Synthesis of Some Novel 1,8-Naphthyridine Chalcones as Antibacterial Agents

INTRODUCTION
Nano-catalysts include metals, metal oxide, and non-oxides with nanoscale size and shape. Due to their unique physical and chemical properties, nanomaterials have been found more attention in process based on organic chemistry [1, 2]. The morphology, size and crystalline structures of nanomaterials-based catalysts lead to their catalytic performance, recyclability, and selectivity. The structural and morphological engineering of nanomaterials lead to higher catalytic activity. Till now, various nanostructures have been applied as a heterogeneous catalyst for organic process [3-5].
Heterocyclic compounds are the most important branches of organic chemistry, which are widely distributed in nature and are essential to life fund as potential [5], anticancer agent [6] and antimalarial agent [7]. 1,8-naphthyridine derivates possessing a wide spectrum of biological activities such as antibacterial [8], antibiotic of group is being widely used for the diagnostics and chemotherapy of infection disease of human including aids [9]. Chalcones and pyrimidine derivatives of this group also useful drugs with many biological, pharmaceutical and therapeutically activities and used as analgesics, antiviral, anti-inflammatory agent, antibacterial and anti-tuberculosis agent [10-12]. As well as its proposal as a drug against the emerging corona virus, by studying a theory using the Docking program [13]. There are two general methods available for the preparation of 1,8-naphthyridinenamely the Skraup and Friedlander reaction [14] but the vilsmeier – Haack reagent used as a versatile reagent for the preparation of 1,8-naphthyridine [15,16]. Recently, a substance has been used nanomaterial namely nano-[SiO2@R-Im-SO3H][CF3COO] (NSRISC) Thereafter, the solvent-free production of bis-coumarins (from aryl aldehydes and 4-hydroxycoumarin) was catalysed by NSRISC for condensation reactions[17,18]. Antimicrobial-resistant microorganisms, such as bacteria and fungi, necessitate the continual discovery and development of novel antibacterial agents through the modification and synthesis of new classes of molecule due to the obtained resistance encoded by the resistance issue [19, 20]. There are six distinct isomeric forms of naphthyridines [21].  The 1,8-naphthyridines are the most investigated of them because of their scientifically verified biological actions. Antiparasitic, antibacterial, anti-inflammatory, anti-allergic, and anticancer properties have been demonstrated for these compounds [22]. This study aimed to Synthesis Novel 1,8-Naphthyridine Chalcones derivatives and detect their antibacterial activity against Gram and Negative bacteria

MATERIALS AND METHODS
Electro-thermal (CIA9300) device was used to evaluate the Melting Points of various organic compounds (open capillary tube method). The melting point is the simplest and most significant way to distinguish one substance from another. IR spectra were recorded in KBr powder on tensor 27 , Bruker Co. Germany. The 1H, 13C NMR spectra were measured in DMSO-d6 solvent on JEOL 400 MHz.

Synthesis of NSRISC
Stirred for 12 hours in reflexed toluene (15 mL) with a mixture of imidazole and (3-chloropropyl)trimethoxysilane (0.99 g, 5 mmol). I was produced by distilling the solvent at 100oC under vacuum. It took 18 hours for me to make II by refluxing nano-SiO2 in EtOAc (15 mL) for 18 hours. After a 4-hour stirring period, dry CH2Cl2 (15 mL) was progressively added to an aqueous solution of ClSO3H (0.34 mL, 5 mmol) in order to create III. This was followed by stirring III for 9 hours at room temperature and 3 hours under reflux for the last time, giving NSRISC, which was obtained by slowly adding 0.46 mL (six molecular weight) of CF3CO2H solution in dry CH2Cl2 (15 mL). It was dried after being cleaned with the solvent and then rinsed (Fig. 1).

Synthesis of 2-chloro-1,8-naphthyridine-3-carboaldehyde (1)[23,24]
N-(pyridine -2- yl) acetamide (1 mol) equivalent was taken in around bottom flask (250 ml) to this add (3 mol) equivalent of dimethyl formamide (DMF) the round bottom flask in then equipped with a dropping funnel filled with (7 mol) equivalent POCl3 (phosphoryl chloride), the whole assembly was placed on an ice bath and (0-5 oC) temperature was maintained. then drop wise adding of POCl3 was done with continuous stirring after the drop-wise addition, dropping funnel was removed and immediately the contents of reaction mixture in round bottom flask were equipped with a reflux condenser and then the mixture was stirred at (80-90 oC) for (15 hrs) then the reaction mixture was immediately poured in ice cold water and stirred for half an hour. the reaction mixture was filtered on Buchner funnel; the crystals obtained were washed with cold water and dried. the recrystallization was done by ethanol (Fig. 2).
The melting point was found to be 165-167 oC , The % Yield obtained was 65% , Color pale yellow. F.T. IR (KBr. ν , cm-1) 3055(Ar-H), 2780 (C-H), 1685 (C=O), 1565 (C=N), 775 (C-Cl), 1H NMR spectra (400 MKz, DMSO-d6, δ , ppm) 8.39-8.41 (d, 1H, C-7), 7.26-7.29(d, 1H, C-6), 7.48-7.50(d, 1H, C-5), 7.91(s, 1H, C-4), 10.45(s, 1H, CHO).

Synthesis of 2-morpholeno-1,8-naphthyridine-3-carboaldehyde (2)[25]
Morpholine (0.01 mol) and 2-chloro-3-formyl-1,8-napthyridine(1) (0.01 mol) in DMF (10 ml) were stirred at 35 oC for 6 hrs. the PH was controlled using sodium bicarbonate . after cooling, the precipate was washed with cold water. the recrystallization was done by ethanol to obtained  above compound(2) . yield 55% as yellow , m.p. 187-189 oC . F.T. IR (KBr. ν , cm-1) 3055(Ar-H), 1685 (C=O), 1577 (C=N), 1045 (C-O-C), 760 (C-Cl), 1H NMR spectra (400 MKz, DMSO-d6, δ , ppm) 3.82-3.75 (t, 4H, CH2), 3.98-3.88 (t, 4H, CH2), 8.63-8.62 (d, 1H, C-7), 7.02-7.04 (d, 1H, C-6), 7.95-7.93 (d, 1H, C-5), 6.57-6.65 (s, 1H, C-4), 10.19 (s, 1H, CHO).

Synthesis of E-3-(2-morpholino-1,8-naphthyridine-3-yl)-1-(4-aminophenyl)prop-2-en-1-one(3)[26]
A mixture of 2-morpholino-3-formyl-1,8-naphthyridine (0.01 mol) and p-aminoacetophenone (0.01mol) and  NSRISC (0.04 g) were stirred by a rod at 100ºC. TLC revealed the end point reaction, the mixture was cooled, EtOAc (30 mL) was added, and stirred followed by refluxing for 5 minutes, the precipitate was produced, washed with cold water, dried, and recrystallized from ethanol; yield 70%, yellow solid m.p. 135-137 oC ; F.T. IR   (KBr. ν, cm-1) 3342 (N-H), 1655 (C=O), 19595 (C=C), 1565 (C=N), 1055 (C-O-C), 1H NMR spectra (400 MKz, DMSO-d6, δ , ppm) 3.92 (s, 2H, NH2), 3.78-3.75 (t, 4H, CH2), 3.88-3.86 (t, 4H, CH2), 7.18-7.21 (d, 1H, CHα), 7.96-7.94 (d, 1H, CHβ), 8.65-8.64 (d, 1H, C-7), 7.68-7.67 (d, 1H, C-6), 6.69-6.67 (d, 1H, C-5), 7.16 (s, 1H, C-4), 7.13-9.91 (m, 4H, Ar-H).

Synthesis of 1,3-diphenyl prop-2-en-1-one[27]
Acetophenone (10 m.mol) was mixed with benzaldehydes (10 m.mol) and desolved in 50 ml of ethanol. The mixture was then stirred at room temperature for half an hour. after that drop wise addition of (4 ml) of (40%) aqueous sodium hydroxide (NaOH), stirred continuously for two hours and then kept at 0 oC for 24 hours. The precipitate was washed with cold water. The recrystallization was done by ethanol (scheme-5-).

Synthesis of 2-amino-4,6-diphenyl-1,3,4-dihdropyrimidine [28,29]
Around bottom flask with two necked (250 ml) equipped with a dropping funnel which charged with a solution of sodium hydroxide (0.4 gm in 5 ml of water). A mixture of 1,3-diphenyl prop-2-en-1-one (0.0048 mol) and quinidine hydrochloride (0.0048 mol) in 20 ml of ethanol was refluxed while the solution of sodium hydroxide was added drop by drop with stirring during 2 hrs. Then mixture was reflux for 10 hrs. let the mixture at (25 oC) then dilute with water and left overnight, precipitate was formed washed with 20 ml of mixture of water / ethanol (1:1 v/v) then recrystallized frombenzene.

Synthesis of 2-chloro-N-(4,6-diphenylpyrimidin-2-yl)acetamide(4)
A chloroacetylchlorid (0.03 mol, 3.38 gm , 2.4 ml) and three drops of acetic acid was stirred at (60 oC) for 20 min. 2-amino-4,6-diphenyl-3,4-dihydropyrimidine (0.03 mol) in 5 ml ethanol was added drop wise and the mixture was refluxed for 5 hrs. Then the white solid crystals were filtered and washed with (10 ml) benzene, dried and recrystallized from ethanol to give compound(4). 

Synthesis of (E)-N-(4,6-diphenylpyrimidine-2-yl)-2-((4-(3-(2-morpholino-1,8-naphthyridin-3-yl)allyl)amino)acetamide (5)[30]
A mixture of compound (3) (10 m.mol) andcompound (4) (10 m.mol) in (15 ml) dimethyl formamide. small quantity of sodium bicarbonate was added heated at 90 oC for 10 hrs and then the reaction mixture was immediatcly poured in ice cold water and stirred continuously for half an hrs , the reaction mixture was filtered on buchner funnel , the crystals obtained were washed with ethanol and dried. The recrystallization was done by acrylonitrile ; yield 60% as brown solid, m.p. 196-199 oC; F.T. IR (KBr. ν, cm-1): 3325 (N-H), 1665,1680 (C=O), 1565 (C=C), 1623 (C=N), 1123 (C-O-C), 1H NMR spectra (400 MKz, DMSO-d6, δ , ppm) 3.42-3.44 (t, 4H, CH2), 3.82-3.85 (t, 4H, CH2), 3.89 (s, 2H, CH2), 6.34 (s, 1H, NH), 9.90 (s, 1H, NH), 7.12-7.15 (m, 14H, Ar-H), 7.06-7.08 (d, 1H, CHα), 7.98-7.99 (d, 1H, CHβ), 6.96 (s, 1H, CH), 8.32-8.35(d, 1H, H-7), 6.64-6.66 (t, 1H, H-6), 6.64-6.66 (d, 1H, H-5), 6.94 (s, 1H, H-4).

Preparation of N-chloroacetylaniline [31]
A Chloroacetylchloride (0.03 mol) and 2-3 drops of acetic acid at (60 oC) for 15 min. a solution of aniline (0.02 mol) was added drop wise and reflux for 3 hrs. Then white precipitate was formed washed with benzene and recrystallized from ethanol.

Preparation of (E)-2-((4-(3-(2-morpholine-1,8-naphthyridine-3-yl)acryloyl)phenyl)amino)-N-phenylacetamide(6)
A mixture of compound (3) (10 m.mol) and N-chloroacetylaniline (10 m.mol) in (25ml DMF) in the presence of small quantity of sodium bicarbonate was heated at 90-100 oC for 10 hrs. then the result was immediately poured in ice cold water and stirred continuously for half an hour. then brown parcipate was formed, washed with water and recrystallized from ethanol to give compound(6). Yield 55% as brown , m.p. 226-228 oC;  stirred for F.T. IR (KBr. ν, cm-1): 3355 (N-H), 1645,1680 (C=O), 1595 (C=C), 1125 (C-O-C), 1H NMR spectra (400 MKz, DMSO-d6, δ , ppm) 3.46-3.42 (t, 4H, CH2), 3.91-3.88 (t, 4H, CH2), 3.98 (s, 2H, CH2), 5.99 (s, 1H, NH), 8.73-6.63 (m, 5H, Ar-H), 6.94-6.99 (t, 1H, H-6), 7.14-7.12 (d, 1H, H-5), 7.73-7.75 (d, 1H, CHα), 8.57-8.45(d, 1H, CHβ), 7.91-7.92 (s, 1H, H-4), 7.58-7.63 (m, 14H, Ar-H), 8.84-8.85 (d, 1H, H-7) , 10.08 (s, 1H, N-H).

Antibacterial activity of synthesized compounds
The newly synthesized compounds (5,6) were screened for their antibacterial activity against [32,33] two-gram positive organism Staphylococcus aurous and Staphylococcus epidermises and two-gram negative such as Escherichia coli and Proteus vagaries at three concentration 500, 250 and 100 mg using disc diffusion methods [34-36]. Ampicillin was used as standard for comparison by measuring the diameter of inhibition zone at the end of 24 hrs. at 37 oC. the newly synthesized compounds were screened for them for antibacterial activity against two gram positive and two gram negative with these three concentrations. 

RESULTS AND DISCUSSION

Fig. 3 shows the SEM image of applied SiO2 nanoparticles. As well as seen, the homogenous spherical SiO2 nanoparticles with average particle size of 28 nm was provided. 
In this work, 2-chloro-1,8-naphthyridine-3-carboaldehyde (1) was prepared from reaction of N-(pyridine-2-yl) acetamide and Vilsmeier reagent, the Vilsmeier cyclization was done by adding POCl3 to the substrate in dimethyl formamide at 0-5 oC then heating to 90 oC for 1 hrs. (Fig. 4)
The IR in this compound (1) showed C=O stretch peak at 1685 cm-1, C=N stretch in ring shows peak at 1565 cm-1 and C-Cl group at 775 cm-1. The 1H NMR spectra for this compound in DMSO showed a significant peak at 10.45 ppm for aldehydic proton. The chlorine group of 1,8-naphthyridine (1) is difficult to displace by N-nucleophiles (such as amine under various condition). However, the reaction morpholine with compound (1) at 90-100 oC result compound (2) (Fig. 5).
The IR in this compound (2) showed C=O stretch peak at 1685 cm-1 and C-O-C group appeared at 1045 cm-1. The 1H NMR spectra of compound (2) showed two triplets at 3.42-3.46 ppm and at 3.96-4.00 ppm for morpholine proton. Compound (3) has been prepared via condensation in nano catalyst / solvent free of compound (2) with 4-aminoacetophenone (Fig. 6).
The IR in this compound (3) showed C=O stretch peak at 1655 cm-1, NH2 group showed at 3342 cm-1 , C=N stretch in ring shows peak at 1565 cm-1, C=C stretch appeared at 1595 cm-1. The 1H NMR spectra of compounds (3) showed a significant peak as the following singlet at 4.16 ppm for NH2 group and two doublets at 7.18 ppm and 7.75 ppm for Hα and Hβ for unsaturated carbonyl group and two triplet at 3.45 and 3.99 ppm for morpholine proton. The treatment of compound (4) which was synthesized according to literature [37-39] with compound (3) in the presence of small quantity potassium carbonate in dimethyl formamide gave compound (5) (Fig. 7 and Fig. 8).
The IR in this compound(5) gave a peak at 3325 cm-1 for NH and at 1565 cm-1 for C=C and two absorption for C=O group at 1665 cm-1 and 1680 cm-1 and at 1623 cm-1 for C=N. the 1H NMR spectra of this compound gave two singlet at 6.03 ppm and at 9.62 ppm for NH group and two doublet at 7.42-7.44 ppm and at 8.11-8.15 ppm for Hα and Hβ for unsaturated carbonyl group . Compound (3) on treatment with N-chloroacetylaniline in the presence of sodium bicarbonate in DMF give compound (6) (Fig. 9)
The IR in this compound(6) showed a peak at 3355 cm-1 for NH and two absorption for C=O group at 1680 cm-1 and 1645 cm-1 and at 1123 cm-1 for C-O-C. the 1H NMR spectra of compound (6) showed two singlet signal at 5.95 ppm and 9.64 ppm for NH group and two triplet at 3.60-3.63 and 3.83-3.86 ppm for CH2 for morpholine ring and singlet at 4.08 ppm for CH2 and two doublet at 7.44-7.46 and at 8.06-8.08 ppm for Hα and Hβ.

Antibacterial Activity
Antibiotic resistance in bacteria isolated from diverse environments is the most important public health problem [40], So, new antibacterial drugs have become necessary due to the rapid spread of antibiotic resistance [41]. in this study the two compounds (5,6) were found show excellent activity against gram positive bacteria and good activity against gram negative bacteria as shown in the table 1. A study found that 1,5-naphthyridinone derivatives kill germs by inhibiting bacterial topoisomerase [42]. E. coli and Staphylococcus aureus strains have been shown to be resistant to naphthyridinone-based antibacterials, according to another study [43]. Additionally, Gençer et al. [44] used in silico research to show that 1,8-naphthyridine derivatives block DNA gyrase (topoisomerase), such as fluoroquinolones, and they acquired a strong antibacterial impact against Gram-positive and Gram-negative microorganisms through in vitro investigations.

CONCLUSION
Naphthyridine compounds are considered heterocyclic organics of great importance. In this work, we were able to prepare new substituted compounds (5,6) that showed high activity to some types of bacteria, compared with the known antibiotics (ampicillin) Therefore, we infer that 1,8-naphthyridine derivatives enhance the effectiveness of fluoroquinolone antibiotics against multi-resistant bacterial strains, and thus intriguing candidates for the development of medications against bacterial illnesses caused by multidrug-resistant strains. And the molecular structure of the prepared compounds was diagnosed through NMR spectroscopy.

ACKNOWLEDGEMENT
We would like to express our heartfelt appreciation to the University of Mosul for its assistance in completing this research.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.