Unexpected Tellurohalogenation of Terminal N‐Alkynyl (Alkenyl) Derivatives of 4‐Functionalized Pyrazoles

Abstract

Abstract The efficient synthesis of terminal N‐alkenyl and N‐alkynyl derivatives of pyrazole‐4‐carboxylic acids and their methyl esters as substrates for studying electrophilic heterocyclization has been reported. The regiochemistry and stereoselectivity of the tellurium‐induced heterocyclization of 1‐pentynyl (butynyl, butenyl) substituted pyrazole‐4‐carboxylic acids and their methyl esters under the action of tellurium (IV) oxide in hydrohalic acid were determined. This electrophilic heterocyclization leads to the formation of tetrahydropyrazolo[1,2‐a]pyridazinium and dihydropyrazolo[1,2‐a]pyrazolium intramolecular salts, as confirmed by X‐ray diffraction (XRD) and comprehensive spectral analysis.

Keywords

• Alkynyl(alkenyl)pyrazole
• Dihydropyrazolo[1.2-a]pyrazolium salts
• Electrophilic heterocyclization
• Tetrahydropyrazolo[1,2-a]pyridazinium salts
• Zwitterion