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

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.