 7.4 Hz), 7.68–7.72 m (2H, C6H4), 8.80 br.s (5H, NH, NH2). 13C NMR spectrum (DMSO-d6), δC, ppm: 66.82, 83.90, 90.49, 115.15, 128.06, 130.21, 131.34, 131.91, 132.97, 146.95, 148.70, 154.93, 157.80. Mass spectrum: m/z 286/284 (Irel 33/100%). Found, %: C 54.73; H 3.25; N 29.60. C13H9ClN6. Calculated, %: C 54.84; H 3.19; N 29.52. M 284.71.

3,4-Diamino-6-(3,4-dichlorophenyl)-1H-pyra­zolo­[3,4-b]pyridine-5-carbonitrile (1e). Yield 0.287 g (90%), mp 322–323°C (decomp.). IR spectrum, ν, cm–1: 3466, 3335, 3161 (NH, NH2), 2216 (C≡N), 1657 (C=C). 1H NMR spectrum (DMSO-d6), δ, ppm: 5.74 br.s (2H, NH2), 7.36 s (2H, NH2), 7.73 d.d (1H, C6H3, J = 8.3, 2.1 Hz), 7.77 d (1H, C6H3, J = 8.4 Hz), 7.95 d (1H, C6H3, J = 2.1 Hz), 12.27 br.s (1H, NH). 13C NMR spectrum (DMSO-d6), δC, ppm: 80.73, 91.62, 118.54, 129.34, 130.86, 131.00, 131.39, 132.55, 139.81, 148.72, 153.50, 153.91, 158.80. Mass spec­trum, m/z (Irel, %): 320 (65), 318 (18), 316 (100). Found, %: C 48.80; H 2.60; N 26.39. C13H8Cl2N6. Calculated, %: C 48.92; H 2.53; N 26.33. M 319.15.

3,4-Diamino-6-(3-nitrophenyl)-1H-pyrazolo­[3,4-b]pyridine-5-carbonitrile (1f). Yield 0.338 g (85%), mp 243–244°C (decomp.). IR spectrum, ν, cm–1: 3460, 3343, 3185 (NH, NH2), 2210 (C≡N), 1672 (C=C). 1H NMR spectrum (DMSO-d6), δ, ppm: 7.84 t (1H, C6H4, J = 8.1 Hz), 8.22 d.t (1H, C6H4, J = 7.8, 1.4 Hz), 8.39 d.d (1H, C6H4, J = 8.1, 2.4 Hz), 8.56–8.61 m (1H, C6H4). Mass spectrum: m/z 295 (Irel 100%). Found, %: C 52.77; H 3.00; N 33.32. C13H9N7O2. Calculated, %: C 52.88; H 3.07; N 33.21. M 397.44.

CONCLUSIONS

2,4-Diamino-6-arylpyridine-3,5-dicarbonitriles have been synthesized, and their electronic absorption and luminescence spectra have been recorded. The synthesized compounds show fluorescence in solution with an emission maximum located at λ 484 to 548 nm and do not fluoresce in the solid state.