Chemical characterization of decomposition products of pyrazolidinedione-3,5 derivatives. Part 2: Chemical transformations of 4-prenyl-1,2-diphenyl-3,5-pyrazolidinedione (prenazone) and its main decomposition product N,N'-diphenylhydrazide-2-carboxy-5-methyl-4-hexenoic acid in sulphuric acid concentrated.

4-Prenyl-1,2-diphenyl-3,5-pyrazolidinedione (prenazone) in the medium of sulphuric acid concentrated (92-98%) undergoes an intramolecular rearrangement to 6',6'-dimethyl-4',5'-dihydropyrane-[2',3'-d]-1,2-diphenyl-3-pyrazolinone. The main decomposition product of prenazone, formed due to the hydrolytic process in the above conditions, undergoes changes according to two parallel reactions. First, it is hydrolyzed to prenylmalonic acid and hydrazobenzene which is subsequently transformed to benzidine. The second reaction proceeds via cyclization to form prenazone as well as 6',6-dimethyl-4',5'-dihydropyrane [2',3'-d]-1,2-diphenyl-3-pyrazolinone.