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Reaction of diketene with N-substituted hydrazones of aldehydes and sterically crowded ketones leads to the corresponding N-acetoacetylhydrazones. With hydrazones of other ketones, this reaction yields cyclic isomers of N-acetoacetylhydrazones, namely, 2,5,5trisubstituted 4-acetylpyrazolidin-3-ones. The products where characterized by H and C NMR and electron ionization (EI) mass spectrometry. The EI mass spectra of the latter compounds and those of linear N-acetoacetylhydrazones show that in the gas phase their molecular ions attain predominantly similar forms as the respective neutral compounds in solutions. The mass-spectrometric fragmentation of both forms is characterized by retrosynthetic cleavages with the loss of neutral acetylketene. The imines produced from 2,5,5trisubstituted 4-acetyl-3-pyrazoli-dinones and primary amines predominantly exist in solutions in the enamine form. The EI mass spectra of these condensation products show that in the gas phase their molecular ions exist as pyrazolidinones. Characteristic fragmentation paths under the EI conditions include the pyrazolidine ring cleavages at the N1-C5 and C3-C4 bonds, with the positive charge mainly localized in the enamine moiety. There is no evidence for ring-chain isomerism of the compounds studied.

Synthesis and structural characterization of N-acetoacetylhydrazones and 2,5,5-trisubstituted 4-acetyl-3-pyrazolidinones