Solvent-Free Synthesis of 1 , 8-Dioxo-octahydroxanthenes and 14-Aryl-14 H-dibenzo [ a , j ] xanthenes using Saccharin Sulfonic Acid as an Efficient and Green Catalyst

Saccharin sulfonic acid (SaSA) is utilized as a highly efficient, green and inexpensive sulfonic acid-containing catalyst for the following onepot multi-component organic transformations: (i) the condensation of dimedone (2 eq.) with aromatic aldehydes (1 eq.) leading to 1,8-dioxooctahydroxanthenes, and (ii) the reaction of 2-naphthol (2 eq.) with arylaldehydes (1 eq.) leading to 14-aryl-14H-dibenzo[a,j]xanthenes. In these protocols, the title compounds are produced in high to excellent yields and in relatively short reaction times under solvent-free conditions.


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, KAl(SO 4 ) 2 .12H 2 O 23 , silica sulfuric acid 24 and TaCl 5 25 .Nevertheless, most of the reported methods for the synthesis of the xanthenes derivatives are associated with one or more of the following drawbacks: (i) moderate yields, (ii) long reaction times, (iii) the use of large amount of catalyst, and (iv) the use of expensive, non-available or toxic catalysts, and (v) no agreement with the green chemistry protocols.
In recent years, the use of sulfonic acid-containing catalysts and reagents has gained considerable attention in organic synthesis, due to their unique properties such as enhanced reactivity as well as selectivity, efficiency, straightforward workup, easy availability of their starting materials, eco-friendly reaction conditions and ability to promote a wide range of reactions [26][27][28][29][30][31][32][33][34][35][36][37][38][39] .They are also non-toxic, non-corrosive and inexpensive.Saccharin sulfonic acid (SaSA) is certainly one of the most interesting SO 3 H-containing catalysts which has been recently prepared, and used in some organic transformations including chemoselective trimethylsilylation of alcohols 36 , acetylation of alcohols, phenols and amines 37 , N-Boc protection of amines and formation of tert-butyl ethers from alcohols 38 , and preparation as well as deprotection of 1,1-diacetates 39 .The structure of saccharin sulfonic acid (SaSA) is shown in Figure 1.Nevertheless, in most of the existing processes in organic synthesis, the use of toxic and volatile organic solvents as reaction media is inevitable, and these are environmentally unacceptable from green chemistry view point.One of the most effective techniques to solve this problem is solvent-free conditions which makes synthesis simpler, saves energy and prevents solvent waste, hazards, and toxicity [40][41][42][43][44][45] .Consequently, it is noteworthy that the combination of safe catalysts with the use of solvent-free technique represents a suitable way toward the so-called "ideal synthesis" [40][41][42][43][44][45] .

Figure 1
Multi-component reactions (MCRs) have drawn great interest enjoying an outstanding status in modern organic synthesis and medicinal chemistry because they are one-pot processes bringing together three or more components and show high atom economy and high selectivity [44][45][46][47] .Moreover, MCRs offer the advantage of simplicity and synthetic efficiency over conventional chemical reactions [44][45][46][47] .
Having the above subjects in mind, we report here a new, highly efficient and simple method for the one-pot multi-component synthesis of 1,8-dioxo-octahydroxanthenes from dimedone (2 eq.) and aromatic aldehydes (1 eq.), as well as 14-aryl-14H-dibenzo[a,j] xanthenes from 2-naphthol (2 eq.) and arylaldehydes (1 eq.) using SaSA as an inexpensive and green SO 3 H-containing catalyst in the absence of solvent (Schemes 1 and 2).It is worth noting that this method has none of the above-mentioned drawbacks at all.

Experimental
All chemicals were purchased from Merck or Fluka Chemical Companies.All known compounds were identified by comparison of their melting points and spectral data with those reported in the literature.Progress of the reactions was monitored by TLC using silica gel SIL G/UV 254 plates.The 1 H NMR (500 or 300 MHz) and 13 C NMR (125 or 75 MHz) were run on a Bruker Avance DPX-250, FT-NMR spectrometer (δ in ppm).Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.

Procedure for the preparation of saccharin sulfonic acid (SaSA)
A flask (500 mL) charged with saccharin (17.1 g, 0.1 mol) was equipped with a constant pressure dropping funnel containing chlorosulfonic acid (11.65 g, 0.1 mol) and a gas outlet tube which was dipped into water to dissolve the generated HCl gas during the reaction.The flask was put into an ice bath and chlorosulfonic acid was added dropwise over a period of 10 min, and the resulting mixture was stirred slowly for another 10 min.The temperature of the mixture was brought up to the room temperature and was stirred for an additional 30 min.The mixture was triturated with n-hexane (10 mL) and then filtered.The solid residue was washed with n-hexane (10 mL) and dried under vacuum to give SaSA [36][37][38][39] .

General procedure for the synthesis of 1,8-dioxo-octahydroxanthene derivatives
To a mixture of dimedone (0.28 g, 2 mmol) and aldehyde (1 mmol) in a 10 mL roundbottomed flask connected to a reflux condenser, SaSA (0.04 g, 0.15 mmol) was added.The resulting mixture was stirred in an oil-bath (90 ºC), and the reaction was monitored by TLC.After completion of the reaction, EtOAc (10 mL) was added to the reaction mixture, stirred for 2 min and filtered.The solvent of the filtrate was evaporated and the resulting solid (crude product) was recrystallized from EtOH (96%) to afford the pure product.

General procedure for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene derivatives
A mixture of 2-naphthol (0.288 g, 2 mmol), aldehyde (1 mmol) and SaSA (0.04 g, 0.15 mmol) in a 10 mL round-bottomed flask connected to a reflux condenser, was stirred in an oil-bath (120 °C).After completion of the reaction, as monitored by TLC, EtOAc (10 mL) was added to the reaction mixture, stirred for 2 min and filtered.Then, the EtOAc from the filtrate was evaporated, and the resulting solid (crude product) was recrystallized from isopropanol/CHCl 3 (4/1) to afford the pure product.

Results and Discussion
As above mentioned, more recently, SaSA has been introduced as an efficient SO 3 Hcontaining catalyst in organic synthesis [36][37][38][39] .In addition, xanthenes derivatives have various industrial and pharmaceutical applications [1][2][3][4][5][6] , and the reported methods for the preparation of these compounds have several drawbacks.These subjects encouraged us to examine applicability of SaSA in the synthesis of two important types of xanthene derivatives including 1,8-dioxo-octahydroxanthenes and 14-aryl-14H-dibenzo[a,j]xanthenes, and introduced new efficient procedures for the preparation of these compounds.For this purpose, at first, the one-pot multi-component reaction of dimedone (2 mmol) with 4-methoxybenzaldehyde (1 mmol) was studied in the presence of different amounts of SaSA at range of 60-100 °C under solvent-free conditions to provide compound 1b.The respective results are summarized in Table 1.As Table 1 indicates, 15 mol% of the catalyst was sufficient to perform the reaction efficiently at 90 °C.Moreover, increment the amount of SaSA, the temperature and the reaction time did not improve the yield.a Isolated yield.
To recognize efficiency and scope of the method for the synthesis of 1,8-dioxooctahydroxanthenes, dimedone was reacted with different aromatic aldehydes under the optimal reaction conditions (Table 2).As it is shown in Table 2, all aldehydes including benzaldehyde and aromatic aldehydes possessing electron-donating substituents, electronwithdrawing substituents or halogens on their aromatic rings afforded the desired 1,8-dioxooctahydroxanthene derivatives in high to excellent yields and in short reaction times.Thus, the protocol was efficient and general.After the successful application of SaSA in the preparation of 1,8-dioxooctahydroxanthenes, we checked its applicability to catalyze the preparation of 14-aryl-14Hdibenzo[a,j]xanthenes, by studying the multi-component condensation of 2-naphthol (2 mmol) with 4-methoxybezaldehyde (1 mmol) using different molar ratios of SaSA at range of 90-130 °C in the absence of solvent to provide compound 2b.The results are displayed in Table 3.As it can be seen in Table 3, higher yield and shorter reaction time were obtained when the reaction was carried out in the presence of 15 mol% of the catalyst at 120 °C.In another study, the efficiency and generality of the method were evaluated by the reaction of 2-naphthol with arylaldehydes bearing electron-donating substituents, electronwithdrawing substituents or halogens on their aromatic rings (Table 4).As Table 4 indicates, the method was general and efficient; all reactions were performed successfully to furnish the corresponding 14-aryl-14H-dibenzo[a,j]xanthenes in high to excellent yields and in relatively short reaction times.

Conclusion
In summary, we have developed new protocols for the synthesis of 1,8-dioxooctahydroxanthenes and 14-aryl-14H-dibenzo[a,j]xanthenes using SaSA under solvent-free conditions.The advantages of these methods are efficiency, generality, high yield, relatively short reaction time, cleaner reaction profile, simplicity, ease of preparation of the catalyst, ease of product isolation, and compliance with the green chemistry protocols.

15 a
Isolated yield.

Table 1 .
The reaction of dimedone with 4-methoxybenzaldehyde using different molar ratios of SaSA at range of 60-100 °C.

Table 3 .
Effect of amount of SaSA and temperature on the condensation of 2-naphthol with 4-methoxybenzaldehyde.
a Isolated yield.