Synthesis and Pharmacological Evaluation of 7-chloro-6-fluoro-2-( 5-aminooxazol / thiazol-2-aryldenylamino )-benzo ( 1 , 3 ) thiazoles

New series of 7-chloro-6-fluoro-2-(5-aminooxazol/thiazol-2-aryldenylamino)-benzo (1,3)thiazoles (Schiff’s base)(5a-g, 6a-g) were synthesized from 7-chloro-6-fluoro-2-(5amino-oxazol/thiazol-2-amino)-benzothiazole (4a-b). Structures of the compounds were verified by H-NMR spectra. The synthesized compounds were screened for in vivo analgesic and antiinflammatory activities. Some of the compounds showed moderate activities.


Materials and Methods
The purity of all the synthesized compounds was confirmed by using thin layer chromatography (TLC) and melting point.TLC of the compounds was run by using silica gel G as stationary phase and by using suitable solvent mixture.Different analytical techniques such as IR and NMR were used to characterize the synthesized compounds.The IR spectra were recorded on Shimadzu FT-IR 8400-S spectrophotometer by KBr pellet technique.The 1 H-NMR spectra were recorded on AMX-400 NMR spectrophotometer at 400 MHz using DMSO/CDCl 3 as the solvent and TMS as internal standard.Chemical shifts are expressed in ppm.

Methods
The title compounds were prepared in following steps (scheme is also shown): To a cooled solution of 7-chloro 6-fluoro 2-amino benzothiazole (2) [11] in ethanol (10.15 g, 0.05 mol), chloroacetylchloride (5.8 ml, 0.05 mol) was added drop wise for 1 h while stirring.Further the mixture was stirred for 2 h and refluxed for 1 h.After completion of the reaction (monitored by TLC), the reaction mixture was cooled and poured onto crushed ice.The separated solid was filtered, washed with water and recrystallized from methanol.
The reaction mixture was cooled and poured onto crushed ice to obtain the solid, which was dried and crystallized by DMSO.Physical and analytical data of synthesized compounds is summarized in Table 1 and characterization data in Table 2.

In vivo anti inflammatory activity
In vivo Anti-inflammatory activity of synthesized compounds was evaluated using carrageenan induced rat hind paw oedema [12].The animals were divided into control, standard and test groups, each consisting of six animals.The first group was treated with Tween-80 (1%) suspension which served as control, second group was administered with a dose of 20 mg/kg suspension of diclofenac sodium intraperitoneally which served as standard and other groups were treated with 30 mg/kg of suspension of test compounds in Tween-80.After 30 min, the rats were injected with 0.1 ml of carrageenan (1% w/v) to the sub plantar region of left paw of the rats.The volume of paw was measured using mercury displacement technique with the help of plethysmograph both in control and animals treated with standard and test compounds at 0, 1, 2 and 3 h after injection of carrageenan.The percentage inhibition of oedema was calculated by using formula, percentage inhibition = (1-Vt/Vc) 100, where Vt is the mean paw volume of the test drug, Vc is the mean paw volume of the control.The results are recorded in Table 3.

In vivo analgesic activity
The animals were divided into four groups of six animals each.The animals, which showed reaction time of 2-3 s, were selected for experiment and analgesic activity of synthesized compounds was studied by tail flick method [13].The tail received radiant heat from a wire, which is heated by passing a current of 6 mA.The time taken for the withdrawal of tail was recorded before the administration of the compounds and for 30, 60, 90 and 120 min after administration of compounds.The cut off time for determination of latent period was taken as 40 s to avoid the injury to the skin and based on our pilot studies.One group served as a standard (pentazocine hydrochloride) with dose of 10 mg/kg body weight and another group served as control (1% Tween-80) and rest of the groups used for the test drugs.The test compounds and pentazocine hydrochloride were suspended in 1% Tween-80 which was used as vehicle for the control group.The tested compounds were administered at the dose of 30 mg/kg body weight in the form of suspension and administered intraperitoneally.The results of analgesic activity are shown in Table 4.The animal experimental protocols were approved by Institutional Animal Ethical Committee (IAEC).
The compounds 5f, 5g, 6f and 6g were found to possess moderate anti-inflammatory activity having percentage of inhibition to the extent of 38.30%, 40.41%, 42.38% and 43.75% as compared with diclofenac sodium, which showed percentage inhibition of 54.20%.The compounds 5f, 5g, 6f and 6g were found to show considerable analgesic activity while the remaining compounds were found to be exhibit less active when compared to standard.Hence electron donating group at position 4 of benzylidene-(7-chloro-6fluorobenzothiazol-2-yl)oxazol/thiazol-2,5-diamine is necessary for anti-inflammatory and analgesic activity.

Table 1 .
Physical and analytical data of synthesized compounds.

Table 2 .
Characterization data of the synthesized compounds.

Table 3 .
Results of Antiinflammatory activity of the synthesized compounds.

Table 4 .
Results of analgesic activity of the synthesized compounds.
1H-NMR spectrum was recorded in CDCl 3 exhibited a singlet at 8.4 ppm due to -NH integrating for proton, aromatic protons resonated as multiplet at 7.3-8.0ppm and -CH 2 protons appear as singlet at 4.8 ppm.The compound 3 was refluxed with urea and thiourea separately to produce 7-chloro-6-fluoro-2-(5-aminooxazol-2-amino) benzothiazole (compound 4a) and 7chloro-6-fluoro-2-(5-aminothiazol-2-amino) benzothiazole (compound 4b) respectively.The IR spectrum of 4a exhibited absorbance band at 3473 cm -1 due to -NH 2 group.Its 1 H-NMR spectrum was recorded in CDCl 3 exhibited, a singlet integrating at 9.2 ppm due to NH, a broad singlet at 5.2 ppm for two protons of NH 2 and aromatic protons resonated at 7.3 ppm and 7.9 ppm.Similarly, the IR spectrum of compound 4b exhibited -NH 2 absorbance band at 3475 cm -1 .Its 1 H-NMR spectrum was recorded in CDCl 3 exhibited a singlet integrating at 9.2 ppm due to NH, a broad singlet at 5.1 ppm for two protons of NH 2 .The aromatic protons resonate between 7.3-7.9ppm.Schiff bases were prepared from 4a and 4b by reacting with appropriate aromatic aldehydes in absolute alcohol in the presence of concentrated sulfuric acid.IR spectrum of benzylidene-(7-chloro-6-fluorobenzothiazol-2-yl) oxazole-2,5-diamine (compound 5a) exhibited -NH absorbance band, C-H stretching, C-S band and N=CH band at 3308, 3010, 2366 and 1647 cm -1 , respectively.The 1 H-NMR spectrum of 5a was recorded in DMSO exhibited singlet at 8.5 ppm due to NH integrating for one proton.The aromatic protons resonate between 7.1-8.2ppm.Whereas N=CH proton appeared at 3.8 ppm.In the similar way, remaining Schiff bases were identified.The spectral data of the synthesized compounds are given in Table one