Synthesis and Antimicrobial Evaluation of Novel 4-phenylPyrrole-2-Carboxamides

Ten novel 4-phenylpyrrole-2-carboxamide derivatives (5a-j) were synthesized and evaluated for their in vitro antibacterial activity. Among the tested compounds, the most effective were 5c and 5e with MIC value in the range of 6.05-6.25 μg/mL against Gram-negative bacterial strains. Further, the synthesized compounds have been screened for their in vitro antifungal activity. In the present study, novel 1-(4-chlorobenzyl)-4-phenyl-1H-pyrrole-2-carboxamides as antibacterial agents have been disclosed.


Introduction
Drug resistance infection always creates a threat to healthcare.
It becomes more acute due to the rapid development of resistance against conventional chemotherapy [1]. The necessity for more potent antimicrobial agents has become vital because of emerged resistance to the currently used antibiotics. In the last few decades, rapid scientific progress has been made in the treatment of infectious diseases. However, they still remain a serious and challenging health problem due to several factors which have led to the re-emergence of these diseases. Antibiotic resistance, population increase, international travel, migration, increase in the number of immune-suppressed patients, and climate change are some of the factors that play a significant role in the battle against infectious diseases [2][3][4][5][6]. In order to keep microorganisms resistance under control, careful use of existing antimicrobial drugs and the design of novel drugs with different modes of action e.g. linezolid [7][8][9] are required [10][11][12][13]. Pyrrole and its derivatives are ever present in nature. Pyrrole subunit has diverse applications in therapeutically active compounds including fungicides, antibiotics, anti-inflammatory drugs [14], antitumor agents [15], cholesterol reducing drugs [16] and many more. They are known to inhibit reverse transcriptase [Human immunodeficiency virus type 1 (HIV-1)] and cellular DNA polymerases protein kinases. Moreover, they are also a component of polymers [17], indigoid dyes and of larger aromatic rings [18]. In catalytic reactions, pyrroles are well utilized as catalyst for polymerization process [19], preservative [20], solvent for resin [21], corrosion inhibitor [22], terpenes and in metallurgical process [23].
One approach is to improve the activity of natural anti-microbial substances by synthesis of analog compounds of naturally produced organohalogens [24]. Our curiosity in halogenated pyrrole derivatives led to synthesis and antimicrobial evaluation of some analogues of pyoluteorin [25]. In continuation to this we found that bromopyrrole alkaloids; a family of marine alkaloids represents a fascinating example of the large variety of secondary metabolites formed by marine sponges. These compounds are involved in the sponge's defense mechanism against fishes. Also, several pharmacologically important bromopyrrole congeners have been previously described as having antihistaminic, antiserotonergic and antineoplastic activity. Furthermore, these natural products also possess antibacterial, antifungal and anitibiofilm activity [26][27][28]. Most of these compounds are defined by the signature of bromopyrrole carboxamide with oroidin as their prototype alkaloid reported for its antibiofilm activity [26]. Structure-activity relationship performed on synthetic library of oroidin derivatives indicated that N-methylation of the pyrrole ring led to increased antibiofilm activity against medically relevant Gram-negative c-proteobacterium Pseudomonas aeruginosa, as indicated by the most active member of the library, Dihydrosventrin (DHS) [29].
The formation of compound 5a was confirmed by 1H NMR, 13C NMR and mass spectral analysis. In the 1H NMR spectrum of compound 5a, N-H proton of amide group observed at δ 9.3 ppm.
The methylene groups attached to nitrogen of pyrrole ring and amide amide functionality showed singlet at δ 4.2 and 4.8 ppm respectively. In addition to this, the signal observed at δ 3.7 ppm indicates the presence of the -OMe group on the benzene ring. In the 13C NMR spectrum of compound 5a, the signal at δ 162.4 ppm is due to amide carbonyl carbon. The signals at δ 42. 8  The results of antibacterial activity of the tested compounds (5a-j) are shown in Table 1. The antibacterial activities were tested using

Experimental Material and Methods
Reagents and solvents were purchased from commercial in DMF (35 ml). The reaction mixture was stirred at 60oC for 6 h (monitored by TLC). The reaction mixture was poured into water (25 mL) and extracted with EtOAc (3x20 mL). The organic layer was washed with saturated aq. NaHCO3 (1x20 mL), water (3x10 mL), brine (1x10 mL), dried over anhydrous Na2SO4 and concentrated in vacuum. The product was isolated by silica gel chromatography using Hexane: EtOAc (90:10) to afford 3.78 g (85%) of compound (2) as white solid. The product was confirmed by spectral analysis.

Conclusion
Ten novel 4-phenylpyrrole-2-carboxamide derivatives (5a-j) were prepared from corresponding carboxylic acid and different aromatic or aliphatic or cycloaliphatic amines by employing an efficient amide coupling protocol. The antibacterial and antifungal activity of these compounds (5a-j) was studied against pathogenic Gram-negative bacteria and fungi. The results exhibit that compounds 5c and 5e moderate antibacterial activity relative to standard drugs. The compound 5c displayed moderate antibacterial activity relative to standard against Escherichia coli and Pseudomonas aeruginosa strains with MIC value of 6.05 µg/ mL. None of these compounds show significant antifungal activity.
These results note that presence of lipophilic phenyl group on pyrrole decreases antibacterial activity.