Ethyl Vinyl Ether - An Agent for Protection of the Pyrazole NH-Fragment. A Convenient Method for the Preparation of N-Unsubstituted 4-Alkynylpyrazoles

Abstract

Abstract – N -Unsubstituted 4-iodopyrazole ( 1) is easily converted into 4-alkynylderivatives ( 5 ) in moderate to good overall yields by using intermediateprotection of the nitrogen atom of the pyrazole ring by ethyl vinyl ether. INTRODUCTION Acetylenic derivatives of pyrazoles are attractive synthons as well as promising biologically activecompounds. 1,2 Special interest is shown by pyrazolylacetylenes bearing a formyl group, which gives thepossibility to create stable radicals, such as nitronylnitroxides derived from 2-imidazolines. 3,4 We havereported the synthesis of some acetylenyl- N -alkylpyrazolylnitronylnitroxides, which showed interestingoptical and magnetic properties. 3,4 However, more promising for the design of high dimension molecularferromagnetics are N- unsubstituted pyrazolyl halides due to the possibility to use the NH-fragment, forinstance to form N–H···N aggregates in the solid state. From the various papers dealing with the synthesisof alkynylpyrazoles it may be concluded that the Pd-Cu-catalyzed cross-couplings of 1-alkynes withpyrazolyl halides is the most convenient method for synthesizing the above mentioned compounds.