Xanthene and its derivatives have attracted considerable attention in the areas of pharmaceutical and medicinal chemistry due to their wide range of pharmacological properties such as antibacterial,antiviral,anti-inflammatory,and anticancer activities ^{[1, 2, 3]}. In addition,they have been used as synthetic precursors for many valuable organic compounds ^[4],dyes ^[5],compounds used in laser technologies ^[6],and fluorescent materials for visualization of biomolecules ^[7] and photodynamic therapy (PDT) ^[8]. There are several methods reported for the preparation of various classes of xanthene derivatives,most of which have been carried out in the presence of protonic acids ^[9] or Lewis acids like InCl₃·4H₂O ^[10],FeCl₃·8H₂O ^[11],NaHSO₄ ^[12],and tetrabutylammonium hydrogen sulfate (TBAHS) ^[13]. Some heterogeneous catalysts,such as silica sulfuric acid ^[14],polyaniline p-toluenesulfonate ^[15],PPA-SiO₂ ^[16],TiO₂/SO₄ ^[17],amberlyst-15 ^[18], SbCl₃/SiO₂ ^[19],Fe³⁺-montmorillonite ^[20],alumina-sulfuric acid ^[21],silica bonded N-propyl sulfamic acid ^[22],nanosized MCM- 41-SO₃H ^[23],PMA-SiO₂ ^[24],NBS ^[25],[Bmim][HSO₄] ^[26], [Hmim]TFA ^[27],[TMPSA]HSO₄ ^[28],and [DDPA][HSO₄] ^[29] have also been employed for this conversion. However,due to the difficulties encountered in the above cases,the search for new,readily available,green catalysts is still in progress. In this investigation,we have employed ceric ammonium nitrate supported HY-zeolite as an efficient heterogeneous catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes under solvent-free conditions. 2. Experimental

All the solvents and reagents were commercially available and were freshly used after being purified by standard procedures. Reactions were monitored by TLC using silica gel coated plates with ethyl acetate/hexane solutions as the mobile phase. Melting points are uncorrected and were measured using Electrothermal 9100 apparatus. NMR spectra were obtained on an FT-NMR Bruker Spectro Spin DRX-400 MHz instrument with DMSO-d₆ as the solvent,and the chemical shifts are expressed in δ units with Me₄Si as the internal standard. 2.1. Preparation of 30 mol% CAN supported HY-zeolite

About 0.148 g of ceric ammonium nitrate (30 mol%) was dissolved in 5 mL acetone. To this solution,about 0.1 g of activated HY-zeolite was added and stirred until complete dryness to afford the 30 mol% CAN supported HY-zeolite. The same procedure has been followed for the preparation of all the other metal nitrate supported HY-zeolites. 2.2. Synthesis of 3,3,6,6-tetramethyl-9-(phenyl)-1,8-dioxooctahydroxanthene (3a)

Dimedone (0.25 g,1.8 mmol),benzaldehyde (0.095 g, 0.9 mmol),and 0.1 g of 30 mol% of CAN supported HY-zeolite were ground in a mortar and transferred to a reaction vessel. Then, the reaction mixture was heated at 80℃ until the completion of the reaction. The progress of the reaction was periodically monitored by TLC. After completion of the reaction,15 mL of chloroform was added to the reaction mixture,which was stirred for 10 min. The catalyst was removed by simple filtration and the filtrate was evaporated to half of its volume,to which hexane was added to generate the crude product. The crude product was further purified by recrystallization with hot ethanol. The same procedure has been followed for the synthesis of other 1,8-dioxooctahydroxanthenes.

Characterization data of 3a: Light yellow solid; mp 202-204℃; ¹H NMR (400 MHz,DMSO-d₆): δ 7.21-7.42 (m,5H),4.66 (s,1H), 2.45 (s,4H),2.20 (s,4H),1.12 (s,6H),0.98 (s,6H); ¹³C NMR (100 MHz,DMSO-d₆): δ 196.5,162.3,144.2,128.5,128.1,126.5, 115.8,50.8,40.9,32.3,29.4,27.4; IR (KBr,cm^-1): v_max 2963,2952, 1662,1198. 3. Results and discussion

The synthesis of 1,8-dioxo-octahydroxanthenes has been achieved by employing CAN supported HY-zeolite as a novel supported catalyst under solvent-free conditions (Scheme 1).

Scheme 1

	Download: JPG larger image
Scheme 1.Synthesis of 1,8-dioxo-octahydroxanthenes.

In order to optimize the reaction conditions,initially we investigated the effect of different solvents on the reaction rate and yield of the products. In this regard,we have chosen the condensation of dimedone with p-chlorobenzaldehyde as a model with 30 mol% CAN supported HY-zeolite as catalyst in various solvents,including methanol,ethanol,isopropanol,THF,acetonitrile, chloroform,and dichloromethane under reflux conditions. In aprotic solvents like acetonitrile,chloroform,tetrahydrofuran,and dichloromethane,the reaction was very slow,resulting in poor yields (54-68%). Performing the reaction in protic solvents improved both the reaction rate and yield of the product (72- 82%). Due to the longer reaction time in the solvent media,the reaction was conducted under solvent-free conditions to make the procedure simpler,save energy,and prevent solvent wastes, hazards,and toxicity. Further reactions were conducted under solvent-free conditions (Table 1).

Table 1

Table 1 Effect of reaction medium on the cyclocondensation reaction of dimedone with pchlorobenzaldehyde in presence of CAN supported HY-zeolite as a catalysta.a

Table 1 Effect of reaction medium on the cyclocondensation reaction of dimedone with pchlorobenzaldehyde in presence of CAN supported HY-zeolite as a catalysta.a

Next,to optimize the mol% of metal nitrate,the above cyclocondensation reaction was performed with different mol% of CAN such as 10,30,60,80,and 100 mol%. When the reaction was carried out with 10 mol% of the CAN supported HY-zeolite, the reaction was completed in 3 h with 65% yield,but the same reaction when performed with 30 mol% of CAN supported HYzeolitewas completed in 1 hwith higher product yield (85-93%). Further increase in the mol% of the CAN caused no considerable change in the yield of the product and rate of the reaction. To compare the efficiency of the catalyst with other similar catalysts,the same condensation reaction was carried out with othermetal nitrate supported HY-zeolite like copper,iron,nickel, cobalt,bismuth,cadmium,and zinc,and it was observed that ceric ammonium nitrate supported HY-zeolite provided better results,whereas the other metal nitrate supported HY-zeolites required longer reaction times even to produce poorer yields (Table 2).

Table 2

Table 2 Effect of different catalysts for the cyclocondensation reaction of dimedone with pchlorobenzaldehyde under solvent free conditions.

Table 2 Effect of different catalysts for the cyclocondensation reaction of dimedone with pchlorobenzaldehyde under solvent free conditions.

The effect of different substituents on the aromatic ring of aldehyde was studied and it can be observed that aromatic aldehydes with electron-withdrawing substituents like chloro, bromo,and nitro groups gave excellent yields in shorter times, whereas aromatic aldehydes with electron-donating substituents required longer reaction times even to produce poorer yields (Table 3).

Table 3

Table 3 Cyclocondensation of dimedone with substituted aromatic aldehydes using 30 mol% CAN supported HY-zeolite.

Table 3 Cyclocondensation of dimedone with substituted aromatic aldehydes using 30 mol% CAN supported HY-zeolite.

Next,the reusability of CAN supported HY-zeolite was examined by the model condensation of dimedone with p-chlorobenzaldehyde under solvent-free conditions. Aftercompletion of the reaction,the reaction mixture was stirred with ethanol,and the catalyst was recovered by filtration. The catalyst was then washed thoroughly thrice in ethanol,dried,and activated before use in the next run. The catalyst could be effectively recycled at least five timeswith no appreciable change in its catalytic activity. 4. Conclusion

In conclusion,30 mol% CAN supported HY-zeolite has been efficiently used as a catalyst for the condensation reaction of dimedone with aromatic aldehydes under solvent-free conditions at 80℃ to afford the corresponding 1,8-dioxo-octahydroxanthenes in good to moderate yields. We have compared the catalytic efficiency of CAN supported HY-zeolite with other different metal nitrate supported HY-zeolites. We have evaluated the suitable mol% of the metal nitrate needed for the cyclocondensation reaction. The reusability of the catalyst after simple activation was the added advantage of the method. Operational simplicity coupled with excellent yields makes this protocol an economically viable one. Acknowledgment

We are pleased to acknowledge the financial support received from Council of Scientific and Industrial Research (No. 02(0025)/ 11/EMR-II),New Delhi. O