One-Pot Tandem Synthesis of Tetrasubstituted Pyrazoles via 1,3-Dipolar Cycloaddition Between Aryl Hydrazones and Ethyl But-2-ynoate

Abstract

1,3,4,5-Tetrasubstituted pyrazoles are rapidly and regioselectively synthesized in a one-pot, three-step sequence consisting of condensation, nitrilimine generation, and cycloaddition using mercuric acetate. Newly synthesized compounds were characterized by spectral studies. Regiochemistry of compounds 6a and 8a was determined as 1,4- and 1,5-regioisomers respectively by X-ray crystallography.

GRAPHICAL ABSTRACT

Keywords:

• 1,3-Dipolar cycloaddition
• ethyl but-2-ynoate
• regioselective
• tetrasubstituted pyrazoles
• X-ray crystallography

ACKNOWLEDGMENTS

The authors are thankful to K. Yeshoda Nanjappa, assistant professor of English, Yuvaraja's College, University of Mysore (UOM), Mysore, for proofreading the article and to the principal, Yuvaraja's College, UOM, Mysore, for providing necessary facilities and constant encouragement.

Notes

^a Isolated yields; NR, no reaction.

Note. All reactions were carried out in 1-mmol scale of reactants in 5 mL of EtOH.

^a Reaction conditions: benzaldehyde 1 1.0 mmol, phenylhydrazine 2 1.1 mmol, ethyl but-2-ynoate 5 1.5 mmol, and EtOH 5 mL.

^b Isolated yield.

^a Reaction conditions: benzaldehyde 1 1.0 mmol, phenylhydrazine 2 1.1 mmol, ethyl but-2-ynoate 5 1.5 mmol, and EtOH 5 mL.

^b The ratio of 6:8 was determined by the isolated yields of 6 and 8.

^c Isolated yields.

^d No isomer observed.

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