Axial/equatorial populations in alpha-hetero-substituted cyclohexanone oximes and O-methyl oximes

Axial equatorial populations were determined for (E)-2-X-cyclohexanone oximes and O-methyl oxime ethers in chloroform by the Eliel method [X = F, Cl, Br, OCH3, N(CH3)(2), SCH3]. A novel approach is presented, which uses H-1 NMR data from the protons bonded to C-6. The conformational proportions were also obtained from the C-4 chemical shifts, the Z-isomer spectral parameters being taken as reference for calculation. For both series, all substituents adopt preferentially the axial conformation (86-96%), but the O-methyl oxime ethers present an enhanced axial population compared with the corresponding oximes, owing to the accepted occurrence of a pi(C=NOH)/sigma(CX)* stabilizing interaction. Copyright (C) 2000 John Wiley & Sons, Ltd.38862763