Benzimidazoles. In: Preston PN, editor. Chemistry of Heterocyclic Compounds. Hoboken: John Wiley & Sons, Inc.; 1981. pp. 1-285. [DOI: 10.1002/9780470187159.ch1]

Synthesis, Spectroscopic Characterization and DFT/TD-DFT Calculations of new Fluorescent Derivatives of Imidazo[4',5':3,4]Benzo[c]Isoxazole

An increasingly wide variety of fluorescent compounds is used in biotechnology, genomics, immunoassays, array technologies, imaging, and drug discovery. Therefore, synthesis of fluorophores with novel structural features can be interesting and useful in various fields. In this paper, four new fluorescent heterocyclic compounds with high quantum yields are introduced. These new fluorophores are synthesized in moderate to high yields via regioselective nitration of 3-alkyl-8-(4-chlorophenyl)-3 H-imidazo[4',5':3,4]benzo[c]isoxazoles. The latter compounds are obtained from the reaction of 1-alkyl-5-nitro-1 H-benzoimidazoles with (4-chlorophenyl)acetonitrile in basic MeOH solution. Physical spectral (UV-vis, IR, (1)HNMR, (13)C NMR, NOESY and fluorescence) and analytical data have established the structures of synthesized compounds. The fluorescence properties of new fluorescent heterocyclic compounds are studied. The fluorescence of all compounds is very intense and fluorescence quantum yields are high (> 0.52). Density functional theory (DFT) calculations are performed to provide the optimized geometries, relevant frontier orbitals and the prediction of (1)H NMR chemical shifts for confirming the exact structure of fluorescent compounds. Calculated electronic absorption spectra were also obtained by time-dependent density functional theory (TD-DFT) method.

Topoisomerase I inhibition and cytotoxicity of 5-bromo- and 5-phenylterbenzimidazoles

Topoisomerase I is an enzyme that is essential for maintaining the three-dimensional structure of DNA during the processes of transcription, translation and mitosis. With the introduction of new clinical agents that are effective in poisoning topoisomerase I, this enzyme has proved to be an attractive molecular target in the development of anticancer drugs. Several terbenzimidazoles have been identified as potent topoisomerase I poisons. Structure-activity data on various terbenzimidazoles have revealed that the presence of lipophilic substituents at the 5-position of various terbenzimidazoles correlates with enhanced cytotoxicity. While the effect of having substituents at both the 5- and 6-positions had not been evaluated, previous studies did indicate that the presence of a fused benzo-ring at the 5,6-position results in a significant decrease in topoisomerase I poisoning activity and cytotoxicity. In the present study we investigated whether substituents at both the 5- and 6-positions of varied terbenzimidazoles would allow for retention of topo I poisoning activity. The 6-bromo, 6-methoxy, or 6-phenyl derivatives of both 5-bromo- and 5-phenylterbenzimidazole were synthesized and evaluated for topo I poisoning activity, as well as their cytotoxicity toward human lymphoblastoma cells. The data indicate that such derivatives do retain similar topo I poisoning activity and possess cytotoxicity equivalent to either 5-bromo- or 5-phenylterbenzimidazole. Significant enhancement in the topoisomerase I poisoning activity and cytotoxicity of 5-phenylterbenzimidazole is observed when the 2"-position is substituted with either a chloro or trifluoromethyl substituent. The influence of such substituents on the biological activity of 5.6-dibromoterbenzimidazole (6a) was also explored. In the case of either 2"-chloro-5,6-dibromoterbenzimidazole (6b) or 2"-trifluoromethyl-5,6-dibromoterbenzimidazole (6c), topoisomerase I poisoning was not enhanced relative to 6a. While cytotoxicity toward RPMI 8402 was also not significantly affected, comparative studies performed against several solid human tumor cell lines did reveal a significant increase in cytotoxicity observed for 6c as compared to 6a.

2"-Substituted 5-phenylterbenzimidazoles as topoisomerase I poisons

5-Phenylterbenzimidazole (1) is active as a topoisomerase I poison (topo I) and is cytotoxic to human tumor cells. No cross-resistance was observed for 1 when it was evaluated against the camptothecin-resistant cell line, CPT-K5. Derivatives of 1 substituted at the 2"-position, however, did exhibit cross-resistance to this cell line. The basis for the resistance of this cell line towards CPT is that it possesses a mutant form of topo I. These results suggest that substituents at the 2"-position may be in proximity to the wild-type enzyme. Therefore, we hypothesized that terbenzimidazoles with 2"-substituents could be capable of interacting with the enzyme and thereby influence activity within this class of topo I poisons. 5-Phenylterbenzimidazoles with a hydroxy, hydroxymethyl, mercapto, amino, N-benzoylaminomethyl, chloro, and trifluoromethyl group at the 2"-position were synthesized. In addition, several 2"-ethyl-5-phenylterbenzimidazoles were prepared containing either a methoxy, hydroxy, amino, or N-acetylamino group at the 2-position of the ethyl side-chain. These 2"-substituted 5-phenylterbenzimidazoles were evaluated as topo I poisons and for cytotoxic activity. The presence of a strong electron-withdrawing group at the 2"-position, such as a chloro or trifluoromethyl group, did enhance both topo I poisoning activity and cytotoxicity. Studies on the relative DNA binding affinity of 1 to its 2"-amino and 2"-trifluoromethyl derivatives did exhibit a correlation with their relative differences in biological activity.