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Reaction of hydrazones derived from ketones bearing an acceptor substituent adjacent to the carbonyl group (α,α,α-trifluoroacetone and ethyl pyruvate) with Vilsmeier-Haack reagent was studied. In most cases, the method allows for regioselective preparation of 1,3-disubstituted pyrazole-4-carbaldehydes – the products of initial  C -electrophilic attack, although in one case, the product resulting from concurrent  N - and  C -attacks of the electrophile at the hydrazone moiety is observed. Under analogous conditions, the reaction of  N -arylhydrazone derived from butanedione leads to the formation of 3-chloro-3-(1-arylpyrazol-3-yl)acrylaldehyde – the product of double formylation at both ketone and hydrazone moieties of the starting material.

Reaction of hydrazones derived from electron-deficient ketones with Vilsmeier-Haack reagent