4-acyl-5-methyl-2-phenylpyrazolones: NMR and X-ray structure investigations

H-1- and C-13-NMR investigations with 4-acyl-5-methyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-ones (1-6) are presented, indicating these compounds to exist predominantely as hydroxypyrazoles in CDCl3 or benzene-d(6) solution, whereas in DMSO-d(6) also a considerable amount of NH tautomer is present. X-Ray crystal analyses revealed that in the solid state the 4-propionyl compound (2) is present as hydroxypyrazole, the 4-(2-thienyl) derivative (6) as NH isomer and the 4-cinnamoyl product (4) to have an exocyclic double bond structure stabilized by an intramolecular hydrogen bond. Cyclisation of the latter compound (4) in acidic medium leads to the formation of 3-methyl-1,6-diphenyl-5,6-dihydro-1H-pyrano[2,3-c]pyrazol-4-one (7) in very low yields.