Microwave-Assisted Synthesis of Some 3,5-Arylated 2-Pyrazolines

Condensation of 2-acetylnaphthalene with benzaldehydes under microwave irradiation affords chalcones which undergo facile and clean cyclizations with hydrazines RNHNH2 (R= H, Ph, Ac) to afford 3,5-arylated 2-pyrazolines in quantitative yields, also under microwave irradiation and in the presence of dry AcOH as cyclizing agent. The results obtained indicate that, unlike classical heating, microwave irradiation results in higher yields, shorter reaction times (2-12 min.) and cleaner reactions.


Results and Discussion
As a result of our studies related to the development of synthetic protocols using microwave irradiation, we now report a novel and easy access to 3,5-arylated 2-pyrazolines using a one-pot procedure and demonstrate its superiority over our previously reported classical heating method [8]. We report in this paper some aldol condensation reactions between 2-acetylnaphthalene (1) and benzaldehydes 2a-e in the presence of KOC 2 H 5 /C 2 H 5 OH to give intermediate chalcones 3a-e which undergo a rapid cyclization with hydrazines 4f-h under microwave irradiation at 80 º C to yield 2-pyrazolines 5af-eh quantitatively in 2-12 minutes (Scheme 1). The heterocyclic products were characterized on the basis of their IR, 1 H-NMR, 13 C-NMR, MS spectral and elemental analysis (

Conclusions
In summary, this work demonstrates a rapid, efficient and environmentally friendly method of synthesis of 3,5-arylated 2-pyrazolines under microwave heating, and the results obtained confirm the superiority of the microwave irradiation method over the classical heating one.

Acknowledgements
The authors wish to acknowledge the financial support of the Bu-Ali Sina University, Chemistry Department, to carry out this research.

General
All melting points were determined on a Büchi 530 melting point apparatus, and are uncorrected. The 1 H-NMR spectra were recorded for deuteriochloroform solutions using tetramethylsilane as the internal standard on Jeol FX (at 90 MHz) and Brüker AM (at 200 MHz) spectrometers at ambient temperature. IR spectra were recorded on a Shimadzu IR-435U-04 instrument using potassium bromide pellets. Elemental analyses were performed at the Iran Polymer Research Center, Karaj, Iran.
In the first step the chalcones 3a-e are prepared by the reaction between 2-acetylnaphthalene (1.70 g, 10 mmol), dissolved in 1M KOC 2 H 5 /C 2 H 5 OH solution (10 mL), and the corresponding benzaldehyde (10 mmol) under microwave irradiation (300 watt) for a few min. The resulting crude yellow solid is filtered, washed successively with dilute HCl solution and distilled water and finally recrystallized from ethanol (95%) to give the pure chalcones 3a-e in 80-95 % yield. In the second step, the hydrazine reagent (12 mmol) is then added dropwise to a stirring solution of the chalcones 3a-e (10 mmol) in glacial AcOH (10 mL). The mixture is subjected to microwave heating for several min. using a domestic microwave oven (300 watt) to afford 2-pyrazolines, which were recrystallized from ethanol (95%) to give pure compounds 5af-eh with a 82-99% yield. Analytical data for the prepared compounds is given in Table 1. Spectroscopic data is summarized in Table 2.