Pyrazole-3-carbaldehyde hydrazones

Hydrazones (166) react with chloromethyleneiminium salt (I) giving, after hydrolysis, aldehydes (167 Scheme 12). In this transformation the hydrazones (166) are clearly behaving as aza-enamines. - Hydrazones (168) and (169) react with chloromethyleneiminium salt (1) at both the methyl group and at nitrogen to yield the corresponding iminium salts (170) and (171), which cyclize with loss of dimethylamine to provide iminium salts (172) and (173), respectively. Hydrolysis of these salts (172) and (173) affords pyrazole-4-carbaldehyde derivatives (174) and (175 72-96% Scheme 13). Pr uct (174) can also be prepared from semicaibazone (176). 

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