Synthesis of quinolines and their In vitro antioxidant activities under solvent free conditions by using the SiO 2 – Zn – MgO as a novel and reusable catalyst

M. Brahmayya*, B. Venkateswara rao, U. Viplavaprasad, M.V. Basaveswara Rao, K. Raghu Babu, B. Kishore babu, K. Rajkumar, Ch. Praveen, N. Giribabu, M. Vijaya, Ch. V. Padmarao, N. Srinivasa Rao Department of Engineering Chemistry, Andhra University, Visakhapatnam-530003, India. Department of Organic Chemistry and FDW, Andhra University, Visakhapatnam-530003, India. Department of Chemistry, Krishna University, Machilipatnam – 521 001, India. Pharmacology Division, AU College of Pharmaceutical Sciences, Andhra University, Visakhapatnam-530 003, India.


INTRODUCTION
Quinolines are the naturally occurring alkaloids containing a hetero atom (Nitrogen) in the skeleton.Friedlander annulation is the well known named reaction for the preparation of quinolines from the past 100 years (Wu et al., 2005;Narasimhulu et al., 2007).For the condensation of quinolines so much acid catalysts were applied in friedlander reactions, which were most effective in such conditions like sulfamic acid (Yadav et al., 2005), phosphoric acid (Das et al., 2007), HCl (Wang et al., 2006) and other more catalysts like triflates (Atechian et al., 2007), ionic liquids (Palimkar et al., 2003).Somehow, these all above mentioned catalysts undergo from poor yields, relatively expensive, lengthy times, difficulties to work-up, and tedious procedures (Ghassamipour and Sardarian, 2009).
To handle this particular condensation in smooth way, we have developed a neutral, heterogeneous solid catalyst under solvent free conditions with effect of inexpensive and the yields were at 98%.The increasing yields were due to the large surface area, surface activity of the catalyst with effect of reduced time to get the products.Quinolines are having wide applicability towards pharmacological, antihypertensive, antimalarial, antibacterial and insecticidal agents (Kalluraya and Sreenivas, 1998;Doube et al., 1998;Chen et al., 2001;Rajanarendar et al., 2012).However, the role of reactive oxygen species (ROS) has been implicated in several diseases including cancer, diabetes, and cardiovascular diseases (Valko et al., 2007).Antioxidants are vital substances which possess the ability to protect the body from damage caused by free radical induced oxidative stress (Sherwin, 1990).There is an increasing interest in antioxidants which are might help in preventing oxidative damage.Therefore, the purpose of this present study was to investigate quinolines synthesis by an eco-friendly, effective, inexpensive and smooth rea ction handlings in the presence of SiO 2 -Zn-MgO as a reversible catalyst under solvent free conditions and antioxidant activity by using different in vitro methods.

Catalyst Preparation
Chemicals like SiO 2, Zn (powder), MgO were purchased from Merck, Darmstadt, Germany.In the experiment, three components SiO 2 (0.5 gm), Zn (powder) (0.5gm), MgO (0.5gm) were dissolved in 100 ml of deionized water at room temperature to form a homogeneous mixture.Then it was refluxed for 8 hours and cooled.
It was centrifuged and washed with deionized water and absolute ethanol and dried at 75 º C for 6 hours.The coordinated SiO 2 ---Zn---MgO catalyst was obtained by calcining the precursor in a furnace in air at 400 º C for 1hour.

General procedure synthesis of quinoline derivatives
To a mixture of α-methylen carbonyl compound (1.8 mmol) and 2-amino aryl ketone (1.2 mmol), SiO 2 -Zn-MgO (0.006 gm, 10 mol%) was added.This reaction mixture was heated at 100 º C and the reaction was monitored by TLC, G.C.After the completion of the reaction, Ethylacetate (EtOAc) was added to the reaction mixture and centrifuge to decant the catalyst.
The organic layer was evaporated under reduced pressure.It was purified by chromatography on silica gel (70:30 n-Hexane: Ethylacetate), the light yellow solid and was recrystallized by ethanol was obtained.All the products were characterized by 1 HNMR, FTIR, Mass spectra.The selected compounds 3d, 3e, 3j were given as spectral data.In the view of fluoro and non-fluoro examples 3e and 3j were examined for their biological activities given at table number 3

In vitro antioxidant studies
The DPPH free radical scavenging activity of 3e and 3j was determined by the method of Bilos, (1958).The superoxide radical scavenging activity was determined using NBT (nitroblue tetrazolium reagent) method as described by Nishikimi et al., (1972).The hydroxyl radical scavenging activity was determined using modified method of Halliwell et al., (1987).The ability of the 3e and 3j to hydrogen peroxide scavenging activity was assayed according to the method of Ruch et al., (1989).

RESULTS AND DISCUSSION
In the view of the good results, the catalyst acquired additional surface area with the support of SiO 2 (Silica gel-GF 254) along with Zn powder, as well as commercially available MgO towards the synthesis of biologically active quinolines and their derivatives, we have shown a lot of continued effort.In this way, α-amino acetophenone (1a) was reacted with ethylacetoacetate (2a) in the presence of metal oxide as shown in the scheme-1 (Table-1).Two kinds of metal oxides SiO 2 -Zn-MgO, SiO 2 -MgO were used.We examined the reaction under organic solvents and water which requires longer reaction times and afford low yields of the product (entries -2,4), while the reaction was failed in water (entry 8) The reaction was explored under solvent free conditions, affords the best results, shown in table (1), table (2), and applied the same reaction procedures to obtain 3a-3l series (3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l) under solvent free conditions (Scheme-1).We had not found any other isomeric products other than 3a-3l series in our experiment with 1 HNMR of the crude product mixture.All the compounds were well characterized by melting point, 1 HNMR, Mass spectroscopy and elemental analysis with high regiospecificity.The catalytic activity of the SiO 2 -Zn-MgO had shown high regioselectivity for the yields compared to SiO 2 -MgO which was examined and neglected due to low yields (20 %).The Lewis base sites (O 2-) and Lewis acid sites (Mg + ) Si + , Zn + from the source of the catalyst was one of the major reason for the good yields.
The in vitro antioxidant activity of 3e and 3j was shown in table 3. The IC 50 value of 3e and 3j of DPPH was calculated to be 3.39 µg/ml, 2.54 µg/ml.While for the standard Vitamin C it was 35.13µg/ml respectively.The IC 50 value of the 3e and 3j of the superoxide radical was found to be 8.22 µg/ml, 6.54 µg/ml and it was lower than that of the standard drug Vitamin C (27.34 µg/ml).The IC 50 value of the 3e and 3j of the hydroxyl radical scavenging activity was 5.67 µg/ml, 4.11 µg/ml and the IC 50 value of 3e and 3j was lower than the standard, Vitamin E (41.82 µg/ml).The Hydrogen Peroxide scavenging activity IC 50 values for 3e, 3j were found to be 4.10 µg/ml and 2.93 µg/ml respectively, and it was lower than that of the standard drug Vitamin C (23.34 µg/ml).In the present study 3e (example of fluoro-compound) and 3j (example non-fluoro compound ) was able to show the antioxidant properties like DPPH radical scavenging activity, superoxide radical scavenging activity, hydroxyl radical scavenging activity, hydrogen peroxide scavenging activity.The 3e and 3j was found to be efficacy against free radicals due to the presence of different substituents {3e } which are different from the rest of the synthesized compounds and might be useful in preventing free radical induced disorders like cardiovascular disease, diabetes and cancer etc (Ghinet et al., 2012).

CONCLUSION
The following conclusions are drawn from the above experimental observations.By supporting Zn (powder) on SiO 2 , MgO a considerable amount of Lewis acid sites and Lewis base sites could be generated due to the formation of coordinating interaction between metal oxides, due to the reaction the selectivity of the products were increased.Quinolines are act like antioxidants and prevent oxidative stress induced disorders.

ACKNOWLEDGEMENT
Authors are thankful to Department of Engineering chemistry, Andhra University, University of Hyderabad, India to carry out this research and grateful to AU College of Pharmaceutical Sciences for helping us in In vitro antioxidant assay and their kind encouragement.

Table . 1
: Optimization of the Synthesis of quinolines (3a a ).