Expeditious microwavable one-pot synthesis and biological exploration of spiro[indoline-3,4′-pyrazolo[3,4-b] pyridine derivatives

A rapid and efficient microwave-promoted one-pot strategy has been utilized to synthesize differentially functionalized spiro[indoline-3,4′-pyrazolo[3,4-b] pyridine derivatives. Different optimized reaction conditions have been exploited when one-pot four-component (hydrazines, 1,3-dicarbonyl compounds, isatin, and malononitrile) reactions took place to afford satisfying yields of compounds [5–28]. Among different methodologies, the catalyst-free microwave-assisted fusion process has been proved to be the strategy with desirable traits to give good-to-excellent yields of the target compounds. Structural features of the obtained compounds were confirmed by the basis of elemental analyses, mass, 1H-NMR, and 13C-NMR spectrometry.


Introduction
Multicomponent reactions (MCRs) have increasingly emerged as a useful tool for the synthesis of chemically and biologically important compounds because of their convergence, atom economy, and green chemistry point of view [1][2][3]. Interestingly, a large number of organic reactions carried out under these conditions afforded higher yields with shorter reaction time under milder conditions [4][5][6]. According to literature, reactions involving four different simple substrates have been proved to be powerful means for expedient building up of molecular complexity and diversity [7][8][9] through the facile formation of several new covalent bonds in a one-pot transformation.
As anticipated, in other ethanolic patches, the same previous components were mixed and refluxed in presence of different Lewis acid catalysts such as AlCl 3 , ZnCl 2 , FeCl 3 , CAN, and nano ZnO. From Table 1, entries (7)(8)(9)(10)(11) showed improvements in the overall yields (35 up to 50%) compared with the catalyst-free approach. Pursuing excellent yields, we sought to endeavor another route; therefore, the starting four components were mixed and refluxed in the presence of basic catalysts such as NaOH, KOH, and TEA. The yields were not improved in such a satisfactory trend as depicted in entries (12, 13, and 14) (Table 1).
Further optimization of the experimental conditions accompanied by the green chemistry approach led us to explore the combination of solvent/catalyst-free conditions under microwave-assisted irradiation. The reaction mixture components were fused. Following the reaction progress by TLC examination showed that the reaction duration was dramatically improved down to 60 s at 100 °C accompanied by an astonishing yield of 97%.
The advantageous microwave-assisted synthesis of derivative 5 as stated vide supra, made this approach a reliable and universal methodology in providing a facile and ultrafast technique to obtain the remaining spiro compounds 6-28 ( Table 2).

Antibacterial activity evaluation
The antimicrobial activities of the novel compounds  have been screened using the agar diffusion method- [35,36] against a panel of bacterial strains Staphylococcus aureus (G +ve), Pseudomonas aeruginosa (G −ve) as well as two different fungi: Candida albicans (yeast) and Aspergillus niger (fungus). The obtained results are compiled in Table 3. Most of the compounds showed moderate to high antimicrobial activities. In particular, compounds 5, 9, 12, 14, 17, and 23 showed remarkable antimicrobial activity which suggests that those compounds might have promising chances to be plausible antimicrobial curative potencies.

Determination of total antioxidant capacity (TAC)
The total antioxidant capacity (TAC) of the tested compounds was estimated via the phosphomolybdenum antioxidant technique. The TAC values are referred to as ascorbic acid in mg AAE/g compound are presented in From the above-mentioned results, we can conclude that the tested compounds have different abilities to reduce Mo(VI) into Mo(V) and subsequent formation of a green phosphate/Mo(V) complex at pH acidic; which may be due to the variation in the chemical structures of tested compounds and to the presence or absence of certain substituents like (carbonyl, hydroxyl, methoxy, and carboxylic groups).

Conclusions
An environmentally benign and efficient microwaveassisted methodology has been developed for the synthesis of spiro[indoline-3,7′-pyrazolo[3,4-b]pyridines derivatives ; in which solvent/catalyst-free one-pot multi-component conditions were utilized. This method which is categorized as one of the green chemistry approaches showed many advantages as swift progress, short reaction time, and non-tedious work up. Moreover, excellent yields of the target compounds were affordable. These spiro-compounds might

Experimental
All melting points were measured on a Gallenkamp electric melting point apparatus are uncorrected. The infrared spectra were recorded in potassium bromide disks on a pye-Unicam SP-3-300 and Shimdazu FT IR 8101 PC infrared spectrophotometers at the central laboratory of the faculty of science, Ain Shams University. The 1 H-NMR and 13 C NMR spectra were recorded on a Varian Mercury VX-300 MHz using TMS as an internal standard in deuterated dimethylsulphoxide (DMSO-d 6 ). Chemical shifts are measured in ppm. The mass spectra were recorded on a Shimadzu GCMS-QP-1000EX mass spectrometer At 70 e.v. Elemental analyses were carried out at the Microanalytical center of Ain Shams University. All the reactions and the purity of the new compounds were followed and cheeked by TLC using TLC aluminum sheets silica gel F 254 .

General procedure for the synthesis of spiro[indolin e-3,4′-pyrazolo[3,4-b] pyridines (5-28)
A mixture of hydrazine derivatives (1 mmol), malononitrile/ ethyl cyanoacetate (1 mmol), and a suitable 1,3-dicarbonyl compound (1 mmol) was added to isatin (1 mmol). The reaction mixture was heated under microwave irradiation at 100 °C for 1 min after completion of the reaction monitored by TLC, the reaction mixture was cooled to room temperature, the precipitate filtered off, and washed with methanol to obtain the pure spiro-heterocycles 5-28 as yellow solids. Spectroscopic data for all these compounds are given below.

5′-Benzoyl-3′-imino-2′-methyl-6′-phenyl-2′,3′,3a′ ,7′-tetrahydrospiro[indoline-3,4′-pyrazolo [3,4-b] pyridin]-2-one (10)
Isolated as a yellow solid, yield: 90%, mp 202-204 °C (recrystallization from EtOH/DMF mixture); 1 H-NMR Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.