Irradiation of Tungsten Light: A Useful Energy Source for Synthesis of 4,5-Dihydro-pyrazole-1-carbaldehyde Derivatives

In the present communication, reported the easy, convenient route for the synthesis of N-formyl pyrazolines from α,β-unsaturated ketone. The aqueous 2-methoxyethanol was used as reaction media for the synthesis of titled compounds. All synthesized compounds were confirmed on the basis of IR, 1H NMR, 13C NMR and CHN analysis.

The three components one step synthesis has great current interest towards development of novel synthetic organic compounds. One of the three component one step reaction involve, synthesis of Nformyl pyrazolines in which α,β-unsaturated ketone reacts with hydrazine hydrate and formic acid [12][13][14]. In view of these observations and in continuation of earlier research work on synthesis of substituted 4,5-dihydro-pyrazole-1-carbaldehyde derivative [15][16][17], herein first time reported the synthesis of titled compound using irradiation of tungsten light as a useful energy in organic synthesis.

Chemical
All chemicals, solvents and reagents used in the present study were of analytical grade purchased from Sigma, SD Fine, or Spectrochem.

Instrumentation
Melting points were determined in an open capillary tube and are uncorrected. The reactions were carried out in aqueous 2-methoxyethanol solvent (10 mL:10 mL, v/v) using 200 watt tungsten bulb light. Purification of the compounds was indicated using TLC (mixture of ethyl acetate and hexane, 0.20 mL:0.20 mL, vv). IR spectra were recorded in KBr pellets on a Perkin-Elmer FT-IR Shimadzu spectrometer. 1 H and 13 C NMR spectra were obtained in DMSO-d6 on Avance 300 MHz spectrometer using TMS as an internal standard. The mass spectra were recorded on EI-Shimadzu-GC-MS spectrometer. Elemental analyses were performed on a Carlo Erba 106 Perkin-Elmer model 240 analyzer.
General Procedure for synthesis of 4,5-dihydropyrazole-1-carbaldehyde derivative: In 50 mL beaker, a mixture of α,β-unsaturated ketones 1 (0.01 mol), hydrazines hydrate 2 (0.01mmol) and formic acid 3 (0.01 mmol) was dissolved in aqueous 2-methoxyethanol (20 mL) by warming. To this hot reaction solution acetic acid (0.001 mmol) was added and irradiated under tungsten light (200 Watt) for 40-54 min, and progress of reaction was monitored on TLC. After completion of reaction, the resultant mixture was poured with stirring into water (20 mL). The precipitate formed was filtered through simple büchner funnel under vacuum pressure and crystallized from ethanol to yield 2-pyrazolines (Scheme 1).

RESULTS AND DISCUSSION
Recently we reported the synthesis of 2pyrazolines from 2'-hydroxychalcones in presence of catalytic amount of acetic acid under irradiation of tungsten light [18]. In view these observations we tried to implement the same reaction condition for cyclisation of α,β-unsaturated ketone to afford Nformyl pyrazolines. Initially we attempted the condensation of 1,5-Bis-(4-fluoro-phenyl)-penta-1,4dien-3-one with hydrazine hydrate and formic acid using acetic acid in aqueous 2-methoxyethanol in combination with irradiation of tungsten light. The reaction went to completion within 40 min and corresponding product 4a obtained in 88% yield (Table 1).
In order to optimize the reaction conditions, we made comparison between by carried out the above reaction through conventional pathway, and we found that the method using irradiation of tungsten light is efficient in terms of clean reaction conditions, not expensive, yields and environmentally ecofriendly.

CONCLUSION
In summary, present communication reported the synthesis of 4,5-dihydro-pyrazole-1-carbaldehyde derivatives starting from α,β-unsaturated ketone in aqueous 2-methoxyethanol under irradiation of tungsten light. The advantage of methods in comparison with classical synthesis, which includes clean reaction procedure, easy isolation of products, short reaction time and no need of special apparatus device.

ACKNOWLEDMENTS
Author is very thankful to Director, Indian Institute of Chemical Technology, Hyderabad for providing spectral analysis.