13C Nuclear Magnetic Resonance Studies. 34. The 13C Spectra of Several Methylcyclopentanones and -cyclohexanones

Abstract

The ¹³C spectra of 16 methyl substituted cyclohexanones and 15 methylcyclopentanones have been determined. These series were chosen as model systems for the study of steric and conformation effects on ¹³C shieldings. Complete assignments of the individual signals were accomplished by intercomparison of the shielding data within each series and the trends observed are readily interpreted in conformational terms. Each of the cyclohexanones exists preferentially in chair conformations although there is evidence of ring distortion in the tetramethyl derivatives. The cyclopentanones apparently strongly favor half-chair forms with maximum puckering at C-3 and C-4. In general, shielding differences between cis and trans isomers are pronounced and the assignment of stereochemistry for some cis–trans dimethyl derivatives is illustrated.Further examples of marked deshielding trends associated with sterically crowded δ nuclei in syn-axial arrangements are described.