Quinolines have received lots of attention from biologists and chemists as they are significant elements in the synthesis of dyes, fragrances, and natural products with biological activities (Michael, 2001; Michael, 2002; Michael, 2003; Michael, 2004; Michael, 2005; Michael, 2007; Michael, 2008; Isaac-Márquez et al., 2010; Cai et al., 2011; Russ et al., 2012). In pharmaceuticals, they have been outlined as, antibiotic (Mahamoud et al., 2006), anticancer (Insuasty et al., 2013; Sun et al., 2013), anti-inflammatory (Leatham et al., 1983), antimalarial (Nasveld and Kitchener, 2005), antihypertensive (Muruganantham et al., 2004), anti-HIV (Strekowski et al., 1991; Wilson et al., 1992), inhibition of Platelet-derived growth factor (PDGF) (Maguire et al., 1994), and anti-tuberculosis (Lilienkampf et al., 2009) agents. In other words, pyrrolidones are also typical buildings in several important categories of bioactive compounds (Jouyban et al., 2010; Kato and Nagao, 2012). Molecules bearing a pyrrolidone structure, are used in dye-sensitized solar cells and several natural products with biologically activeties (Daly et al., 1999; Dewick, 2009; Ikai et al., 2012). For instance, arcyria rubin A and its derivatives show potent antiviral activities (KIM et al., 1995; Slater et al., 1995), antimicrobial (Mahboobi et al., 2006), and powerful protein kinase C inhibitors (Davis et al., 1992).

The merger of these outstanding heterocycles, pyrrolidone, and quinolone is promising classes of pharmaceutical frameworks with antifungal (Chen et al., 2004), anti-inflammatory (Kategaonkar et al., 2010), anticancer (Eswaran et al., 2010), anti-tuberculosis (Tseng et al., 2010), anti-Alzheimer (Tseng et al., 2009), anti-HIV, anti-hypertension, and anticancer activities (Camps et al., 2009; Sharma et al., 2018). They also have inhibitory activities versus hepatitis C virus (HCV) polymerase (Thomas and Tallman, 1981; Summa et al., 2009), ADAMTS-5 (A disintegrin and metalloproteinase with thrombospondin motifs 5) and ADAMTS-4 (A disintegrin and metalloproteinase with thrombospondin motifs 4) (Sharma et al., 2008; Cappelli et al., 2010).

In this regard, the pyrrolo[3,4-c]quinoline-1,3-dione segment (1) exhibits a perfect range of pharmacologically and biologically enjoyable activities (Figure 1) (Okun et al., 2006a; Okun et al., 2006b; Segura-Cabrera et al., 2011; Mollin et al., 2012). For example, pyrrolo[3,4-c]quinoline (2) is a potent inhibitor of caspase-3 (Kravchenko et al., 2005a), which plays a clef role in apoptosis (Porter and Jänicke, 1999; Hentze et al., 2003). Caspases are interesting goals for therapeutic intervention in neurodegenerative, cardiovascular and metabolic disorders (Lockshin et al., 1998; Lockshin and Zakeri, 2004). In particular, caspase-3 inhibitors have been reported as powerful hepatoprotectants (Lockshin et al., 1998; Hoglen et al., 2001; Segawa et al., 2001; Lockshin and Zakeri, 2004; Meki et al., 2004), cardioprotectants (Chapman et al., 2002; Isabel et al., 2003), and neuroprotectants (Scott et al., 2003). Also, compound (3) has inhibitory activity against HCV polymerase (Di Francesco et al., 2009). Furthermore, alpkinidine (4) has shown potent therapeutic efficacy in vivo in HCT-116-bearing mice (Valeriote et al., 2012).

Because of broad applications in medicinal chemistry, the synthesis of these fused interesting heterocycles has specific importance to the pharmaceutical and organic chemists. Newly, there has been increasing attentiveness in the construction of pyrrole-fused-quinolines, and various procedures have been reported. Main synthetic approaches include Lewis acid-catalyzed electrophilic cyclization (Aggarwal et al., 2012), copper (Kiruthika et al., 2014) and palladium-catalyzed (Chai and Lautens, 2009; Shukla et al., 2012; Kiruthika et al., 2014) reactions, DDQ-mediated intramolecular cyclization (Wald et al., 1980), allene-based reaction cascades (Baumann and Baxendale, 2015), photo substituted reactions and flash vacuum pyrolysis. Although the majority of synthetic plans were applied for the synthesis of pyrrolo[3,2-c]quinoline and pyrrolo[1,2-a]quinoline analogs. Few synthetic relate have been released on the synthesis of pyrrolo[3,4-c]quinolones, and only multi-step synthetic methods are known to date.

In this matter, there are notable examples based on the cyclo condensation of b-keto amides and 2-amino-5-fluorophenyl glyoxylic acid (Ivachtchenko et al., 2003), Pfitzinger reaction (Mortoni et al., 2004; Kravchenko et al., 2005b), the one-pot two-component method by DMAP-catalyzed (Avula et al., 2013), the BF₃Et₂O-catalyzed isocyanide-based cycloaddition reaction (Li et al., 2013), and microwave-assisted reaction methods (Xia et al., 2014)^, which all are multi-step reactions (Scheme 1). However, these procedures are limited by low yields, harsh reaction conditions, the long reaction time, and their complexity.

Multicomponent reactions (MCRs) have become increasingly popular as a simple and powerful tool for the rapid formation of new scaffolds from simple starting materials with structural diversity and molecular complexity in a convergent manner (Mashayekh and Shiri, 2019; Sedighian et al., 2021). MCRs are one-pot strategies exploiting three or more simple substrates where most of the reactant atoms are incorporated into the final desired product (Chen et al., 2017; Kurhade et al., 2019; Shiri and Aboonajmi, 2020; Yavari and Safaei, 2020). In comparison to the traditional multistep sequential assembly of target compounds, MCRs manifest several advantages including easy handling, selective bond formation, time-saving, high atom economy, fewer purification steps and structural variability (Younus et al., 2021; Shiri et al., 2022).

Due to our experience and interest in the synthesis of novel heterocycles, we became engrossed in how Knoevenagel product obtained from isatine and pyrazole could be in situ trapped by keto amides resulting from diketene and primary amines to give a heterocycle product. We considered the utilization of diketene as starting material and reagent because it is extensively used for the generation for a diverse range of different heterocycles. For this purpose, in continuation of our successive attempts towards the synthesis of heterocycles by multicomponent strategies, (Rezvanian et al., 2018a; Talaei et al., 2018; Rezvanian et al., 2020a; Rezvanian et al., 2020b), especially using diketene reactions (Alizadeh et al., 2012; Rezvanian, 2015; Rezvanian, 2016; Rezvanian et al., 2017; Rezvanian et al., 2018b; Rezvanian et al., 2019; Rezvanian et al., 2020c; Rezvanian et al., 2020d; Rezvanian et al., 2020e; Rezvanian et al., 2021a; Rezvanian et al., 2021b), we herein explain an efficient approach to synthesize pyrrolo[3,4-c]quinoline-1,3-diones 7 from the reaction of isatin, diketene, and primary amines based on the unique reactivity of pyrazole as a promoter in high yields (Scheme 3).

At the outset of our investigation, the reaction of hydrazine, ethyl acetoacetate, isatine 5, diketene, and primary amine 6 in the lack of any catalyst at room temperature was designed. To study this new process, isatin 5a, ethyl amine 6a, and ethyl acetoacetate were selected as model reactions (Scheme 2). In this route, firstly, hydrazine (1 mmol), ethyl acetoacetate (1 mmol), and isatin 5a (1 mmol) in ethanol (4 ml) were stirred at room temperature for 1 h, which afforded the Michael adduct 8a. Next, ethylamine 6a (1 mmol) and diketene (1 mmol) were added to the reaction mixture. The advance of the reaction was followed by TLC (1-6 ethyl acetate-hexane). Unfortunately, no product was obtained at room temperature after 48 h (Table 1, entries 1). However, when the mixture reaction was heated at 70°C gratifyingly, we observed that the acceptable product was formed in an isolated yield of 84% within 5 h (Table 1, entry 2). Upon the construction of the desired outcome, immediately the precipitated solid was filtered off, washed with ethanol, and crystallized from hot ethanol in excellent yield. Amazingly, instead of the expected spiro pyridine product 11 (Scheme 4), we observed an unanticipated process leading to pyrrolo[3,4-c]quinolone 7a in excellent yield (Scheme 2). On the other hand, post-workup product 7a, the solvent was evaporated. We observed the precipitated white solid of pyrazole (from hydrazine and ethyl acetoacetate) slowly settling in which it was filtered and washed with water. The absolute structure of the newly synthesized product 7a was explicitly confirmed by X-ray analysis.

Then, in a controlled testing, the reaction of this five-component manufacturing process proceeds in the absence of pyrazole resulting from hydrazine and ethyl acetoacetate in which the effect of pyrazole was evaluated for this reaction. We concluded that the response could not advance without pyrazole under these conditions, and when the reaction mixture was carried with pyrazole (1 mol), the objective compound 7a obtained an 84% yield. Also, the change in amounts of pyrazole was explored for the reaction. The best result (90%) of the product was formed when (1 mol) of the pyrazole was exploited. By decreasing the amount of pyrazole to (0.7 and 0.5 mol), the development was accomplished at 35% and 27%, and it was observed that increasing the pyrazole loading had a considerable effect on the formation product. However, without using pyrazole, the reaction failed to develop even after 48 h.

Thus, to increase the yield of pyrrolo[3,4-c]quinoline-1,3-dione 7a and minimize reaction time, four-component reactions between isatin 5a, pyrazole, diketene, and ethylamine 6a were designed (Table 1), because of the success achieved using pyrazole promoted response. We found pyrrolo[3,4-c]quinoline-1,3-dione 7a as the only product when the reaction mixture was composed of a 1:1:1:1 variety of compounds.

In this reaction, solvent and temperature were examined to optimize the reaction conditions. Due; to the impossibility of carrying out the reaction at ambient temperature, the response was performed under reflux conditions. Also, it was observed that increasing the temperature above 70°C has no significant effect on the product yield. Despite obtaining good results, organic solvents did not improve much compared to water, including acetonitrile, methanol, water, water/ethanol, and tetrahydrofuran). Therefore, all reactions were performed under reflux conditions at 70°C in water to give satisfactory and excellent results.

Reaction conditions: pyrazole (1.0 mmol), isatine 5a (1.0 mmol), diketene (1.0 mmol), ethyl amine 6a (1.0 mmol), solvent (4.0 ml). [b] Isolated yield.

Having identified the best available conditions, to explore the efficiency and generality of this approach, the reactions between another isatine 1 and primary amines 2 were conducted, and the outcomes are shown in Scheme 3. The corresponding functionalized pyrrolo[3,4-c]quinoline-1,3-diones 7 were obtained in excellent yields at 70°C in ethanol in the presence of pyrazole (1 mmol) as a promoter. Various primary amines (6a-f) reacted with isatines to generate corresponding pyrrolo[3,4-c]quinoline-1,3-diones 7a-h.

The skeleton of all synthetic compounds 7a-h was elucidated by ESI-MS, IR, ¹HNMR, ¹³CNMR spectroscopy, and X-ray analysis. FTIR of 7a exhibited absorption bands in 1764, 1705, and 1622 due to the two CO and C=N stretching frequencies. In the ¹H-NMR spectrum of 7a, triplet and quartet in δ = 1.31 (³ J _H-H = 7.2 Hz) and δ = 3.77 (³ J _H-H = 7.2 Hz) ppm are due to CH₃ and CH₂ groups. The Singlet peak in δ = 3.01 is due to the CH₃ group. Also the signals in the aromatic section confirmed the presence of the four aromatic hydrogens of the aromatic ring. The presence of 14 apparent signs in the ¹³C-NMR spectrum is in accordance with the suggested structure of 7a. The highlighted areas in the ¹³C NMR are due to two CH₃, two CH₂, and two C=O groups, which are evident at δ = 13.94, 22.03, 33.01, 168.06, 168.33 ppm. Single crystal X-ray crystallography structure of 7b was certified as the product structure (Figure 2).

This reaction is a particular case, and a probable tool is illustrated in Scheme 4 for the generation of compound 7. It is advisable to suggest that the first stage starts via a Knoevenagel-type condensation of isatine and pyrazole to provide the intermediate 8 as the Michael acceptor. Then, nucleophilic attack of the amine to diketene and ring-opening of diketene pursue by proton transfer to give β-ketoamide 9. After the formation of adduct 8, nucleophilic addition enol form 10 on the Michael acceptor 8 afforded intermediate 11 via Michael addition. At this stage, attending to the preceding articles, we expected adduct 11 with O-cyclization, attacking the carbonyl group, and tautomerization to give the desired heterocyclic compound 13. But contemplating X-ray diffraction and ¹H and ¹³C NMR spectra, product 12 was not formed, and another extraordinary incident happened. In truth, in this step, nucleophilic addition of the enol form 10 is followed the elimination of pyrazole to give 14 via proton transfer. Then, in intermediate 14, with an intramolecular cyclization via N-nucleophilic attack of the amide group and proton transfer to form middle 15, ring opening and proton transfer gives the medium 17. Finally, nucleophilic attack of the amine 17 to C=O bond, intramolecular cyclization to form middle 18, elimination of H₂O, and deprotonation product 7 are created.

As a result, we have described an unusual three-component reaction to construct novel molecules containing a pyrrolo[3,4-c]quinoline-1,3-dione core from readily available reagents of pyrazole, isatine, diketene, and primary amine in ethanol. The most significant aspects of the process are the accessibility of the starting precursors, mild reaction conditions, short reaction times, high yields of the synthesized products, and easy operation at the manufacturing scale. The overall process of reaction includes all the aspects of green chemistry and has new portals for the growth of more sustainable multicomponent reactions. This category of heterocycles with several pharmacophores may be interesting for medicine and pharmacology.