Synthesis and Antimicrobial Activity of Some New 1 , 4-Benzothiazine Containing Thiosemicarbazides and 1 , 3 , 4-Oxadiazole Derivatives

A series of novel 3methyl-7-substituted-4H-1,4-benzothiazine-2carbohydrazide (3a-e) and corresponding thiosemicarbazides (4-a-q); 2-[3-methyl-7substituted4H -1, 4-benzothiazine-2-yl]-N-(aryl) hydrazine carbothiamide have been synthesized. The thiosemicarbazide when cyclized with iodine via intramolecular cyclisation gave benzothiazonyl oxadiazoles (5-a-q); 5-(3-methyl -7-substitued-4H1,4-benzothiazin-2-yl)N –aryl1,3,4oxadiazol -2-amine and the compounds were tested for antibacterial and antifungal activities against different microorganisms.


Introduction
The chemistry of heterocyclic compounds has been an interesting field of study for a long time.The synthesis of novel oxadiazole derivatives and investigation of their chemical and biological behaviour have gained more importance in recent decades for medicinal and agricultural reasons.Different classes of oxadiazole compounds possess an extensive spectrum of pharmacological activities, particularly in compounds bearing 1, 3, 4-oxadiazole nucleus which are known to exhibit unique antiedema and anti-inflammatory activity 1,2 .Substituted oxadiazole moiety has been found to have important activities such as analgesic 3,4 , antimicrobial 5,6 , antitumor 7 , antimalerial 8 and antihepatities 9 .In some cases 1,4-benzothiaazine are also known for their utility as dyes, photographic developers, ultraviolet light absorbers and antioxidants 10 .
The multifarious applications of 1, 4-benzothiazine have directed organic chemists to synthesize new 1, 4-benzothiazine bearing heteryl pharmacophores.In view of this, we are working on the synthesis of new 1, 4, benzothiazine containing 1, 3, 4-oxadiazole derivatives, because it is common observation that combination of two or more biologically active heterocyclic rings in some compounds results in enhancement of biological profiles of such compounds by many folds.

Experimental
Melting points were determined in open capillaries in a liquid paraffin bath and are uncorrected.The purity of compounds was checked by TLC.IR spectra were recorded in Nujol on a Perkin-Elmer FT-IR spectrophotometer and 1 HNMR spectra in DMSO-d 6 using TMS as internal standard.

Step I: Preparation of 2-carboethoxy-3-methyl-7-substituted-1,4-benzothiazine (2a-e)
A mixture of 2-amino-5-substituted-thiophenol (1a-e) (0.1 mol) and hydrazine hydrate (0.01 mol) was heated at 100 0 C for 2-3 minutes before introducing the ethyl acetoacetate (0.1 mol) and warming the reaction mixture to 100 0 C for further 10 minutes.After cooling to room temperature, 4-5 mL of ethyl alcohol was added.The solid that separated was filtered and recrystallised from ethyl alcohol.The compounds (2a-e) were prepared in same fashion and their physical constants are given in Table 1.

Antimicrobial screening
All the newly synthesized compounds (5a-q) were evaluated for in vitro antibacterial activity against gram positive and gram negative bacterial strains such as Bacillus subtilis, Bacillus pumilus, Escherichia coli and pseudomonas aureginosa at concentration 100 µg/mL by disc diffusion method 11 by using DMSO as solvent control and nutrient agar was employed as culture media.After 24 hours of incubation at 37 0 C, the zone of inhibition were measured in mm.The activity was compared with known antibiotic ciprofloxacin.All these compounds were also screened (doses of 100 µg) for their antifungal activity against Aspergillus niger using Greseofulvin as a standard.The results of antibacterial to antifungal screening studies are reported in Table 4.

Conclusion
All the 17 newly synthesized compounds were screened for antibacterial and antifungal studies.The data in the table indicates that among the synthesized compounds 5c, 5f, 5g, 5h, 5j, 5p and 5q has found to broad spectrum of activity.However, the activities of tested compounds are much less than those of standard antibacterial and antifungal agents used.

Table 2 .
Characterization data for the synthesized compounds (4a-q)

Table 3 .
Characterization data for the synthesized compounds (5a-q)