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Paper | Special issue | Vol 50, No. 2, 1999, pp.799-818
Published online, 1st January, 1970
DOI: 10.3987/COM-98-S(H)74
4-Acyl-5-methyl-2-phenylpyrazolones: NMR and X-Ray Structure Investigations

Wolfgang Holzer,* Kurt Mereiter,* and Brigitte Plagens

*Institute of Pharmaceutical Chemistry, University of Vienna, Pharmaziezentrum, Althanstrasse14, A-1090 Vienna, Austria


1H- and 13C-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-d6 solution, whereas in DMSO-d6 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.