Azoles are a large class of five-membered ring heterocyclic compounds containing at least one nitrogen atom. In particular, azoles and their derivatives have attracted increasing interest as versatile intermediates for the synthesis of biologically active compounds. In recent times the environmental impact has become very important in design of new synthetic methods in organic and medicinal chemistry. Therefore, the use of alternative activation factors like microwaves (MW), ultrasounds (US) or their cross-combination (SMUI - Simultaneous Microwave and Ultrasound Irradiation) became very promising and desirable synthetic methodologies in efficient and fast active structures formation. A comparative study between different activating factors for obtaining derivatives of pyrazole and isoxazole systems were described, including microwave irradiation, ultrasonic action and SMUI method and also classical reflux. Employing, the reactions of appropriate β-dicarbonyl compounds (e.g. 2,4-pentadione, ethyl l,3-oxobutanoate and their alkyl derivatives) respectively with hydrazine or hydroxylamine went to completion in 5-10 minutes and afforded the heterocyclic products in good yields. Based on good results of the above reactions, the next β-dicarbonyl structure modified in to heterocyclic derivatives under SMUI condition was curcumin, natural compound that is a candidate for therapeutic use. The pyrazole and isoxazole heterocyclic derivatives of curcumin were obtained in short time 10-20 minutes with good yield. SMUI was found to be the most efficient activating factor, followed by microwave and ultrasonic mode. Both microwave and ultrasonic factors promoted the heterocyclization reaction but the use of SMUI proved to be fundamental for reducing the reaction time.
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Synthesis of selected azoles derivatives using the cross-combination of microwave and ultrasound factors
Published: 03 November 2017 by MDPI in The 21st International Electronic Conference on Synthetic Organic Chemistry session Symposium on Microwave Assisted Synthesis
Keywords: SMUI, microwaves, ultrasounds, azoles, pyrazole, isoxazole, β-dicarbonyl compounds, curcumin