Poly(4-Vinylpyridinium)Hydrogen Sulfate Catalyzed an Efficient and Ecofriendly Protocol for the One-Pot Multicomponent Synthesis of 1,8-Acridinediones in Aqueous Medium

Highly efficient, ecofriendly, and improved protocol for the synthesis of 1,8-acridinediones has been developed via one-pot multicomponent condensation of 1,3-cyclohexanedione/dimedone, aromatic aldehydes, and ammonium acetate utilizing poly(4vinylpyridinium)hydrogen sulfate as catalyst in aqueous medium. Excellent yields in shorter reaction time, simple work-up procedure, easy recovery, and reusability of the catalyst are attractive features of this green protocol.


Introduction
Multicomponent reactions (MCRs) have emerged as an efficient and powerful tool in modern organic chemistry towards the generation of highly diverse and complex product from readily available substrates in a single operation without isolation of intermediates in minimal time with maximum selectivity, high atom economy, and high purity with excellent yields [1].In addition, reactions under aqueous medium have received considerable attention in organic synthesis, because of both economic and environmental safety reasons [2].Moreover, heterogeneous catalysts have advantages over homogeneous catalysts in terms of efficiency, operational simplicity, recovery from the reaction mixture, and reusability [3].
In view of the above applications, many methods have been developed including microwave irradiation [13,14], using various catalytic systems such as p-toluenesulfonic acid [15], ceric ammonium nitrate [16], InCl 3 [17], [20], MCM-41-SO 3 H [21], Amberlite IR-120H [22], methanesulfonic acid [23], silica bonded s-sulfonic acid [24], silica-supported preyssler nanoparticles [25], p-dodecylbenzenesulfonic acid [26], carbon based solid acid [27], and ionic liquids [28,29].However, most of these methods suffer from drawbacks such as low yield of the product, longer reaction time, multistep process, formation of side products, critical isolation procedure, use of hazardous organic solvents and expensive catalysts, difficult in recovery and reusability of the catalyst.Thus, there is a need for the development of new and efficient methods for the preparation of such compounds in high yields and under mild reaction conditions.
In continuation of our studies towards the development of novel methodologies for the synthesis of biologically  active heterocyclic compounds [30,31], herein we report an ecofriendly method for the synthesis of 1,8-acridinediones using poly(4-vinylpyridinium)hydrogen sulfate [P(4-VPH)HSO 4 ] as an efficient, heterogeneous, and recyclable catalyst in aqueous medium.
In order to find out the optimal conditions, a model reaction was carried out by the condensation of dimedone (2) with p-chlorobenzaldehyde (3b) and ammonium acetate (4) in aqueous medium at different temperatures by varying the amount of catalyst, and the results were summarized in Table 1.From Table 1, we found that only 0.02 g of P(4-VPH)HSO 4 at 80 ∘ C is sufficient for completion of the reaction with maximum yield (Table 1, Entry 7).We also observed that as the amount of catalyst increases >0.02 g and the temperature >80 ∘ C, the yield of the product has decreased due to the unidentified impurities.
At these optimal conditions (0.02 g of P(4-VPH)HSO 4 , 80 ∘ C, aqueous medium), we have synthesized various 1,8acridinediones (5a-k and 6a-k) using different aromatic aldehydes with excellent yields in shorter reaction times (Table 2).All the synthesized compounds were characterized by their spectral studies and compared with the literature values where both were in good agreements.After completion of the reaction, the catalyst was recovered, washed with dichloromethane, dried, and reused for subsequent reactions for additional five times, and a slight decrease in its activity in terms of product yield was observed (Table 2, Entry 13).
A plausible mechanism for the formation of 1,8acridinediones catalysed by P(4-VPH)HSO 4 is shown in Scheme 2. In the presence of catalyst, the electrophilicity of aldehydic carbonyl carbon increases and readily reacts with the enolic form of 1,3-dicarbonyl compounds which resulted the formation of chalcone derivative [C].Compound [C] reacts with aminated 1,3-dicarbonyl compound [B] which is formed in situ by the reaction of 1,3-dicarbonyl compound with ammonium acetate that furnished the corresponding 1,8-acridinediones through the cyclisation followed by the dehydration.

Experimental
All the chemicals were procured from Aldrich/Merck, and solvents were used without further purification.

General Procedure for the Synthesis of 1,8-Acridinediones.
To a mixture of 1,3-cyclohexanedione/dimedone (2 mmol), arylaldehyde (1 mmol), and ammonium acetate (3 mmol) in 5 mL of water, 0.02 g of poly(4-vinylpyridinium)hydrogen sulfate was added and stirred at 80 ∘ C for an appropriate time as shown in Table 2.After completion of the reaction (TLC monitoring), the solid separated out was filtered, washed with water, dried, and recrytallized from ethanol/acetic acid to afford the pure 1,8-acridinediones in excellent yields.Aqueous layer containing catalyst was recovered under reduced pressure, dried, and reused for additional five times for subsequent reactions.

Conclusion
In conclusion, we have developed a simple, mild, and efficient protocol for the synthesis of 1,8-acridinediones utilizing poly(4-vinylpyridinium)hydrogen sulphate as catalyst in aqueous medium.Excellent yields in shorter reaction times, easy work-up procedure, environmentally begin nature, recovery, and reusability of the catalyst are the obvious advantages of this methodology.

Table 1 :
Optimizing the reaction conditions a .

Table 3 :
Comparing the catalytic efficiency of P(4-VPH)HSO 4 for the synthesis of 6b with some reported catalysts.