Pyrazoles and pyridazines from hydrazine and dicarbonyl compounds
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص767-768
2025-07-15
562
Pyrazoles and pyridazines from hydrazine and dicarbonyl compounds
Disconnection of pyridazines reveals a molecule of hydrazine and a 1,4-diketone with the proviso that, just as with pyridines, the product will be a dihydropyrazine and oxidation will be needed to give the aromatic compound. As with pyridines, we prefer to avoid the cis double bond problem.
As an example we can take the cotton herbicide made by Cyanamid. Direct removal of hydrazine would require a problematic cis double bond in the starting material.
If we remove the double bond fi rst, a much simpler compound emerges. Note that this is a ketoester rather than a diketone.
When hydrazine is added to the keto-ester an imine is formed with the ketone but acylation occurs at the ester end to give an amide rather than the imino-ester we had designed.
Aromatization with bromine gives the aromatic pyridazolone by bromination and dehydro bromination, and now we invoke the nucleophilic substitution reactions introduced. First we make the chloride with POCl3 and then displace with methanol.
The five-membered ring pyrazoles are even simpler as the starting material is a 1,3-di carbonyl compound available from the aldol or Claisen ester condensations.
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