Abstract
The dialkyl 2,6-dialkylsubstituted 4-piperidone-3,5-dicarboxylates were synthesized by a Mannich procedure. Depending on the substitution at the nitrogen keto-enol-tautomerism and a configurational isomerism at C 2 is observed. The structure of the N-substituted piperidone 24 E (C18H29NO5) has been determined by X-ray analysis: it is characterized by an enol structure of the β-ketoester and an axial position of the alkyl group at C 2 and an equatorial one of the alkyl group at C 6. The O–H···O hydrogen bond shows characteristic values of a strong hydrogen bond. The N-unsubstituted piperidones adopt a ketone structure with the allequatorial position of all substituents. This stereochemistry is confirmed for other enolic and ketone analogues by NMR spectroscopic methods. To work out the reason for the different thermodynamic stabilities of the different stereochemical structures of N-substituted and N-unsubstituted piperidones, various semiempirical calculations were done for series of simple pairs of carbonyl/enol tautomers, substituted acetoacetates, oxoglutarate as well as of systematically varied substituted cyclohexanone, 4-oxacyclohexanone and 4-piperidone derivatives.
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