Einfluss der A-spannung auf die stereochemie von oximen und semicarbazonen

In piperidone-(4)-oximes, the A^1,3 strain between the substituent at the exocyclic double bond and a Me group causes the sole formation of anti-isomers. A change of the ring conformation avoids the A^1,3 strain in 1-methyl-2,6-diphenyl-3,5-dimethyl-piperidone-(4)-oxime. trans-Configuration of the Me substituents at C-3 and C-5 can be shown in a correspondingly substituted semicarbazone; in this example, the A^1,3 strain is avoided by epimerization. This semicarbazone can be transformed into the 3,5-trans-disubstituted piperidone by cerium(IV) oxidation.