Sodium Saccharin as an Effective Catalyst for Rapid One-pot Pseudo-five Component Synthesis of Dihydropyrano[ 2,3-g]chromenes under Microwave Irradiation

One-pot microwave-assisted synthesis of dihydropyrano[2,3-g]chromenes catalyzed by sodium saccharin as an efficient, mild and green catalyst was studied. The method presented is a safe and eco-friendly approach for the multicomponent synthesis of dihydropyrano[2,3-g]chromene derivatives with many merits including short reaction times (in comparison with other reported results), high yields and easy work up.


Introduction
One pot multicomponent reactions (MCRs) with high atom economy 1,2 play an important role in combinatorial chemistry.4][5] During MCRs, target compounds are produced with greater efficiency by generating structural complexity in a single step from three or more reactants. 6,7hromenes and their derivatives have a wide range of applications in various fields of chemistry, biology and pharmacology. 8][11] During attempts to synthesize the title compounds, some shortcomings were observed, such as long reaction time.Consequently, to overcome this drawback, the microwave irradiation method was used in the present study.Microwave irradiation (MW) as a form of electromagnetic energy that falls at the lower-frequency end of electromagnetic spectrum (300-300000 MHz), uses the ability of some liquids and solids to transform electromagnetic radiation into heat to drive chemical reactions. 12,13[16] Development of catalytic systems composed of lowcost, clean, environmentally benign and commercially available materials, has been a challenge in organic synthesis.Saccharin as an artificial sweetener with no food energy, has been employed extensively in a variety of foods and beverages, such as drinks, juices, cookies, medicines, toothpaste and gelatin.][19][20] Recently, sodium saccharin as a basic green and easy available compound was used as catalyst in some organic syntheses. 21,22We now report a new efficient and simple synthetic method, taking place via addition and subsequently cyclization of 2,5-dihydroxy-1,4-benzoquinone, arylaldehydes and malononitrile in the presence of a catalytic amount of sodium saccharine under microwave irradiation.Green and low-cost catalyst, easy workup, short reaction time in comparison with other reported results, 23 are just some of the advantages of the presented method.
termined in an open capillary using a Thermo Scientific 9300 apparatus.FTIR spectra were recorded with a Perkin-Elmer FTIR 550 spectrometer. 1H NMR and 13 C NMR spectra were recorded in DMSO-d 6 using Bruker DRX-400 spectrometer operating at 400 and 100 MHz, respectively.The elemental analyses (CHN) were obtained from a Carlo Erba model EA 1108 analyzer carried out on Perkin-Elmer 240c analyzer.Microwave-assisted reactions were performed with a Milestone ETHOS EZ apparatus, keeping irradiation power fixed and monitoring the internal reaction temperature.Mass spectra were recorded on a Finnigan MAT 44S by Electron Ionization (EI) mode with an ionization voltage of 70 eV.

Results and Disscussion
In the initial stages of the presented research optimization of the catalyst amounts, solvent and power of microwave irradiation were investigated.In the first step, catalyst optimization was studied.For determining the best quantity of sodium saccharin, 2 mmol of 4-nitrobenzaldehyde, 2 mmol of malononitrile and 1 mmol of 2,5-dihydroxybenzoquinone were used (as a model reaction).The reaction was performed in the presence of various amounts of the catalyst.As shown in Table 1, it is clearly observed that 0.019 g (10 mol%) of sodium saccharin lead to the highest yield of 4a (Table 1, entry 5).According to the optimzation results, the procedure was extended to various aldehydes (with electron withdrawing and electron donating groups).As can be seen in Table 4, aldehydes containing electron withdrawing groups (especially at the para position), have the highest yields (Table 4, entries 1, 8), also low yield products were obtained using aldeydes with a group at the ortho position (Table 4, entries 2, 3, 7).As a result, the reaction proceeded very efficiently and led to the formation of dihydropyrano[2,3-g]chromene derivatives 4a-l in high yields and in short reaction times.The structures of 4a-l was deduced from their FTIR, mass spectroscopy, elemental analysis, 1 H NMR and 13 C NMR techniques.The obtained results show that excellent yield was achieved using 10 mol% (0.019 g) of sodium saccharin (Table 1, entry 5).It should be considered that due to the very low solubility of products in general organic solvents, homogeneous catalysts are the best choice for the synthesis of dihydropyrano[2,3-g]chromenes.
In continuation of the research, the power of microwave irradiation was examined.Model reaction was run at several powers.Results in Table 2 show that the power of 100 W was the optimized condition for the synthesis of 4a (Table 2, entry 4).In last step, the effect of the solvent on the time and yield of the reaction was studied.Depending on the results presented in Table 3, in contrast to other solvents, H 2 O/Et-OH in ratio of 1:1 is the proper solvent for the synthesis of 4a in high yield (Table 3, entry 4).A plausible mechanism of the reaction is presented in Scheme 2. In the first step of the reaction, acidic proton of malononitrile has been separated by sodium saccharin as a basic catalyst.Then, the Knovenagel condensation of malononitrile anion and aldehyde leads to the intermediate I.In the next step, cyclization and tautomerization lead via II to the product III.The same process occurs on the other side of 2,5-dihydroxy-1,4-benzoquinone (due to its special structure) and finally, a dual chromene structure is produced.
The efficiency of sodium saccharin in comparison with other catalysts in the synthesis of dihydropyrano[2,3g]chromenes was also investigated.Results presented in Table 5 clearly show that sodium saccharin can be mentioned as a powerful, highly efficient catalyst for the synthesis of dihydropyrano[2,3-g]chromenes under microwave irradiation.

Conclusion
We have developed an efficient, novel and ecofriendly method for the synthesis of dihydropyrano[2,3g]chromenes in the presence of sodium sacchrin as the catalyst under microwave irradiation.The procedure described provides clean reaction conditons with easy work-up, simple filtration and short reaction times with high yields of products.
Scheme 1.Moradi and Sadegh: Sodium Saccharin as an Effective Catalyst for Rapid ...

Table 1 .
Effect of catalyst amounts on the time and yield of 4a a a 1 mmol of 1, 2 mmol of 2 and 2 mmol of 3a in 5 mL EtOH/H 2 O (1:1) and power of 100 W. b Isolated yield.c Thermal condition (35 C). d Time in hours.

Table 3 .
Solvent effect on synthesis of 4a a b Isolated yield.

Table 2 .
Optimization of microwave power a a 1 mmol of 1, 2 mmol of 2 and 2 mmol of 3a in the presence of 10 mol% of catalyst in EtOH/H 2 O (1:1).b Isolated yield.

Table 5 .
The effect of various catalysts on the synthesis of 4a under optimized conditions a Isolated yield.b 0.02 g.