Synthesis, Characterization and Biological Activity Evaluation of Some Pyrazoles, Thiazoles and Oxazoles Derived from 2-Mercaptoaniline

Synthesis of 2-mercaptobenzothiazole (A1) is performed from the reaction of o-aminothiophenol and carbon disulfide CS2 in ethanol under basic condition. Compound (A1) is reacted with chloro acetyl chloride to give compound (A2). Hydrazide acid compound (A3) is obtained from the reaction of compound (A2) with hydrazine hydrate in ethanol under reflux in the presence of glacial acetic acid .The reaction of hydrazide acid compound (A3) with ethyl acetoacetate gives pyrazole compound (A4). The new hydrazone compound (A5) was prepared from the reaction of compound (A3) with benzaldehyde. Reaction of compound (A3) with thiourea dissolved in ethanol gave 2-amino thiazole compounds(A6) which was used the reaction with 4– N,N-dimethyl benzaldehyde to yield compound hydrazone (A7).While, the reaction of compound (A2) with urea in the presence of ethanol gave 2-amino oxazole compounds (A8) which was used in the reaction with 3-hydroxy – 4 -methoxy benzaldehyde to yield hydrazone (A9). The structures of the prepared compounds were established by spectral (H-NMR,Elemental analysis (C.H.N),and FT-IR. In addition to systematic characterization of some active functional groups in these compounds, antibacterial activity (Esheriechia coli, Bacillus subtilis ) for some of the synthesized compounds were evaluated against two types of fugal (Candida albicans ) , the synthesized compounds.


Introduction:
Heterocyclic compounds contain nitrogen and sulfr. They play an important role, not only for life sciences, but also in many other industrial fields. Benzoxazole contains a benzene fused to an oxazole ring. (1). Heterocyclic compounds, particularly five and six member heterocyclic, brought the attention of pharmaceutical community over the years because of their therapeutic importance .Benzothiazole and its derivatives nucleus are important heterocyclic compounds and because of their synthetic utility and broad range of biological applications , such as antitumor (2) antimicrobial (3) anthelmintic (4) antileshmanial, (5) anticonvulsant (6) anti-inflammatory (7) and antihumane rhinovirus (HRV) activities(8) antibiotic (9) antifungal (10) anticancer (11) antiparkinson (12) anti-HIV (13) antioxidant (14), trypanocidal agent (15), hypoglycemic (16), antidiabetic (17) antituberculosis ,anti-urease (18) and inhibitor of α-glucosidase .They have also been used as ligands for asymmetric transformations (19). Moreover, some derivatives have anti-oxidant and radioprotective effects (20). Farhan reported the preparation and fungicidal activity of 2mercaptobenzothiazole tyrosine methyl ester derivative of 2-Mercaptobenzothiazole which is found to have an excellent antifungal activity most of all against Candida albicans (21) This research including preparation new derivatives for heterocyclic compounds are 2mercaptobenzothiazole derivatives.Studying the biological activities of the prepared compounds as antibacterial , antifungal activities .The structure of these newly synthesized compounds were established on the basis of elemental analysis, FT-IR, 1 HNMR.

Synthesis of Hydrazones(A7, A9) (23)
A mixture of compound A 6 or A 8 (0.004 mol) withsubstituted benzaldehyde (0.004 mol) was dissolved in absolute ethanol (15 mL) of few drops of acetic acid were added .The reaction mixture was refluxed for about 4 hrs, then reaction was concentrated to a brown precipitates which was filtered and recrystallized from ethanol .

Result and Discussions
All the compounds (A1-A9) were synthesized shown of the following scheme 1.

Preparation of 2-Mercptobenzothiazole (A1)
The compound 2-MBT was prepared according to the reaction of 2-Mercptoaniline with the carbon disulfide (CS 2 ). The reaction was followed up by using lead acetate paper which changes its color to black paper because of H 2 S liberation when the reaction takes place. The FT-IR spectrum of compound (A1) showed an absorption band at υ(2539) cm -1 due to (S-H) stretching. other bands shows at υ (3113) cm -1 was attributed to C-H stretching of aromatic ring , υ (1593) cm -1 due to (C-H) aliphatic. stretching; (1496) cm -1 due to (C=N) stretching and υ (752) cm -1 due to (C-S-C) stretching. The 1 H-NMR spectrum of compound (A1)showed the following characteristic chemical shift ,the (S-H) proton was resonated at (11.96) ppm, in additional to signals at δ= (7.20-7.50) ppm due to aromatic protons

Synthesis of 2-[(benzothiazol-2-yl)thio] Acetyl Chloride (A2)
The compound(A2) was synthesized by the treatment of 2-MBT with the chloroacetyl chloride, the success of the reaction was proved by the changes in the physical properties .The silver nitrate test confirmed the presence of chlorine group. The FT-IR, Fig. 1 spectrum absorption bands that showed disappearance of υ (2550)cm -1 due to(-SH) and the appearance strong bands at υ (1643) cm -1 .which was attributed to (C=O )group stretching, υ (848 ) cm -1 due to (C-Cl ) stretching. The 1 HNMR spectrum of (A2)which is depicted in Fig.2, supported the expected structure by presenting chemical shifts δ (7.2 -7.4) ppm due to aromatic ring hydrogen ,peak at δ 4.7 ppm (2H,s) which was attributed to (CH 2 ).

Synthesis of Hydrozones (A5,A7and A9)
The final step of this work deals with the reactions of compounds (A5, A7 and A9) by condensation reaction with substituted benzaldehyde to come up with the required benzothiazole linked to Schiff-base through amino group. The first stage in the condensation reaction between aromatic amine compound and various aromatic aldehydes consists nucleophile ,adding compounds containing amine (NH 2 ) group to carbonyl ( C=O)group producing hydrazones which exclude (H 2 O)water molecular to afford Schiff's base compounds. So the changes in physical properties and the FT-IR characterization showed disappearance of NH 2 group which is good sign that the reaction took place. The FT-IR spectrum for compound A5 in Fig. 7 was υ (3112) cm -1 due to NH stretching , υ (3076) cm -1 was attributed to C-H aromatic rings stretching ,2893 cm -1 due to C-H aliphatic was due to stretching ,1681cm -1 due to C=O stretching , υ (1627) cm -1 was attributed to C=C stretching, and υ (1575) cm -1 was due to C=N stretching of compound (A7).The 1 HNMR spectrum of (A7)which is depicted in Fig. 8, supported the expected structure by presenting chemical shifts δ 6.7-7.7ppm for aromatic hydrogen the singlet also appeared at 6.52ppm attributed to one proton of C=CH. ,signal at δ 9.7ppm for NH hydrogen(1H)and signal at δ 3.0ppm.(6H,singlet) was attributed to (NMe 2 ) protons.
The 1 H-NMR spectra of compound A8 in Fig.10 showed a signals at reign (7.1-7.9) ppm of four aromatic ring protons and the peak at δ 7.3 ppm (2H), which was due to (NH 2 ) protons, as well as peak at δ 4.63 ppm (s, 2H) was due to (-CH 2 ). The synthesized compounds in this work were expected to show biological activity since they have active groups in their molecules all of the tested compounds were studied at different concentration of using DMSO as a solvent (0.05, 0.001, 0.075, 0.005, 0.0025 mg/mL). Thus a preliminary evaluation of antibacterial and antifungal activity for some of the new 2-MBT compounds were tested against types of bacteria like Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) and against Candida albicans fungus. The results showed that most of the tested compounds have good antibacterial and antifungal activity those kinds of bacteria and fungus have been chosen because of their wide importance in the clinical field so they cause many diseases in addition to their various resistance of the antibiotic and chemical drugs. So their biological activity illustrated in Table 1 which shows antifungal activity and antibacterial activity.

Open
The result in Table 1 shows that the synthesized compounds have biological activity against the chosen fungus and bacteria because they have ability of inhibing the chosen bacteria and fungi by choosing different concentrations of the compounds, the inhibition zone is from (16 mm the lowest inhibition zone to 36 mm the highest inhibition zone of Fungus), but for bacteria it is about (10 mm the lowest inhibition zone to 32 mm the highest inhibition zone of bacteria). From the outcome it is also clear that the tested compounds(A3-A6) and (A8,A9) showed difference toxicity against different fungus and one of type bacteria.This difference in toxicity may be due to change in functional group or structures.as shown in picture 1.

Conclusion:
The present work deals with the synthesis of some benzothiazole derivatives that was achieved with substituted aromatic aldehydes in presence of ethanol to obtained Schiff bases (A 5 ,A 7 and A 9 ) . All the derivatives prepared by this method are analyzed by 1 HNMR and IR . The data in the table indicate that the synthesized compounds A 6 and A 8 showed moderate antibacterial activity while A 5 and A 9 showed good biological activity. From the results of various biological activity it is clear that these compounds would be of better use in drug development.