Design And Synthesis Of Novel Naphthalimide Imidazoles And Their Antimicrobial Researches

Naphthalimide is an important nitrogen-containing aromatic heterocycle with the wide medicinal applications.Especially as anticancer drugs,many naphthalimide derivatives such as amonafide and mitonafide have already entered clinical trials.Recently,along with the abuse of antibiotics and synthetic drugs and the appearance of multi-drug resistant strains,naphthalimide-based compounds were found to have the great potential as antibacterial and antifungal agents.Much literature has shown that naphthalimide derivatives could prevent deoxyribonucleic acid(DNA)replication by interacting with DNA or topoisomerase to inhibit the growth of the bacteria.This encouraged us with overwhelming interest to design and synthesis of novel naphthalimide-based antibacterial and antifungal drugs.Imidazole nucleus is present in numerous natural products such as histamine,vitamin B12,DNA and hemoglobin,and the exploitation of the imidazole-based active molecules is exceptionally active.It is well known that imidazoles as antibacterial and antifungal agents have made considerable achievements,many imidazole drugs have been used in clinic such as metronidazole,ornidazole,secnidazole,miconazole and ketoconazole and so on.All mentioned above showed the great exploitation value and extensive potential application of the imidazole compounds in the antimicrobial field.This thesis based on the current situation in the research of naphthalimides and our group's research for the imidazoles,a series of new naphthalimide imidazoles were designed and synthesized.The preparative methods and conditions of the target compounds were explored,and their antibacterial and antifungal activities and structure-activity relationships were discussed.Bacterial resistance research,time-kill kinetic assay and cytotoxicity were also done.Meanwhile,the mechanism of antibacterial activity of highly active target compounds and their in vitro transport behaviors by human serum albumin were investigated by UV-vis absorption and fluorescence spectroscopy.The main work was summarized as follows:(1)Preparation of novel aliphatic amino naphthalimide metronidazole derivatives: Commercially available 4-bromo-1,8-naphthalic anhydride II–1 was treated with aqueous ammonia to produce intermediate II–2,and was further reacted with chloroacetone in DMF under the presence of potassium carbonate as base to afford compound II–3.The latter was brominated to generate the corresponding compound II–4 in the yield of 53%.The prepared bromide II–4 was coupled with 2-methyl-5-nitroimidazole in DMF at 40 oC using potassium carbonate as base to give the desired compound II–5.Subsequently,the reduction of compound II–5 by sodium borohydride afforded the metronidazole derivative II–6.The further N-alkylation of compound II–6 with various alkyl chain amines in dimethyl sulphoxide using potassium carbonate as base and copper(I)oxide as catalyst under nitrogen atmosphere produced target compounds II–7a–g.(2)Preparation of novel alicyclic amino naphthalimide metronidazole derivatives: Naphthalimide metronidazole intermediate II–6 was synthesized via multi-step reactions from commercially available 4-bromo-1,8-naphthalic anhydride II-1.The further N-alkylation of compound II–6 with different alicyclic amines in 2-methoxyethanol at 125 oC under nitrogen atmosphere to give III–7a–d.Piperazine was reacted with alkyl bromides in ethanol to give the corresponding compounds III–8a–j,and then further treated with intermediate II–6 to give the target compounds III–9a–j.(3)Preparation of novel naphthalimide imdazoles: The prepared bromide II–4 was reacted with different imidazoles in DMF at 0 oC using potassium carbonate as base to give the target compounds IV–5a–f.Subsequently,the reduction of compound IV–5a–f by sodium borohydride afforded the corresponding imidazole derivative IV–6a–f.(4)The newly synthesized compounds were characterized by NMR,IR and HRMS spectra.(5)The in vitro antibacterial and antifungal activities of intermediates and target compounds was evaluated.Biological results showed that the ethylamine modified naphthalimide metronidazole II–7b had the best antibacterial activities,especially for P.vulgaris and S.dysenteriae with MIC values of 0.002 and 0.01 ?mol/mL,respectively.In comparision with the aliphatic amine modified maphthalimides,decyl piperazine metronidazole compound III–9e also possessed good broad-spectrum and efficient antibacterial activities.The anti-K.pneumoniae and anti-A.baumannii potency of compound III–9e was the strongest with MIC values of 0.013 ?mol/mL.Naphthalimide imidazole compounds IV–5f and IV–6d gave relatively moderate antibacterial and antifungal activities,but were much better than metronidazole intermediates II–5 and II–6.Thus,further modification of alkyl imidazole appended maphthalimides might have the large possibility to develop excellent antimicrobial agents.(6)Structure-activity relationships revealed that amino group displayed a positive effect on aliphatic amine metronidazoles and could greatly enhance the antibacterial and antifungal activities of target compounds.The length of the alkyl chain had a certain effect on the biological activity.Moreover,the introduction of hydroxyl into the alkyl chain or the ethylation of secondary amino group could not effectively improve the antibacterial and antifungal activities.In addition,the introduction of double bond or triple bond could significantly decrease the biological activities of metronidazole compounds.Imidazole,2-methylimidazole and 2-ethylimidazole naphthalimides displayed no obvious antibacterial and antifungal activities,while 2-propylimidazole and 2-ethyl-4-methylimidazole modified naphthalimide compounds exhibited relatively good antibacterial and antifungal activity.(7)In order to evaluate the antibacterial potency,the development of resistance and bactericidal kinetics of II–7b were studied,and the results showed that II–7b showed low resistance and rapidly bactericidal rate.Meanwhile,the preliminary antibacterial mechanism of action investigated by UV spectroscopy revealed that II–7b could intercalate into calf thymus DNA to form a steady supramolecular complex and thus block DNA replication.Molecular docking showed that compound II–7b could interact with topoisomerase II by hydrogen bonds.In addition,the interaction between II–7b and human serum albumin(HSA)was peformed by fluorescence spectroscopy.The results showed that II–7b could be effectively transported by human serum protein through the formation of supramolecular complex.(8)Further experiments confirmed that compound III–9e could rapidly kill the Acinetobacter baumanii(A.baumanii)bacteria and did not trigger the development of drug resistance.Drug combination data of compounds III–9e with Chloromycin,Norfloxacin and Clinafloxacin showed better antimicrobial efficiency against A.baumanii bacteria with less dosage and broader spectrum than their separated use.Besides,compound III–9e could effevtively permeabilize the membrane of A.baumanii bacteria.Molecular docking indicated that compound III–9e could bind to bacterial DNA gyrase B through hydrogen bonds,which correlated with the strong inhibitory effect.Quantum chemical calculations also confirmed the binding mode obtained from above docking study.In addition,compound III–9e could intercalate into A.baumanii DNA to display the antibacterial activity.Forty eight compounds were successfully synthesized in this thesis,forty seven of them were new,including fourteen intermediates,seven aliphatic amino naphthalimide metronidazole derivatives,fourteen alicyclic amino naphthalimide metronidazoles,and twelve naphthalimide imdazoles.