Microwave Irradiated and Conventional Synthesis, Antibacterial Activity Evaluation Studies of Tryptamine-Azole-Fluoroquinolone Conjugates

Tryptamine was converted to the corresponding 1,2,4-triazole, 1,3,4-oxadiazole, 5-oxo-1,3-thia(oxa) zolidine and 5-(4-chlorophenyl)-1,3-thia(oxa)zole derivatives via several steps. 1,3,4-oxadiazole and 1,2,4-triazoles were then converted to the corresponding Mannich bases containing fl uroquinolone core using a one-pot three-components procedure. Conventional and microwave-assisted methods were applied for all syntheses. All the newly synthesized compounds were screened for their antibacterial and most of them were found to have good–moderate antibacterial activity. Research Article Microwave Irradiated and Conventional Synthesis, Antibacterial Activity Evaluation Studies of Tryptamine-AzoleFluoroquinolone Conjugates Yıldız Uygun Cebeci1, Serap Basoglu Özdemir1, Ahmet Demirbas1, Neslihan Demirbas1* and Sengül Alpay Karaoglu2 1Department of Chemistry, Karadeniz Technical University, Trabzon, Turkey 2Department of Biology, Recep Tayyip Erdogan University, Trabzon, Turkey Received: 10 October, 2018 Accepted: 20 December, 2018 Published: 21 December, 2018 *Corresponding author: Neslihan Demirbas, Department of Chemistry, Karadeniz Technical University, Trabzon, Turkey, Tel: +90 462 3774252; Fax: +90 4623253196; E-Mail:


Introduction
The increasing community-and hospital-acquired infectious diseases caused by resistant bacteria to most classes of antibacterial drugs resulted in a pressing and urgent need for designing of new antibiotic candidates. The declaration of The European Centre for Disease Prevention and Control (ECDC) reporting "Every year, the infections caused by resistant bacteria gives rise to 25,000 deaths with a cost of over 1.5 billion Euro because of healthcare spending and labor losses in the Europa" reveals that this is a public health problem with also socio-economic loses [1][2][3][4][5].
In recent years, to overcome the drug resistance problem, the concept of hybrid molecules, which contain two or more pharmacophore groups binding together covalently in one molecular framework, has been introduced in the medicinal chemistry fi eld. These compounds that are obtained by molecular hybridization of several pharmacophore groups, act by inhibiting two or more conventional targets simultaneously, and this multiple target strategy has resulted in the development of a number of bioactive hybrid molecules with desired pharmacokinetic profi le, therapeutic index and more importantly less tendency to resistance [6][7][8][9][10][11].
Heterocyclic compounds accepted as medicinally important products have given a new direction to new drug design and discovery studies. 1,2,4-Triazole, 1,3,4-oxadiazole and 1,3-thiazole derivatives which attract great attention by synthetic and medicinal chemists due to their distinct structures have been regarded as useful tools for drug discovery processes [12][13][14]. The compounds containing these heterocyclic units in their structures have been reported to possess a wide range of biological activities, such as antibacterial, antitubercular, antiviral, analgesic, antioxidant, anticancer etc [15][16][17][18]. Another class of bioactive compounds, indoles exhibit several biological activities with high bioavailability and relatively low toxicity. Indole unit constitutes a part of several natural compounds isolated from marine creatures, medicinal plants or microorganisms. Although a number of studies devoted on the modifi cations of indole unit was reported, there are only a few studies about indole-azole hybrids. Based on the pharmacodynamic principle of superposition, it is accepted that the indole hybrids containing 1,3,4-oxadiazole and 1,2,4-triazole units at the position 3 exhibit effi cient antibacterial activity [19,20].
In the anti-infective chemotherapy fi eld, fl uoroquinolones which target two type II bacterial topoisomerase enzymes, DNA gyrase and/or topoisomerase IV, constitute a large and constantly expanding group of synthetic antibiotics. They have been attracting major interest due to broad-spectrum of activity towards both Gram (-) and Gram (+) bacteria by inhibiting bacterial DNA replication, and their chemotherapeutic effi cacy [8][9][10][11]. In the concept containing the synthesis of new analogs or modifying existing drug compounds, new fl uoroquinolones including large substituent at the position 7 were reported by several research groups [7,[21][22][23][24][25]. Microwave assisted techniques were reported to be more effective in terms of minimal reaction time and structural complexity. Thus, MCRs are also considered as green chemical processes [27].
Among these, Mannich reaction, a one pot three-component condensation reaction, provide synthetically and biologically important β-aminoalkylated compounds, which are important intermediates for the construction of various nitrogencontaining natural products and pharmaceuticals [28].
Thus, based on the aforementioned statements, this study focused on the design, ecofriendly synthesis, and antibacterial activity evaluation of new indole-azole-fl uoroquinolone hybrids based on pharmacophores hybridization. Indole scaffold was selected as the key prototype structural unit and the integration of indole skeleton, azole and fl uoroquinolone pharmacophores we attempted to conventional and MW mediated green synthesizes of new indole-azole hybrid scaffold in the one molecular frame was performed as shown in fi gure 1 with the aim to prepare new antibacterial agents with preferably therapeutic profi le having less tendency to antibacterial resistance.

Chemistry
In this study, we attempted to conventional and MW mediated green synthesize of new indole-azole-fl uoroquinolone hybrids as possible drug candidates with antibacterial activity. On the basis of 1 H, 13 C NMR, FT IR and EI-MS data, the structure of the target products was established. All synthesized compounds were checked for purity and identity using elemental analysis.
The MICs against clinically important Gram-negative and Gram positive pathogens were determined as well. The synthetic methodologies adopted to obtain the target compounds were depicted in Scheme 1 and Scheme 2.
The treatment of the starting compound, tryptamine which was selected considering its biological importance [29][30][31][32][33] of ester (2). The substitution of ester group by hydrazide generated 2-[2-(1H-indol-3-yl)ethylamino]acetohydrazide (3), which were confi rmed by the appearance of broad signal for -NHNH 2 group in FT IR. Further, the protons of hydrazide function appeared at 3.93 (NH 2 ) and 10.94 ppm (NH) as D 2 O exchangeable singlets confi rming the formation of hydrazide. In order to optimize microwave (MW) irradiation conditions, MW was applied at different power values of 120 and 150 W without any solvent, while the conventional synthesis of compound 3 required ethanol as reaction solvent. The complete conversion of the compound 2 in best yield was observed after microwave irradiation at 220 W maximum power for 6 min. Higher MW power or longer reaction time caused to lower yields. Compound 3 gave the corresponding 1,3,4-oxadiazole derivative (4) upon the treatment with carbon disulfi de in basic media. The 13 C NMR observations revealed the appearance 1,3,4-oxadiazole C-2 and C-5 carbons at the region 166.19 ppm and 163.99 ppm confi rming the formation of 1,3,4-oxadiazole [34,35]. The stretching band characteristic for SH absorption was seen at 2921 cm -1 in the FT IR spectrum. The signal observed at 13.69 ppm in the 1 H NMR spectrum was attributed the SH proton. On the other hand, the treatment of compound 3 with alkyliso(thio)cyanates produced the corresponding hydrazincarbo(thio)amides (6a-c) with both conventional and MW irradiated method. In the conventional method, the reaction yielding compounds 6a-c completed in 24 h in DCM with 86-91 % yield, while reaction time was 8 min with the yield 97-99 %. The structures of hydrazinecarbo(thio)amides were confi rmed by the presence of additional signals at the related chemical shift values originated from alkyliso(thio) cyanate moiety. This compound exhibited mass fragmental and elemental analysis data confi rming the assigned structures.
The basic treatment of compounds 6a-c produced the corresponding 1,2,4-triazoles (7a-c), which can be considered as important tools for further condensation reactions leading to the formation of new bioactive molecules. The reaction was carried out in water-ethanol as a none toxic ecofriendly solvent mixture under refl ux and also microwave conditions. With the use of MW conditions, higher yields and lower reaction times were assessed. Microwave irradiation decreased the reaction time from 6 h to 18 min and increased the yields from 56-63 % to 93-97 %. The optimum reaction condition was assessed at 200 W maximum power ( Table 1).
The FT IR spectra of compounds 7a-c have -C=S or C=O and -NH stretching bands at 1223-1284 cm -1 , 1683 cm -1 and 3163-3397 cm -1 , respectively. In the 1 H NMR spectra, resonances assigned to the -NH proton on 1,2,4-triazole ring were detected at 10.34-11.40 ppm (NH) which are supported by the literature fi ndings [36][37][38][39][40]. 13      with the compounds including a thioamide function is the procedure most often referred to. According to the accepted mechanism, the attack of sulfur atom of thioamide, which is present in its ene-thiole form to the halogen atom of carbonyl compound, and this attack is followed by HBr and H 2 O elimination, which leads to the 1,3thiazole ring formation [36].
As different from the classical Hantzsch reaction, the fact that hydrazinecarbo(thio)amides synthesized in the present study (6a-c) have more than one nucleophilic center and 2-bromo-1-(4-chlorophenyl)ethanone, that is the carbonyl component in this reaction has two position for nucleophilic attacks, can cause the formation of different structural isomers; each of them can exist as their individual E and Z geometrical [3,4,[37][38][39][40].
In our previous studies [37,38,40], to identify the exact structure of type of compounds 10a-c, full geometric optimization of the possible isomer products was obtained by DFT/B3LYP (density functional theory with B3LYP-the hybrid Becke's three parameter functional and Lee-Yang-Parr exchange-correlation potential) method with the 6-31G (d,p) basis set and the structure of the molecules was also investigated in detail. According to the obtained results [37,38], the most stable product is Z geometrical isomer with the calculated relative energy 0.000 kcal mol -1 . Therefore, thermodynamically, the formation of Z isomers is more favorable. Similar case is present for compounds 9a-c.

Antibacterial activity
All newly synthesized compounds were screened for their antibacterial activity and the results obtained were submitted in

Con clusion
This study reports the conventional and successfully developed microwave assisted synthesis of some new hybrid molecules containing several heterocyclic moieties having importance for biological activity. Hence, we combined all these potential chemotherapeutic units, namely indole, 1,3,4-oxadiazole, 1,2,4-triazole, fl uoroquinolone and/or 1,3-thiazole moieties.
The structures of new compounds were confi rmed by IR, 1 H-, 13 C NMR, mass spectroscopic and elemental analysis techniques.
In addition, the newly synthesized compounds were screened for their antimicrobial activities. The antimicrobial screening suggests that the compounds containing fl uoroquinolone core exhibited excellent activities against most of the test microorganisms.

Experimental Section
Chemistry All the chemicals were purchased from Fluka Chemie AG Buchs (Switzerland) and used without further purifi cation.
Melting points of the synthesized compounds were determined

Method 1:
The solution of the corresponding compound 6a-c (10 mmol) in ethanol:water (1:1) was refl uxed in the presence 100 mL of 2 % NaOH for 6 h. Then the resulting solution was cooled to room temperature and acidifi ed to pH 4 with 37% HCl. The precipitate formed was fi ltered off, washed with water, and recrystallized from ethyl acetate to afford the desired compound.

Method 2:
The mixture of the corresponding compound 6a-c (10 mmol) and 2 % NaOH (100 mL) was irradiated in monomod microwave reactor in closed vessel with the pressure control (Table 1). Upon acidifi cation of reaction content to pH 7 with 37% HCl, a white solid appeared. This crude product was fi ltered off, washed with water and recrystallized from ethyl acetate to afford the desired product.

General Method for The Synthesis of Compounds 9a-c
Method 1: A mixture of compound 6 (10 mmol) and ethyl bromoacetate (10 mmol) in absolute ethanol was refl uxed in the presence of dried sodium acetate (50 mmol) for 18 h. The reaction mixture was cooled to room temperature and the salt was separated by fi ltration. After the solvent was removed under reduced pressure, a solid appeared. This crude product was recrystallized from acetone-water (1:1) to afford the desired product.

Method 2:
A mixture of compound 6 (10 mmol) and ethyl bromoacetate (10 mmol) and dried sodium acetate (50 mmol) was irradiated in monomod microwave reactor in closed vessel with pressure control ( Table 1). The solid obtained was treated with 50 mL of water, the precipitate was collected by fi ltration and recrystallized from acetone-water (1:1) to afford the desired product.  Table 1). The solid obtained was treated with 50 mL of water, the precipitate was collected by fi ltration and recrystallized from acetone-petroleum ether (1:1) to afford the desired product.