Aza‐enamines, VIII. — Electrophilic Substitution Reactions at the Azomethine C‐Atom of Aldehyde Dialkylhydrazones: Vilsmeier Formylation and Consecutive Reactions The reaction of hydrazones 1 with the Vilsmeier reagent yields 3‐phenyl‐1,4,5‐triaza‐1,3‐pentadienium salts according to their aza‐enamine character. Hydrolysis of 2 gives 1‐phenylglyoxal 1‐dialkylhydrazones 3, which rearrange in acidic media to 1‐phenylglyoxal 2‐dialkylhydrazones 4. Compound 3h forms the dihydropyrroloimidazole 5 in boiling ethanol. Pyrrolotriazinium salt 6 is obtained by the reaction of 1b with the isolated Vilsmeier reagent from dimethylformamide/oxalyl chloride.
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R. Brehme (1990) studied this question.