Microwave as an energy source in the synthesis of 2-aryl-4-quinolone alkaloids and naphthyridines
Authors:   , Ndaba Hlengiwe Glenrose 
Published: August 2021

One of the greatest medical challenges facing mankind is the Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome (HIV/AIDS) which has now become a major epidemic with more than 40 million people infected worldwide. Of equal concern is its implication in high mortality and the onset of a number of opportunist mycobacterial infections, principally tuberculosis. In spite of the discovery of some relatively effective antiretroviral (ARV) drugs such as Azido Thymidine (AZT), Nevirapine (NVP) and Efavirenz (EFV), its’ application as either a single or combinational form causes side effects by harming the bone marrow. Drug resistance is a key cause of failure for treatment of HIV infection. Hence greater interdisciplinary efforts, involving both natural and social sciences, are needed urgently to combat this HIV/AIDS pandemic. Heterocyclic nitrogen based compounds, obtained from either natural sources or synthesis are adequately documented to have increased biological activity against several diseases. Recently a study of drugs containing the naphthyridine scaffold has acquired increasing attention because of its potential against HIV/AIDS. Generally, naphthyridines demonstrate good potency in both the enzyme and cellular systems and this prompted our interest in the synthesis of naphthyridine derivatives from simple and readily available starting compounds. Furthermore we wanted to form an intermediate quinolone moiety since it has good biological potential. In this study we report the synthesis of three naphthyridine derivatives, i.e. 6-phenyl-dibenzo [b, h] [1, 6] naphthyridine, 4-methyl-6-phenyl-dibenzo [b, h] [1, 6] naphthyridine and 2- methyl-6-phenyl-dibenzo [b, h] [1, 6] naphthyridine from easily available chemicals such as aniline, ortho-toludine, para-toluidine and ethyl benzoylacetate via a five step reaction scheme using either conventional reflux, microwave irradiation or both methodologies. It was found that microwave irradiation was several folds faster than conventional reflux methodology and the yield of the product was higher. The first step of the reaction scheme is a simple condensation reaction: three acrylate derivatives, viz. ethyl-3-aniline-3-phenyl acrylate, ethyl-3-phenyl-3-(ortho-tolylamino) acrylate and ethyl-3-phenyl-3-(para-tolylamino) acrylate were synthesized by refluxing ethyl benzoylacetate in an acidified ethanolic solution with aniline, ortho-toluidine and paratoluidine respectively for three hours; the yields were 95, 87.5 and 80 % respectively. Page v In the second step, thermal cyclisation was achieved for the synthesis of three quinoline derivatives, viz. 2-phenylquinoline-4(1H)-one, 8-methyl-2-phenylquinoline-4(1H)-one and 6- methyl-2-phenylquinoline-4(1H)-one from their respective acrylates under microwave irradiation for 5 minutes at 180 °C and 250 watts; the yields were 92, 84 and 80 % respectively. In the third step of the reaction, synthesis of 4-chloro-2-phenylquinoline, 4- chloro-8-methyl- 2-phenylquinoline and 4- chloro-6-methyl-2-phenylquinoline was achieved from a mixture of POCl3 and their respective quinolines via microwave irradiation for 3 minutes at 75 °C and 150 watts and via conventional reflux for 5 hours. It was found that under microwave irradiation, the reaction occurred nearly 100 fold faster but the % yield of the product was marginally higher. The fourth step of the reaction resulted in the formation of three schiff’s base, viz. 4-(Nphenyl)- 2-phenyl-4-aminoquinoline, 8-methyl-4-(N-phenyl)-2-phenyl-4-aminoquinoline and 6-methyl-4-(N-phenyl)-2-phenyl-4-aminoquinoline from their respective quinolines via microwave irradiation for 20 minutes at 180 °C and 180 watts and via conventional reflux for 2 hours. It was found that under microwave irradiation, the reaction occurred nearly 6 fold faster and the % yield of the product was over 10 % higher. The final step of the reaction was achieved by a Vilsmeir Haack reaction and in situ base catalyzed thermal cyclisation: 6-phenyl-dibenzo [b, h] [1, 6] naphthyridine, 4-methyl-6- phenyl-dibenzo [b, h] [1, 6] naphthyridine and 2-methyl-6-phenyl-dibenzo [b, h] [1, 6] naphthyridine were synthesized from their respective schiffs base via microwave irradiation for 20 minutes at 75 °C at 120 watts and via conventional reflux for 21 hours. It was found that under microwave irradiation, the reaction occurred over 60 fold faster and the % yield of the product was over 20 % higher.The outline for the five step synthesis of the three naphthyridines is presented graphically below: Page vi Key: (a) R1= H; R2=H (b) R1 = H; R2 = CH3 (c) R1 = CH3; R2 =H Reaction Conditions: 1) conc.HCl, EtOH, 3hrs, 50 °C; 2) conc. HCl, hand stirring 10 min; 3) 180 °C, MWI, 250 watts, 5 min; 4) POCl₃, MWI, 75 °C, 150 watts, 2 min; 5) POCl₃, 100 oC, 5 hrs; 6) aniline, t-BuOH, MWI, 180 °C, 180 watts, 20 min; 7) aniline, t-BuOH, 80 °C, 3 hrs; 8) DMF, POCl₃, MWI, 75 °C,120 watts 20 minute; 9) DMF, POCl3, 100 oC, 21 hrs.