1H and 13C NMR spectra of the rotational isomers of N‐hydroxymethylamides and derivatives

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Abstract
A series of N‐hydroxymethylamides, RCONR′CH2OH, and their O‐methyl and O‐acetyl derivatives, have been studied by 13C and 1H magnetic resonance spectroscopy. Signals have been assigned to the E‐ and Z‐isomers on the basis of the analysis of the fully coupled spectra, and by comparison of the chemical shifts with those of model compounds. The introduction of the hydroxy, alkoxy or acetoxy groups at the α‐position of the N‐alkyl moiety causes a significant shift in the equilibrium towards the E‐rotamer compared with the unsubstituted N‐alkylamide. The predominant effect in determining the E: Z ratio appears to be the steric interaction between the carbonyl oxygen and the α‐oxygen in the alkyl moiety; intramolecular hydrogen bonding does not play a significant role in determining the rotamer populations of these molecules.