Efficient Synthesis of Dihydropyrazoles by Halocyclization of β,γ‐Unsaturated Hydrazones

An efficient halocyclization of β,γ‐unsaturated hydrazones with N ‐bromosuccinimide was developed without the addition of any additives under mild reaction conditions to provide facile access to biologically important 4,5‐dihydropyrazoles. Under the optimized conditions, a variety of highly substituted 4,5‐dihydropyrazole derivatives were obtained in generally good yields. Moreover, this reaction can be further applied to the synthesis of pyrazoles in a one‐pot fashion.