Stereochemistry of 2,4-dinitrophenylhydrazones of β,β-dichlorovinyl ketones

1. It was shown by the PMR method that 2,4-dinitrophenylhydrazones of β,β-dichlorovinyl ketones exist in the form of a mixture of two stereoisomers (syn- and anti-forms), the ratio of which is determined by the steric and electronic structure of the C-substituent. 2. The equilibrium content of the syn- and anti-isomers can be changed directedly under chemical and photochemical influences. 3. For the aldehyde derivative the S-cisoid conformations of the molecule predominates. The same conformations is evidently characteristic of alkyl ketone derivatives, while aryl ketones exist in a S-transoid form. 4. The stereochemical peculiarities found for the compounds studied explain their behavior in reactions of cyclization to pyrazoles.