Pentafluoropropionic Acid as a Efficient Catalyst for the One-Pot Synthesis of 14-aryl-14 H-dibenzo [ a , j ] xanthenes Under Solvent Free Conditions

Pentafluoropropionic acid was found to be highly efficient, and green catalyst for the synthesis of dibenzo[a, j] xanthenes by condensation reaction of -naphthol with aryl aldehydes in good to high yields under solvent-free conditions. Present methodology offers several advantages, such as good yields, simple procedure with an easy work-up and the absence of any volatile and hazardous organic solvent.


INTRODUCTION
Xanthenes and their derivatives are of considerable interest as they possess a wide range of biological properties, such as anti-inflammatory 1 , antibacterial 2 , antiviral 3 activities, as well as their use in photodynamic 4 .Such compounds are also utilized for antagonism of the paralyzing action of zoxazolamine 5 .Moreover, xanthene derivatives such as 14-aryl-14H-dibenzo [a, j] xanthenes are of importance as they have various industrial, pharmaceutical and biological applications [5][6][7][8][9] .For example, these compounds have been applied as dyes in laser technology 6 , and as PH sensitive fluoresent materials for visualization of biomolecules 8 .Therefore, the development of new and efficient methodologies for the synthesis of xanthene derivatives will be interesting in both synthetic organic and medicinal chemistry.Dibenzoxanthene derivatives are generally synthesized via one-pot reaction of an aryl aldehyde and 2-naphthol in the presence of several catalysts such as AcOH-H 2 SO 4 8 , P-TSA 10,11 , wet cyano chloride 12 , molecular iodine 13,14 , sulfamic acid 15 , heteropoly acide 16 , silica sulfuric acid 17 , cellulose sulfuric acid 18 , caro ' s acidsilica gel 10 , Sr(OTf) 2 20 , Al(HSO 4 ) 3 21 , LiBr 22 , Cationexchange resines 23 , HClO 4 -SiO 2

Scheme 1: synthesis of 14-aryl-14H-dibenzo [a, j] xanthenes EXPERIMENTAL
All of the chemical material used in this work were purchased from merck and without further purification.Melting points were recorded of Electrothermal type 9100 melting point apparatus.The IR spectra were obtained on a 4300-Shimadzu spectrophotometer in KBr disks.The 1 H NMR(500MHz) spectra were recorded on a Bruker-Ac-500 spectrometer.

General procedure for the synthesis of 14-aryl-14H-dibenzo [a, j] xanthenes (3a-f)
A mixture of β-naphthol (0.288g, 2 mmol), aryl aldehyde (1 mmol) and pentafluoropropionic acid (25 mol% based on aryl aldehyde) was heated on the oil bath at 100ºC for the indicated time.The progress of the reaction was monitored by TLC.After completion of the reaction, mixture was cooled to room temperature and water was added.The precipitated solid was collected by filtration under suction, washed with cold water, and then recrystallized from ethanol to afford pure products 3a-f (Table2) in good yields.All the products were identified by comparison of spectral data (IR, and 1 H NMR), and m.p. with those reported [8, 15, 34, 40].

RESULTS AND DISCUSSION
Solvent-free conditions are especially important for providing an eco-frindly system.One advantage of solvent-free reactions, in comparison to the reaction in molecular solvents, is that the compounds formed are often sufficiently pure to circumvent extensive purification using chromatography.Due to the increasing demand in modern organic processes for avoiding expensive purification, we decided to investigate the efficiency of pentafluoro propionic acid as a catalyst in the synthesis of 14-aryl-14H-dibenzo [a, j] xanthene derivatives under solvent-free conditions.At frist, the synthesis of compound 3a was selected as a model reaction to optimize the reaction conditions.Therefore, mixture of β-naphthol1 (2 mmol), and benzaldehyde2 (1mmol), was heated on the oil bath at different temperatures in the presence of various amount of penta fluoropropionic acid as catalyst under solvent-free conditions(Table1).As can be seen from this table, the yield of compound 3a is affected by the catalyst amount and reaction temperature.No product was obtained in the absence of the catalyst(Entry 1) or in the presence of the catalyst at room temperature (Entry 2) indicating that the catalyst and temperature are necessary for the reaction.Increasing the amount of the catalyst and reaction temperature up to 25mol% and 100ºC, respectively, increased the yield of the product 3a.Further increase in both catalyst amount and temperature did not increase the yield noticeably (Entries 8-11).
To evaluate the generality of this model reaction we then prepared a range of dibenzo [a, j] xanthenes under optimized reaction conditions.In all cases, aromatic aldehydes with substituents carrying either electron-donating or electron-withdrawing groups reacted successfully and gave the products in good yields.The type of aldehydes had no significant effect on the reaction.The results are summarized in table 2.   a 2mmol -naphthol, 1mmol benzaldehyde and 25mol% pentaf luoropropionic acid at 100ºC under solvent-free conditions.b The products were characterized by comparision of their spectroscopic and physical data with authentic samples synthesized by reported procedures.c Isolated yields.

CONCLUSIONS
In conclusion we have successfully demonstrated the catalytic activity of penta fluoropropionic acid in the synthesis of 14-aryl-14Hdibenzo [a,j] xanthenes.Easy simple work-up, short reaction times and good yields are some advantages of this method.

Table 1 :Effect of penta fluoropropionic acid amount on the model reaction a
b Isolated yieldes.