Design And Synthesis Of Novel Naphthalimide Thiazoles And Their Antimicrobial Study

Naphthalimides are one of the most representative cyclic imides containing a naphthalene framework and imides moiety,this unique structure endows its derivatives with desirable large?-conjugated backbone and strong hydrophobicity.Therefore,naphthalimides could easily interact with various biological active sites via diverse weak interactions such as hydrogen bond,coordination,ion-dipole,?-?stacking,hydro-phobic effect,van der Waals force etc.,and exhibit diverse biological activities.Particularly,their interactions with deoxyribonucleic acid?DNA?or topoisomerase?TOP?II could effectively induce single-strand breakage and/or double-strand breakage of DNA,which have drawn great attention in new drug design.Recent researches disclosed that naphthalimides possessed quite large potentiality in the treatment of antibacterial and antifungal infections by targeting bacterial DNA.In addition,the introduction of some nitrogen-containing heterocycles into naphthalimide backbone is much favorable to enhance the biological activity and broaden the antimicrobial spectrum.This encourages our great efforts towards more naphthalimide-based compounds as novel potential antimicrobial agents.Thiazole compounds are an important class of five-membered aromatic heterocycles containing one nitrogen and one sulfur atoms,which endows them with electron-rich system.Thiazole derivatives have received extensive attention in new drug design due to their broad range of biological activities,such as antibacterial,antifungal,anticancer,anti-inflammatory and so on.Especially in antimicrobial field,numerous thiazole-based drugs have been extensively used to treat various types of bacterial and fungal infections in clinic,such as the antibacterial antibiotics cephalosporins cefodizime,cefoselis,and cefmenoxime,the antifungal drugs ravuconazole,abafungin and so on.Studies have shown that thiazole ring possesses high lipophilicity,low toxicity,as well as easy metabolism capacity,and the introduction of thiazole fragment is essential for the antimicrobial activity.In view of this,thiazole core will be incorporated into the naphthalimide backbone with the aim to develop novel and highly efficient anti-infective drugs.Based on the current situation in the researches of naphthalimides and thiazoles,a series of novel naphthalimide thiazole derivatives were designed and synthesized.All the new compounds were investigated for their antimicrobial activity.The preliminary structure-activity relationships and preparation methods were discussed.To evaluate their transportation and pharmacokinetic properties,further binding behaviors between the highly active molecule and human serum albumin?HSA?were investigated by UV-vis absorption and fluorescence spectroscopy.The preliminary antimicrobial mechanism was also discussed.The main work was summarized as follows:?1?Preparation of novel methylene-bridged naphthalimide thiazole derivatives:Naphthalimide derivative II–2 was easily prepared by the reaction of commercial4-bromo-1,8-naphthalic anhydride and aqueous ammonia,and subsequently reacted with chloroacetone in dimethyl formamide at 100 ^oC in the presence of potassium carbonate to smoothly give intermediate II–3.Bromination of compound II–3 in glacial acetic acid yielded derivative II–4,which underwent cyclization with thiourea and thioacetamide in ethanol at 80 ^oC to produce aminothiazole derivatives II–5 and II–6,respectively.Aminothiazoles II–5 and II–6 were treated with alicyclic amines in2-methoxyethan-1-ol at 125 ^oC under nitrogen atmosphere to afford the target molecules II–7a–d and II–8a–d.Naphthalimide aminothiazoles II–9a–c were also formed under the same conditions by the reaction of compound II–5 and aliphatic amines.?2?Preparation of novel amino-bridged naphthalimide aminothiazoles:Condensation of 4-bromo-1,8-naphthalic anhydride and thiosemicarbazide in dimethylformamide at 100 ^oC afforded compound III–2,which was further cyclized with the?-halogenated carbonyl compounds in ethanol at 80 ^oC to give aminothiazole derivatives III–3a–c.The N-alkylation of piperazine with alkyl halides in ethanol at 80^oC generated mono-substituted alkyl piperazines III–6a–e.Compound III–3a was treated with alicyclic amines in 2-methoxyethanol at 125 ^oC under nitrogen atmosphere to achieve the desired compounds III–4a-d.Under the same reaction conditions,the substitution of bromo atom in compound III–3a by piperazinyl derivatives III–6a-e produced the target naphthalimide aminothiazoles 5a-e.?3?All the new synthesized compounds were characterized by ¹H NMR,¹³C NMR,IR,and HRMS spectra.?4?The antibacterial and antifungal activities in vitro for all the new synthesized compounds were evaluated.The antimicrobial assays showed that some prepared compounds in the series II displayed potent inhibitory potency against the tested strains and gave broad antibacterial spectrum,and methylene-bridged naphthalimide aminothiazoles performed better than the naphthalimide thiazole derivatives,some of them were more active than the reference drugs.The morpholine-modified compound II–8c was effective in inhibiting Gram-negative bacteria,especially for E.coli?MIC=2?g/mL?.The piperazine-appended compound II–8d was effective in inhibiting Gram-positive bacteria,especially for MRSA?MIC=4?g/mL?.In the series III,piperazine substituted naphthalimide aminothiazole III–4d also possessed potent inhibitory potency and gave broad antibacterial spectrum,which showed more effective anti-MRSA activity?MIC=4?g/mL?than chloromycin?MIC=16?g/mL?,was further investigated as a potential candidate.?5?The structure-activity relationships?SARs?study revealed that the combination of thiazole fragment and naphthalimide backbone was of great importance to exploit DNA-targeting antimicrobial agents.In the series II,methylene-bridged naphthalimide aminothiazoles performed better than the naphthalimide thiazoles,aminothiazole was more favorable for the antibacterial potency than the thiazole ring.It was noteworthy that naphthalimides bearing alicyclic amino groups on the 4-position exerted better inhibitory efficiency than the aliphatic amines substituted counterparts.In the series III,the modification on the 5-position of the aminothiazole ring might be unfavorable for the antibacterial activity.For the highly active compound III–4d,the introduction of the alkyl groups into the piperazine ring produced conspicuous loss of inhibitory efficacy against the tested pathogens.?6?Time-kill kinetic assays manifested that compound II–8d could rapidly kill MRSA,and bacterial resistance assays against MRSA indicated that MRSA was more difficult to develop resistance against compound II–8d than the clinical drugs norfloxacin and chloromycetin.Drug combination data of compound II-8d with chloromycin and norfloxacin exhibited better antimicrobial efficiency with less dosage than their individual use.The highly bioactive compound II–8d showed lower cytotoxicity against the human embryonic kidney cell lines?HEK 293?than compound II–8c.The preliminary exploration for the antimicrobial mechanism disclosed that compound II–8d could not only permeabilize the membrane of MRSA,but also form a stable II–8d–DNA supramolecular complex with MRSA DNA by intercalation,thus exerting promising antibacterial activity.?7?Compound III–4d also possessed rapid bactericidal efficacy and low propensity to induce bacterial resistance.The active molecule III–4d was further evaluated for its cytotoxicity using the MTT assay,which showed low toxicity to normal rat macrophage cells?RAW 264.7?.Preliminary mechanism research revealed that compound III–4d could not only bind with gyrase-DNA complex through hydrogen bonds,but also form a steady III–4d–DNA supramolecular complex with MRSA DNA by intercalation,which might be responsible for the powerful bioactivity.The binding behavior revealed that the association of HSA with molecule III–4d was spontaneous,and the hydrophobic interactions and hydrogen bonds played significant roles in the transportation of III–4d.This work found that fourteen compounds exhibited more potent antibacterial or antifungal activity than the clinical drugs chloromycin,norfloxacin and/or fluconazole.These compounds were worthy of further studies as potential antibacterial or antifungal candidates.Particularly compounds II–8d and III–4d exhibited efficient and broad-spectrum antibacterial activity,and possessed rapid bactericidal efficacy,low drug resistance and low toxicity.Preliminary research found that both compounds could permeabilize the bacterial inner membrane and interact with bacterial DNA.Thus the highly active compouds II–8d and III–4d could be further optimized as potential anti-MRSA agent.