Phosphonyl hydrazones react with DMF/POC13 to afford 3-phosphonyl pyrazoles. Phosphonyl methylene hydrazones react with DMF/POC13 to provide 4-phosphonyl pyrazoles. 5-Phosphonyl pyrazoles are obtained from the reaction of phosphonyl chlorovinylaldehydes with phenylhydrazine.
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Chen et al. (1999) studied this question.