High-Pressure Metal-Free Catalyzed One-Pot Two-Component Synthetic Approach for New 5-Arylazopyrazolo[3,4-b]Pyridine Derivatives

An appropriate and efficient Q-tube-assisted ammonium acetate-mediated protocol for the assembly of the hitherto unreported 5-arylazopyrazolo[3,4-b]pyridines was demonstrated. This methodology comprises the cyclocondensation reaction of 5-amino-2-phenyl-4H-pyrazol-3-one with an assortment of arylhydrazonals in an NH4OAc/AcOH buffer solution operating a Q-tube reactor. This versatile protocol exhibited several outstanding merits: easy work-up, mild conditions, scalability, broad substrate scope, safety (the Q-tube kit is simply for pressing and sealing), and a high atom economy. Consequently, performing such reactions under elevated pressures and utilizing the Q-tube reactor seemed preferable for achieving the required products in comparison to the conventional conditions. Diverse spectroscopic methods and X-ray single-crystal techniques were applied to confirm the proposed structure of the targeted compounds.


Introduction
Heterocycles are pivotal in diversified domains as they are considered the essential motif in industrial, agricultural, and biological fields. Intriguingly, heterocyclic compounds are prevalent in more than 85% of the pharmacologically active substances, and over 60% of FDA-approved medications possess, in their structure, nitrogen-based heterocycles [1]. Considering that they are contained in numerous therapeutic medicines marketed as anxiolytics, including cartazolate, etazolate, and tracazolate, they have been recognized as potent pharmaceutically important compounds ( Figure 1) [2]. Furthermore, pyrazolo [3,4-b]pyridines are the key components of the cardiovascular therapeutic drug BAY 41-2272 [3] and the Glycogen Synthase Kinase 3 (GSK-3) inhibitor that is effective in the treatment of Alzheimer's disease [4,5]. They are generally utilized to treat pulmonary hypertension as sGC stimulators [6,7]. Interestingly, pyrazolo [3,4-b]pyridines have a substantial inhibitory impact on diverse enzymes, such as Cyclin-Dependent Kinase (CDK) [8], ognized as potent pharmaceutically important compounds ( Figure 1) [2]. Furthermore, pyrazolo [3,4-b]pyridines are the key components of the cardiovascular therapeutic drug BAY 41-2272 [3] and the Glycogen Synthase Kinase 3 (GSK-3) inhibitor that is effective in the treatment of Alzheimer's disease [4,5]. They are generally utilized to treat pulmonary hypertension as sGC stimulators [6,7]. Interestingly, pyrazolo [3,4-b]pyridines have a substantial inhibitory impact on diverse enzymes, such as Cyclin-Dependent Kinase (CDK) [8], Anaplastic Lymphoma Kinase (ALK) [9], nucleotide pyrophosphatase, and human recombinant alkaline phosphatase [10][11][12]. It is noteworthy that the pyrazolo [3,4-b]pyridine motif is a versatile system with various advantages. Antiproliferative, antiviral, antimicrobial [13], anticancer [14], anti-inflammatory [15], anti-HIV [16], antioxidant [17], antiallergic, and antiherpetic [18] biological activities are just a few of these. Due to the diverse applications of the heterocycles that comprise the pyrazolo [3,4-b]pyridine moiety, developing new protocols for their synthesis is challenging for pharmaceutical and organic chemists. The first Q-tube-mediated, high-pressure strategy for the preparation of an unparalleled series of thiazolo [4,5-c]pyridazines was recently published by our group [19]. Consequently, as part of our continued endeavors to develop efficient, environmentally friendly, and expedient protocols, a metal-free catalyst, high-pressure-assisted strategy for synthesizing pyrazolo [3,4-b]pyridines was explored in the present study. High-pressure chemistry (HPC) has been an unconventional, promising, practical, and full-potential approach to organic synthesis since 1981 [20]. Specifically, the use of the Q-tube is regarded as a pioneer in HPC. Notably, the Q-tube-mediated approach has numerous substantial advantages, including high reaction rates, cleaner reaction profiles, smaller reaction volumes, and quantitative conversions. According to the Arrhenius equation, utilizing the Q-tube approach might exponentially enhance the reaction rate as the boiling point is elevated. Furthermore, increasing the pressure inside the Q-tube improves the probability of reactant collision, which accelerates the reaction rate, resulting in the minimization of competitive reagent decompositions and a smoother reaction profile [20]. In addition, the Q-tube is a cost-effective alternative to the costly microwave (MW) technique; it permits the reactions to be performed at a temperature higher than the solvent's boiling point, even for MW-transparent solvents [21]. AlMarzouq et al. reported an intriguing assessment of numerous traditional and alternative heating procedures in 2016. The Q-tube strategy was recommended as the technique of preference for the cleanest, shortest, and most effective preparation of the heterocyclic compounds under study [22]. In other respects, the coupling Q-tube-assisted approach with the one-pot multicomponent reactions (MCRs) strategy is considered to be one of the most advantageous protocols for achieving step efficiency and atom economy [19,[23][24][25][26]. In this study, the coupling of the Q-tubemediated protocol with the two-component reaction (MCR) strategy can be investigated to provide universal access to a series of unreported 5-arylazopyrazolo [3,4-b]pyridines with superior reaction profiles and higher rates and approximately quantitative yields. Furthermore, increasing the pressure inside the Q-tube improves the probability of reactant collision, which accelerates the reaction rate, resulting in the minimization of competitive reagent decompositions and a smoother reaction profile [20]. In addition, the Q-tube is a cost-effective alternative to the costly microwave (MW) technique; it permits the reactions to be performed at a temperature higher than the solvent's boiling point, even for MW-transparent solvents [21]. AlMarzouq et al. reported an intriguing assessment of numerous traditional and alternative heating procedures in 2016. The Q-tube strategy was recommended as the technique of preference for the cleanest, shortest, and most effective preparation of the heterocyclic compounds under study [22]. In other respects, the coupling Q-tube-assisted approach with the one-pot multicomponent reactions (MCRs) strategy is considered to be one of the most advantageous protocols for achieving step efficiency and atom economy [19,[23][24][25][26]. In this study, the coupling of the Q-tube-mediated protocol with the two-component reaction (MCR) strategy can be investigated to provide universal access to a series of unreported 5-arylazopyrazolo [3,4-b]pyridines with superior reaction profiles and higher rates and approximately quantitative yields.

General
The measured melting points were determined by employing a Griffin melting point device, and the results were given incorrectly. The FT-IR spectra (KBr) were obtained utilizing the Jasco FT-IR-6300 spectrometer. The NMR spectra ( 1 H: 600 MHz and 13 C: 150 MHz) were obtained utilizing the Bruker DPX 600 super-conducting spectrometer, where the TMS was used as an internal reference and DMSO-d6 or TFA-d as the solvent. The molecular weights of the synthesized compounds were recorded by employing both a high-resolution GC-MS (DFS) thermo-spectrometer [MS (EI) at 70.1 eV] and the magnetic sector mass analyzer [HRMS (EI)]. Thin layer chromatography (TLC) was used to monitor the progress of the reactions and to ensure product purity. All the reactions were carried out using a Q-tube gas purging kit (180 psi) from Q Labtech (Sigma-Aldrich, St. Louis, MO, USA), which included a catch bottle, PTFE-faced silicone septa, a borosilicate glass pressure tube (35.0 mL), a needle adapter, a Teflon sleeve, and a stainless steel adapter with a pressure gauge (300 psi). Microwave heating was carried out with a single-mode cavity Explorer Microwave synthesizer (CEM Corporation, Matthews, NC, USA), producing continuous irradiation and equipped with a simultaneous external air-cooling system. The Bruker X8 Prospector or Rigaku R-AXIS RAPID II diffractometer (Billerica, MA, USA) was used to record the X-ray crystallographic results. The arylhydrazonal derivatives (2) were synthesized by following up on the reported protocols [27].

Results and Discussion
Owing to the remarkable therapeutic usages of the pyrazolopyridines, it is worthwhile to assemble a unique family of arylazopyrazolo [3,4-b]pyridines (3a−v, Scheme 1, Figures S1-S44), utilizing a safer and greener approach. A series of arylhydrazonals 1a−v was constructed following the reported protocols [27]. The reaction involving 5-amino-2phenyl-4H-pyrazol-3-one (1) and 3-oxo-2-arylhydrazonopropanal (2a) was chosen as a template reaction to evaluate and study the optimal reaction conditions (Scheme 1 and Table 1). Scheme 1. Synthesis of 5-arylazopyrazolo [3,4-b]pyridine derivatives 3a-v.   At the outset, it was observed that refluxing a mixture of 5-amino-2-phenyl-4H-pyrazol-3-one (1, 5.0 mmol) and arylhydrazonal (2a, 5.0 mmol) in various solvents, including polar aprotic solvents (dioxane and CH 3 CN) and polar protic solvents (ethanol and propanol), comprising AcONH 4 or anhydrous AcONa (10.0 mmol) under normal pressure for 12 h, did not produce any new products (Table 1, entries 1-4). They were interesting; utilizing DMF as a reaction solvent produced a new product in a 14% yield within 6 h, while the reaction yield did not increase with the increasing of the reaction duration (Table 1, entry 6). Furthermore, refluxing the selected reactants in acetic acid for 3 h yielded a product of a 45% yield when AcONH 4 was used as an additive and a 30% yield when anhydrous AcONa was employed (Table 1, entries 6 and 7). Consequently, AcONH 4 will be employed as an additive in the subsequent experiments. According to the results of several analyses, the newly obtained products in the cases above (Table 1, entries 5-7) are matched and elucidated to be 2,6-diphenyl-5-(phenyldiazenyl)-2,7-dihydro-3H-pyrazolo [3,4-b]pyridin-3-one (3a) and not the open-chain derivative 4 (Scheme 2). Among these analyses, the high-resolution mass and mass spectrometric analyses (See SI) of 3a exhibited an exact mass of m/z 391.1428 and a mass of m/z 391, respectively, for the related molecular formula of C 24 H 17 N 5 O. The 1 H NMR spectrum of 3a in DMSO-d6 revealed a multiplet at δ 7.27-7.99 ppm due to 15 aromatic protons, a singlet signal for pyridine C-H4 at δ 8.25 ppm, and an abroad singlet assigned for the NH proton at δ 12.34 ppm. Furthermore, as anticipated, the 13 C NMR spectrum of 3a exhibited 18 signals with only one carbonyl signal. panol), comprising AcONH4 or anhydrous AcONa (10.0 mmol) under normal pres for 12 h, did not produce any new products ( Table 1, entries 1-4). They were interes utilizing DMF as a reaction solvent produced a new product in a 14% yield within while the reaction yield did not increase with the increasing of the reaction duration (T 1, entry 6). Furthermore, refluxing the selected reactants in acetic acid for 3 h yield product of a 45% yield when AcONH4 was used as an additive and a 30% yield w anhydrous AcONa was employed (Table 1, entries 6 and 7). Consequently, AcONH4 be employed as an additive in the subsequent experiments. According to the resul several analyses, the newly obtained products in the cases above (Table 1, entries 5-7 matched and elucidated to be 2,6-diphenyl-5-(phenyldiazenyl)-2,7-dihydro-3H-p zolo [3,4-b]pyridin-3-one (3a) and not the open-chain derivative 4 (Scheme 2). Am these analyses, the high-resolution mass and mass spectrometric analyses (See SI) exhibited an exact mass of m/z 391.1428 and a mass of m/z 391, respectively, for the re molecular formula of C24H17N5O. The 1 H NMR spectrum of 3a in DMSO-d6 reveal multiplet at δ 7.27-7.99 ppm due to 15 aromatic protons, a singlet signal for pyridin H4 at δ 8.25 ppm, and an abroad singlet assigned for the NH proton at δ 12.34 ppm. thermore, as anticipated, the 13 C NMR spectrum of 3a exhibited 18 signals with only carbonyl signal. Scheme 2. Reactions of 5-amino-2-phenyl-4H-pyrazol-3-one (1) with arylhydrazonal derivative 2a.
The remarkable results motivated us to investigate the optimal parameters that impact the model reaction in a green and sustainable approach. Additionally, the investigation will be extended to demonstrate a comparative study between the microwave technique and the Q-tube methodology as an economical and affordable alternative to the costly MW. For comparison, we initially employed a MW (250 watts, 140 • C, 30 min) to perform the template reaction by mixing an equimolar amount (2.0 mmol) of compound 1 and 2a in the presence of ammonium acetate (4.0 mmol)/acetic acid (5.0 mL) buffer solution. After the usual working up, compound 3a was delivered in a 66% yield ( Table 1, entry 8). Unfortunately, both the reaction rate and the obtained yield did not improve with the increasing of the reaction temperature and time. Interestingly, on employing the abovementioned reaction utilizing the Q-tube pressure reactor (140 • C, 30 min), the targeted product 3a was obtained with an 85% yield (Table 1, entry 9). It is worthwhile observing that doubling the amount of the substrates yielded 3a with a comparatively similar efficiency and that prolonging the reaction interval would not enhance the reaction yield; therefore, employing the Q-tube provides a cleaner reaction profile and higher yields. Additionally, the Q-tube reactor was employed to perform such reactions safely under high pressure, avoiding the risk of unintentional explosions that could occur when a conventional sealed tube was utilized. After affirming the effectiveness and merits of the Q-tube and the AcOH/AcONH 4 buffer in carrying out the desired reaction (Table 1, entry 9), the study was extended to investigate the impact of temperature on the reaction progress. The obtained results (Table 1, entries [10][11][12] indicated that the temperature considerably affects the reaction efficiency. For example, when the reaction was carried out at 150 • C, the target product was obtained at 92% (Table 1, entry 10); however, when the temperature was raised to 155, 160, and then 165 • C (Table 1, entries 11-13), compound 3a was obtained at 96%, 98%, and 98%, respectively, indicating that 160 • C is the optimized temperature for such a conversion (Table 1, entry 12).
Further investigations were carried out to study the potential, applicability, and limitations of the two NH 4 OAc-prompted successive condensation reactions (Figure 2) under the established optimal conditions (Table 1, entry 12). To achieve this target, a diversity of 3-oxo-arylhydrazonals 2a-v was prepared and introduced in order to evaluate their reactions with 5-amino-2-phenyl-4H-pyrazol-3-one (1) under the specified optimal conditions (Table 1, entry 12). In general, the electronic properties of the aryl motifs attached to 3-oxo-arylhydrazonals (2) had a minimal influence on reaction efficacy [28,29]. The reaction was exceptionally adaptable to both electron-releasing motifs as well as the electron-accepting motifs. Gratifyingly, the naphthyl 2e-i and thienyl 2j-m derivatives had similar, successful, and smooth pathways in the case of 5-amino-2-phenyl-4H-pyrazol-3-one (1), yielding the condensed products in excellent yields ( Figure 2). After several attempts, a suitable crystal for the X-ray single-crystallographic investigations was isolated as 3v to confirm the initial results ( Figure 3, Table 2). Moreover, the obtained single crystallographic data for the derivative 3v ( Figure 3, Table 2) confirmed the proposed structure and verified the regioselectivity of the reaction, yielding only the (E)-isomer of the 5-arylazopyrazolo[3,4b]pyridine derivatives. Scheme 3 depicts a plausible mechanistic approach for synthesizing 5-arylazopyrazolo [3,4-b]pyridines 3a-v. In the presence of acetic acid, the carbonyl groups became more polarized, and thus, their reactivity towards nucleophiles was enhanced. Firstly, the nucleophile generated from compound 1 underwent nucleophilic attack to the protonated carbonyl carbon of derivative 2, to give the adduct A. Subsequently, the adduct (A) was easily converted to the non-isolable intermediary B by removing the good releasing group (OH 2 + ). Secondly, the amino group underwent an intramolecular nucleophilic attack to the second protonated carbonyl carbon to obtain the intermediate C. Finally, the targeted products 3a-v were formed via the exclusion of another water molecule (Scheme 3).