Microwave-assisted solvent-free synthesis of 14-aryl / alkyl-14 H-dibenzo [ a , j ] xanthenes and tetrahydrobenzo [ a ] xanthen-11-ones catalyzed by nano silica phosphoric acid

Article history: Received June 29, 2012 Received in Revised form November 6, 2012 Accepted 16 December 2012 Available online 16 December 2012 Nano silica phosphoric acid (nano SPA) was applied as a catalyst for synthesis of 14-aryl/alkyl-14H-dibenzo[a,j]xanthenes and tetrahydrobenzo[a]xanthen-11-ones in microwave oven under solvent free conditions. High efficiency, easy availability and reusability are some advantages of this catalyst. © 2013 Growing Science Ltd. All rights reserved.


Introduction
Multi-component reactions (MCRs) are special types of synthetically useful organic reactions in which three or more different starting materials react to form a product in one pot procedure 1 .Such reactions are atom-efficient processes by incorporating the essential parts of the starting materials into the final product.MCRs are powerful tools in the modern drug discovery process and allow the fast, automated, and high through put generation of organic compounds 2 .

Results and Discussion
Silica phosphoric acid was prepared by reaction of silica chloride with dry phosphoric acid.It is noted that, silica chloride was prepared via reaction of silica gel and thionyl chloride.By using nano silica gel instead of silica gel, according to above pathway, nano silica phosphoric acid (nano-SPA) was prepared.The particle size of nano-SiO 2 and nano-SPA were measured by SEM and TEM imaging (Fig. 1).
The acid capacity of nano-SPA is 10.32 mmol.g -1 and was determined by titration of 0.2 g of catalyst with standard solution of NaOH.The FT-IR (ATR) spectra of silica chloride, nano-SPA and H 3 PO 4 .SiO 2 were shown in figure 2. In all ATR spectra, the Si-O-H and Si-O-Si stretching bonds are appeared in 900 until 1100 cm -1 .In silica chloride spectrum, the Si-Cl stretching bond is appeared in 700 cm -1 .In ATR spectra of nano-SPA and H 3 PO 4 .SiO 2 , the P-O-H, P=O, P-O stretching bonds are appeared 910-1040, 1637 and 2400-2800 cm -1 respectively.According to above data, we suggested one structure for nano-SPA with PO 3 H 2 on silica gel (Scheme 1).The X-ra SiO 2 XRD p SPA XRD scherrer's fo   The molar ratio of 2-naphthol : benzaldehyde is 2:1 b The molar ratio of 2-naphthol : benzaldehyde : dimedone is 1:1:1 According to the obtained best condition, we have applied 2-naphthol and various aldehydes: a) for the synthesis of 14-aryl/alkyl-14H-dibenzo[a,j]xanthene derivatives (Scheme 3 and Table 2) and b) with dimedone for the synthesis of tetrahydrobenzo[a]xanthen-11-one (Scheme 4 and Table 3).2. Synthesis of 14-aryl/alkyl-14H-dibenzo[a,j]xanthenes (7 a-h )in the presence of nano-SPA under solvent-free conditions and microwave irradiation for 6 min.The ratio of 2-naphthol (mmole): aldehydes (mmole):dimedone (mmole): nano-SPA (g) is 1:1:1:0.02 .b Isolated yield.

Conclusions
We have demonstrated a simple method for the synthesis of tetrahydrobenzo[a]xanthen-11-ones and 14-aryl/alkyl-14H-dibenzo[a,j]xanthenes using nano-SPA as eco-friendly and efficient catalyst.Short reaction times, high yields, a clean process, simple methodology, easy work-up and green conditions are advantages of these protocols.

Materials and Methods
The materials were purchased from Merck Company and were used without any additional purification.Products were characterized by FT-IR, 1 H-NMR and comparison of their physical properties with those reported in the literature.FT-IR (ATR) spectra were run on a Bruker, Eqinox 55 spectrometer.A Bruker (DRX-400 Avance) NMR was used to record the 1 H NMR spectra.The SEM and TEM of nano particles were determined with a VEGA/TESCAN scanning electron microscope and Leo 912AB OMEGA transmission electron microscopy, respectively.The X-ray diffraction (XRD) patterns of materials were recorded by employing a Philips Xpert MPD diffract meter equipped with a Cu Kα anode (λ = 1.54 A°) in the 2θ range from 5 to 80°.

Typical procedures for the preparation 14-aryl/alkyl-14H-dibenzo[a,j]xanthenes under microwave condition in the presence of nano-SPA
In a 5 ml microwave reactor vessel, a mixture of of 2-naphthol (2 mmol), aldehyde (1 mmol) and nano-SPA (0.02 g) was irradiated in microwave oven for 6 minutes.After completion of the reaction, the product was dissolved in hot ethanol and filtered to recover the catalyst.The solvent of filtrate was evaporated to obtain the product.The crude product was recrystallized from ethanol.

Typical procedures for the preparation of tetrahydrobenzo[a]xanthenes-11-one under microwave condition in the presence of nano-SPA
In a 5 ml microwave reactor vessel, a mixture of 2-naphthol (1 mmol), aldehyde (1 mmol), 1,3diketone (1.2 mmol), and nano-SPA (0.02 g) was irradiated in microwave oven for 6 min.After completion of the reaction, the product was dissolved in hot ethanol and filtered to recover the catalyst.The solvent of filtrate was evaporated to obtain the product.The crude product was recrystallized from ethanol.

Fig. 3 .
Fig. 3. X-ray d investigati the sy then-11-one timize the mount of cat roducts wer ntries 5 and sability of n nd filtered.T ar from the although a

Table 1
known and characterized by FT-IR, 1 H-NMR, and the physical properties of those reported in the literature.

Table 1 .
Optimization of the reaction conditions for preparation of (3) and (5) in microwave oven under solvent free conditions