Unusual rearrangement in the reactions of phenylmalonic acid dihydrazide with 1, 3-Diketones

Phenylmalonic acid dihydrazide reacted with 2,4-pentanedione to give, unexpectedly, 5,7-dimethyl-1,3-dioxo-2-phenyl-2,3-dihydro-1H-pyrazolo[1,2-a]pyrazole-4-ylium 5. The structure of the product is confirmed by X-ray crystallography.