Sigma Aldrich has been selected as vendor for sourcing the chemicals. All chemicals were purchased, which is having the purity not less than 99.9%. Analytical Thin Layer Chromatography (TLC) was carried out by using silica gel 60 F254 pre-coated plates. Visualization was accomplished with UV lamp. All the products were characterized by their IR, ¹H NMR and Mass spectra. ¹H NMR was recorded on 300 MHz in CDCl₃/DMSO, and the chemical shifts were reported in parts per million (ppm, δ) downfield from the Tetramethyl silane (TMS).

Nickel-Cobalt Ferrites with formula Ni_xCo_1−xFe₂O₄ (x = 1, 0.75, 0.5, 0.25 and 0). In this Ni_0.5Co_0.5Fe₂O₄ (x = 0.5) has been chosen for the study and was synthesised by a chemical sol-gel co-precipitation method. In order to prepare Ni_0.5Co_0.5Fe₂O₄ nanoparticles, 0.05 moles of nickel nitrate, 0.05 moles of cobalt nitrate and 2 moles of iron nitrate are dissolved separately in a little amount of deionised water and then citric acid solution was prepared stoichiometric proportions. These two solutions were added in a 1:1 molar ratio and P^H adjusted to 7 by the addition of ammonia and ethylene glycol is added. The aqueous mixture was heated to 60˚C, it was converted to gel and then temperature increased to 200˚C finally to get powder. That powder was calcined to 600˚C and then characterised with XRD and SEM.

Aromatic aldehyde (5 mmol) and dimedone (10 mmol) and nano nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄, 20 mol%) catalyst was taken in a round bottomed flask and the contents are dissolved in 5 mL of ethanol and 5 mL of water. Then the reaction mixture was stirred for 25 min at reflux temperature (Scheme 1). The completion of the reaction was monitored by TLC (n-hexane: ethyl acetate 4:1). After completion of the reaction the catalyst was separated by using an external strong Neodymium 35 magnet. Then 10 mL of ethanol was added to the reaction mixture and removal of solvent by rota vapor. After, the dried product was recrystallized from hot ethanol for several times to get the corresponding pure product dioxo-octahydroxanthenes in excellent yields. The purity of products was confirmed by IR, ¹H NMR and Mass spectras.

Aromatic aldehyde (5 mmol), dimedone (10 mmol), ammonium acetate (5 mmol) and 20 mol% nano nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) were taken in a 100 mL round bottomed flask and the contents are dissolved in 5 mL of ethanol and 5 mL of water. Then the reaction mixture was stirred for 40 min at reflux temperature (Scheme 2). After completion of the reaction the catalyst was separated by using an external strong Neodymium 35 magnet. Then 10 mL of ethanol was added to the reaction mixture and removal of solvent by rot a vapor. After, the dried product was recrystallized from hot ethanol for several times to get the corresponding pure product dioxohexahydroacridines in excellent yields. The purity of products was confirmed by IR, ¹H NMR and Mass spectras.

Scheme 1. A generalized scheme for synthesis of 1,8-dioxo-octahydroxanthene in presence of nano nickel cobalt ferrite catalyst by cyclocondensation of dimedone and aromatic aldehyde in water and ethanol solvent medium. (R = a) H, b) 4-OCH₃, c) 4-Cl, d) 4-NO₂, e) 2-NO₂, f) 4-(2-pyridyl)).

Initially a model reaction is conducted by using different solvents and different mol% of catalyst for synthesis of 1,8-dioxo-octahydroxanthenes (Scheme 3) and 1,8-dioxohexahydroacridines (Scheme 4) to investigate the feasibility of the reaction.

First synthesis of 1,8-dioxo-octahydroxanthenes (Scheme 3), in this dimedone and benzaldehyde were taken in different solvents (CH₂Cl₂, CCl₄, CH₃CN, and H₂O, ethanol and ethanol + water) in the presence of nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) NPs and results were captured in Table 1. It was clearly observed that low yield of products were obtained with CH₂Cl₂, CCl₄, and CH₃CN) (35%, 55% and 40%, Table 1, entry 1 - 3) respectively even after 7 hrs stirring. From Table 1, it is evident that low product yields (60% and 65%, entries 4 and 5) were obtained with water and ethanol independently. In a combination of 1:1

Scheme 2. A generalized reaction for synthesis of 1,8-dioxohexahydroacridines in presence of nano nickel cobalt ferrite catalyst by cyclocondensation of dimedone, aromatic aldehyde and ammonium acetate in ethanol and water solvent medium. (R = a) H, b) 4-Cl, c) 2-NO₂).

Scheme 3. A model reaction for synthesis of 1,8-dioxo-octahydroxanthene in presence of nano nickel cobalt ferrite catalyst by cyclocondensation of dimedone and benzaldehyde.

Scheme 4. A model reaction for synthesis of 1,8-dioxohexahydroacridines in presence of nano nickel cobalt ferrite catalyst by cyclocondensation of dimedone, benzaldehyde and ammonium acetate.

H₂O and EtOH, good yields of dioxo-octahydroxanthene derivatives were obtained with in 25 min (95 %, Table 1, entry 6).

Upon identifying the suitable solvent, next stage is to study the catalyst role on the reaction rate and product yield. To identify the appropriate catalyst, a series of parallel reactions were carried out with the catalytic amounts of different catalysts and the results are summarized in the Table 2. When the reaction with SiCl₄, SmCl₃, Amberlyst-15, ZnO-acetyl chloride and p-dodecyl benzenesulfonic acid gave capable results with better yields (Table 2, entries 1, 2, 6, 7 and 8), and the best result was obtained with nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) nano particles in the H₂O-EtOH solvent system (Table 2, entry 10), and the reaction was completed within 25 min.

After finding the suitable solvent and catalyst the model reaction is performed with different mol% of Ni_0.5Co_0.5Fe₂O₄ catalyst and observed that 20 mol% suitable to obtained maximum yield at neat condition (Table 3, entry 4). No change was observed on further enhancing the catalyst mol%.

With the optimised reaction conditions in hand, the reaction was performed with different benzaldehydes (Scheme 1) with 20 mol% Ni_0.5Co_0.5Fe₂O₄ NPs to explore the scope and generality of the present protocol and the results of these observations are summarized in Table 4. From the results, the aromatic aldehydes containing both electron releasing and withdrawing groups give products

with good yields, while electron withdrawing groups give products slightly better than releasing groups. The structures of synthesized 1,8-dioxo-octahydroxan- thenes derivatives were confirmed by IR, H¹ NMR and Mass spectral analysis.

The plausible mechanism for the formation of 1,8-dioxo-octahydroxanthenes by using Nickel cobalt ferrite NPs is shown in Figure 1.

On the other hand a model reaction is conducted for synthesis of 1,8-dioxo- hexahydroacridines (Scheme 4), in this dimedone benzaldehyde and ammonium acetate were taken in different solvents (n-hexane, 1,4-dioxane, diethyl ether, H₂O, ethanol and ethanol + water) in the presence of Ni_0.5Co_0.5Fe₂O₄ NPs and results are summarized in Table 5. It was clearly observed that low yield of product was obtained with n-hexane, 1,4-dioxane and diethyl ether (35%, 40% and 55%, Table 5, entry 1 - 3) respectively even after 10 hrs stirring. From Table 5, it is evident that low product yields (65% and 70%, Table 5, entries 4 and 5) were obtained with water and ethanol independently. In a combination of 1:1 H₂O and EtOH, good yields of dioxohexahydroacridine derivatives were obtained with in 40 min (96 %, Table 5, entry 6).

From the Table 5, 1:1 mixture of H₂O and EtOH is a suitable solvent and then we have to find out the suitable catalyst. In order to find a suitable catalyst, a series of parallel reactions were carried out with the catalytic amounts of different catalysts and the results are summarized in the Table 6. When the reactions are with silica supported sulphuric acid, methanesulfonic acid and CuSO₄∙5H₂O gave capable results with better yields (Table 6, entries 2, 4 and 5), and the best result was obtained with Ni_0.5Co_0.5Fe₂O₄ NPs in the H₂O-EtOH solvent system (Table 6, entry 6), and the reaction was completed within 40 min with 96% yield.

After finding the suitable solvent and catalyst the model reaction is performed with different mol% of Ni_0.5Co_0.5Fe₂O₄ catalyst and observed that 20 mol% suitable to obtained maximum yield at neat condition (Table 7, entry 4). No change was observed on further enhancing the catalyst mol%.

Based on above results, this method extend to synthesis of different substituted 1,8-dioxohexahydroacridines with different aromatic aldehydes (Scheme 2), dimedone and ammonium acetate in presence of Ni_0.5Co_0.5Fe₂O₄ (20 mol%) catalyst in H₂O-EtOH solvent system as shown in the Table 8.

The plausible mechanism for the formation of 1,8-dioxohexahydroacridines by using nickel cobalt ferrite NPs is shown in Figure 2.

The reusability of nickel cobalt ferrite NPs is one of the most important advantages of this protocol that makes it useful for practical commercial applications. We have examined the recyclability of nickel cobalt ferrite NPs catalyst for the model reaction. Interestingly, the recovered catalyst could be reused for up to five cycles which is evident from Table 9. The catalyst was separated by using a magnet after completion of the reaction, washed with water followed by chloroform, dried in oven and reused for the next cycle.