A convenient four-component one-pot strategy toward the synthesis of pyrazolo[3,4-d]pyrimidines

An efficient one-pot synthesis of pyrazolo[3,4-d]pyrimidine derivatives by the four-component condensation of hydrazines, methylenemalononitriles, aldehydes and alcohols has been developed via two different reaction pathways. The structures of target products were characterized by IR spectroscopy, NMR (1H and 13C) spectroscopy and HRMS (ESI) spectrometry. The crystal structure of 4-ethoxy-6-(2-nitrophenyl)-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine was determined by single crystal X-ray diffraction.

In our previous studies, dihydropyrimidinone was synthesized through the condensation of 5-aminopyrazole-4-carbonitrile and ketones [29,30]. 5-Aminopyrazole-4-carbonitrile was prepared from the reaction of ethoxymethylenemalononitrile with phenylhydrazine in a step-wise fashion [43][44][45][46][47]. During the course of previous studies, we envisioned that we could combine these reactions and embarked on designing a strategy toward a one-pot synthesis by combining the three reactants. When benzaldehyde was used as the reactant, the target product was obtained (Scheme 1A). But when benzaldehyde was switched to anisaldehyde, the expected product was not

Results and Discussion
Phenylhydrazine, 2-(ethoxymethylene)malononitrile, ethanol and benzaldehyde were selected as the model reactants. The influence of the reaction conditions was studied and the results are summarized in Table 1. No target product was afforded in the presence of an inorganic weak base or without a catalyst (  1, entry 3). This shows that the catalytic properties of sodium hydroxide have some limitations. Fortunately, some strong bases could promote the reaction to produce 5a, though DBU needed a higher reaction temperature (Table 1, entries 4-6). Taking into account the yield of the reaction, sodium alkoxide was the best choice. The reaction performed in alcohol resulted in the highest yield ( Table 1, entries 6-9). The reaction temperature was screened and the appropriate temperature was found to be 60 °C (Table 1, entries 6, 10 and 11). The amount of catalyst had an effect on the reaction and 1.2 equivalents of sodium alkoxide was the most appropriate choice (Table 1, entries 12 and 13). This means that sodium alkoxide is not only a catalyst, but also participates in the reaction.
A series of hydrazines, methylenemalononitriles, aldehydes and alcohols were investigated under the optimal reaction conditions. As shown in Figure 1, under optimal conditions. The corresponding products were obtained in good yield (Figure 1, 5a-g). Then a set of hydrazines were selected and the target products were obtained. However, the yield of aromatic hydrazines bearing electronwithdrawing groups or electron-donating groups was higher than that of methylhydrazine (Figure 1, 5h-j). This is possibly due to the electronic effect of the substituents. Though the steric hindrance could affect the reaction, 3-substituted pyrazolo [3,4d]pyrimidine was also obtained in good yield (Figure 1, 5k). In order to further broaden the scope of this one-pot methodology, a series of alcohols such as methanol, n-butanol, n-propanol and isopropanol were investigated. The expected products were also obtained in good yield (Figure 1, 5i-p). This fact revealed the universality and advantages of this method for the synthesis of pyrazolo [3,4-d]pyrimidines.
With those results in hand, two possible reaction mechanisms were proposed and shown in Scheme 3. 5-Aminopyrazole-4carbonitrile 6 was obtained from the reaction of hydrazine 1 and methylenemalononitrile 2 through nucleophilic addition, cyclization and aromatization. The nucleophilic attack of the amino group of 6 on the carbonyl group of the aldehyde affords 7. Then 7 provides the target product via two different reaction pathways. The first route is that 7 loses a water molecule to afford the Schiff base 8. Then 8 undergoes a Pinner reaction and imine 9 is formed, and then 9 turns into 10 through intramolec-ular cyclization. Finally, 10 is oxidized to give pyrazolo[3,4d]pyrimidine 5. Another route is that 7 undergoes an intramolecular Pinner reaction to form 11. Then 11 rearranges to dihydropyrazolo [3,4-d]pyrimidin-4-ones 13 via Dimroth rearrangement and 13 is oxidized to provide 14 [49]. Finally, 14 undergoes a nucleophilic addition and loses a water molecule to afford the final product 5.
All products were characterized by IR, 1 H NMR, 13 C NMR and HRMS. A final confirmation of the structure of the reaction product 5g was determined by X-ray diffraction ( Figure 2) [50].

Conclusion
In summary, we have disclosed an efficient one-pot fourcomponent synthesis of pyrazolo [3,4-d]pyrimidines. The simplicity of execution, readily available substrates and the potentially important use of the products make this synthetic protocol attractive for academic research and practical applications. Further studies towards the detailed mechanism and synthetic application of this protocol are in progress.

Supporting Information
Supporting Information File 1 Experimental section and copies of 1 H and 13 C NMR spectra of compounds.