Synthesis of Benzo [ g ] quinoxaline-5 , 10-dione Based Pyridine Derivatives and their Antimycobacterial Activity SHIV

Thirteen new compounds belonging to series 2-amino-6-(5,10-dioxo-2,3-diphenyl5,10-dihydrobenzo[g]quinoxalin-7-yl)-4-(substituted)phenylpyridine-3-carbonitrile (6a-m) were synthesized by multistep synthetic scheme. The newly synthesized compounds were screened for their antimycobacterial activity by L.J. Slope (Conventional) Method. Compound 6h with 4-N(CH3)2 group at phenyl ring of above mentioned position has been reported as most active antimycobacterial compound and compound 6k with 4-CH3 substitution at phenyl above mentioned position has been reported as the least active antimycobacterial compound.

Tuberculosis (TB) is a major health problem worldwide now-a-days.In year 2014, there were an estimated 9.6 million new TB cases were reported and 1.5 million deaths were caused due to TB worldwide 16 .Quinoxaline ring is also a an important part of antileprotic drug clofazimine (CZM) which is also widely indicated for treatment of multidrug resistant tuberculosis.In Mycobacterium tuberculosis, CZM is reduced by NADH-dehydrogenase (NDH-2) to release reactive oxygen 17 and CZM also act as competitor with menaquinone (MK-4), which acts as a key factor for its reduction by NDH-2 18 .First line antitubercular drug isoniazid (INH) and second line antitubercular drug ethionamide (Etm) are derivatives of six membered nitrogen containing pyridine ring.Isoniazid and Ethionamide, after bioactivation by mycobacterial catalase-peroxidase enzyme complex, inhibit synthesis of mycolic acids 19- 23 , that are important components of mycobacterial cell wall.

Chemistry
Melting points were measured in open capillary tubes and are uncorrected.All the Fourier-Transform Infra Red (FT-IR) spectra were recorded on Shimdazu FT-8400 Spectrophotometer using KBr pallets.The 1 H-NMR spectra were recorded on Bruker-Spectrospin DCX NMR spectrometer using DMSO-d 6 and CDCl 3 as solvents and tetramethylsilane (TMS) as an internal standard (Chemical shifts expressed in d/ppm The purity of the compounds was checked by thin layer chromatography (TLC) on Merck Silica Gel 60F 254 precoated sheets using Toluene : Ethylacetate : Formic acid (5:4:1) solution as mobile phase.

Synthesis of 2,3-Dichloronaphthalene (1)
2,3-dihydroxynaphthalene (5 gm) was dissolved in phosphorusoxychloride (90 mL) in parts at 0 o -5 o C with occasionally stirring and refluxed for about 4 h till the appearance of clear dark pinkish solution.This solution was cooled and poured into ice-cold water.A dark pinkish solid appeared and was filtered, dried and recrystallized with dimethylformaide (DMF).Pinkish crystalline solid; Yield 85%; R f 0.72; mp 185 o C. IR (n max , KBr, cm -1 ):

Primary screen
In primary screening, the compounds were diluted to 500 µg/mL, 250 µg/mL and 125 µg/ mL concentrations.Compounds found active in this primary screening were further tested in a second set of dilution against Mycobacterium tuberculosis.

Reading Result
The highest dilution showing at least 99% inhibition was considered as Minimum Inhibitory Concentration.The inoculum size of test should contain 10 8 organism/mL.
Unsubstituted phenyl ring (6a) on the 4-position of pyridine ring in above mentioned nucleus exhibited moderate antimycobacterial activity.Substitution with electron donating para-dimethylamino group (6h) on the phenyl ring present at C-4 of pyridine ring increases antimycobacterial activity and produces the most active antimycobacterial compound of the above mentioned newly synthesized series.Substitution with electron withdrawing 4-Cl group (6b) and 2-Cl (6g) at the phenyl ring also increases the antimycobacterial activity but lesser than that of substitution with para-dimethylamino group (6h).Substitution with para-hydroxy (6c) and electron withdrawing para-trifluoromethyl (6i) at the pheny ring exhibits moderate activity lesser than that of compounds 6h, 6g and 6b and more than that of compound with unsubstituted phenyl ring (6a).While, substitution with bulky electron withdrawing group para-nitro (6d), electron donating para-methyl (6k) and bulky electron donating groups 3-OH-4-OCH 3 (6f) and 3,4-dimethoxy (6j) decreases the antimycobacterial activity to great extent and produces the least active compounds of the newly synthesized series.