3.1. Chemistry
In the present study, 10 thiosemicarbazone and thiazole derivatives have been synthesized which are outlined in scheme 1 and scheme 2. The starting material thiosemicarbazone (1) was prepared by the condensation reaction of substituted aldehyde/ketones with substituted thiosemicarbazides in the presence of ethanol/methanol with few drops of glacial acetic acid reluxing with stirring at 80–90ºC for 8–9 hrs. The thiazole derivatives (2) was prepared by the reaction between thiosemicarbazone derivatives and phenacyl bromide refluxing in isopropyl alcohol for 5 hrs. General procedure for the synthesis of thiosemicarbazone and thiazole derivatives (Kp1-10) were achieved through the versatile and efficient synthetic route outlined in scheme 1 and scheme 2. Briefly, the synthesis of thiosemicarbazone and thiazole was achieved by using substituted aldehyde / ketone, in ethanol / methanol. Whereas, substituted thiosemicarbazides were added with stirring to ethanolic warm solution of substituted aldehyde / ketone in presence of glacial acetic acid. The resulting mixture was refluxed with stirring at 80–90°C and in second step thiosemicarbazones was further treated with phenacyl bromide and the purity of the product as well as composition of reaction mixture were monitored by TLC (Thin layer chromatography). The reaction mixture were cooled down to room temperature and then recrystallized with suitable solvent. The purity of the synthesized thiazoles was ascertained by TLC using Pre-coated Merk silica plate and Ethyl Acetate : Hexane (3 : 2) or chloroform : Hexane (3: 2) as the mobile phase. The spots were detected under UV at 245nm and Rf value was calculated. The structures assigned to the compounds were supported by the results of IR, 1H NMR, 13C NMR and mass spectral data.
Structure of all the synthesized compounds was monitored by the FTIR, 1HNMR, 13CNMR, and Mass spectral analysis. Compound Kp-1 showed actual peak of OCH3, C = N, N-H(stretching), C = C, C-H(aromatic), C-H(aliphatic) is at 1258cm-1, 1121 cm-1, 3295 cm-1, 1513 cm-1. Compound Kp-2 showed actual peak of OH, C-H, C-N, N-H, C = C, C-H(aromatic), C-H(aliphatic) are at 3400cm-1, 3150cm-1,1136cm-1, 1121cm-1, 3127 cm-1, 1513 cm-1. Compound Kp-3 showed actual peak of OH, C-H, C-N, N-H, C = C are at 3400cm-1, 3050 cm-1, 1136 cm-1, 1121cm-1, 3125 cm-1, 1513 cm-1. Compound Kp-4 showed actual peak of OH, C-H, C-C are at 3447 cm-1, 3150 cm-1, 2735 cm-1. Compound Kp-5 showed actual peak of C = N, C-H, C-N, O-H are at 2228cm-1, 3108 cm-1, 1226 cm-1, 3400cm-1. Compound Kp-6 showed actual peak of C = C, C-N, O-H, are at 1606cm-1, 1179 cm-1, 3326cm-1, 2940 cm-1. Kp-7 showed actual peak of O-H, C = C, C-N, is at 3400-3650cm-1, 1600&1475cm-1, 1030–1230 cm-1, but their expected values are at 3434cm-1, 1599cm-1, 1230cm-1. Compound Kp-8 actual peak of O-CH3, C = C, C-N, are at 1236cm-1, 1600cm-1, 3147 cm-1, 3334 cm-1. Compound Kp-1 showed 1HNMR chemical shift peak at δ 6.25–7.38 due to aromatic proton. Chemical shift peak at δ 4.02 due to OCH3 group attached to aromatic ring. Compound Kp-2 showed multiplet at δ 6.86–7.81 due to aromatic proton. Compound with OH group attached to aromatic ring showed chemical shift peak at δ 5.50 and chemical shift peak at δ 7.02 due to aliphatic NH. The mass spectra of compound Kp-1 [(E)-2-(2-(4-methoxybenzylidene)hydrazineyl)-4-phenyl-2,3-dihydrothiazole] showed peak at m/z 323.41. Compound Kp-2 [(E)-4-(1-2-(4-phenyl-2,3dihydrothiazol-2yL)hydrazineylidene)ethyl)phenol] showed peak at m/z 311.11.
3.2. In vitro antimalarial activity screening
The antimalarial activity of the synthesized thiazole derivatives (Kp 1–10) was assessed against human pathogenic malarial strain viz. Plasmodium falciparum while quinine was taken as the standard drug. Activity profile of all the compounds screened for antimalarial activity is represented in Table 1. Results antimalarial evaluation revealed that most of the tested compounds exhibited remarkable inhibitory activity against the tested plasmodium strain. Among them, compound Kp-9 was found to be most promising which exhibited strongest inhibitory activity against P. falciparum with an IC50 value of 0.29µg/mL which was higher than the reference drug quinine (1.26µg/mL). Compound 10 also presented excellent inhibition of P. falciparum with IC50 values of 0.59 respectively. Other tested derivatives such as Kp-1, Kp-3 and Kp-4 also exhibited significant antimalarial activity with IC50 values in the range of 0.65, 0.90, 0.85µg/mL. Compound Kp-2 was the least potent candidate of the series with IC50 value of 5.52 µg/mL.
The structure-activity relationship of the various titled compounds (Kp-1-10) screened for antimalarial activity has been analysed using quinine as the reference drug. Results of in vitro evaluation data indicated that inhibitory activity of the various tested compounds against P. falciparum depends upon the nature and type of substituents introduced at the 5th position of the thiazole core. Substitution with methyl group or bulky groups in compound Kp-2, Kp-6 at R decrease the anti-malarial activity whereas substitution with hydrogen group at same position responsible for increase in activity as shown by compound Kp-2, Kp-6 and Kp-7. Substitution with electron withdrawing group at R1 increases anti-malarial activity as shown in compound Kp-9. Substitution at R2 with hydrophobic group responsible for decrease in activity as shown in compound Kp-5, Kp-7 and Kp-8. Compounds having hydrogen or hydroxyl group at R and R2 position shown in compound Kp-1 showed promising anti-malarial activity. The present study highlighted that most of the synthesized thiazole derivatives possessed strong inhibitory potential against human pathogenic malarial strain P. falciparum.