One-pot, multi-component reactions (MCRs) are having momentous meaning due to formulation of mono product with elevated yields by the blending of two or more components in a one step process   . The advantages of MCR’s are atom economy, less time-consuming, easy purification process and avoiding protection-deprotection steps. Therefore, the design and development of efficient and green MCRs focused on a target molecule are one of the most significant challenges in organic synthesis in both medicine and industry.
Xanthenediones and acridinediones are having a significant role in the biological chemistry and organic synthesis  . 1,8-Dioxo-octahydroxanthenes are an important group of oxygen heterocycle family. 1,8-Dioxo-octahydroxanthenes are having phenyl substituted pyran ring which is fused on either side with two cyclohexanone rings. From past several years, biological and pharmaceutical industries are showing more interest towards synthesis of xanthenediones. Derivatives of xanthenediones will act as antiviral  , antibacterial  and anti-in- flammatory  . Xanthenediones are also utilised as antagonists for paralyzing action of zoxazolamine  and in photodynamic therapy  . Xanthenediones are the same structural units constituting various natural products  and used as versatile synthons, because of inherent reactivity of their pyran ring  . 1,8-Dioxohexahydroacridines are an important class of nitrogen heterocycle in which a phenyl substituted pyran ring is fused on either side with two cyclohexanone rings. 1,8-Acridinediones have 1,4-dihydropyridine (1, 4 DHP) parent nucleus, which is used for the treatment of cardiovascular diseases, such as, angina pectoris. These are also used as bronchodilator, anti-atherosclerotic, antitumor, gero-protective, hepatoprotective and antidiabetic agents  .
For synthesis of xanthenes many methods are available; they are categorised based on starting compounds which include synthesis by cyclisation of polycyclic aryl triflate esters  , intra-molecular trapping benzynes by phenols  and reaction of aryloxy magnesium halides with triethyl orthoformate  as well as cyclo-condensation of 2-hdroxyl aromatic aldehyde with 2-tetralone  . Condensation of aromatic aldehydes with 5,5-dimethyl-1,3-cyclohexanedione (dimedone) or 1,3-cyclohexanedione is one of the commonly utilized methods. There are different kinds of catalysts which have been reported for 1,8-dioxooc- tahy-droxanthenes synthesis. Some of the catalysts were specified in this section. They are SiCl4  , SmCl3  , InCl3/P2O5  , triethyl benzyl ammonium chloride (TEBA)  , trichloro isocyanuric acid  , p-dodecyl benzenesulfonic acid  , diammonium hydrogen phosphate  , tetramethylguanidine trifluoroacetate  , SbCl3/SiO2  , trimethyl silyl chloride  , ZnO-Acetyl chloride  , ZrOCl2・8H2O  , Amberlyst-15  , montmorillonite K-10  and cellulose sulfonic acid  . Aromatic aldehydes will condensate with 1,3-cyclo- hexanedione or 5,5-dimethyl-1,3-cyclohexanedione (dimedone) with ammonium acetate which is a most commonly used method for the synthesis of 1,8- dioxo-hexahydroacridines. Several catalysts have been reported for the synthesis of 1,8-dioxohexahydroacridines such as HY-Zeolite  , Silica supported sulphuric acid  , Bronsted acidic ionic liquid ([CMIM][CF3COO])  , methanesulfonic acid  and Poly(4-Vinylpyridinium) Hydrogen Sulfate  .
The methods which are specified above are having its own advantages and merits, however many of these methods are unsatisfactory as they involve the use of halogenated solvents, unsatisfactory yields, harmful catalysts, catalyst loadings up to 30 mol%, prolonged reaction time and tedious experimental procedures. Development of clean and highly yielding and environmentally benign approaches is still desirable and much in demand. Therefore, it is necessary to develop the alternate methods for the synthesis of 1,8-dioxo-octahydroxanthenes and 1,8-dioxohexahydroacridines. The synthesis mechanism should involve simple in process, efficient, eco friendly with high yields with novel catalysts.
In recent years, magnetic nano particles have emerged as a useful group of heterogeneous catalysts. Separation of magnetic nano particles is simple and an attractive alternative to filtration as it prevents catalytic loss and enhances reusability. The use of low cost and readily available species as catalyst plays a significant role for economic feasibility of the chemical process. The greener generation of nanoparticles and their eco-friendly applications in catalysis via magnetically recoverable and recyclable nano-catalysts for a variety of oxidation, reduction, and condensation reactions     , has made an incredible impact on the development of sustainable pathways. Magnetically recyclable nano catalysts and their use in benign media is an ideal merge for the development of sustainable methodologies in organic synthesis.
Therefore, in order to accomplish the novel, high yielding and eco-friendly synthetic process, minimizing the by-products, with minimum number of separate reaction steps, improving the yields, our research work was extended by the application of nano catalysts in MCRs, in this we report a clean and environmentally friendly approach to the synthesis of 1,8-dioxo-octahydroxanthenes and 1,8-dioxohexahydroacridines via multi-component reaction in the presence of nickel-cobalt ferrite nanoparticles.
Nickel cobalt ferrite nano particles are magnetically separable and having effective activity. Nickel cobalt ferrite nano particles are having advantages of multi cycling, easy work-up and clean reaction profiles apart from the lack of necessity ligands and in minimizing the organic waste generation when compared to the conventational catalytic systems. In this, we report xanthenediones and acridinediones synthesis by using magnetically separable nano nickel cobalt ferrite as heterogeneous catalyst. The synthesised derivatives were characterised by IR, 1H NMR and Mass spectral data.
2. Materials and Methods
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).
2.2. Preparation of the Nickel-Cobalt Ferrite Nano Catalyst
Nickel-Cobalt Ferrites with formula NixCo1−xFe2O4 (x = 1, 0.75, 0.5, 0.25 and 0). In this Ni0.5Co0.5Fe2O4 (x = 0.5) has been chosen for the study and was synthesised by a chemical sol-gel co-precipitation method. In order to prepare Ni0.5Co0.5Fe2O4 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 PH 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.
2.3. General Experimental Procedure for Synthesis of 1,8-Dioxo-Octahydroxanthenes
Aromatic aldehyde (5 mmol) and dimedone (10 mmol) and nano nickel cobalt ferrite (Ni0.5Co0.5Fe2O4, 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.
2.4. General Experimental Procedure for Synthesis of 1,8-Dioxohexahydroacridines
Aromatic aldehyde (5 mmol), dimedone (10 mmol), ammonium acetate (5 mmol) and 20 mol% nano nickel cobalt ferrite (Ni0.5Co0.5Fe2O4) 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)).
3. Results and Discussions
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 (Ni0.5Co0.5Fe2O4) 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 (Ni0.5Co0.5Fe2O4) 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 Ni0.5Co0.5Fe2O4 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% Ni0.5Co0.5Fe2O4 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
Table 1. Optimisation of synthesis of 1,8-dioxo-octahydroxanthenes in presence of nickel cobalt ferrite at different solvent medium.
Table 2. Screening of various catalysts with Ni0.5Co0.5Fe2O4 NPs in the synthesis.
Table 3. Effect of Ni0.5Co0.5Fe2O4 catalyst concentration on synthesis of 1,8-dioxooctahy- droxanthenes.
Table 4. Ni0.5Co0.5Fe2O4 catalysed synthesis of 1,8-dioxo-octahydroxanthenes derivatives.
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 Ni0.5Co0.5Fe2O4 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 Ni0.5Co0.5Fe2O4 NPs in the H2O-EtOH solvent system (Table 6, entry 6), and the reaction was completed within 40 min with 96% yield.
Figure 1.Plausible mechanism for the formation of 1,8-dioxo-octahydroxanthenes.
Table 5. Optimisation of synthesis of 1,8-dioxohexahydroacridines in presence of nickel cobalt ferrite at different solvent medium.
Table 6. Screening of various catalysts with nickel cobalt ferrite in the synthesis.
After finding the suitable solvent and catalyst the model reaction is performed with different mol% of Ni0.5Co0.5Fe2O4 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 Ni0.5Co0.5Fe2O4 (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.
3.2. Reusability of the Catalyst
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.
3.3. Spectral Data
3.3.1. Spectral Data of 1,8-Dioxo-Octahydroxanthenes
White solid, 1HNMR (300 MHz, CDCl3) δ = 0.96 (s, 6H), 1.08 (s, 6H), 2.02 (d, J = 16.2 Hz, 2H), 2.28 (d, J = 16.1 Hz, 2H), 2.51 (d, J = 17.3 Hz, 2H), 2.56 (d, J = 17.5 Hz, 2H), 5.12 (s, 1H), 7.05 - 7.28 (m, 5H); FTIR (KBr, cm−1): 3311, 2982, 1794, 1724, 1700, 1654, 1520, 1361, 1199; ESI-MS (m/z): 351 (M+ + 1).
Table 7. Effect of Ni0.5Co0.5Fe2O4 catalyst concentration on synthesis of 1,8-dioxooctahy- droxanthenes.
Table 8. Ni0.5Co0.5Fe2O4 catalysed synthesis of 1,8-dioxohexahydroacridines.
Figure 2. Plausible mechanism for the formation of 1,8-dioxohexahydroacridines.
Table 9. Productivity with re-cycle catalyst.
White solid, 1HNMR (300 MHz, CDCl3) δ = 0.98 (s, 6H), 1.10 (s, 6H), 2.04 (d, J = 16.1 Hz, 2H), 2.26 (d, J = 16.2 Hz, 2H), 2.50 (d, J = 17.4 Hz, 2H), 2.54 (d, J = 17.3 Hz, 2H), 3.82 (s, 3H), 5.46 (s, 1H), 6.84 (d, 2H), 7.02 (d, 2H); FTIR (KBr, cm−1): 3030, 2970, 2873, 1679, 1650, 1511, 1460, 1372, 1261, 1193, 1165, 1138, 1030, 841; ESI-MS (m/z): 381 (M+ + 1).
White solid, 1H NMR (300 MHz, CDCl3) δ = 0.92 (s, 6H), 1.08 (s, 6H), 2.09 (d, J = 16.1 Hz, 2H), 2.24 (d, J = 16.1 Hz, 2H), 2.51 (d, 2H), 2.53 (d, J = 17.6 Hz, 2H), 4.52 (s, 1H), 7.16 (d, J = 8.3 Hz, 2H), 7.24 (d, J = 8.2 Hz, 2H); FTIR (KBr, cm−1): 3020, 2985, 2952, 1680, 1660, 1480, 1361, 1200, 1198, 1160, 1090, 1000, 850; ESI-MS (m/z): 385.5 (M+ + 1)
Yellow solid, 1H NMR (300 MHz, CDCl3) δ = 0.94 (s, 6H), 1.04 (s, 6H), 2.06 (d, J = 16.2 Hz, 2H), 2.26 (d, J = 16.1 Hz, 2H), 2.52 (d, 2H), 2.54 (d, J = 17.6 Hz, 2H), 4.54 (s, 1H), 7.16 (d, J = 8.3 Hz, 2H), 7.26 (d, J = 8.2 Hz, 2H); FTIR (KBr, cm−1): 3030, 2980, 2956, 1685, 1661, 1515, 1465, 1361, 1344, 1292, 1201, 1160, 1090, 850; ESI-MS (m/z): 396 (M+ + 1).
Yellow solid, 1H NMR (300 MHz, CDCl3) δ = 0.92 (s, 6H), 1.08 (s, 6H), 2.02 (d, J = 16.1 Hz, 2H), 2.24 (d, J = 16.2 Hz, 2H), 2.46 (d, J = 17.4 Hz, 2H), 2.52 (d, J = 17.1 Hz, 2H), 5.46 (s, 1H), 7.04 - 7.28 (m, 4H); FTIR (KBr, cm−1): 3020, 2972, 2870, 1680, 1650, 1512, 1466, 1372, 1260, 1190, 1100, 1030, 841, 574; ESI-MS (m/z): 396 (M+ + 1).
6) 3,3,6,6-tetramethyl-9-(4-(2-pyridyl)-phenyl)-1,8-dioxo-octahydroxan- thenes
White solid, 1H NMR (300 MHz, CDCl3) δ = 0.96 (s, 6H), 1.12 (s, 6H), 2.18 (d, J = 16.7 Hz, 2H), 2.23 (d, J = 17 Hz, 2H), 2.52 (d, J = 17.8 Hz, 2H), 2.52 (d, J = 17.5 Hz, 2H), 5.42 (s, 1H), 7.04 - 7.28 (m, 4H), 7.12-7.38 (m, 4H); FTIR (KBr, cm−1): 3030, 2962, 2930, 2872, 1680, 1654, 1610, 1419, 1374, 1299, 1248, 1198, 1160, 1090, 868, 841; ESI-MS (m/z): 428 (M+ + 1).
3.3.2. Spectral Data of 1,8-Dioxohexahydroacridines
1) 3,3,6,6-tetramethyl-9-phenyl-3,4,6,7,9,10 hexahydroacridine-1,8-dione
White solid, 1HNMR (300 MHz, CDCl3) δ = 9.28 (s, 1H), 4.8 (s, 1H), 0.92 (s, 6H), 1.02 (s, 6H), 1.94 (d, J = 21.5 Hz, 2H), 2.12 (d, J = 21.5 Hz, 2H), 2.32 (d, J = 22.8 2H), 2.4 (d, 21.5 Hz, 2H), 7.05 - 7.28 (m, 5H); FTIR (KBr, cm−1): 3433, 3279, 3205, 3063, 2955, 2932, 1630, 1605, 1481, 1365, 1210, 1142; ESI-MS (m/z): 350 (M+ + 1).
2) 3,3,6,6-tetramethyl-9-(4-chlorophenyl)-3,4,6,7,9,10 hexahydroacridine- 1,8-dione
Yellow solid, 1H NMR (300 MHz, CDCl3) δ = 9.16 (s, 1H), 5.02 (s, 1H), 1.02 (s, 6H), 1.10 (s, 6H), 2.02 (d, J = 16.1 Hz, 2H), 2.23 (d, J = 16.3 Hz, 2H), 2.42 (d, J = 17.3 Hz, 2H), 2.52 (d, J = 17.1 Hz, 2H), 7.24 (d, J = 8 Hz, 2H), 7.26 (d, J = 5.7 Hz, 2H); FTIR (KBr, cm−1): 3430, 3275, 3170, 3030, 2955, 1650 1609 1485, 1398, 1360 850; ESI-MS (m/z): 384 (M+ + 1).
3) 3,3,6,6-tetramethyl-9-(2-nitrophenyl)-3,4,6,7,9,10 hexahydroacridine- 1,8-dione
White solid, 1H NMR (300 MHz, CDCl3) δ = 8.98 (s, 1H), 5.0 (s, 1H), 0.98 (s, 6H), 1.12 (s, 6H), 2.04 (d, J = 25.1 Hz, 2H), 2.26 (d, J = 22.6.2 Hz, 2H), 2.50 (d, J = 21.4 Hz, 2H), 2.54 (d, J = 22.3 Hz, 2H), 7.06 - 7.28 (m, 4H); FTIR (KBr, cm−1): 3440, 3274, 3170, 3030, 2970, 2873, 1685, 1650, 1512, 1460, 1375, 1250, 1190,1100, 1030, 841; ESI-MS (m/z): 380 (M+ + 1).
Based on the above conclusion it was concluded that, we have described a novel, efficient, multi-component one-pot green synthetic method using nano nickel cobalt ferrite catalyst and ethanol and water as a solvent. The novel and synthetic utility of this method is established in the efficient synthesis of 1,8-dioxooctahe- draxanthenes and 1,8-dioxohexahydroacridines derivatives. The advantages of this method include its simplicity of operation, cleaner reaction, and being good to excellent yields. Further, the purification of the product is simple involving filtration. The catalyst is easily separated by using external magnet and is reusable up to five cycles.
The corresponding author is grateful to CSIR, New Delhi for supporting through fellowship (JRF & SRF) & Department of Engineering chemistry, AUCE (A), Andhra University, and Visakhapatnam for providing general lab facilities. The corresponding author is also grateful to Prof. K. Raghu Babu for his valuable & constant support.
 Di Stilo, A., Visentin, S., Cena, C., Gasco, A.M., Ermondi, G. and Gasco, A. (1998) New 1,4-Dihydropyridines Conjugated to Furoxanyl Moieties, Endowed with Both Nitric Oxide-Like and Calcium Channel Antagonist Vasodilator Activities. Journal of Medicinal Chemistry, 41, 5393-5401.
Thull, U. and Testa, B. (1994) Screening of Unsubstituted Cyclic Compounds as Inhibitors of Monoamine Oxidases. Biochemical Pharmacology, 47, 2307-2310.
Tu, S., Miao, C., Fang, F., Youjian, F., Li. T., Zhuang, Q., Zhang, X., Zhu, S. and Shi, D. (2004) New Potential Calcium Channel Modulators: Design and Synthesis of Compounds Containing Two Pyridine, Pyrimidine, Pyridone, Quinoline and Acridine Units under Microwave Irradiation. Bioorganic & Medicinal Chemistry Letters, 14, 1533-1536.
 Poupelin, J.P., Saint-Ruf, G., Lacroix, R., Narcisse, G., Foussard-Blanpin, O. and Uchida-Ernouf, G. (1978) Synthesis and Antiinflammatory Properties of Bis(2-Hydroxy, 1-Naphthyl) Methane Derivatives. European Journal of Medicinal Chemistry, 13, 67-71.
 Saint-Ruf, G., Huynh-Trong-Hieu and Poupelin, J.P. (1975) The Effect of Dibenzoxanthenes on the Paralyzing Action of Zoxazolamine. Naturwissenschaften, 62, 584-585.
 Ion, R.M., Planner, A., Wiktorowicz, K. and Frackowiak, D. (1998) The Incorporation of Various Porphyrins into Blood Cells Measured via Flow Cytometry, Absorption and Emission Spectroscopy. Acta Biochimica Polonica, 45, 833-845.
 Hatakeyma, S., Ochi, N., Numata, H. and Takano, S.J. (1988) A New Route to Substituted 3-Methoxycarbonyldihydropyrans; Enantioselective Synthesis of (–)-Methyl Elenolate. Journal of the Chemical Society, Chemical Communications, 17, 1202-1204.
 Shan, R., Velazquez, C. and Knaus, E. (2004) Syntheses, Calcium Channel Agonist-Antagonist Modulation Activities, and Nitric Oxide Release Studies of Nitrooxyalkyl 1,4-Dihydro-2,6-dimethyl-3-nitro-4-(2,1,3-benzoxadiazol-4-yl)pyridine-5-carboxylate Racemates, Enantiomers, and Diastereomers. Journal of Medicinal Chemistry, 47, 254-261.
 Wang, J.Q. and Harvey, R.G. (2002) Synthesis of Polycyclic Xanthenes and Furans via Palladium-Catalyzed Cyclization of Polycyclic Aryltriflate Esters. Tetrahedron, 58, 5927-5931.
 Knight, D.W. and Little, P.B. (1998) The First High-Yielding Benzyne Cyclisation Using a Phenolic Nucleophile: A New Route to Xanthenes. Synlett, 10, 1141-1143.
 Casiraghi, G., Casnati, G. and Cornia, M. (1973) Regiospecific Reactions of Phenol Salts: Reaction-Pathways of Alkylphenoxy-Magnesiumhalides with Triethylorthoformate. Tetrahedron Letter, 14, 679-682.
 Soliman, H.A. and Salama, T.A. (2013) Silicon-Mediated Highly Efficient Synthesis of 1,8-Dioxo-Octahydroxanthenes and Their Transformation to Novel Functionalized Pyrano-Tetrazolo[1,5-a] Azepine Derivatives. Chinese Chemical Letters, 24, 404-406.
 Llangovan, A., Malayappasamy, S., Muralidharan, S. and Maruthamuthu, S. (2011) A Highly Efficient Green Synthesis of 1,8-Dioxo-Octahydroxanthenes. Chemistry Central Journal, 5, 81.
 Verma, G.K., Raghuvanshi, K., Verma, R.K., Dwivedi, P. and Singh, M.S. (2011) An Efficient One-Pot Solvent-Free Synthesis and Photophysical Properties of 9-Aryl/Alkyl-Octahydroxanthene-1,8-Diones. Tetrahedron, 67, 3698-3704.
 Wang, X.S., Shi, D.Q., Li, Y.L., Chen, H., Wei, X.Y. and Zong, Z.M. (2005) A Clean Synthesis of 1-Oxo-Hexahydro-Xanthene Derivatives in Aqueous Media Catalyzed by TEBA. Synthetic Communications, 35, 97-104.
 Bigdeli, M.A., Nemati, F., Mahdavinia, G.H. and Doostmohammadi, H.A. (2009) Series of 1,8-Dioxooctahydroxanthenes Are Prepared Using Trichloroisocyanuric Acid. Chinese Chemical Letters, 20, 1275-1278.
 Jin, T.S., Zhang, J.S., Xiao, J.C., Wang, A.Q. and Li, T.S. (2006) Clean Synthesis of 1,8-Dioxo-Octahydroxanthene Derivatives Catalyzed by p-Dodecylbenezenesulfonic Acid in Aqueous Media. Ultrasonics Sonochemistry, 3, 220-224.
 Darvish, F., Balalaei, S., Chadegani, F. and Salehi, P. (2007) Diammonium Hydrogen Phosphate as a Neutral and Efficient Catalyst for Synthesis of 1,8-Dioxo-Octahydroxanthene Derivatives in Aqueous Media. Synthetic Communications, 37, 1059-1066.
 Rahmati, A. (2010) A Rapid and Efficient Method for the Synthesis of 14H-Diben-zo[α.j]Xanthenes, Aryl-5H-Dibenzo [b.i]Xanthene-5,7,12,14-(13H)-Tetraone and 1,8-Dioxo-Octahydroxanthenes by Acidic Ionic Liquid. Chinese Chemical Letter, 21, 761-764.
 Zhang, Z.H. and Liu, Y.H. (2008) Antimony Trichloride/SiO2 Promoted Synthesis of 9-Ary-3, 4, 5, 6, 7, 9-Hexahydroxanthene-1,8-Diones. Catalysis Communications, 9, 1715-1719.
 Maghsoodlou, M.T., Habibi-Khorassani, S.M., Shahkarami, Z., Maleki, N. and Rostamizadeh, M. (2010) An Efficient Synthesis of 2,2’-Arylmethylene Bis(3-Hydroxy-5,5-Di-Methyl-2-Cyclohexene-1-One) and 1,8-Dioxo-Octahydroxanthenes Using ZnO and ZnO-Acetyl Chloride. Chinese Chemical Letters, 21, 686-689.
 Lü, H.Y., Li, J.J. and Zhang, Z.H. (2009) ZrOCl2·H2O: A Highly Efficient Catalyst for the synthesis of 1,8-Dioxo-Octahydroxanthene Derivatives under Solvent-Free Conditions. Applied Organometalic Chemistry, 23, 165-169.
 Das, B., Thirupathi, P., Mahender, I., Reddy, V.S. and Rao, Y.K. (2006) Amberlyst-15: An Efficient Reusable Heterogeneous Catalyst for the Synthesis of 1,8-Dioxo-Octahydroxanthenes and 1,8-Dioxo-Decahydroacridines. Journal of Molecular Catalysis A: Chemical, 247, 233-239.
 Oskooie, L.H., Tahershamsi, A., Heravi, M.M. and Baghernejad, B. (2010) Cellulose Sulfonic Acid: An Efficient Heterogeneous Catalyst for the Synthesis of 1,8-Dioxo-Octahydroxanthenes. E-Journal of Chemistry, 7, 717-720.
 Nikpassand, M., Mamaghani, M. and Tabatabaeian, K. (2009) An Efficientone-Potthree-Component Synthesis of Fused1, 4-Dihydropyridines Using HY-Zeolite. Molecules, 14, 1468-1474.
 Sheik Mansoor, S., Aswin, K., Logaiya, K. and Sudhan S.P.N. (2014) Aqua-Mediated Synthesis of Acridinediones with Reusable Silica-Supported Sulfuric Acid as an Efficient Catalyst. Journal of Taibah University for Science, 8, 265-227.
 Patil, D., Chandam, D., Mulik, A., Patil, P., Jagadale, S., Rajni, K., Gupta, V. and Deshmukh, M. (2014) Novel Brønsted Acidic Ionic Liquid ([CMIM][CF3COO]) Prompted Multicomponent Hantzsch Reaction for the Eco-Friendly Synthesis of Acridinediones: An Efficient and Recyclable Catalyst. Catalysis Letters, 144, 949-958.
 Janardhan, B., Rajitha, B. and Peter A.C. (2013) Poly (4-Vinylpyridinium) Hydrogen Sulfate Catalyzed an Efficient and Ecofriendly Protocol for the One-Pot Multicomponent Synthesis of 1,8-Acridinediones in Aqueous Medium. Journal of Chemistry, 2013, Article ID: 850254.
 Polshettiwar, V., Baruwati, B. and Varma, R.S. (2009) Magnetic Nanoparticle-Supported Glutathione: A Conceptually Sustainable Organocatalyst. Chemical Communications, 14, 1837-1839.
 Baig, R.B.N. and Varma, R.S. (2013) Organic Synthesis via Magnetic Attraction: Benign and Sustainable Protocols Using Magnetic Nanoferrites. Green Chemistry, 15, 398-417.
 Polshettiwar, V., Baruwati, B. and Varma, R.S. (2009) Nanoparticle-Supported and Magnetically Recoverable Nickel Catalyst: A Robust and Economic Hydrogenation and Transfer Hydrogenation Protocol. Green Chemistry, 11, 127-131.
 Polshettiwar, V. and Varma, R.S. (2010) Nano-Organocatalyst: Magnetically Retrievable Ferrite-Anchored Glutathione for Microwave-Assisted Paal-Knorr Reaction, Aza-Michael Addition, and Pyrazole Synthesis. Tetrahedron, 66, 1091-1097.