Synthesis and Antimicrobial Screening of Novel 4-Substituted Phenyl-5-[1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-yl]-2H-1,2,4-triazole-3-thiones

The paper describes a convenient method for the preparation of 4-substituted phenyl-5-[1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-yl]-2H-1,2,4-triazole-3-thiones. The structures of the synthesized compounds are established by the results of LCMS, 1H NMR, 13C NMR, and IR and elemental analyses. The mercaptotriazoles are indicated to be in thione form by 1H NMR spectra. All the synthesized compounds have been screened for antibacterial and antifungal activities. Compounds 12d and 12h exhibit encouraging results, while the remaining compounds show moderate activities. On the basis of spectral studies, formation of 2-amino-1,3,4-thiadiazoles from the isobenzofuran acyl thiosemicarbazides 11(a–h) is ruled out.

An attempt to prepare 2-amino-5-substituted-1,3,4-thiadiazoles from substituted acyl thiosemicarbazides 11 has failed to furnish the corresponding products. The decomposition of the starting material is indicated by 1 H NMR and 13 C NMR spectra. This reaction in acidic medium is dependent on both pH of the medium and substituents of acylthiosemicarbazides. The probable reason for the failure may be the steric hindrance posed by bulky isobenzofuran component for cyclization pathway.
This paper portrays the synthetic tactic for the preparation of 3-mercapto-1,2,4-triazole derivatives bearing isobenzofuran moiety and an assessment of their antimicrobial action. as an internal standard (Chemical shift in ppm). Mass spectral (MS) analysis is carried out on Agilent 6520 ESIQTOF MS with Ionization source ESIQTOF and acetonitrile as solvent (110 Volts). Elemental analyses are obtained on a Vario-EL instrument. Thin layer chromatography is conducted on 0.25 mm silica gel plates (60F 254 , Merck). Visualization is made with ultraviolet light. All extracted solvents are dried over Na 2 SO 4 and evaporated with a BUCHI rotary evaporator. Reagents are obtained commercially and used as received.

Procedure for the Synthesis of 5-Amino
To a cooled solution of SnCl 2 (1.0 g, 5.27 mmol) in Conc. HCl (20 mL) add 4-nitrophthalimide 1 (5.0 g, 0.026 mol) slowly under stirring. After complete addition, the reaction mass is stirred for 3 h at r.t. The reaction mass is cooled and the solid formed is filtered and washed with chilled water. The wet solid 2 (4 g, 0.024 mol) is dissolved in 30% aq. NaOH solution (20 mL). Zn dust (2 g, 0.030 mol) is added to the mixture and refluxed for 4 h. The residue is filtered and the filtrate is acidified with Conc. HCl (20 mL). This mass is refluxed again for 2 h. Thereafter, the reaction mass is cooled and the pH is adjusted to neutral using liq. NH 3 . The solid formed is filtered and recrystallised from boiling methanol to afford 5-amino-3H-isobenzofuran-1-one 3 as off white solid, yielding 3.0 g (83%), m.  (7). Under nitrogen atmosphere p-fluoro phenyl magnesium bromide (7.5 g, 37.73 mmol) is added dropwise to a stirred solution of 1-oxo-1, 3-dihydro-isobenzofuran-5-carbonitrile 4 (5 g, 31.44 mmol) in THF (50 mL) at 0-5 ∘ C. The mixture is stirred for 1 h at r.t. Add saturated NH 4 Cl to facilitate decomposition of magnesium complex. Put in some more THF and wash with water. Remove off THF and pour residue 5 into methanol (10 mL). NaBH 4 (0.58 g, 15.72 mmol) is added slowly for 30 min to the reaction mass and stirred for 2 h at r.t. Solvents are removed to get diol 6 which is refluxed with 2N HCl (50 mL) for 2 h. The reaction mass is cooled and diluted with water. The product is extracted into methylene chloride, washed with water, and dried (anhy. Na 2 SO 4 ). The solvent is removed and the residue is crystallized in IPA to provide 1-(4fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile 7 as pale yellow solid yielding 5.  (8). 1-(4-Fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile 7 (5 g, 20.92 mmol) is added to methanol (50 mL) and 1N NaOH (20 mL). The mixture is refluxed for 3 hr. Completion of reaction is marked by TLC with mobile phase toluene : ethyl acetate = 7.5 : 2.5. After completion of reaction, the mass is cooled and acidified with 1N HCl. The solid formed is filtered, washed with water, and recrystalized from ethanol to give 8 as crystalline white solid with a yield of 4. The mixture is refluxed for 5 h. The progress of the reaction is monitored by TLC (toluene : ethyl acetate = 7.5 : 2.5). To stop the back reaction, excess of alcohol is evaporated off by rotavap. Residue is extracted with diethyl ether. Ether layer is washed with 5% sodium bicarbonate solution and thereafter evaporated to yield corresponding aromatic ester 9. Compound 9 (4 g, 0.013 mol) is directly refluxed with 98% hydrazine hydrate (4 mL) in ethanol (4 mL) for 3 h. The progress of the reaction is monitored by TLC (toluene : ethyl acetate : diethylamine = 7.5 : 2.5 : 1). Reaction mixture is cooled and the solid formed is filtered and washed with ethanol to get 1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbohydrazide 10 as white solid with a yield of 3.
The same procedure is used in all the following cases.

Biology.
As per NCCLS document M62-A7 Protocols, the preliminary antimicrobial activity is performed using the disc diffusion method [22]. Four bacterial and four fungal strains are chosen for the study. The bacterial inoculum is prepared by suspending in 9 mL of sterile water for colonies from 24 h culture on MH agar medium. For the fungi, the inoculum is prepared with spores derived from 48 h-96 h culture on SD agar medium.
To standardize the inoculum density for a susceptibility test, a BaSO 4 turbidity standard, equivalent to a 0.5 McFarland is used. The turbidity of the actively growing microbial culture is adjusted with sterile saline to attain turbidity optically comparable to that of the 0.5 McFarland standards. Within 15 minutes after adjusting the turbidity of the inoculum suspension, the petri plates are inoculated with a standardized suspension of the tested microorganisms. The predetermined series of antimicrobial discs is allotted onto the surface of the inoculated agar plate. Whatman filter paper number 1 is used to prepare discs approximately 6 mm in diameter, which are sterilized in a hot air oven before placing in petri plates. These paper discs are placed in a circular pattern in each inoculated plate and then injected with the specific amount of tested compound using micropipettes.
All the newly synthesized mercaptotriazoles are dissolved in dimethyl sulfoxide (DMSO) to prepare chemicals of stock solution of 10 mg/1 mL for antibacterial study and 25 mg/1 mL for antifungal study. Gentamicin and fluconazole are used as standard antibacterial and antifungal drugs, respectively. Dimethyl sulfoxide is used as solvent control. The plates are incubated and the inhibitory activity is measured (in mm) as the diameter of the observed inhibition zones. The tests are repeated to confirm the findings and the average of the readings is taken into account. The results of study are tabulated in Tables 1 and 2. Following M62-A7 Protocols, broth dilution test by doubling dilution of the antibiotics is carried out to examine the minimum inhibitory concentration of the most promising compound 12d against the tested microbes. The antibacterial and antifungal assays are performed in Mueller-Hinton broth and Sabouraud Dextrose broth, respectively. MIC for microbes is to be observed between 250 g/1 mL and 7.81 g/1 mL. Gentamicin (1 mg/2 mL) and fluconazole (1 mg/2 mL) are used as standard antibacterial and antifungal drugs, respectively. DMSO is the solvent control.
Compound 7 is hydrolysed in aq. methanolic NaOH to provide 1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carboxylic acid 8 in good yield. This acid is esterified and the ester 9 is converted to 1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbohydrazide 10. The synthesized acid hydrazide is treated with different substituted phenyl isothiocyanates (a-h) to yield 1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-acyl thiosemicarbazides 11(a-h). Cyclodehydration of the thiosemicarbazides 11(a-h) in basic medium (5% NaOH) leads to the formation of 4-substituted phenyl-5-[1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-yl]-2H-1,2,4triazole-3-thiones 12(a-h) (Scheme 1). Purity of the compounds is analyzed by TLC, LCMS, and elemental analyses. 1 H NMR, 13 C NMR, and IR spectra have backed up the characterization. The IR spectra of the mercaptotriazoles 12(a-h) mark the absence of carbonyl absorption around 1750 cm −1 . The existence of triazoles in thione form is indicated by the absence of SH stretching in the characteristic region of 2540 cm −1 . The 1 H NMR spectrum of every compound unveils the deshielding of NH of the triazole ring as anticipated in the range 14.00-14.50. The signal for NH appearing downfield than 12.0 authenticates the existence of the ring in thione form. The characteristic fluorocoupling is observed for the aromatic protons of the phenyl ring attached to the 1st position of isobenzofuran moiety. In the 13 C NMR spectra of triazol-3-thiones, values close to 168 and 150 are the characteristic signals for C-3 and C-5 carbons, respectively. Disappearance of signal around 161-163 for carbonyl carbons indicates the formation of target compounds 12(a-h). The molecular ion peak in the mass spectrum of C 22 H 16 FN 3 OS (12a) is found to be at m/z 390.09. This is in agreement with its calculated formula weight 389.44. This confirms the formation of the product. The base peak is observed at m/z 267.07 for C 15 H 11 N 3 S fragment.
Many research articles on heterocycles have explicated the synthesis of 2-amino-5-substituted-1,3,4-thiadiazole derivatives from substituted acyl thiosemicarbazides. This reaction runs parallel to the preparation of 3-mercapto-1,2,4triazoles from the same starting material. Here, we have followed the classical methods of cyclization with cold concentrated H 2 SO 4 . Moreover, the use of novel reagents like saccharin and p-toluene sulfonic acid has also failed to afford the required products. The spectra have disclosed the decomposition of substituted thiosemicarbazides. Therefore, this plan of synthesis is discontinued. Similarly, spectral characterization reveals that 2-substituted acyl thiosemicarbazides have failed to form the required mercaptotriazoles. Greater steric hindrance of the bulky isobenzofuran substituents may be the rationalization for both failures.
Disc diffusion method is followed to observe the antimicrobial action. The results of the preliminary testing of the antimicrobial activity of the novel compounds are tabulated in Tables 1 and 2. Varying grades of inhibition against the growth of the tested microbes by the synthesized compounds can be noticed. Broadly, the inhibitory activity against the Gram-positive bacteria is higher than against the Gramnegative bacteria. Compound 12d shows significant inhibition against all the tested microorganisms owing to the presence of chloro group on the benzene ring attached to the 4th position of triazole ring. The better activity of 12e, 12g, and 12h can be attributed to the presence of nitro, methoxy, and bromo groups, respectively, at para positions of benzene ring. Compound 12a, which has unsubstituted benzene at 4th position of triazole ring, also shows better activity than the triazoles with benzyl and phenylethyl groups. The fluoro substituent attached to the para position of benzene ring, which is connected to the 1st position of the isobenzofuran core, appears to be the reason behind the antimicrobial activities displayed by the target compounds. Minimum inhibitory concentration (MIC) is inspected for 12d by broth dilution approach. For Gram-positive bacteria, the values are less than 62.5, but for Gram-negative bacteria the results are 125 ≤ MIC > 62. 5. For all the fungal strains, the values are less than 62.5.