In Situ Generation and [3 + 2] Annulation Reactions of PropiolaldehydeA Metal-Free, Cascade Route to Pyrazole and Bipyrazole Carboxaldehydes in One Pot

Propiolaldehyde, the most reactive
yet less explored
electrophilic
acetylene, was generated in situ via a base-free
Kornblum oxidation from propargyl tosylate and successfully intercepted
with hydrazones affording pyrazole-4-carboxaldehyde in one-pot by
a [3 + 2] annulation reaction. Further, the pyrazole-4-carboxaldehyde
endured a unique cascade reaction with phenylhydrazine and propargyl
tosylate, yielding synthetically challenging bipyrazole carboxaldehydes.
The method is free of any metal catalyst, base, or additives and offers
a convenient protocol to handle the reactive and volatile propiolaldehyde
under ambient conditions.