<i>N</i>‐Tosylhydrazones as <i>[N,N]</i> Synthons in Heterocyclic Chemistry: Synthesis of 3,5‐Disubstituted N‐Alkenyl‐1H‐pyrazoles

A new mode of reactivity of N‐sulfonylhydrazones towards alkynes is described: ketone‐derived N‐tosylhydrazones (NTHs) bearing α‐H atoms behave as [NN] synthons in their reactions with alkoxy alkynyl Fischer carbene complexes (FCCs) and only the two N atoms are incorporated into the newly formed pyrazole ring. The scope of the reaction, leading to 5‐methoxy‐N‐alkenyl pyrazoles, proved to be wide regarding both partners; thus, a variety of aryl and primary, secondary, and tertiary alkyl substituents are tolerated within the FCC, while aryl alkyl NTHs, symmetric and asymmetric dialkyl NTHs and NTHs derived of cyclic ketones can be also employed. In addition, the reaction can be performed at large scale and applied for late‐stage diversification of natural products. A reasonable reaction mechanism is also proposed, which is supported by deuteration experiments and DFT calculations.