Synthesis of Bioactive Fluorinated 10 H-Phenothiazines and their Sulfone Derivatives

The present communication deals with the synthesis of a series of fluorinated 10H-phenothiazines. 10H-phenothiazines is prepared by Smiles rearrangement of substituted 2-foramido-2 ́-nitrodiphenylsulfide. Substituted 2-foramido-2 ́-nitrodiphenylsulfide were obtained by the reaction of 2-amino-3fluorobenzenethiol with o-halonitrobenzenes followed by formylation and 1-nitro/1-halo-10H-phenothiazines have been prepared by the reaction of substituted 2-aminobenzenethiols with reactive o-halonitrobenzene containing a nitro group or halogen atom at o-position to the reactive halogen atom directly yielded 1-nitro/1-halo-10H-phenothiazines in situ. 10H-phenothiazine sulfone derivatives have been synthesized by the oxidation of 10H-phenothiazines by 30% hydrogen peroxide in glacial acetic acid. The structure of the synthesized compounds has been characterized by spectroscopic data and elemental analysis. Antimicrobial studies of the synthesized compounds have also been included.

A slight change in the substitution pattern of phenothiazine nucleus causes a marked difference in their biological activities.So it has been considered worthwhile to extend our efforts to synthesize phenothiazines and their sulfone derivatives to make them available for biological screening in order to study the effects of structural change with the biological activities and to obtain the drugs of improved therapeutic effects and with minimum undesirable side effects.Kerby Bauer procedure 14,15 have been used for antimicrobial activity of newly synthesized 10H-phenothiazines and sulfone derivatives.

Experimental
All the melting points were determined in open capillary tubes and are uncorrected.IR spectra were recorded in KBr on NICOLET-MEGNA FT-IR 550 spectrometer and the 1 H NMR spectra were recorded on JEOL AL-300 spectrometer (300 MHz) in CDCl 3 /DMSO-d 6 using TMS, as an internal standard (chemical shifts are measured in δ ppm).Mass spectra were recorded on JEOL SX 102/DA 6000 using Argon/Xenonas FAB gas.The purity of the synthesized compounds were checked by TLC using silica gel "G" as adsorbent, visualizating these by UV light or Iodine chamber.

Synthesis of 2-amino-2´-nitrodiphenylsulfides (III a-b )
To the refluxing solution of 2-amino-3-fluorobenzenethiol I (0.01 mole in 20 mL ethanol) and anhydrous sodium acetate (0.01 mole in 5 mL ethanol) was added in alcoholic solution of halonitrobenzene II a,b (0.01 mole in 20 mL of ethanol), refluxing was continued for four hours.The resultant solution was concentrated, cooled and kept overnight in an ice bath.The solid separated out was filtered, washed with 30% ethanol and recrystallized from methanol.

Synthesis of 2-formamido-2´-nitrodiphenylsulfides (IV a-b )
The diphenylsulphides III a-b (0.01 mole) obtained was refluxed for 4h in 90% formic acid (20 mL).The contents were then poured into a beaker containing crushed ice, a solid separated out was filtered, washed with water until the filtrate was neutralized and crystallized from benzene.

Synthesis of 10H-phenothiazines (V a-b )
To a refluxing solution of formyl derivatives IV a-b (0.01 mole) in acetone (15 mL) was added an alcoholic solution of potassium hydroxide (0.2 gm in 5 mL ethanol) was added.The contents were heated for 30 minutes.A second lot of potassium hydroxide (0.2 gm in 5 mL ethanol) was added to the reaction mixture and refluxed for 4h.The contents were poured into beaker containing crushed ice and filtered.The residue obtained was repeatedly washed with cold water and finally with 30% ethanol and then crystallized from benzene.

Synthesis of phenothiazines (V c-e ) by one step process
A mixture of reactive o-halonitrobenzene II c-e (0.01 mole), 2-amino-3-fluoro/2-amino-3isopropyl and 2-amino-5-isopropylbenzenethiols I (0.01 mole), sodium hydroxide (0.01 mole) and absolute alcohol (25 mL) was refluxed for two hours.The reaction mixture was concentrated on water bath, cooled and filtered.The precipitate was washed with hot water and ethanol and crystallized from acetone.

Synthesis of 10H-phenothiazine sulfones (VI a-e )
To a solution of substituted 10H-phenothiazines V a-e (0.01 mole) in 20 mL of glacial acetic acid, 5 mL of 30% hydrogen peroxide was added and refluxed for fifteen minute.Heating was stopped and another lot of hydrogen peroxide (5 mL) was added.The reaction mixture was again refluxed for 3-4h.The contents were poured in a beaker containing crushed ice.The yellowish residue obtained was filtered and washed with water and recrystallized with ethanol.

Antimicrobial activity
Newly synthesized compounds were screened for their antibacterial activity against Staphyllococcus aureus and Pseudomonas flueroscence bacterial strain by Kerby Bauer procedure using Streptomycin as standard drugs.Compounds are also screened for their antifungal activity against Aspergillus flavus and Aspergillus niger.The activity of each compound was compared with that of flukanozole as a standard drug.The results of such studies are given in Table 2. Antimicrobial activities are given in term of activity index.

Results and Discussion
2-Amino-3-fluorobenzenethiol I was condensed with o-halonitrobenzenes II a,b (2chloronitrobenzene and 2,4-dinitrochlorobenzene) to gave substituted 2-amino-2´nitrodiphenyl sulfides III a,b , which on formylation with 90% formic acid and substituent treatment with alcholic KOH underwent smiles rearrangement yielding fluorinated 10H-phenothiazines.When reactive o-halonitrobenzene (containing a nitro group as well as halogen atom at ortho position to the reactive halogen atom) condensed with 2aminobenzenethiols, yielded 10H-phenothiazines in a single step.Here cyclisation take place with the preference of halogen elimination and Smiles rearrangement as well as ring closure occurs simultaneously and in situ.Fluorinated 10H-phenothiazines 5 c-e have been prepared by condensation of 2-amino-3-fluoro/2-amino-3-isopropyl and 2-amino-5isopropylbenzenethiols with reactive halonitrobenzene II c-e .Substituted 10H-phenothiazine (V a-e ) converted into corresponding sulfones (VI a-e ) by oxidation with 30% hydrogenperoxide in glacial acetic acid (Scheme 1).The structures of all the synthesized compounds are characterized by spectroscopic data and elemental analysis.Kerby-Bauer procedure (Filter paper disc method) has been used for study of antimicrobial activity of newly synthesized compound.
The structure assignment of these compounds was made on basis of elemental analysis (Table 1) and spectral data (Table 2).The characteristic IR bands and 1 H NMR data of compounds (V a-e ) and (VI a-e ) are presented in Table 2.

IR spectra
Compounds (V a-e ) and (VI a-e ) exhibit a single sharp peak in the region 3410-3275 cm -1 due to N-H stretching vibration.Compound V b and VI b exhibit peaks 1585-1580 cm -1 and 1385-1380 cm -1 due to assymetric and symmetric stretching vibrations of -NO 2 group.Compounds (V a-e ) and (VI a-e ) also exhibit a single peak in the region 1295-1260 cm -1 due to C-F stretching vibrations.Sulfones derivatives of these 10H-Phenothiazines exhibit three characteristic absorption peaks as symmetric stretching v 1 (1195-1140 cm -1 ), Asymmetric stretching v 3 (1380-1370 cm -1 ) and banding vibration v 2 (575-520 cm -1 ) in chloroform solution.

NMR Spectra
All the synthesized compounds show a singlet in the region δ 9.72-9.18ppm is due to N-H protons.Multiplet due to aromatic protons appeared in the region δ 8.40-6.32ppm.(Table 2).

Mass spectra
In mass spectra molecular ion peak are in accordance with their molecular weight.

Conclusion
The structure proposed to the synthesized compound is well supported by spectroscopic data and elemental analysis.From the antimicrobial activity data (Table 2), it may be concluded that all the synthesized compounds showed good and moderate activity against the antimicrobes.

Table 1 .
Characterization data of phenothiazines and sulfones.

Table 2 .
R and 1 H NMR spectral data and antimicrobial activity of synthesized compounds.