Chemo-, Regio-, and Stereoselective N-Alkenylation of Pyrazoles/Benzpyrazoles Using Activated and Unactivated Alkynes.

Abstract

Transition-metal-free chemo-, regio-, and stereoselective synthesis of (Z) and (E) styryl pyrazoles and benzpyrazoles by the addition of N-heterocycles onto functionalized terminal and internal alkynes using a super basic solution of KOH/DMSO has been described. The stereochemical outcome of the reaction was governed by time and quantity of the base. The reaction of pyrazoles and benzpyrazoles onto alkynes takes place chemoselectively without affecting the free -NH2 group of pyrazoles and -OH group of alkynes. The designed protocol was well implemented on alkynes bearing long alkyl chain, an alicyclic ring, hydroxy, ether, and ester functionality, which offer the N-alkenylated products in good yields. This developed methodology also provides easy access for the synthesis of bis-vinylated heterocycles. The presence of free -NH2, -OH, -COOR, and halo group in styryl pyrazoles, could be further utilized for synthetic elaboration, which is advantageous for biological evaluation. For the first time, we have disclosed the base-mediated conversion of (Z)-styryl pyrazoles to (E)-styryl pyrazoles in KOH/DMSO system. The cis-trans isomerization was supported by the control experiments and deuterium labeling studies.