Synthesis and characterization of new diiodocoumarin derivatives with promising antimicrobial activities

A series of 6,8-diiodocoumarin-3-N-carboxamides (4–11) were prepared. Treatment of ethyl 6,8-diiodocoumarin-3-carboxylate (1) with ethyl cyanoacetate/NH4OAc gave ethyl 2-(3-carbamoyl-6,8-diiodocoumarin-4-yl)-2-cyanoacetate (12) and 2-amino-4-hydroxy-7,9-diiodocoumarino[3,4-c]pyridine-1-carbonitrile (13), and treatment with acetone in the presence of NH4OAc or methylamine gave the ethyl 4-oxo-2,6-methano-2-methyl-3,4,5,6-tetrahydro-8,10-diiodobenzo[2,1-g]-2H-1,3-oxazocine-5-carboxylate derivatives 14a,b. All compounds were evaluated for their antimicrobial activity and the compounds 12–14a,b exhibited a pronounced effect on all tested microorganisms.


Introduction
Coumarins and their derivatives are biologically and pharmaceutically interesting compounds known for their use as additives in food, perfumes, cosmetics, pharmaceuticals, platelet aggregation and agrochemicals [1,2]. Coumarins have also been reported to exhibit several biological activities, such as antimicrobial, anticancer, antifungal, anti-HIV and antioxidant properties [3][4][5][6], and they also served as versatile precursors for many organic transformations in the synthesis of a number of drug-like molecules [7,8]. Moreover, coumarin-based dyes and pigments are organic fluorescent materials exhibiting unique photochemical and photophysical properties, which render them useful in a variety of applications such as dye lasers, anion sensors, organic light-emitting diodes and solar cells [9,10].
In view of the important biological properties of the diiodocoumarin derivatives and iodo-organic compounds as medical agents, we planned to synthesize some new diiodocoumarin derivatives bearing side chains with different structures, as such derivatives could possess interesting and useful biological properties.
Reaction of 1 with acetone in the presence of NH 4 OAc or methylamine at room temperature for 7 days gave 1,3-oxazocine-5-carboxylate derivatives (14a,b) [16-18] (Scheme 5). The formation of 14 indicates that the activated methylene compounds attack at the C3-C4 olefinic bond in 1 under Michael reaction conditions to yield a cyclic Michael adduct, which underwent hydrolysis by NH 3 or MeNH 2 and cyclization through the elimination of H 2 O (Scheme 5).
The structure of compound 14a was established by 13 C NMR, which showed δ at 42.5 (CH 2 (c)), 168.4 cm -1 (CONH), and 170 cm -1 (CO). The structures of all newly synthesized compounds were confirmed by IR, 1 H NMR, 13 C NMR and mass spectrometry.
The inhibitory effects of the synthetic compounds against these organisms are given in Table 1, Figure 1 and Figure 2. Among the series tested, compounds 12-14a,b exhibited excellent antibacterial activity, better than the standard ampicillin, against two species of Gram-positive bacteria, Staphylococcus aureus  (NCTC-7447), Bacillus cereus (ATCC-14579) and two Gramnegative bacteria, Escherichia coli (NCTC-10410) and Serratia marcescens (IMRU-70), while the same compounds showed moderate antifungal activity against the tested organisms. Compounds 9-11 exhibited comparable activity to ampicillin against the tested bacteria and moderate to weak antifungal activity against the tested organisms. Furthermore, compounds 1-8 showed moderate to weak activities against all the tested bacteria and fungi, compared with the standards ampicillin and calforan. In addition, compounds 2 and 5 in the series were found to be inactive against Escherichia coli (NCTC-10410), while compound 11 was inactive against Serratia marcescens

Experimental General methods
Melting points were determined on a Stuart melting point apparatus and are uncorrected; IR spectra were recorded in KBr on a FT-IR 5300 spectrometer and Perkin Elmer spectrum RXIFT-IR system (ν, cm −1

8-11
To a well-stirred solution of 3 (0.46 g, 1 mmol) in dry dichloromethane (DCM) containing a few drops of triethylamine (TEA) an equivalent amount of an ambient nucleophile [4-aminophenylethanol, p-aminophenol, anthranilic acid and p-aminophenylacetic acid (1.2 mmol)] was added. The reaction mixture was stirred at room temperature under dry conditions for 3 h. DCM was removed under reduced pressure until dryness, the obtained solid was then washed with 10% HCl and the remaining solid recrystallized from dioxane.     General procedure for the synthesis of ethyl cyanoacetate and pyridine derivatives 12 and 13 General procedure for the synthesis of 1,3oxazocine-5-carboxylate derivatives 14a,b
tested compounds were dissolved in N,N-dimethylformamide (DMF) to give a solution of 1 mg mL −1 . The inhibition zones (diameter of the hole) were measured in millimeters (6 mm) at the end of an incubation period of 48 h at 28 °C; N,N-dimethylformamide showed no inhibition zone.

Conclusion
It was interesting to note that four of the new compounds (12, 13 and 14a,b) were found to have an antimicrobial activity greater than that of the standard antibiotic ampicillin or the standard antifungal claforan, while compounds 1-11 were either inactive or only weakly active against the tested microorganisms. The presence of fused diiodocoumarino [3,4-c]pyridine and diiodobenzo[2,1-g]-2H-1,3-oxazocine nucleus increased the antimicrobial activity, whereas the presence of diiodocoumarin-3-carboxamides decreased the antimicrobial activity.