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

New tellurium-containing tetrahydropyrazolo[1,2-a]pyridazines and dihydropyrazolo[1,2-a]pyrazoles were obtained by electrophilic heterocyclization in the form of intramolecular salts, and optimal conditions for their synthesis were found. The heterocyclization process is regio- and stereoselective, which is confirmed by spectral studies, and by X-ray structural analysis. The mechanism of tellurium-induced heterocyclization is proposed.

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.

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