Microwave Synthesis , Characterization , and Antimicrobial Activity of Some Novel Isatin Derivatives

1Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia 2Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia, Alexandria 21321, Egypt 3Bioproducts Research Chair, Zoology Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia 4Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Saudi Arabia 5X-Ray Crystallography Unit, School of Physics, Universiti Sains Malaysia (USM), 11800 Penang, Malaysia


Introduction
The development of new therapeutic agents is one of the essential goals in medicinal chemistry.In the recent years there has been a spectacular increase in the use of microwave heating within the pharmaceutical industry, because it facilitates the synthesis of new chemical entities with reduced reaction time [1].Microwave irradiation is considered as an alternate source of heating and it has been proven recently that microwave heating improves the efficiency of the reaction and reduces the reaction time [2][3][4][5][6][7].
We report here the synthesis of three different series of isatin derivatives (hydrazone, thiosemicarbazone, and imino carboxylic acid derivatives) employing microwave irradiation.The prepared compounds were subjected to in vitro screening for their antimicrobial activities against representatives of pathogenic Gram-positive and Gram-negative bacteria and fungi.

Experimental Section
2.1.Chemistry 2.1.1.Materials.The solvents used were of HPLC reagent grade.Melting points were determined with a Mel-Temp apparatus and are uncorrected.Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet 560 spectrometer.Nuclear magnetic resonance spectra ( 1 H NMR and 13 C NMR) were recorded on a JEOL 400 MHz spectrometer with chemical shift values reported in ppm ( units) relative to an internal standard.The microwave irradiation was conducted using a multimode reactor (Synthos 3000, Anton Paar GmbH, and 1400 W maximum magnetron).Reactions were performed in teflon vessels (capacity 10 mL).The target temperature was set and fixed during the irradiation.Settings and readings for power (W) and pressure were taken from the instrument.Elemental analyses were performed on a Perkin-Elmer 2400 elemental analyzers, and the values found were within ±0.3% of the theoretical values.Follow-up of the reactions and checks of the purity of the compounds were done by thin layer chromatography (TLC) on silica gelprotected aluminum sheets (GF254, Merck), and the spots were detected by exposure to UV-lamp at  254 nm for a few seconds.The compounds were named using Chem.Draw Ultra version 11, Cambridge Soft Corporation.The X-ray diffraction measurements of compound 5b were performed using Bruker APEX-II D8 Venture diffractometer.

Synthesis of Isatin Derivatives (1b-1f) .
A mixture of isatin (5 mmol) and potassium carbonate (8 mmol) in DMF (10 mL) was stirred for 10 minutes at room temperature.Alkyl halides (6 mmol; benzyl bromide for preparation of 1c, 1d, and 1e; CH 3 I for preparation of 1b; and 1,3-dibromoethane for preparation of 1f) were added dropwise to the reaction mixture and then the reaction was microwave irradiated using a multimode reactor (Synthos 3000, Anton Paar GmbH, Graz, Austria) (1,400 W maximum magnetron).The vessels were heated for 5 minutes at 80 ∘ C and held at the same temperature for a further 5 minutes (400 W).Cooling was accomplished by a fan (5 minutes).The final product was dried and recrystallized from ethanol.All the spectral data for the products obtained were in good agreement with the reported data.

Microwave Method for the Synthesis of (3a-3n).
A multimode reactor (Synthos 3000 Anton Paar, GmbH, 1400 W maximum magnetron) was used.The initial step was conducted with 4-Teflon vessels rotor (MF 100) that allows the reactions to process under the same conditions.Isatin derivatives 1a-1f and hydrazine hydrate 2a, thiosemicarbazide 2b, or 4-aminobenzamide 2c were mixed together in a small portion of ethanol and then subjected to microwave irradiation (400 watt).The vessels were heated for 5 min at 100 ∘ C and held at the same temperature for another 5 min.cooling was accomplished by a fan (5 min).The final product was washed with cold ethanol and then dried under vacuum to afford the products 3a-3n in high yield and purity.

Microwave Method for the Synthesis of (5a-5d
). Isatin derivatives 1a-1d and 4-aminophenyl acetic acid 4 were mixed in small amount of ethanol as a solvent in the presence of glacial acetic acid (2-3 drops).The individual vessels were heated for 5 min at 100 ∘ C and held at the same temperature for another 5 min; cooling was accomplished by a fan (5 min).The final product was washed with cold methanol and then dried under vacuum to afford products 5a-5d in a pure state, as observed from their spectral data.
(1) 2-     Pages 33 and 34).The product was obtained as reddish brown crystals from ethyl acetate in 86% yield; mp 180-182 ∘ C. 1 H NMR (d 6 -DMSO):  = 3.64 (s, 2H, CH 2 ), 4.99 (s, 2H, CH 2 ), 6.50 (d, 1H, J = 7.3 Hz, Ar), 6.78 (t, 1H, J = 8.0 Hz, Ar), 6.96-7.07(m, 3H, Ar), 7.29-7.46(m, 8H, Ar) ppm.13 C NMR (d 6 -DMSO):  = 40.3,43.4, 11.2, 114.3, 115.9, 117.9, 122.9, 125.7, 127.9, 128.The antimicrobial activities of the prepared compounds were tested against representative pathogenic Gram-positive, Gram-negative, and filamentous fungus test strains by using the paper disc diffusion method [31].In this method, three agar media, nutrient agar [32] for bacteria, Sabouraud agar [33] for the yeast strain, and Czapek Dox agar for the fungus strain [34], were prepared and sterilized by autoclaving at 120 ∘ C and 1.5 atm for 20 min.The agar plates were poured, left to cool down and after solidification they were inoculated with the bacterial and fungal strains by streaking.The synthesized compounds were dissolved in dimethylsulfoxide (DMSO) at a final concentration of 10 mg/mL and 5 L of each compound was loaded on a sterile filter paper disc (5 mm diameter; 50 g per disc).The filter paper disc was then transferred aseptically into the inoculated agar plates along with commercially available tetracycline and erythromycin discs as positive controls and DMSO as the negative control for comparison.The plates were then incubated at 37 ∘ C for 24 h for bacteria and at 30 ∘ C for 48-72 h for fungi.After incubation, diameters of the inhibition zones around the paper discs were measured in mm as an indication of the antimicrobial activities of the compounds.
The IR spectra of 3a-3e showed absorption bands in the regions 3398 to 3217 cm −1 corresponding to (NH), a band at 1688 cm −1 corresponding to the C=O group, and a peak around 1584 cm −1 that reveals the formation of the C=N bond.The NMR spectra of all the products 3a-3e are in a good agreement with their structures [28][29][30].
The IR spectra of the series 3f-3j showed absorption bands in the region 3453 cm −1 related to the asymmetric (N-H) vibration of the terminal NH 2 group.The other bands at 3336 and 3256 cm −1 may be due to the symmetric (N-H) vibrations of the amide and amino groups.The peaks at 1680 and 1588 cm −1 correspond to C=O and C=N group, its structure, where two signal were observed at  = 26.1 and 40.0 ppm assigned to the methyl and benzylic protons, respectively.The observed peak at  = 154.1,162.2, and 172.8 ppm is related to the C=N, C=O, and the carbonyl of the carboxylic groups, respectively.Single crystal X-ray crystallography is without doubt a decisive analytical tool which confirms the configuration of the imine double bond in the target compounds 5a-5d.Fortunately, we have succeeded in getting single crystals of compound 5b that was suitable for X-ray crystallography.The solved structure is a representative example for the synthesized target compounds 5a-5d.The assigned (E)configuration of compound 5b was established via its single crystal X-ray structure (Figure 1).
Molecular formula of 5b is as follows: Crystallographic data have been deposited with the Cambridge Crystallographic Data Center (supplementary publication number CCDC-1004251).Copies of the data may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, UK.

Biology.
The antimicrobial activities of the prepared compounds against a panel of pathogenic tested organisms are collected in Table 1.The results revealed that all the Nalkyl isatin derivatives 1b-1f were biologically active with different spectrum activity.The isatin itself 1a exhibited average antibacterial activities against the Gram-negative test organisms only, but the activity decreased with the presence of the methyl group in 1b.On the other hand, the introduction of a benzyl group in 1c decreased the activity against Gram-negative bacteria and confers activity against Gram-positive bacteria, namely, Bacillus subtilis.However, the presence of ethyl bromide group in 1f expands the spectrum activity to be active against Bacillus subtilis and two Gram-negative tested bacteria and against the fungal pathogens in study.
Differently, most of the 3-hydrazino and 3-thiosemicarbazino isatin derivatives 3a-3n were biologically inactive and generally the active derivatives showed weak to moderate activity against at least two of the tested Gram-positive test bacteria.Interestingly, the activity was associated with the presence of the bromine or chlorine in the isatin moieties 3d, 3e, and 3n; however, their effect cannot be confirmed in the activity of 3h and 3i because 3f was originally active.
Among the imino isatin carboxylic acid derivatives, 5d which has bromine at position 5 showed good to high antimicrobial activity compared to the positive control (erythromycin and tetracycline) against the Gram-positive and fungal test organisms.The presence of the methyl group in 5b was associated with the activity against the Gram-negative bacteria and, similarly as in the N-alkyl isatin derivatives, compound 5c, having a benzyl group, showed antimicrobial activity against Bacillus subtilis and Candida albicans.These results obtained are similar to the reported results by Singh et al. and Patel et al. [28,39].

Conclusions
Three series of isatin derivatives (3-hydrazino, 3-thiosemicarbazino, and 3-imino carboxylic acid derivatives) were synthesized by employing microwave irradiation in good yields and purities.The prepared compounds were evaluated for their antimicrobial activity against Gram-positive and Gram-negative bacteria and fungal pathogenic organisms.In general, the N-alkyl isatin derivatives were biologically active, while some of the 3-hydrazino and 3-thiosemicarbazino isatin derivatives were active against at least two of the three used Gram-positive test organisms.It was found that the activity was associated with the presence of the bromine or chlorine in the isatin moiety of the prepared compounds.The highest activity was achieved with one of the imino isatin carboxylic acid derivatives that showed good to high activity towards the three Gram-positive bacteria and the two fungal tested microorganisms.