THE SYNTHESIS, CHARACTERIZATION AND BIOLOGICAL EVALUATION OF A STABLE PHOSPHORUS YLIDE AND AN IMIDAZOLE AS NOVEL COMPOUNDS

A convenient one pot synthesis of two novel compounds including a stable phosphorus ylide and an imidazole from electron-poor acetylenes in fairly good yields by the condensation of triphenylphosphine and acetylene derivatives, in the presence of dimethyl thiourea from the 1:1:1 addition reactions is described. The structures of the synthesized compounds were characterized by IR, 1H-NMR, 13C-NMR and elemental analysis. The newly synthesized compounds were screened for antimicrobial on Staphylococcus aureus, Bacillus sublitis, Escherichia coli and Pseudomonas aeruginosa and antifungal activities on Candida albicans, patient isolate Candida glabrata and Candida krusei. The results showed that these compounds have activity against all the tested bacteria and fungi with a sufficient minimum inhibitory concentration.


INTRODUCTION
The chemistry of phosphorus ylides is very significant in the twentieth century which takes part in many valuable reactions in organic synthesis because of its applications in the synthesis of organic products [1][2][3][4][5]. Phosphorus ylides are interesting synthetic targets because of their importance in a variety of industrial, biological and chemical synthetic usages. These compounds have pharmaceuticals properties, are widely used as anti-inflammatory, anticancer, analgesic, and antimicrobial activity [6][7][8][9][10]. Imidazole nuclei are also an important heterocyclic ring and a wide variety of imidazole derivatives are known for their chemotherapeutic importance especially antifungal activity [11][12][13]. Phosphorus ylides are reactive systems and several methods have been developed for the preparation of phosphorus ylides [4,14,15]. In general, phosphorus ylides are usually prepared by deprotonation of phosphonium salts which can be prepared most often by the reaction of triphenylphosphine and an alkyl halide [16][17][18] that often obtained in excellent yields from the 1:1:1 addition reaction between triphenylphosphine, dialkyl acetylendicarboxylates, in the presence of CH, SH, NH or OH-acid. Phosphonium salts can also prepared by Michael addition of phosphine to activated olefins. Michael addition of phosphorus(III) compounds such as triphenylphosphine to acetylenic esters leads to reactive 1,3-dipolar intermediate betaines which are not detected even at low temperature [4,19,20].
Here we describe a rapid and efficient one-pot synthesis method for the preparation of a stable phosphorus ylide (I) and an imidazole (II) using triphenylphosphine, acetylene derivatives (methyl acetylene carboxylate and dibenzoyl acetylene, respectively) and dimethyl thiourea (Scheme 1) and then synthesized compounds are tested for evaluation of their antibacterial and antifungal activities, respectively.   Screening for antibacterial activity. The antimicrobial activities were determined using disc diffusion method [21] by measuring the zone of inhibition in mm. Newly synthesized compound was screened in vitro for its antibacterial activity against two Gram-positive strains (Staphylococcus aureus and Bacillus sublitis) and two Gram-negative strains (Escherichia coli and Pseudomonas aeruginosa) at concentration of 500 µg/mL. Ciprofloxin (10 µg/disc) was used as a standard drug for antibacterial screening. New synthesized compound exhibited sufficient antibacterial activities. Each experiment was done in triplicate and the average reading was taken. The results are tabulated in Table 1. Table 1. Results of antimicrobial activity of the tested compound.

Key to symbols: Highly active = +++ (inhibition zone > 12 mm). Moderately active = ++ (inhibition zone 9-12 mm). Slightly active = + (inhibition zone 6-9 mm). Inactive = -(inhibition zone < 6 mm).
Screening for antifungal activity. The yeasts Candida albicans, patient isolate Candida glabrata and Candida krusei were grown on Sabouraud Dextrose Broth (Difco); the yeasts were incubated for 48 h at 25.91 o C. The antifungal activity test was carried out at pH 7.4 in Sabouraud Dextrose Broth and the 2-fold dilution was applied. A set of tubes containing only inoculated broth was kept as controls. After incubation for 48 h at 25.91 o C, the last tube with no yeast growth was recorded to represent minimum inhibitory concentration (MIC), expressed in µg/mL. The results are given in Table 2. Antifungal activities of imidazole compound were carried out by Disc Diffusion Technique [22].

RESULTS AND DISCUSSION
Chemistry. On the basis of the well established chemistry of trivalent phosphorus nucleophiles it is reasonable to assume that phosphorus ylides result from the initial addition of triphenylphosphine to the electron-poor acetylenes and a concomitant protonation of the 1:1 adduct by dimethyl thiourea. The reaction between methyl acetylene carboxylate and 1,3dimethyl-thiourea in the presence of triphenylphosphine proceeded in dichloromethane and dioxane and was complete after 2 hours. The yellow powders separated from the reaction mixture were identified as phosphorus ylide (I) and imidazole (II). The 1 H and 13 C NMR spectra of the crude product clearly indicated the formation of stable phosphorus ylide and imidazole. No product other than I and II could be detected by NMR spectroscopy. The structures of compounds I and II were deduced from their elemental analyses, IR, 1 H and 13 C NMR spectra. The ylide moiety of these compounds is strongly conjugated with the adjacent carbonyl group.

Compound
Antibacterial activity S.aureus B.subtilis E.Coli P.aeruginosa C25H27N2O2PS ++ ++ --Ciprofloxacin +++ +++ +++ +++ exhibited singlet (for CH 3 -N, imide group, δ = 2.16 ppm). The phenyl residues gave rise to characteristic signals in the aromatic region (δ = 7.47-7.68 ppm) of the spectrum. The 13 C NMR spectrum for I showed eight distinct resonances and for II showed five distinct resonances, as expected for the phosphorus ylide and imidazole structures. Partial assignment of these resonances is given in the Experimental section. The structural assignment of compound I and II made on the basis of the 1 H NMR and 13 C NMR spectra is supported by its IR spectrum, which in the carbonyl region displayed distinct absorption bands for the stretchings of the C=O groups.
Pharmacology. To check the biological activity of the compounds, the two compounds I and II were screened for in vitro antimicrobial activity and antifungal against a variety of bacteria and fungi. The antimicrobial activity was determined using disc diffusion method [21] by measuring the zone of inhibition in mm. The compound I was assayed for antibacterial activity against two Gram-positive strains (Staphylococcus aureus and Bacillus sublitis) and two Gram-negative strains (Escherichia coli and Pseudomonas aeruginosa). From the data presented in Table 1, the preliminary screening results for the compound I established that compound I was showing a sufficient activity against all the tested bacteria.
The in vitro antifungal activity of the compound II was tested by the tube dilution technique [22]. The test compound and standards Miconazole, Fluconazole and Cotrimoxazole were dissolved in 10% DMSO, at concentrations of 100 µg/mL. Further dilutions of the compound and standards in the test medium were prepared at the required quantities of 50, 25, 12.5, 6.25, 3.125, 1.5 and 0.78 µg/mL concentrations. The final inoculums size was 10 5 CFU/mL. The MICs were defined as the lowest concentrations of the compounds that prevented visible growth. It was determined that the solvent had no antifungal activity against any of the test microorganisms. All the compounds were tested for their in vitro growth inhibitory activity against C. albicans, patient isolate C. glabrata and C. krusei (Table 2). Compounds II possessed comparable activity to Fluconazole and Cotrimoxazole against C. albicans with a MIC of 12.5 µg/mL. However the compound was not superior to the standards used against any fungi.

CONCLUSION
The present method may be considered as a practical route for the synthesis of the stable phosphorus ylides and imidazole using a one-pot reaction between triphenylphosphine and acetylenes in the presence of NH compounds such dimethyl thiourea. This procedure has advantages of high yield, mild reaction conditions, and simple experimental and work-up conditions and may be an acceptable method for the preparation of phosphorus ylide and imidazole. Our studies clearly demonstrate that novel synthesized compounds had significant pharmaceutical properties. As a consequence, we can conclude that newly synthesized can be used for the development of new drugs in future.