Synthesis and antimicrobial activity of bischalcone derivatives

Several bischalcones (2a-h and 5a-e) and flavones (3a-f) were synthesized and evaluated for their antimicrobial actions. Bischalcones were prepared by condensing 1,1'-(4,6-dimethyl-1,3-phenylene)diethanone (1) or 1-(5-acetyl-2,4-dimethoxyphenyl)-1-ethanone (4) with arylaldehydes. Bischalcones were cyclized in presence of iodine to give corresponding flavones (3a-f). An alternative route to synthesize the flavones consisted in preparing the diester derivatives (6a-f) of (1) with different aromatic acids, which could be converted to beta-diketones followed by cyclization to give the corresponding flavones. However, all the attempts in this direction were unsuccessful and it could not be possible to proceed beyond diester stage; six diester derivatives (6a-f) were synthesized. The structures of the synthesized compounds were assigned on the basis of H-1 NMR, mass spectral data and microanalyses results. The antimicrobial screening was performed at a concentration of 100 mu g/mL by cup plate method; the compounds inhibiting growth of one or more of the microorganisms were further tested for their minimum inhibitory concentration (MIC) by turbidity method. Preliminary antimicrobial results revealed that the compounds 2a-h and 3a-f were significant in their antibacterial and antifungal activities. MICs results showed that the compound 2f exhibited very good activity against E. coli, P. aeruginosa, and C. albicans with MIC-12.5 mu g/mL. Similar type of activity was shown the compound 3a against S. aureus and C. albicans with MIC-12.5 mu g/mL. Another compound, 3f, was active against P. aeruginosa and C. albicans with MIC-12.5 mu g/mL. Methylation of the two chelated hydroxyls (5a-e) significantly reduced the activity. However, oxidative cyclization of bischalcones resulted in compounds (3a-f) which were found to be considerably active. Diesters (6a-f) were insignificant in their antimicrobial activities.