Introduction

Benzofuran compounds are ubiquitous in nature, particularly among plant kingdom. often such natural products possessing benzofuran nucleus are endowed with useful pharmacological properties. This has generated enormous interest in synthetic products containing benzofuran nucleus and has resulted in the development of benzofuran chemistry during the last several years. Due to wide scope for synthetic investigation leading to more potent synthetic leads, voluminous synthetic work has been done. Several benzofuran compounds are reported to posses antibacterial^1-4, antifungal², antitumor^5-6 anti-inflammatory⁷, antidepressant⁸, analgesic⁹ and hypoglycemic¹⁰activities. Substituted triazoles have been reported for antimicrobial^11-14 activity. The substituted oxadiazoles are heterocyclic compounds, which serve both as biomimetic and reactive pharmacophores and many are key elements with potential biological activities such as CNS stimulant, anti-inflammatory, hypotensive,^15a insecticidal,^15b bactericidal,^15c hypoglycemic,¹⁶ analgesic, anticonvulsive, antiemetic, diuretic,¹⁷ muscle relaxant¹⁸ and fungicidal¹⁹ activities.

Experimental

All the melting points were determined in open capillaries. Thin layer chromatography was performed on microscopic slides (2x7.5cm) coated with silica-Gel-G and spots were visualized under resembling iodine. IR spectra of all compounds were recorded in KBr on FT-IR spectrophotometer using KBr. The ¹H-NMR was recorded on Bruker advanced NMR 400MHz instruments using CDCl₃ as solvent. Mass spectra were obtained using mass spectrometer.

Preparation of 3-methyl-6-hydroxy-2-ethoxy carbonyl benzofuran

2, 4 dihydroxy actophenone (3ml) was taken in round bottom flask and 10 ml of acetone and K₂CO₃ (3gm) crystals were added to it, the reaction mixture was stirred for 5 minutes in ice-bath. To this reaction mixture ethyl bromo acetate (2.8ml) was added drop by drop from dropping funnel about 10 minutes. Further whole reaction mixture was allowed to stir for 20 minutes with catalytic amount of potassium iodide. The resultant solution was poured in crushed ice, the solid obtained was filtered and recrystalized from ethanol to produced 3-methyl-6-hydroxy-2-ethoxy carbonyl benzofuran .

Figure 1 Click here to view figure

Preparation of 3-methyl-6-hdroxy-2-benzofuran carbohydrazide :

3-methyl-6-hydroxy-2-ethoxy carbonyl benzofuran (3gm) was taken in a round bottom flask dissolved in 25ml ethanol and catalytic amount of acetic acid was added. To the above solution , hydrazine hydrate (1ml) was added drop wise. The mixture was stirred at room temperature for about 2 hours. After completion of the reaction as indicated by thin layer chromatography (1:1:: Benzene: Hexane solvent system), the 3-methyl-6-hdroxy-2-benzofurancarbohydrazide formed was recrystallized from ethanol.

Figure 2   Click here to view figure

Preparation of 3-methyl-6-hyroxy –N'- (phenylmethylene) benzofuran-2- carbohydrazide:

3-methyl-6-hydroxy-2-benzofurancarbohydrazide (1.9gm) was treated with  benzaldehyde (1ml) in ethanol, in presence of catalytic amount of acetic acid producing 3-methyl-6-hyroxy –N'- (phenylmethylene) benzofuran-2- carbohydrazide .

Figure 3   Click here to view figure

Synthesis of 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-thiol -1,3,4 oxadiazole:

3-methyl-6-hdroxy-2-benzofurancarbohydrazide (1.9gm) was refluxed with alcoholic potassium hydroxide (0.56 gm) and carbon disulphide (10 ml) for 10 hours. TLC was taken at regular intervals (1:1:: Benzene: Hexane solvent system). The reaction mixture then cooled and acidified with concentrated HCl. The compound 5-(3-methyl-6-hydroxybenzofuran-2-yl)-2-thiol-1,3,4 oxadiazole  separated as solid and was filtered, washed with cold water and recrystallised using dimethyl formamide - methanol mixture. Yield: - 2.5 gms, Melting Point: - 195⁰C was reported.

Figure 4 Click here to view figure

Photochemical reaction of 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-thiol -1,3,4 oxadiazole  with ultraviolet light:

A sample of 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-thiol -1,3,4 oxadiazole (0.5gms) was dissolved in a small volume of dry benzene and then made up to 500 ml of benzene. Benzophenone (0.01gm) was added to the solution as sensitizer. The solution was irradiated with a low pressure mercury lamp, which had been placed inside the immersion well. Progress of the reaction was followed by TLC analysis (1:1:: Benzene: Hexane solvent system). After 10 hrs no new product were observed and starting compound was recovered unchanged. Again taken sample of 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-thiol -1,3,4 oxadiazole (0.5gms) was dissolved in doubly distilled water(100ml) and distilled  benzene (100ml) in double walled beaker. Benzophenone (0.01gm) was added to the solution as sensitizer. The solution was irradiated with a low pressure mercury lamp, which had been placed inside the immersion well. Air was continuously bubbled through the solution with the help of an aerator. The temperature of the reaction mixture was kept constant by continuous water circulation. Progress of the reaction was followed by TLC analysis (1:1:: Benzene: Hexane solvent system). Starting material was almost completely consumed in 10 hrs. Solvent was removed under reduced pressure and residue was chromatographed over silica gel. Elution of the column with a mixture (8:2) of benzene and hexane gave a new product identified as the 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-sulphonate -1,3,4 oxadiazole .Yield: - 0.3 gms, Melting Point: - 235-237⁰C was reported.

Figure 5   Click here to view figure

The fragmentation pattern is as follows: - Molecular formula- C₁₁H₈N₂O₆S

Figure 6 Click here to view figure

The fragmentation pattern of 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-sulphonate -1,3,4 oxadiazole

Results and Discussion

Characterization Data of the Synthesized Compound: 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-thiol -1,3,4 oxadiazole

The IR spectra (KBr) of  the compound shows absorption at 1099.35 cm¹,2921.96 cm¹, 3068.53 cm¹, 2729.09 cm¹, 1643.24 cm¹ 2852.52 cm¹ due to CH, aromatic-OH, >C-SH, >C=N,  -CH₃ stretch respectively.

The ¹HNMR in CDCl₃ shows in the region 5.10 δ (1H) due to  aromatic-OH, 7.08-7.80 δ (3H) due to aromatic-H, 1.79 δ (3H) due to furan-CH₃ and 8.01 δ (1H) due to SH.

The Mass spectrum shows molecular ion peak at m/z 248 (Molecular formula- C₁₁H₈N₂O₃ S).

Characterization Data of the Synthesized Compound: 5-(3-methyl-6-hydrxy  benzofuran-2-yl)-2-sulphonate -1,3,4 oxadiazole 

The IR spectra (KBr) of the compound shows absorption at 1080.06 cm¹, 2894.95 cm¹,  3080.11 cm¹, 1338.51 cm¹, 1633.59 cm¹ and 2860.24 cm¹ due to CH, aromatic-OH, S=O, >C=N and -CH₃ stretch respectively.

The ¹HNMR in CDCl₃ shows in the region 5.07 δ (1H) due to aromatic-OH, 7.34 δ (3H) due to aromatic-H, 1.71 δ (3H) due to furan-CH₃, 2.50 δ (1H) due to SO₃H.

The Mass spectrum of 5-(3-methyl-6-hydrxy benzofuran-2-yl)-2-sulphonate -1,3,4 oxadiazole exhibits the molecular ion peak at m/z 296,which is the molecular weight of the compound and other fragments at m/z 215,187,147,132,115 .

Conclusion

The purpose of present work was to synthesized, characterised and study the Photochemical reaction  of  benzofuran derivative during this period I was able to successfully synthesized derivative and study its photochemical reaction. The compounds were characterized by melting point, TLC, FT-IR, ¹H NMR and Mass spectral analysis.

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