Introduction

Development of novel fluorescent molecules as chemosensors for the detection of chemically, biologically, and environmentally important functional molecules has received much attention in recent years [1–4]. The first naphthyridine derivatives were prepared in 1893 by Reissert who suggested the name naphthyridine [5]. From literature it was also noted that 1,8-naphthyridine derivatives were not only use as luminescence materials in molecular recognition because of their rigid planer structure [6–8], possess antibacterial [9], antymicobacterial [10], antitumoral [11], anti-inflammatory [12], antiplatelet [13], gastric antisecretary [14], antiallergic [15] and local anaesthetic [16]. Recently, our research group reported systematic studies about the fluorescence properties of differently substituted pyrazolofused heterocycles [17–22] and benzo naphthyridines [23–26]. Annulation reactions with hetrocyclic aminoaldehydes provides synthectic entry into hetrocyclic systems, fused to a pyridine or pyrimidine nucleus by Friedlander condensation reactions with reactive methylenes. Thus a suitable blue-emitting material with high brightness and good thermal stability still remains to be developed. These literature reports and our ongoing research in this area prompted us to synthesize new pyrazolo[3,4-b]pyridine and pyrazolo[3,4-b][1,8]naphthyridines.

Results and Discussion

The key starting material 6-amino-3-(3-bromo-4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carboxamide 1 (Scheme 1) containing two bifunctional groups was prepared by Friedlander condensation of 5-amino-3-(3-bromo-4-methoxyphenyl)-1-phenyl-1H-pyrazole-4-cabaldehyde with cyanoacetamide [27].

Scheme 1
scheme 1

Synthesis of 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbaldehyde

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Our initial aim in this work was to design a short and efficient synthetic strategy for synthesis of 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbaldehyde 4. For preparing pyrazolo[3,4-b]pyridine was used a known synthetic sequence, typically via reaction the 5-aminopyrazole with diethyl ethoxy methylenemalonate. This is general, easy and extensible technology for the construction of this heterocycles system [28–32]. A convenient route was successfully developed for the synthesis of novel synthon 4 with amino and aldehyde functionality adjacent to each other which is depicted in (Scheme 1).

The synthesis begins with o-aminocarboxamide 1. The hydrolysis of o-aminocarboxamide 1 is difficult in low boiling alcoholic solvents because of low solubility. We had tried hydrolysis in methanol, ethanol, isopropanol and tert-butanol. Hydrolysis of o-aminocarboxamide compound 1 under reflux conditions in n-butanol was successful and gave colourless solid with 93 % yield. The obtained solid was characterized by spectral and analytical data and assigned 6-amino-3-(3-bromo-4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid 2.

Compound 3 was obtained by reduction of acid 2 using vitride in 93 % yield in which unexpected debromination was observed (Reductive dehalogenation). The same reduction was also carried out by using different reducing agents such as Lithium aluminium hydride (LiAlH4), Diisobutylaluminium hydride (DIBAL-H), but starting material was not consumed. After completion of reaction, Al-complex was hydrolysed with aqueous ammonium acetate solution. Inorganic salts were filtered through celite. Aqueous layer was extracted with ethyl acetate, the organic solvent was evaporated under reduced pressure and furnished a pale yellow solid. It was crystallized from methanol and characterized by spectral and analytical data and assigned (6-amino-3-(4-methoxy-phenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridin-5-yl) methanol 3.

Then the target synthon 4 was achieved by oxidation of primary alcohol to aldehyde is a useful reaction in organic chemistry. A variety of reagents are reported for the oxidation of primary alcohol to aldehyde this conversion several oxidizing agents are known such as pyridinium dichromate (PDC), pyridinium chlorochromate (PCC) and Dess-Martin periodonate. Oxidation of alcohol to aldehyde is partial oxidation; aldehydes are further oxidized to carboxylic acid. Hence, we have carried out oxidation at room temperature for 24 h. Then the dark brown colored solution was filtered through celite. The solvent was removed from the colourless filtrate which yielded a colourless solid. It was crystallized from methanol and characterized by spectral and analytical data and assigned 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbaldehyde 4. Thus versatile synthon was utilized for the synthesis of pyrazolo[3,4-b][1,8]naphthyridines 6.

For the annulations of heterocyclic system o-aminoaldehyde has wide applicability. From literature it was noted that o-aminoaldehyde such as 4 has wide applicability. The first and best known member of this class of compounds has been utilized for synthesis of various heterocycles [33–37]. Recently, we have applied Friedlander reaction to 5-aminopyrazole-4-carbaldehyde and found that reactions with active methylene compounds are useful for the synthesis of pyrazolo[3,4-b]pyridines [27]. On the extension of this reaction we wish to describe synthesis of pyrazolo[3,4-b]- [1,8]naphthyridines 6 a tricyclic heterocycles with four or more nitrogen atoms. The Friedlander reaction is initiated by attack of a carbanion formed from an active methylene or methyl neighbouring to the carbonyl function on the formyl carbon atom of o-aminoaldehyde 4 followed by cyclization to afford the pyrazolo[3,4-b]- [1,8]naphthyridine skeleton such as 6.

Thus, the Friedlander condensation of 6-aminopyrazolo[3,4-b]pyridine-5-carbaldehyde 4 with reactive methylenes was studied. Initially condensation was performed with cyanoacetamide 5a in refluxing ethanolic potassium hydroxide solution for 2 hours (Scheme 2). It was observed that the solid crystallized out from the yellow colored reaction mass at reflux temperature itself, which was cooled to room temperature and isolated by filtration. The obtained solid was purified by crystallization from ethanol as yellow colored solid in 88 % yield. The obtained solid was characterized by spectral and analytical data and assigned as 7-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b][1,8]naphthyridine-6-carboxamide 6a. Analogously, malononitrile was condensed with o-aminoaldehyde 4 under similar reaction conditions to yield 7-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo- [3,4-b][1,8]naphthyridine-6-carbonitrile 6b and characterized by spectral and analytical data.

Scheme 2
scheme 2

7-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo- [3,4-b][1,8]naphthyridine derivatives 6a and 6b

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After successful condensation of cyanoacetamide and malononitrile we extend our strategy towards cyclic ketone. We performed condensation of 6-aminopyrazolo- [3,4-b]pyridine-5-carbaldehyde 4 with 1-indanone 5c (Scheme 3) which yielded yellowish brown solid under similar reaction condition described for synthesis of 6a-b. The obtained solid was purified characterized by spectral and analytical methods and assigned 3-(4-methoxyphenyl)-1-phenyl-1,6-dihydroindeno[1,2-b]pyrazolo- [4,3-g][1,8]naphthyridine 6c. Thus we can achieve synthesis of polycyclic heterocycles using ketones as condensation partner.

Scheme 3
scheme 3

3-(4-methoxyphenyl)-1-phenyl-1,6-dihydroindeno[1,2-b]pyrazolo- [4,3-g][1,8]naphthyridine 6c

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Analogously cyclohexanone 5d was condensed with o-aminoaldehyde 4 to achieve tetracyclic heterocycles 6d (Scheme 4). It was also characterized by spectral and analytical methods and assigned 3-(4-methoxyphenyl)-1-phenyl-6,7,8,9-tetrahydro-1H-benzo[b]pyrazolo[4,3-g][1,8]naphthyridine 6d.

Scheme 4
scheme 4

3-(4-methoxyphenyl)-1-phenyl-6,7,8,9-tetrahydro-1H-benzo[b]pyrazolo[4,3-g][1,8]naphthyridine 6d

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Further the condensation of o-aminoaldehyde 4 was studied with acetophenones and alkyl ketones in ethanolic potassium hydroxide yielded pyrazolo naphthyridine derivatives 6e-j in good yield (Scheme 5). Allthese compounds were characterized by spectral and analytical data.

Scheme 5
scheme 5

6,7-disubstituted pyrazolo [3,4-b][1,8]naphthyridine derivatives 6e-j

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We tried the Friedlander reaction using various organic bases such as pyridine, triethylamine, di-isopropylamine as well as inorganic bases such as sodium carbonate, potassium carbonate but it was observed that no product formation in the reaction that may be due to weak basicity of organic and inorganic bases. So further we planned reaction in strong bases such as NaOH, KOH and potassium tert-butoxide. Among all the strong bases KOH found to be effective for this condensation and gave better yield. Moreover, we found that the yield of product was decreased when reaction was carried out using NaOH and potassium tert-butoxide gave unwanted product. Friedlander reactions were carried out using potassium hydroxide under reflux in absolute ethanol. The use of 2 % of ethanolic potassium hydroxide solution was sufficient to promote the reaction with excellent yield.

Photophysical Properties

To the best of our knowledge, fused polycycle especially the fused heterocycles has efficient luminescence. All of our products 1, 2, 3, 4 and 6a-j exhibit strong fluorescence which can be distinguished by our eyes readily in DMSO or under UV light (360 nm), so the fluorescence properties of the synthesized compounds were evaluated, and the relationship between the observed fluorescence and the substitution pattern on the naphthyridine ring were deduced. Usually naphthyridine compounds are highly fluorescent after excitation to the locally exited state and some of the naphthyridine derivatives show interesting photo-induced properties. Therefore, we have tried to measure the absorption and fluorescence emission spectral properties of all pyrazolopyridines on Shimadzu’s RF-3100 Spectrophotometer and Spectrofluorometer respectively, for 1x10−3M concentration in polar aprotic solvent DMSO. The results obtained are summarized in Table 1. The photophysical properties of compounds 1, 2, 3, 4 and 6a-j were determined with respect to quinine sulphate, which were used as a reference standard for the present study. We have calculated their fluorescence quantum yield and are listed in Table 1.

Table 1 The Photophysical data for electronic absorption (λabs. max.), Fluorescence (λem. max.) and quantum yield (Фf) of pyrazolo[3,4-b][1,8]naphthyridines
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Calculation of the Fluorescence Quantum Yield

The fluorescence quantum yield is the important characteristic of fluorophores. The quantum yield (ΦF) is the number of emitted photons relative to the number of absorbed photons. Substances with largest quantum yields, near to unity, such as rhodamines, display the brightest emission. The measurement of absolute quantum yields requires more sophisticated instrumentation. It is easier to determine relative quantum yields of fluorophores by comparing it with reference standard. The most commonly used reference standards are cresyl violet, fluorescein, quinine sulphate, tryptophan, L-tyrosine etc. Herein, we used quinine sulphate as a reference standard and calculated the quantum yields of pyrazolo[3,4-b][1,8]naphthyridines derivatives 6a-j by using equation,

$$ Q={Q}_R\frac{I}{I_R}\frac{O{D}_R}{OD}\frac{n^2}{n_R^2} $$

Where Q is the quantum yield, I is the integrated intensity, n is the refractive index, and OD is the optical density. The subscript R refers to the reference fluorophore of known quantum yield. It was noted that the incorporation of constituent on to the pyrazolo[3,4-b]- [1,8]naphthyridines 6a-j in the aryl ring at C6 and C7 position of pyridine nucleus has profound influence on the absorption and emission properties Fig. 1. These findings reveal that pyrazolo[3,4-b][1,8]naphthyridines 6a-j substituted with electron donating substituent’s such as alkyl group in the aryl ring at C6 and C7 of pyridine, enhanced the UV absorption in the range of 467–476 nm and fluorescence maxima in the range of 545–559 nm as well as quantum yields in the range of 0.30-0.34. Dialkyl substituted pyrazolo[3,4-b][1,8]naphthyridines 6 g-h showed comparatively slight less fluorescence than mono alkyl substituted pyrazolo[3,4-b][1,8]naphthyridines 6i-j. When the substituents is electron withdrawing such as aryl ring in the pyrazolo[3,4-b][1,8]naphthyridines 6e,f reduces the UV absorption in the range of 441 and 444 nm and fluorescence maxima in the range of 513 and 516 nm along with quantum yields in the range of 0.28 and 0.29 respectively. Quantum yield results are summarized in Table 1 indicates that compound 6a-j shown high quantum efficiency as compared to compound 1–4 may be due to conjugation through aromatic system. The comparative absorption and emission spectra of 6a-j are graphically represented in Fig. 2.

Fig. 1
figure 1

Representative structure of compounds 6

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Fig. 2
figure 2

The comparative absorption and fluorescence spectra of compounds 6a–j

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Conclusion

In conclusion, we have described simple and efficient approach for the synthesis of highly functionalized 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]- pyridine-5-carbaldehyde 4 and it was utilized as synthon for Friedlander condensation with active methylenes to obtained novel tetracyclic or pentacyclic heterocycles i.e., pyrazolo[3,4-b][1,8]naphthyridines 6a-j. The synthesis of new 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbaldehyde 4 was carried out through a three stage process which involves the hydrolysis of o-amino carboxamide 1 followed by the reduction of carboxylic acid compound 2 with vitride to gave o-amino alcohol 3 and further oxidation with MnO2 yielded the important key synthon i.e., o-aminoaldehyde 4. All synthesized compounds were characterized by spectral and analytical data. It was observed that the pyrazolo[3,4-b]-[1,8]naphthyridines 6a-j showed strong fluorescence, so we further studied the photophysical properties of these compounds in organic polar aprotic solvent DMSO. Potential correlations between structure and optical properties are discussed. The spectroscopic data are compiled and the quantum yields calculated are impressive. All these compounds emit high blue, yellow and green fluorescence with 0.17–0.34 quantum yields. All these findings reveal that pyrazolo[3,4-b][1,8]naphthyridine 6a-j are very useful because of their stability and their photophysical properties. Moreover, these strongly fluorescent compounds may find application in opto-electronic devices and are addition in the library of new heterocyclic compounds.

Experimental

Melting points were determined on a Gallenkamp melting point apparatus, Model MFB595 in open capillary tubes and are uncorrected. The 1H NMR and 13C NMR spectra were recorded on a Varian XL-300 spectrometer. Chemical shifts were reported in ppm relative to tetramethylsilane (TMS), and multiplicities are given as s (singlet), bs (broad singlet), d (doublet), t (triplet), q (quartet), or m (multiplet). Infrared spectra were recorded as KBr pellets on a Shimadzu FTIR-408 spectrophotometer. Mass spectra were recorded on a Shimadzu LC-MS: EI QP 2010A mass spectrometer with ionization potential 70 eV. Elemental analyses were performed on Thermo Quest Flash 1112 Series EA analyzer. Reactions were monitored by thin layer chromatography (TLC), carried out on 0.2 mm silica gel 60 F254 (Merck) plates using UV light (254 and 366 nm) for detection and compounds were purified by column chromatography by using silica gel of 5–20 μm (Merck, 60–120 mesh). Column dimension 39 × 2 cm and elution volume used is about 200–400 mL for each product. Common reagent grade chemicals are either commercially available and were used without further purification or were prepared by standard literature procedures.

Synthesis of 6-amino-3-(3-bromo-4-methoxyphenyl)-1-phenyl-1H-pyrazolo- [3,4-b]pyridine-5-carboxylic acid, 2

A solution of amide 1 (15 g, 34.22 mmol) in n-butanol (150 ml) was refluxed with potassium hydroxide (15 g, 267.3 mmol in 15 ml water) for 4 h. The reaction mixture was stirred for about 4 hours at reflux empeature. After completion of the reaction as indicated by TLC (eluent chloroform/methanol, 9:1), and concentrated under reduced pressure. The reaction mixture was left to cool at room temperature; water (300 ml) was added to the solid residue to get clear solution. It was then acidified with hydrochloric acid to pH of about 2, solid obtained was filtered, washed with water to furnished 2.

Colorless solid; (Yield = 14 g, 93 %); mp 308-312 °C; IR (KBr, cm−1): 3470, 3342, 1618, 1499; 1H NMR (DMSO-d 6) δ: 13.26 (s, 1H), 8.76 (s, 1H), 8.32 (d, 2H, J = 7.84 Hz), 8.15 (d, 1H, J = 2.04 Hz), 7.97 (dd, 1H, J = 8.56 Hz), 7.77 (s, 2H), 7.52 (t, 2H, J = 7.84 Hz), 7.31 (m, 2H), 3.94 (s, 3H). 13C NMR (DMSO-d 6) δ: 168.31, 159.16, 156.14, 152.44, 143.86, 139.14, 136.26, 131.05, 129.17 (2C), 127.93, 125.89, 125.87, 120.66 (2C), 113.29, 111.47, 107.21, 104.76, 56.57; MS (EI): m/z = 440 (M+1). Anal. Calcd. for C20H15BrN4O3: C, 54.69; H, 3.44; N, 12.75. Found: C, 54.70; H, 3.28; N, 12.63.

Synthesis of (6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridin-5-yl)methanol, 3

To a solution of acid 2 (13 g, 29.61 mmol) in toluene, Vitride (52 ml) was charged at room temperature in one lot, with vigorous stirring, leads to an exothermic reaction. And the reaction mixture was stirred for one hour. Ammonium acetate solution in water (30 %) was added to quench the reaction mixture. Solid obtained was filtered through celite and washed with toluene. Filtrate transferred in seperatory funnel, aqueous layer was extracted with ethyl acetate (3 × 20 ml). Then the combined organic layers were washed with water (3 × 20 ml), dried over anhydrous sodium sulfate and concentrated in vacuum to furnished 3 pale yellow solid.

Pale yellow solid; (Yield = 7.5 g, 73 %); mp 194-197 °C; IR (KBr, cm−1): 3450, 3348, 1622, 1494; 1H NMR (DMSO-d 6) δ: 8.39 (d, 2H, J = 6.80 Hz), 8.15 (s, 1H), 7.95 (d, 2H, J = 6.80 Hz), 7.53 (m, 2H), 7.26 (m, 1H), 7.12 (d, 2H, J = 6.80), 6.52 (s, 2H), 5.34 (t, 1H, J = 5.20), 4.51 (d, 2H J = 5.20), 3.85 (s, 3H). 13C NMR (DMSO-d 6) δ: 159.95, 157.91, 151.13, 143.77, 140.36, 129.35 (2C), 128.86, 128.48 (2C), 125.96, 125.27, 120.38 (2C), 119.05, 114.87 (2C), 107.15, 60.72, 55.71; MS (EI): m/z = 347 (M+1), 369 (Na+). Anal. Calcd. for C20H18N4O2: C, 69.35; H, 5.24; N, 16.17. Found: C, 69.27; H, 5.30; N, 16.12.

Synthesis of 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]pyridine-5-carbaldehyde, 4

To the solution of 3 (7 g, 20.23 mmol) in dichloromethane (35 ml) was added Manganese (IV) dioxide (5.27 g, 60.59 mmol) at room temperature. The reaction mixture was stirred for 24 h at room temperature. After completion of the reaction as indicated by TLC (eluent ethylacetate/hexane, 1:1), the reaction mixture was filtered through celite and evaporated dichloromethane under reduced pressure. The crude product obtained was purified by crystallization from methanol to furnished 4 as pale yellow solid.

Pale yellow solid; (Yield = 6.2 g, 89 %); mp 196-197 °C; IR (KBr, cm−1): 3415, 3348, 1657, 1506; 1H NMR (DMSO-d 6) δ: 9.95 (s, 1H), 8.90 (s, 1H), 8.29 (t, 2H, J = 7.68 Hz), 8.00 (dd, 2H, J =1.84 Hz), 7.93 (s, 2H), 7.52 (m, 2H, J = 7.68), 7.32 (d, 1H, J = 7.40 Hz), 7.10 (d, 2H, J = 8.84), 3.84 (s, 3H). 13C NMR (DMSO-d 6) δ: 193.18, 160.20, 158.09, 158.02, 152.17, 145.71, 142.97, 139.05, 129.11, 128.54 (2C), 125.89, 124.25, 120.77 (2C), 114.58, 112.62, 112.59, 108.03, 55.39; MS (EI): m/z = 345 (M+1). Anal. Calcd. for C20H16N4O2: C, 69.76; H, 4.68; N, 16.27. Found: C, 69.88; H, 4.73; N, 16.60.

Synthesis of 3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b][1,8]- naphthyridine derivatives, 6a–j

General Procedure

To a solution of 6-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b]- pyridine-5-carbaldehyde 4 (2 mmol), aliphatic or aromatic ketones or cyclic ketones (5a–j) (2 mmol) was added ethanolic potassium hydroxide solution (10 ml, 2 %). The reaction mixture was refluxed for 2 h. After completion of the reaction as indicated by TLC (eluent ethylacetate/hexane, 1:1), the reaction mixture was cooled to room temperature. The precipitated solid was collected by filtration, washed with ethanol, dried and crystallized from ethanol.

7-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b][1,8]naphthyridine-6-carboxamide, 6a

This compound was synthesized by using cyanoacetamide; yellow solid (Yield = 0.52 g, 88 %); mp > 315 °C; IR (KBr, cm−1): 3074, 3004, 1610, 1492; 1H NMR (DMSO-d 6) δ: 9.00 (s, 1H), 8.73 (s, 1H), 8.51 (d, 2H, J = 7.76 Hz), 8.26 (s, 1H), 8.11 (d, 2H, J = 8.76 Hz), 7.87 (s, 2H), 7.69 (s, 1H), 7.58 (t, 2H, J = 8.28 Hz), 7.31 (t, 1H, J = 7.36 Hz), 7.17 (d, 2H, J = 8.80 Hz), 3.88 (s, 3H). 13C NMR (DMSO-d 6) δ: 168.93, 160.14, 160.09, 156.14, 152.72, 144.48, 140.84, 139.38, 133.71, 129.02 (2C), 128.48 (2C), 125.21, 124.29, 119.95 (2C), 114.61 (2C), 114.54, 113.32, 112.64, 55.33. MS (EI): m/z = 411 (M+1). Anal. Calcd. for C23H18N6O2: C, 67.31; H, 4.42; N, 20.48. Found: C, 67.36; H, 4.27; N, 20.29.

7-amino-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b][1,8]naphthyridine-6-carbonitrile, 6b

This compound was synthesized by using malononitrile; yellow solid (Yield = 0.48 g, 86 %); mp > 315 °C; IR (KBr, cm−1): 3336, 3247, 2212, 1656, 1552; 1H NMR (DMSO-d 6) δ: 9.16 (s, 1H), 8.91 (s, 1H), 8.47 (d, J = 7.88 Hz, 2H), 8.10 (d, J = 8.68 Hz, 2H), 7.67 (s, 2H), 7.59 (t, J = 7.62 Hz, 2H), 7.48 (s, 1H), 7.18 (d, J = 8.76 Hz, 2H), 3.88 (s, 3H). 13C NMR (DMSO-d 6) δ: 165.35, 160.60, 153.24, 156.92, 150.67, 145.81, 141.10, 139.33, 131.84, 129.47 (2C), 128.54 (2C), 125.21, 124.29, 120.26 (2C), 116.37, 115.11, 114.54 (2C), 81.36, 25.82. MS (EI): m/z = 393 (M+1). Anal. Calcd. for C23H16N6O: C, 70.40; H, 4.11; N, 21.42. Found: C, 70.51; H, 4.35; N, 21.33.

3-(4-methoxyphenyl)-1-phenyl-1,6-dihydroindeno[1,2-b]pyrazolo[4,3-g][1,8]- naphthyridine, 6c

This compound was synthesized by using indanone; yellowish brown solid (Yield = 0.47 g, 74 %); mp 310–312 °C; IR (KBr, cm−1): 3469, 3327, 1642, 1509; 1H NMR (DMSO-d 6) δ: 8.17 (s, 2H), 7.87 (d, 2H, J = 8.15 Hz), 7.91 (m, 2H), 7.37 (m, 2H), 7.31 (m, 4H), 7.28 (dd, 1H, J = 8.40 Hz), 7.16 (m, 2H) 3.88 (s, 2H), 3.75 (s, 3H).13C NMR (DMSO-d 6) δ:160.35, 158.42, 155.12, 149.11, 145.98, 140.46, 139.33, 137.80, 135.74, 133.21, 131.96, 129.65, 129.40 (2C), 128.61 (2C), 127.44, 127.83, 126.05 (2C), 125.74, 120.19, 120.88 (2C), 114.67, 114.80 (2C), 55.62, 36.08; MS (EI): m/z = 441 (M+1). Anal. Calcd. for C29H20N4O: C, 79.07; H, 4.58; N, 12.72. Found: C, 78.94; H, 4.37; N, 12.07.

3-(4-methoxyphenyl)-1-phenyl-6,7,8,9-tetrahydro-1H-benzo[b]pyrazolo[4,3-g]- [1,8]naphthyridine, 6d

This compound was synthesized by using cyclohexanone; yellow solid (Yield = 0.49 g, 84 %); mp 246–249 °C; IR (KBr, cm−1): 3439, 3340, 1611, 1496; 1H NMR (DMSO-d 6) δ: 8.62 (s, 2H), 8.42 (d, 2H, J = 8.00 Hz), 8.24 (d, 1H, J = 6.00 Hz), 7.91 (m, 2H), 7.52 (t, 2H, J = 4.40 Hz), 7.27 (m, 1H), 7.01 (dd, 1H, J = 8.40 Hz), 3.96 (s, 3H), 3.09 (m, 2H). 2.95 (m, 2H), 1.91 (m, 4H).13C NMR (DMSO-d 6) δ: 160.91, 156.33, 156.02, 149.64, 146.95, 139.22, 135.10, 134.00, 132.87, 129.14 (2C), 128.35 (2C), 126.94, 125.30, 124. 17, 120.96 (2C), 114.28, 114.02 (2C), 55.48, 31.90, 31.88, 23.74 (2C); MS (EI): m/z = 407 (M+1). Anal. Calcd. for C26H22N4O: C, 76.83; H, 5.46; N, 13.78. Found: C, 76.82; H, 5.81; N, 13.40.

7-(3-methoxyphenyl)-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b][1,8]- naphthyridine, 6e

This compound was synthesized by using 3-methoxy acetophenone; yellow solid (Yield = 0.54 g, 82 %); mp 214–217 °C; IR (KBr, cm−1): 3476, 3321, 1609, 1502; 1H NMR (DMSO-d 6) δ: 8.45 (d, 2H, J = 7.60 Hz), 8.43 (s, 1H), 8.22 (m, 2H), 8.08 (m, 2H), 7.89 (s, 1H), 7.61 (dd, 1H, J = 4.40), 7.52 (m, 2H), 7.27 (m, 2H), 6.99 (m, 3H), 3.94 (s, 3H), 3.88 (s, 3H). 13C NMR (DMSO-d 6) δ: 162.32, 160.57, 160.20, 154.85, 152.42, 144.73, 140.14, 139.56, 139.33, 131.98, 129.79, 129.14 (2C), 128.82 (2C), 125.73, 124.82, 121.23 (2C), 120.67, 118.20, 118.01, 117.04, 116.98, 114.62 (2C), 112.93, 55.46, 30.94; MS (EI): m/z = 459 (M+1). Anal. Calcd. for C29H22N4O2 : C, 75.97; H, 4.84; N, 12.22. Found: C, 75.91; H, 4.95; N, 12.10.

3-(4-methoxyphenyl)-1-phenyl-7-(thiophen-2-yl)-1H-pyrazolo[3,4-b][1,8]- naphthyridine, 6f

This compound was synthesized by using 2-acetyl thiophene; yellow solid (Yield = 0.53 g, 85 %); mp 298–301 °C; IR (KBr, cm−1): 3453, 3320, 1631, 1491; 1H NMR (DMSO-d 6) δ: 9.18 (s, 1H), 8.55 (d, 2H, J = 8.42 Hz), 8.47 (s, 1H), 8.22 (m, 2H), 7.71 (m, 2H), 7.60 (m, 1H), 7.30 (m, 2H), 7.28 (m, 1H), 7.11 (d, 1H, J = 7.68 Hz), 7.03 (m, 2H), 3.89 (s, 3H). 13C NMR (DMSO-d 6) δ: 160.52, 158.94, 152.06, 146.88, 148.87, 139.90, 139.33, 135.88, 135.27, 129.61 (2C), 128.41 (2C), 128.11, 127.95, 126.36 (2C), 125.89, 124.66, 121.47, 120.44 (2C), 115.72, 114.93 (2C), 55.74; MS (EI): m/z = 435 (M+1). Anal. Calcd. for C26H18N4OS: C, 71.87; H, 4.18; N, 12.89. Found: C, 71.75; H, 4.39; N, 12.91.

7-ethyl-3-(4-methoxyphenyl)-6-methyl-1-phenyl-1H-pyrazolo[3,4-b][1,8]- naphthyridine, 6 g

This compound was synthesized by using 3-pentanone; green solid (Yield = 0.42 g, 75 %); mp 217–219 °C; IR (KBr, cm−1): 3430, 3355, 1667, 1495; 1H NMR (DMSO-d 6) δ: 9.35 (s, 1H), 8.52 (d, 2H, J = 8.52 Hz), 8.36 (s, 1H), 8.17 (m, 2H), 7.63 (m, 2H), 7.36 (m, 1H), 7.18 (m, 2H), 3.89 (s, 3H), 3.02 (q, 2H), 2.57 (s, 3H), 2.52 (s, 3H), 1.37 (t, 3H, J = 7.40 Hz). 13C NMR (DMSO-d 6) δ: 161.42, 157.98, 154.90, 149.62, 145.77, 139.01, 135.44, 132.91, 131.80, 130.96, 128.31, 126.55, 125.78, 122.03, 120.58 (2C), 114.60, 114.04 (2C), 53.84, 25.37, 19.44, 15.60; MS (EI): m/z = 395 (M+1). Anal. Calcd. for C25H22N4O: C, 76.12; H, 5.62; N, 14.20. Found: C, 76.46; H, 5.81; N, 14.57.

3-(4-methoxyphenyl)-6,7-dimethyl-1-phenyl-1H-pyrazolo[3,4-b][1,8]- naphthyridine, 6 h

This compound was synthesized by using methyl ethyl ketone; yellowish green solid (Yield = 0.43 g, 78 %); mp 237–239 °C; IR (KBr, cm−1): 3447, 3315, 1640, 1494; 1H NMR (DMSO-d 6) δ: 8.77 (s, 1H), 8.62 (d, 2H, J = 8.40 Hz), 8.06 (m, 2H), 8.02 (s, 1H), 7.54 (m, 2H), 7.27 (s, 1H), 7.10 (dd, 2H, J = 6.80, 4.80 Hz), 3.93 (s, 3H), 2.82 (s, 3H), 2.51 (s, 3H). 13C NMR (DMSO-d 6) δ: 183.49, 165.17, 160.10, 151.42, 144.01, 139.38, 137.34, 132.14, 129.12, 128.83 (2C), 128.45 (2C), 125.06, 120.02 (2C), 118.06, 115.47, 114.38 (2C), 55.15, 30.54, 23.92, 18.80; MS (EI): m/z = 381 (M+1). Anal. Calcd. for C24H20N4O: C, 75.77; H, 5.30; N, 14.73. Found: C, 75.90; H, 5.28; N, 14.55.

7-isopropyl-3-(4-methoxyphenyl)-1-phenyl-1H-pyrazolo[3,4-b][1,8]- naphthyridine, 6i

This compound was synthesized by using 3-methyl-2-butanone; yellowish green solid (Yield = 0.45 g, 80 %); mp 180–183 °C; IR (KBr, cm−1): 3450, 3338, 1626, 1491; 1H NMR (DMSO-d 6) δ: 8.89 (s, 1H), 8.61 (dd, 2H, J = 8.40 Hz), 8.34 (d, 1H, J = 8.40 Hz), 8.09 (dd, 2H, J = 10, 7.20 Hz), 7.56 (t, 2H, J = 7.60 Hz), 7.45 (d, 1H, J = 8.80 Hz), 7.27 (s, 1H), 7.12 (dd, 2H, J = 6.80, 4.80 Hz), 3.94 (s, 3H), 3.46 (m, 1H), 1.47 (d, 6H, J = 7.20 Hz). 13C NMR (DMSO-d 6) δ: 173.10, 162.45, 153.94, 151.50, 144.18, 139.44, 139.20, 133.40, 128.90 (2C), 128.50 (2C), 125.29, 124.11, 120.34 (2C), 118.74, 117.57, 115.67, 114.40 (2C), 55.16, 37.01, 21.96; MS (EI): m/z = 395 (M+1). Anal. Calcd. for C25H22N4O: C, 76.12; H, 5.62; N, 14.20. Found: C, 76.03; H, 5.84; N, 14.18.

3-(4-methoxyphenyl)-7-methyl-1-phenyl-1H-pyrazolo[3,4-b][1,8]-naphthyridine, 6j

This compound was synthesized by using acetone; green solid (Yield = 0.46 g, 88 %); mp 233–235 °C; IR (KBr, cm−1): 3472, 3319, 1644, 1502; 1H NMR (DMSO-d 6) δ: 8.35 (s, 1H), 8.62 (d, 2H, J = 7.6), 8.24 (d, 1H, J = 8.40), 8.06 (d, 2H, J = 8.80), 7.55 (t, 2H, J = 7.60), 7.27 (m, 2H), 7.10 (d, 2H, J =8.80), 3.92 (s, 3H), 2.88 (s, 3H). 13C NMR (DMSO-d 6) δ: 160.47, 158.13, 157.08, 149.77, 145.30, 139.28, 135.11, 134.06, 129.99 (2C), 128.37 (2C), 126.61, 125.92, 124.26, 121.55, 120.55 (2C), 114.73, 114.09 (2C), 55.40, 24.68; MS (EI): m/z = 367 (M+1). Anal. Calcd. for C23H18N4O: C, 75.39; H, 4.95; N, 15.29. Found: C, 75.69; H, 4.70; N, 15.52.