MICROWAVE-ASSISTED SYNTHESIS AND EVALUATION OF ANTIBACTERIAL ACTIVITY OF 2 , 2 '-( NAPHTHALENE-2 , 7-DIYLBIS ( OXY ) ) BIS ( N '-SUBSTITUTED ACETOHYDRAZIDE ) DERIVATIVES

A series of Schiff base of 2,2'-(naphthalene-2,7-diylbis(oxy))bis(N'-substituted acetohydrazide) (4a-m) derivatives has been synthesized by the acid catalyzed condensation of aryl/hetero aromatic aldehydes with 2,2'-(naphthalene-2,7-diylbis(oxy))diacetohydrazide (3) under microwave irradiation and conventional method for comparison. The structures of all the newly synthesized compounds have been characterized by IR, H NMR, C NMR and Mass spectra. All the synthesized compounds have been screened for their in vitro antibacterial activity.

Microwave-assisted organic synthesis (MAOS) has been known since 1986 [22].This "nonconventional" synthetic method has shown broad applications as a very efficient way to accelerate the course of many organic reactions, producing high yields and higher selectivity, lower quantities of side products and consequently, easier work-up procedures and purification of the products.MAOS is considered as "green" technology, principally since many organic reactions can be carried out in solvent-free conditions [23].In recent years, researchers have applied microwave as a tool in order to reduce reaction time, avoid side products, increase yield and simplify the course of reaction for combinatorial chemistry [24].Solvent-free reactions are of interest not only from an ecological point of view but in many cases they also offer considerable advantages in terms of yield, selectivity and simplicity.
In many cases reactions that normally require many hours at reflux temperature under normal synthetic conditions can be completed within several minutes or even seconds in a microwave oven.Recent simplification of microwave organic reaction enhancement (MORE) technique has increased safety and practical utility of the microwave oven for their use in organic laboratories without any modification.
Hence, in this paper, we are reporting eco-friendly synthesis of some new Schiff bases by condensing 2,2'-(naphthalene-2,7-diylbis(oxy))diacetohydrazide with aryl/hetero aromatic aldehydes using microwave irradiation technique and the yields are compared with traditional method and their characterization carried out using IR, 1 H NMR, 13 CNMR and Mass spectra.All the synthesized compounds have been screened for their antibacterial activity.

Screening of antibacterial activity
The newly synthesized Schiff base compounds (4a-m) were screened for their antibacterial activity against Escherichia coli (ATTC-25922) (E.coli), Staphylococcus aureus (ATTC-25923) (S. aureus) and Pseudomonas aeruginosa (ATCC-27853) (P.aeruginosa) bacterial strains by agar well diffusion method in Muller Hinton agar medium.Twenty milliliters of agar media was poured into each Petri dish and plates were dried by placing in an incubator at 37 °C for an hour.Using a punch, wells were made on these seeded agar plates and concentrations of the test compounds in dimethylsulfoxide (DMSO) were added into each labelled well.A control was also prepared for the plates in the same way using solvent DMSO.All the plates were incubated at 37 °C for 24 hrs for incubation.The degree of effectiveness was measured by determining the diameters of the zone of inhibition caused by the compounds.Activity of each compound was compared with standard drug tetracycline which is available in the market.

General
All reagents were purchased from Aldrich, SD Fine Chemicals and Qualigens and used without further purification.For the microwave irradiation experiments described below a conventional (unmodified) household microwave oven equipped with a turntable was used (LG, MG-395 WA, 760 W) and operating at 2450 MHz.The melting points were determined by open capillaries and are uncorrected.Infrared (IR) spectra were recorded at room temperature from 4000 cm -1 to 400 cm -1 with KBr pellets at a resolution of 4 cm -1 , using Avatar 330 equipped with DTGS detector. 1H NMR and 13 C NMR spectra were recorded on a Bruker AV-500 MHz, spectrometer in DMSO-d 6 , CDCl 3 using TMS as internal standard and the values are expressed in ppm.Mass spectra were obtained by using HRMS.

Procedure for the preparation of compound 2,2'-[naphthalene-2,7-diylbis(oxy)]diaceto hydrazide (3)
Compound 2 (0.01 mole) in ethanol (20 mL) were stirred at room temperature for 20 min.To this mixture hydrazine hydrate (0.03 mol) was added and stirring continued for 15 more min.The resultant mixture was refluxed for 6 h and the separated solid was filtered using a sintered glass funnel.The residue was dried and then desiccated to afford a crystalline powder.The powder was recrystallized from chloroform/methanol and gave half white solid.Yield of the product was 88-90%.M.

Method B.
A mixture of 2,2'-(naphthalene-2,7-diylbis(oxy))diacetohydrazide 3 (0.01 mol), aryl/hetero aromatic aldehyde (0.02 mol) and catalytic amount of glacial acetic acid were taken in DMSO (2 mL) in a beaker and then the reaction mixture was subjected to microwave irradiation at an interval of 1 min at 180 W for about 1-2 min; progress of the reaction was monitored by TLC.After the completion of the reaction, the obtained product was poured into ice cold water stirred well and the solid separated was filtered, washed with excess of methanol and then dried at room temperature., J = 14.5 Hz, OCH2), 2.98 (q, 12H, J = 19.0Hz, CH3); 13

RESULTS AND DISCUSSION
In the present study, diethyl 2,2'-(naphthalene-2,7-diylbis(oxy))diacetate (2) was synthesized from 2,7-dihydroxy naphthalene 1 on reacting with ethyl chloroacetate in anhydrous potassium carbonate and dry DMF at 80ºC for 16 hours resulting in 82% yield (Scheme 1) [25].The IR spectrum of compound 2 exhibited absorption frequency at 1760 cm -1 for carbonyl group.In 1 H NMR of compound 2 the characteristic signals of an ester moiety confirm the presence of ester group in the structure by resonating as quartet and triplet for -CH 2 and -CH 3 at δ 4.31 ppm and δ 1.32 ppm respectively, which indicates acetate formation.Treatment of compound 2 with hydrazine hydrate in ethanol and refluxing for 6 hours resulted in the formation of 2,2'-(naphthalene-2,7-diylbis(oxy))diacetohydrazide 3. The structure of compound 3 was confirmed by IR, 1 H NMR and Mass spectra.IR spectrum showed the absence of ester stretching frequency, instead it gave band at 1699 cm -1 for carbonyl group and showed two sharp bands in the region of 3311 and 3206cm -1 due to -NH and -NH 2 groups.Compound 3 was treated with substituted aromatic aldehyde in presence of catalytic amount of acetic acid in (1:1) methanol/chloroform mixture and DMSO under reflux temperature and microwave irradiation to synthesise 2,2'-(naphthalene-2,7-diylbis(oxy))bis(N'-substituted acetohydrazides 4a-m.The IR spectra of the compound 4a revealed two strong absorption bands at 1629 cm -1 and 1691 cm -1 for C=O and C=N group respectively.The 1 H NMR spectra of compound (4a) displayed two doublets due to methylene protons at δ 5.25 & 4.77 ppm and the NH group was resonated as triplet at δ 11.61 ppm.The peaks of naphthalene aromatic and phenyl group protons appeared between 7.05-7.83ppm.The signals belonging to -N=CH protons were observed as doublets at δ 8.03 & 8.34 ppm, the disappearance of NH 2 signal of compound 3 in compound 4a indicates the functionalization of hydrazide to hydrazone.The structures of all the new Schiff base derivatives (4a-m) were confirmed by the spectral characterization using FT-IR, 1 H NMR, 13 C NMR and mass spectra.Shorter reaction times were observed for the formation of compounds 4a-m under microwave irradiation rather than the reflux temperature.Reaction time for the formation of compound 4a is one minute under microwave irradiation and 180 minutes under reflux temperature with slightly improved yields, respectively.Comparative yields of all the synthesized products with respect to reaction time and reflux temperature as well as microwave irradiation and are given in Table 1.

Scheme 1 .
Scheme 1. Synthetic pathway for the preparation of compounds 4a-m.

Table 1 .
Data of synthesized Schiff base compounds 4a-m.NMR spectrum of compound 3 exhibited no peak corresponding to ester, instead it showed signals at δ 9.41 ppm and δ 4.58 ppm for -NH and -NH 2 of hydrazide.

Table 2 .
. Antibacterial activities of the Schiff base compounds 4a-m.