Comparison of Conventional and Microwave Synthesis of Phenyl-1H-pyrazoles and Phenyl-1H-pyrazoles-4-carboxylic Acid Derivatives.

Background: Privileged scaffolds are of high importance for molecules containing the pyrazole subunit due to their broad spectrum of pharmacological activities. For this reason, a method that is more efficient needs to be developed for the preparation of pyrazole derivatives.

Objective: The purpose of this study was the optimisation of the conventional synthesis of the pyrazole ring and the oxidation of phenyl-1H-pyrazole-4-carbaldehyde to phenyl-1H-pyrazole-4-carboxylic acid through Microwave- Assisted Organic Synthesis (MAOS).

Methods: We performed a comparison between conventional synthesis and conventional synthesis with microwave heating using the synthesis method of pyrazole ring described by Finar and Godfrey and for the oxidation of phenyl-1H-pyrazole-4-carbaldehyde, the method described by Shriner and Kleiderer was used.

Results: MAOS reduces the reaction time to obtain all compounds compared to conventional heating. At a temperature of 60°C, 5 minutes of reaction time, and power of 50 W, the yield of phenyl-1H-pyrazoles (3a-m) compounds was in the range of 91 - 98% using MAOS, which is better than conventional heating (72 - 90%, 75ºC, 2 hours). An improvement in the yield for the oxidation reaction was also achieved with MAOS. The compounds (5a-m) were obtained with yields ranging from 62 - 92% (80ºC, 2 minutes, 150 W), while the yields with conventional heating were in the range of 48 - 85% (80ºC, 1 hour). The 26 compounds were achieved through an easy work-up procedure with no chromatographic separation. The pure products were characterised by the spectral data obtained from IR, MS, H and C NMR or HSQC/HMBC techniques.

Conclusion: The advantages of MAOS include short reaction time and increased yield, due to which it is an attractive option for pyrazole compounds synthesis.