Microwave-Assisted Solution Phase Synthesis of Novel 2-{ 4-[ 2-( N-Methyl-2-pyridylamino ) ethoxy ] phenyl }-5-Substituted 1 , 3 , 4-Oxadiazole Library

A new series of 2-{4-[2-(N-methyl-2-pyridylamino)ethoxy]phenyl}-5-substituted 1,3,4-oxadiazoles were synthesized by the oxidative cyclization of hydrazones derived from 4-[2-(methylpyridin-2-ylamino)ethoxy]benzaldehyde and aryl hydrazines using chloramine-T as an efficient catalyst. All steps were assisted by microwave irradiation. Microwave enables all these reactions to be simple, rapid, high yielding, and avoid chromatograph purification and led environmentally benign total synthesis of focused oxadiazole library. All the synthesized compounds were isolated in good yield and characterized by 1H NMR, 13C NMR, and elemental analyses. The title compounds represent a novel class of biologically active heterocycles.

Meanwhile, microwave-assisted organic synthesis is a rapidly growing field in organic chemistry [12][13][14].In the early days, domestic microwaves were used with poor reproducibility.Moreover, accidents were common due to the lack of control.The introduction of dedicated equipment by CEM, milestone, biotage, plazmatronika, Anton Paar and domestic companies Shikoku Instrumentation, EYELA and so on allowing for the online monitoring of temperature, power and pressure had a large impact on the further development of this research field [15].In particular, the pharmaceutical industry requires the production of a higher number of novel chemical entities, which requires chemists to employ a number of resources to reduce the time for the production of compounds.In general, most organic reactions have been heated using traditional heat transfer equipment such as oil baths, sand baths, and heating jackets.These heating techniques are, however, rather slow, and a temperature gradient can develop within the sample.In addition, local overheating can lead to product, substrate, and reagent decomposition.Microwave radiation provides an alternate to conventional heating as it utilizes the ability of liquids or solids to transform electromagnetic energy into heat.

Results and Discussions
All the reactions were carried out in CEM Discover microwave apparatus.Acid hydrazides 3 (a-t) were synthesized from corresponding acids by microwave irradiation for 20-50 min in the yield range 60-88%.Alcohol 6 was synthesized in 92% yield in 10 min at 140 • C, which was converted to aldehyde 7 in 20 min at 100 • C to yield 90%. Aldehyde 7 was converted to hydrazones 8 (a-t) in 20-30 min to yield 70-85%.Hydrazones 8 (a-t) were oxidatively cyclised under microwave irradiation in 20-50 min to gave title compounds 9 (a-t).Chloramine-T [19,20] was used as an efficient reagent for the oxidative cyclization of aroyl hydrazones 8 (a-t) to give 1,3,4-oxadiazoles 9 (a-t) in good yield and purity, without column purification in any stage (Table 1).However, by conventional method, the reaction takes 6-10 h heating and results in impure product due to side reactions, which needs column purification or further crystallization, and yields are generally low.Though there are several reagents for cycloaddition, chloramine-T is found to be effective reagent since it is mild, easy to handle, commercially available, nontoxic, and water tolerant.All the synthesized 1,3,4-oxadiazoles were characterized by 1 H NMR 13 C NMR and elemental analyses. 1HNMR of all the synthesized oxadiazoles showed singlet in the region δ 3.0-3.4due to -NCH 3 group, and two triplets in the region δ   3.9-4.2and 4.1-4.4 are due to two methylene groups.The aromatic protons and other protons are observed in the expected region. 13CNMR of all the synthesized oxadiazoles showed peaks in the expected region.

Conclusion
In conclusion, we have developed an efficient microwaveassisted method for the synthesis of 2,5-disubstituted 1,3,4oxadiazoles.Chloramine-T was used as an efficient catalyst for cycloaddition under microwave irradiation.The method is quicker, high yielding, environment friendly, and cleaner chemistries.The isolated products are very pure and do not need any column purification.The method is very useful for combinatorial synthesis new libraries in drug discovery.
The full paper and biological evaluation will be published elsewhere in future.

General Procedure for the Synthesis of Aroyl Hydrazines 3 (a-t).
A solution of respective aromatic acid 1 (a-t) (0.5 g), ethanol (2 mL), and catalytic amount of conc.H 2 SO 4 were inserted into CEM Discover microwave vessel, sealed and irradiated microwave at 100 • C for 10-20 min.After the completion of the reaction by TLC (chloroform-ethyl acetate: 1 : 1), ethanol was removed and the product was extracted with dichloromethane, washed with dil.NaHCO 3 water and dried.Solvent was evaporated and the residue was dissolved in ethanol (2 mL), and 98% hydrazine hydrate (0.5 mL) was added.The mixture was irradiated at 120 • C for 20-30 min.After the completion of the reaction by TLC (toluene/ethyl acetate/DEA, 7.5 : 2.5 : 1), the reaction mixture was cooled, and the solid formed was filtered and washed with chilled ethanol (1 mL) to get corresponding aroyl hydrazine 3 (a-t).

2-(Methyl-pyridin-2-ylamino)ethanol 6.
A mixture of 2-chloropyridine 4 (1.00 g, 8.81 mmol) and 2-(N-methylamino)ethanol 5 (1.33 g, 17.7 mmol) were introduced into a CEM discover vessel equipped with a magnetic stirrer.The vessel was sealed, and the reaction mixture was irradiated microwave (300 W) for 10 min at 140 • C twice successively.The completion of the reaction was monitored by TLC (Toluene-ethyl acetate, 1 : 1).The reaction mass was cooled at room temperature, diluted with 2 mL water, and extracted into 10 mL ethyl acetate twice.The combined ethyl acetate layer was washed with water, saturated brine, dried over anhy.Na 2 SO 4 and concentrated in vacuo to give 6 (1.23 g, 92%) as a pale yellow oil.

General Procedure for the Synthesis of Aroyl Hydrazones 8 (a-t).
An equimolar mixture of aroyl hydrazine 3 (a-t) and 4-[2-(methyl-pyridin-2-ylamino)ethoxy]benzaldehyde 7 in isopropyl alcohol (4 mL) were inserted into CEM microwave vessel and irradiated for 20-50 min at 100 • C. The progress of the reaction was monitored by TLC (Chloroform-ethyl acetate, 1 : 1).After the completion of the reaction, the mass was cooled and the solid formed was filtered to give aroyl hydrazones 8 (a-t) to yield 70-90%, which were directly used for next stage.